gsm and map info by b050653

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									Planning & BSS

Submitted By:Saurabh Mehra & Aniket Varadpande GET „09

Air interface
The Air Interface carries the Radio Waves.The Um interface is the interface between the MS and the BTS. Voice is modulated on a radio frequency carrier and transmitted on the Air Interface. The frequency used in GSM is in UHF range i.e. 30 -3000 MHz. Ultra high frequency radio waves are typically generated by oscillating charges on a transmitting antenna. In the case of a radio station, the antenna is often simply a long wire (a dipole) fed by an alternating voltage/current source, that is, charge is placed on the antenna by the alternating voltage source. We can think of the electric field as being disturbances sent out by the dipole source and the frequency of the oscillating electric field (the electromagnetic wave) is the same as the frequency of the source. Each antenna has a unique radiation pattern. This pattern can be represented graphically by plotting the received time-averaged power, as a function of angle with respect to the direction of maximum power in a log-polar diagram. The pattern is representative of the performance of the antenna in a test environment. However, it only applies to the freespace environment in which the test measurement takes place. Upon installation, the pattern becomes more complex, due to the extra factors affecting propagation under field conditions. Thus, the real effectiveness of any antenna is measured in the field.

Antenna Basics An antenna is a device that is made to efficiently radiate and receive radiated electromagnetic waves. There are several important antenna characteristics that should be considered when choosing an antenna for your application as follows: • Antenna radiation patterns • Power Gain • Directivity • Polarization

Antenna Radiation Patterns An antenna radiation pattern is a 3-D plot of its radiation far from the source. Antenna radiation patterns usually take two forms, the elevation pattern and the azimuth pattern. The elevation pattern is a graph of the energy radiated from the antenna looking at it from the side as can be seen in Figure (a) . The azimuth pattern is a graph of the energy radiated from the antenna as if you were looking at it from directly above the antenna as shown in fig (b). When you combine the two graphs you have a 3-D representation of how energy is radiated from the antenna as shown in fig (c)

Power Gain The power gain of an antenna is a ratio of the power input to the antenna to the power output from the antenna. This gain is most often referred to with the units of dBi, which is logarithmic gain relative to an isotropic antenna. An isotropic antenna has a perfect spherical radiation pattern and a linear gain of one. Gain (with reference to the isotropic radiator dBi) = Gain (with reference to /2-Dipole dBd) + 2.15 dB

Directivity The directive gain of an antenna is a measure of the concentration of the radiated power in a particular direction. It may be regarded as the ability of the antenna to direct radiated power in a given direction. It is usually a ratio of radiation intensity in a given direction to the average radiation intensity.

Polarization Polarization is the orientation of electromagnetic waves far from the source. There are several types of polarization that apply to antennas. They are Linear, which comprises, Vertical, Horizontal and Oblique, and circular, which comprises, Circular Right Hand (RHCP); Circular Left Hand (LHCP), Elliptical Right Hand and Elliptical Left Hand. Polarization is most important to get the maximum performance from the antennas. For best performance the polarization of the transmitting antenna should be matched to that of the receiving antenna. Half-Power-Beam-Width This term defines the aperture of the antenna. The HPBW is defined by the points in the horizontal and vertical diagram, which show where the radiated power has reached half the amplitude of the main radiation direction. These points are also called 3 dB points.

VSWR An impedance of exactly 75 Ohm can only be practically achieved at one frequency. The power delivered from the transmitter can no longer be radiated without loss because of this incorrect compensation. Part of this power is reflected at the antenna and is returned to the transmitter the forward and return power forms a standing wave with corresponding voltage minima and maxima (Umin/Umax). This wave ratio (Voltage Standing Wave Ratio) defines the level of compensation of the antenna. A VSWR of 1.5 is standard within mobile communications. In this case the real component of the complex impedance may vary between the following values: Maximum Value: 50 Ohms x 1.5 = 75 Ohms

Minimum Value: 50 Ohms : 1,5 = 33 Ohms VSWR= [1+ (Reflection Coefficient)]/[1-(Reflection Coefficient)] Since Reflection Coefficient is the magnitude of ratio of the V(Reflected)/V(Transmitted) , its value is always>= 0. The above implies that VSWR is always >= 1 Ideally, VSWR should be = 1, when Reflection Coeffient is equal to 0, i.e. no signal is being reflected which is practically not possible.

Different antennas and their comparison:

Antenna downtilting The problem often faced is that the base station antenna provides an overcoverage. If the overlapping area between two cells is too large, increased switching between the base station (handover) occurs.There may even be interference of a neighbouring cell with the same frequency. Downtilting the antenna limits the range by reducing the field strength in the horizon. Antenna downtilting is the downward tilt of the vertical pattern towards the ground by a fixed angle measured w.r.t the horizon.with appropriate downtilt, the received signal strength within the cell improves due to the placement of the main lobe within the cell radius and falls off in regions approaching the cell boundary and towards the reuse cell. There are two methods of downtilting

– –

Mechanical downtilting Electrical downtilting.

Mechanical downtilting It consists of physically rotating an antenna downward about an axis from its vertical position. In a mechanical downtilt as the front lobe moves downward the back lobe moves upwards. This is one of the potential drawback as compared to the electrical downtilt because coverage behind the antenna can be negatively affected as the back lobe rises above the horizon. Additionally , mechanical downtilt does not change the gain of the antenna at +/- 90deg from antenna horizon.

Electrical Downtilt Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards. This allows the the antenna to be mounted vertically. Electrical downtilt affects both front and back lobes. If the front lobe is downtilted the back lobe is also downtilted by equal amount. Electrical downtilting also reduces the gain equally at all angles on the horizon. .

Antenna diversity In a typical cellular radio environment, the communication between the cell site and mobile is not by a direct radio path but via many paths. Hence the signal that arrives at the receiver is either by reflection from the flat sides of buildings or by diffraction around man made or natural obstructions. When various incoming radiowaves arrive at the receiver antenna, they combine constructively or destructively, which leads to a rapid variation in signal strength. These signal fluctuations are known as „multipath fading‟. Multipath fading causes rapid changes in signal strength over a short distance or time,random frequency modulation due to Doppler Shifts on different multipath signals and time dispersion caused by multipath delays.

Diversity techniques have been recognised as an effective means which enhances the immunity of the communication system to the multipath fading. GSM therefore extensively adopts diversity techniques that include 1. 2. 3. 4. interleaving in time domain frequency hopping in frequency domain spatial diversity in spatial donmain polarization diversity in polarization domain

Spatial and polarisation diversity techniques are realised through antenna systems. A diversity antenna system provides a number of receiving branches or ports from which the diversified signals are derived and fed to a receiver. The receiver then combines the incoming signals from the branches to produce a combined signal with improved quality in terms of signal strength or signal-to-noise ratio (S/N).

Spatial diversity antenna systems The spatial diversity antenna system is constructed by physically separating two receiving base station antennas.Once they are separated far enough, both antennas receive independent fading signals. As a result, the signals captured by the antennas are most likely uncorrelated.The further apart are the antennas, the more likely that the signals are uncorrelated. The types of the configuration used in GSM networks are:  horizontal separation  vertical separation

Two antenna spatial diversity

Polarization diversity antenna systems A dual-polarisation antenna consists of two sets of radiating elements which radiate or, in reciprocal, receive two orthogonal polarised fields. The antenna has two input connectors which separately connect to each set of the elements. The antenna has therefore the ability to simultaneously transmit and receive two orthogonally polarised fields.

Channels in GSM
In GSM frequency division duplex is used for duplex transmission. Uplink refers to signal transmission from MS to BTS and downlink refers to signal transmission from BTS to MS. for GSM 900 890-915 MHz for uplink 935-960 MHz for downlink. 1710-1785 MHZ for uplink 1805-1880 MHz. for downlink

for GSM 1800

In GSM the frequency band is divided into channels of 200 KHz each. Hence 125 carriers in GSM 900 and 375 carriers in GSM 1800.the duplex distance in GSM 900 is 45 MHz and that of GSM 1800 is 95 MHz. FDM is combined with TDMA to increase the no of users. In TDMA each radio frequency channel is divided into consecutive periods of time known as time slots.In GSM each radio channel is divided into 8 time slots.Hence each time slot per user is allotted.these TDMA timeslots are called physical channels.Each time slot lasts for 0.577 μsec thus 8 time slot last for 4.615 ms. These time slots are used for traffic as well as signalling.the TDMA frame cyclicaly repeat time after time. The longest recurrent time period of the structure is called hyperframe and has the duration of 3 h 28 min 53 sec 760 ms. The TDMA Frame Numbers (FN) are numbered from 0 to 2 715 647. One hyperframe is divided into 2048 superframes, which have duration of 6.12 seconds. The superframe is itself subdivided into multiframes. There are two types of multiframes in the system:  26 frame multiframe (51 per superframe) with a duration of 120 ms, comprising 26 TDMA frames. This multiframe is used to carry the logical channels TCH, SACCH and FACCH,  51 frame multiframe (26 per superframe) with a duration of 235.4 ms, comprising 51 TDMA frames. This multiframe is used to carry the logical channels FCCH, SCH, BCCH,CCCH, SDCCH, SACCH, and CBCH .

A variety of information is transmitted between the BTS and the MS. The information is grouped into different logical channels. Each logical channel is used for a specific purpose such as paging, call set-up and speech. For example, speech is sent on the logical channel Traffic Channel (TCH). The logical channels are mapped onto the physical channels.

Logical Channels The logical channels can be separated into two categories. They are traffic channels and signaling channels. There are two forms of TCHs: · full rate TCH (TCH/F) - this channel carries information at a gross rate of 13 kbit/s. · half rate TCH (TCH/H) - this channel carries information at a gross rate of 6.5 kbit/s. Signaling channels are subdivided into three categories: · Broadcast CHannels (BCH) · Common Control CHannels (CCCH) · Dedicated Control CHannels (DCCH)

The following sections describe specific channels within these categories.

BROADCAST CHANNELS (BCH) Frequency Correction CHannel (FCCH) On FCCH, bursts only containing zeroes are transmitted. This serves two purposes. First to make sure that this is the BCCH carrier, and second to allow the MS to synchronize to the frequency. FCCH is transmitted downlink only. Synchronization CHannel (SCH) The MS needs to synchronize to the time-structure within this particular cell, and also ensure that the chosen BTS is a GSM base station. By listening to the SCH, the MS receives information about the frame number in this cell and about BSIC of the chosen BTS. BSIC can only be decoded if the base station belongs to the GSM network. SCH is transmitted downlink only. Broadcast Control CHannel (BCCH) The MS must receive some general information concerning the cell in order to start roaming, waiting for calls to arrive or making calls. The needed information is broadcast on the Broadcast Control CHannel (BCCH) and includes the Location Area Identity (LAI), maximum output power allowed in the cell and the BCCH carriers for the neighboring cells on which the MS performs measurements. BCCH is transmitted on the downlink only. Using FCCH, SCH, and BCCH the MS tunes to a BTS and synchronized with the frame structure in that cell. The BTSs are not synchronized to each other. Therefore, every time the MS camps on another cell, it must listen to FCCH, SCH and BCCH in the new cell. COMMON CONTROL CHANNELS (CCCH) Paging Channel (PCH) At certain time intervals the MS listens to the PCH to check if the network wants to make contact with the MS. The reason why the network may want to contact the MS could be an incoming call or an incoming short message. The information on PCH is a paging message, including the MS‟s identity number (IMSI) or a temporary number (TMSI). PCH is transmitted downlink only. Random Access Channel (RACH) The MS listens to the PCH to determine when it is being paged.When the MS is paged, it replies on the RACH requesting a signaling channel. RACH can also be used if the MS wants to contact the network. For example, when setting up a mobile originating call. RACH is transmitted uplink only.

Access Grant Channel (AGCH) The networks assigns a signaling channel (Stand-alone Dedicated Control CHannel (SDCCH)) to the MS. This assignment is performed on the AGCH. AGCH is transmitted downlink only. DEDICATED CONTROL CHANNELS (DCCH) Stand alone Dedicated Control Channel (SDCCH) The MS as well as the BTS switches over to the assigned SDCCH. The call set-up procedure is performed on the SDCCH, as well as the textual message transmission (short message and cell broadcast) in idle mode. SDCCH is transmitted both uplink and downlink.When call set-up is performed, the MS is told to switch to a TCH. Slow Associated Control Channel (SACCH) The SACCH is associated with SDCCH or TCH (i.e. sent on the same physical channel). On the uplink, the MS sends averaged measurements on its own BTS (signal strength and quality) and neighboring BTSs (signal strength). On the downlink, the MS receives information concerning the transmitting power to use and instructions on the timing advance. SACCH is transmitted both uplink and downlink. Fast Associated Control Channel (FACCH) If a handover is required the FACCH is used. FACCH works in stealing mode meaning that one 20 ms segment of speech is exchanged for signaling information necessary for the handover. Under normal conditions the subscriber does not notice the speech interruption because the speech coder repeats the previous speech block. Cell Broadcast Channel (CBCH) CBCH is only used downlink to carry Short Message Service Cell Broadcast (SMSCB) and uses the same physical channel as the SDCCH.

BURST FORMATS The bit rate over the air interface is 270.8 kbps. This gives a bit time of 3.692 ms (48/13 ms). The time interval of a TS thus corresponds to 156.25 bits. The physical content of a TS is called a burst. There are five different types of bursts. 1. Normal Burst (NB): This burst is used to carry information on traffic and control channels. For TCH it contains 114 encrypted bits, and includes a guard time of 8.25 bit duration (30.46 ms). The stealing flag is relevant only for TCH 2. Frequency correction Burst (FB): This burst is used for frequency synchronization of the MS. It consists of zeroes only. 3. Synchronization Burst (SB): This burst is used for time synchronization of the MS. It contains a long training sequence and carries the information of the TDMA Frame Number (FN) and Base Station Identity Code (BSIC). 4. Access Burst (AB): This burst is used for random access and handover access. It is characterized by a long guard period (68.25 bit duration or 252 ms), to cater for burst transmission from an MS that does not know the timing advance at the first access (or at handover). This allows for a cell radius of 35 km. The access burst is used on the Random Access CHannel (RACH) and on the Fast Associated Control CHannel (FACCH) at handover. 5. Dummy Burst: This burst is transmitted when no other type of burst is to be sent. This means that the base station always transmits on the frequency carrying the system information, thus making it possible for the MSs to perform power measurements on the BTS in order to determine which BTS to use for initial access or which to use for handover. In order to achieve this, a dummy page and a dummy burst is defined in the GSM recommendations. CCCH is replaced by the dummy page, when there is no paging message to transmit. This dummy page is a page to a non-existing MS. In the other TSs not being used, a dummy burst with a pre-defined set of fixed bits is transmitted.

Call flow

1. Mobile originated call

2. Mobile terminated call

Handover
The GSM handover process uses a mobile assisted technique for accurate and fast handovers, in order to maintain the user connection link quality and manage traffic distribution. Prior to handover following process takes place: 1. Measurement of radio subsystem downlink performance and signal strengths received from surrounding cells is made in the MS. 2. These measurements are sent to the BSS for assessment. 3. The BSS measures the uplink performance for the MS being served and also assesses the signal strength of interference on its idle traffic channels. 4. During its idle time (the remaining seven timeslots), the MS switches to the BCCH of the surrounding cells and measures its signal strength. 5. The signal strength measurements of the surrounding cells, and the signal strength and quality measurements of the serving cell are reported back to the serving cell via the SACCH once in every SACCH multiframe. 6. This information is evaluated by the BSS for use in deciding when the MS should be handed over to another traffic channel.

The following measurements is be continuously processed in the BSS: i) Measurements reported by MS on SACCH - Down link RXLEV - Down link RXQUAL - Down link neighbor cell RXLEV ii) Measurements performed in BSS - Uplink RXLEV - Uplink RXQUAL - MS-BS distance - Interference level in unallocated time slots Handover is done on five conditions – Interference – RXQUAL – RXLEV – Distance or Timing Advance – Power Budget

The following are the types of handovers 1. Intracell When handover takes place between two sectors of same cell .it is within same BSC 2. Intercell intra BSC When handover takes place between two cells but within same BSC

3. Intercell inter BSC When handover take place between two cells which are located in diffent BSCs 4. Inter MSC When handover takes place between two cells which located in differnt MSCs

Frequency Hopping
The Frequency Hopping function permits the dynamic switching of radio links from one carrier frequency to another. Frequency Hopping changes the frequency used by a radio link every new TDMA frame in a regular pattern. The reasons of using Frequency Hopping are:  1. Decreasing the probability of interference Frequency Hopping will spread the annoyance of interference over different mobile stations in a particular cell  2. Suppressing the effect of Rayleigh fading Rayleigh fading (or multipath fading) is caused by different paths followed by the radio signal. Rayleigh fading can cause coverage holes.Rayleigh fading is location and frequency dependent. When the mobile station is stationary or moves at a slow speed, Frequency Hopping will significantly improve the level of the air-interface performance. However, when the mobile station moves at a high speed, Frequency Hopping does not harm, but does not help much either. The more frequencies are used in a particular cell, the more Frequency Hopping can gain in suppressing the effect of Rayleigh fading. Process : The regular pattern by which a radio link changes carrier frequency, is described by the hopping sequence. The hopping sequence can have a cyclic pattern or a pseudo-random pattern. In order to calculate the hopping sequence, a function is used which maps a particular TDMA frame to a radio frequency within the set of frequencies, using parameters such as TDMA frame number and number of frequencies in the set of frequencies. Both the uplink and the downlink use the same hopping sequence. For this purpose the parameters used to calculate the hopping sequence are also transferred from the BTS to the mobile station. To reduce complexity of the GSM system, the common channels(BCCH, FCCH, SCH, PAGCH and RACH) do not hop.

There are two types of frequency hopping schemes 1. Basebang frequency hopping 2. Synthesized frequency hopping Some terms: HSN(Hopping Sequence Number): The HSN specifies the order in which the frequencies within the set of frequencies are going to hop.if HSN is 0 then hopping takes place is cyclic fashion .fi it between 0 to 63 then hopping takes place in random manner. MAIO(mobile allocation index offset) : Mobile Allocation Index Offset (MAIO) is a frequency offset set for all Basic Physical Channels . Manual MAIO planning prevents adjacent channel interference within a cell as well as co- and adjacent channel interference in co-sited cells when using frequency hopping BA(BCCH allocation): These are the frequencies allocated for BCCH.these are the fixed frequencies and do not hop . MA(mobile allocation): These frequencies are allocated for traffic channels to hop.

Base band frequency hopping

As shown in fig the BCCH time slot does not hop.timeslot 0 of TRX 2-4 hop over MA(f2,f3,f4).this hopping group uses HSN-1.all timeslots 1-7 hop over MA(f1,f2,f3,f4).this hopping group uses HSN-2.

Frequency synthesized hopping:

In this the BCCH TRX does not hop. MAIOs are different for different TRXs within the same hopping group hence no collisions.in this scheme only one HSN is allocated.

Transmission
The inability of a BTS to cater more than a threshold of subscribers, and Radio Waves (900 MHz /1800MHzGSM frequency) to reach beyond a certain distance without interference, forces us to put a huge number of sites to cover our entire subscriber base. Now, to control these sites, they need to be connected to BSC. These BSCs have to be further connected to MSCs. This connection can be made by any type of transmission medium i.e. it can be Optical Fiber, Microwave link, Satellite Communications, or Coaxial Cable. The following parameters are considered in transmission planning.

Free space loss
The microwave antennas used for point to point links fall into the category of “aperture” antennas, the parabolic dish antenna being the most common example. A propagating electromagnetic wave has a power density Pd (in watts per square metre) associated with it. The aperture (known for these purposes as the “effective aperture” Ae) of the antenna is measured in square metres and the antenna serves to convert the power density into an actual power Pr (the suffix “r” standing for “received”) in accordance with the formula Pr = Pd Ae
Given that the surface area of a sphere of radius r is equal to 4πr2, it is possible to say that the power density Pd is related to the power transmitted Pt by the equation

the power density at same distance is given by

The received power is given by

The effective aperture of isotropic antenna is given by

Our formula for received power now becomes

Expressing in decibals Pr (dBm) = Pt (dBm) + Gt (dBi) + Gr (dBi) − 20 log10 (4π ) − 20 log10 (r ) + 20 log10 (λ ) Now (Pt-Pr = Path loss). So Path Loss = 20 log10 (4π ) + 20 log10 (r ) − 20 log10 (λ ) − Gt − Gr Solving we get Path Loss = 92.4 + 20 log10 d + 20 log10 f − Gt − Gr

When gain of antenna is 0 dBi then loss is called as free space loss FSL = 92.4 + 20 log10 d + 20 log10 f − Gt − Gr

Fade Margin
The Radio Link is usually designed in such a way that the Received Power PR (Normal propagation conditions) is much greater than the Receiver Threshold PTH. The Fade Margin FM is defined as : FM (dB) = PR (dBm) - PTH (dBm) A Fade Margin is required to compensate for the reduction in Rx power caused by Propagation Anomalies.

Link budget
A more complete Link Budgetexample (7GHz, 50 km link) is :

power Transmitted power Tx feeder and branching losses Tx antenna gain Free space loss Additional propagation loss Rx antenna gain Rx feeder and branching loss 30 dBm

Gain

losses

1.4 dB 42.5 dB 143.3 dB 3.0 dB 42.5 dB 1.4 dB

Net Path Loss Received Power PR

64 dB - 34 dBm

Assuming the RX Threshold PTH = -77 dBm, then the Fade Margin is : FM = PR - PTH = 43 dB

Equivalent earth curvature
An "Equivalent Earth Curvature" can be defined by altering the real Earth Curvature in order that the radio ray path be straight. In the Equivalent Earth representation the Earth Radius R is multiplied by a factor k. The value of the k-factor depends on the curvature of the radio ray. The k-factor is a measure of the ray curvature effect, produced by the variation in the Atmosphere Refraction Index with height. So, the k-factor is related to the

Vertical Refractivity Gradient G. The k-factor indicates the atmosphere state at a given time and its effect to the radio ray curvature.In a well-mixed atmosphere (Standard Atmosphere), the Refractivity decreases with height at a constant rate. This corresponds to the so-called Standard Condition, with a stable k-factor, equal to about 4/3. Other k-factor conditions are : k < 4/3 Sub-refractive Atm. (Ray Path closer to the earth. ) The lowest k value corresponds to the highest probability that the radio ray be obstructed by the ground. k > 4/3 Super-refractive Atm.( Ray Path more distant from the earth.) The range of the radio transmission can be significantly expanded. Unexpected interference can be observed.

Freznel zone
A Fresnel zone is a three-dimensional body, bounded by ellipsoids that have their focal points at the transmitter and the receiver antennas. The sum of the distances from a point (P) on the ellipsoid to the transmitter (T) and to the receiver (R) is n/2 wavelengths longer than the LOS path (S): distance (P - T) + distance (P - R) = S + nλ/2, where n is the number of the Fresnel zone.

First Fresnel zone

For the first Fresnel zone, n = 1. The radius of the first Fresnel zone is rF1. To keep out of this zone, the distance r from the optical LOS should be:

where d1 is the distance between the obstacle and the receiver. The obstacles may be hills, buildings, or vegetation. The following figure represents the 1st Fresnel zone:

Diffraction effect

Shadowing does not always mean that no signal is received behind an obstacle. Radio waves may bend around obstructions to a certain extent. This effect is called diffraction. The diffraction effect depends on the wavelength λ in relation to the size of the obstacle. The diffraction effect is greater if the wavelength increases.

Knife-edge diffraction

The following figure shows a simple model for a single knife-edge

diffraction:

Because of diffraction loss, only a fraction of the transmitted power in A will arrive at the receiver in B. The parameter v is calculated using the distances d1[m] and d2[m] from the knife edge to the cell site (BTS) and the MS, respectively, the height of the knife-edge h [m], and the wavelength [m]:

After calculating v, the diffraction loss Ldiffr can be found from the curve in the graph shown in the figure below

.

Multipath propagation
The radio wave may be reflected, from a hill, a building, a truck, an airplane, or a discontinuity in the atmosphere. In some cases, the reflected signal is significantly attenuated, while in others almost all the radio energy is reflected and very little absorbed. The result is that not one but many different paths are followed between transmitter and receiver. This is known as multipath propagation. Reflection and multipath propagation can cause positive and negative effects:  Ducting Multipath propagation may imply coverage extension by allowing radio signals to reach behind hills and buildings and into tunnels.The latter effect is known as ducting.  Constructive and destructive interference The interference due to multipath propagation manifests itself in the following ways: 1. Rayleigh fading: random phase shift creates rapid fluctuations in the signal strength 2. Delay spread in the received signal 3. Intersymbol interference: the delay spread in the received signal causes each symbol to overlap with adjacent symbols 4. Doppler shift: the shift in frequency on different paths causes random frequency modulation.

Network Optimization
The optimization of the network, is done to check the performance of the network, just after it is made operational and to get best possible quality of service. The objective of optimization procedure is:    To check whether the network meets the customer‟s given requirements, on the basis of which network was designed. To check whether the parameters and configurations are defined correctly or not. To find out and suggest changes in the defined parameters and configurations to achieve best possible quality of service.

Quality of service can be characterized by factors such as contiguity of coverage, accessibility to the network, speech quality and number of dropped and blocked calls. A number of parameters are checked as a measure of quality of service by using a drive test system. Drive Test system comprises of a test mobile phone, software to control and log data from the phone and a Global positioning system receiver for position information. A drive test system can only indicate the type of problem in the network that exists, it doesn‟t indicate cause of the problem but with the help of knowledge of possible reasons of a problem, one can trace the cause. Following steps are taken to fulfill the objective of network optimization using a drive test tool. 1. Collection of Data and extraction of relevant information from it. 2. Analysis of the extracted data. 3. Suggesting changes in the network configurations based on the analysis. Collection of Data and extraction of relevant information: Drive test involves setting up a call to best carrier and driving along the roads. While driving the radio parameters and air interference signal data are collected as a log file. In general following parameters are checked during the drive test for different categories of terrains like dense urban, sub-urban, rural, highways and for different clutters like in building, residential areas, commercial areas, industrial areas etc. 1. 2. 3. 4. 5. 6. Rx Level. Rx Quality Timing Advance Handover parameters Data of six best neighbor cells. Layer 2 and layer 3 messages.

From the data collected various information can be extracted which depict the performance of BTS sites and the network as a whole. Following information can be extracted from this drive test data. 1. 2. 3. 4. 5. 6. Coverage edge probability Coverage area probability Speech quality Frequency and BSIC reuse Neighbor cell definition details. Handover details.

Edge Probability: To get an idea of coverage area, coverage boundary of all the cells based on received signal level (RXLEV), is obtained and is plotted over the geographical map of the area. The coverage boundary of a cell is considered to be made up of equal received level points on the field. With the help of this coverage plot the edge probability or the probability of getting a signal level better than a specific value over the boundary of all cells is obtained which helps in determining the performance of the network with respect to coverage boundary requirements given by the customer. Area Probability: The obtained signal levels from the cells at all the points of the network, are then used to make, a best server plot. This best server plot is drawn by categorising it on the basis of in building coverage, in car coverage and on street coverage. These categories are defined on the basis of the coverage area where a good quality conversation is required. The details about threshold defined for these categories are discussed in „coverage planning report‟. Speech Quality: Speech quality is a very important aspect for determining the quality of service for whole of the network. Speech quality is inferred by the RXQUAL measurements during the drive test. RXQUAL, is the Bit error rate (BER) derived from the 26 bits midamble on TDMA burst. Its level characterizes speech quality where 0 indicates the highest quality and 7 the worst. Thus during drive test, poor quality areas can be found and marked by looking over the quality on the scale of 0 to 7. RXQUAL can be poor due to poor RXLEV, Co-channel interference, adjacent channel interference or multipath. RXQUAL is measured and tested for all the categories of clutter and terrain. Frequency and BSIC reuse: From the collected data the frequency reuse pattern with the BSIC (Base Station Identity Codes) planning of all the cells of the network can be obtained. The reuse distance for all the reused frequencies can be determined Neighbour cells definition details: With the help of collected data 6 best serving neighbours of all the cells can be determined. The drive test window of the antenna system gives details of 6 best neighbours at an instance. Handover details: There are certain other very important parameters which has to be checked during drive test as these parameters directly reflects on the performance of the

network, like handover margin, handover threshold, values of handover timers, offset and penalty for the handovers. With a call established, and measuring on the cell edge, we can display the phone measurements of serving and neighbour cells. The difference between the RXLEV of the server and that of neighbours can be monitored on the amplitude and time scale. At some point on the drive-test route, one of the neighbour‟s RXLEV will become stronger than the server‟s signal level and when this difference of the two exceeds the handover margin, for atleast a timing set in the „handover required‟ counter in BSS, a handover will occur. Thus by simultaneously monitoring RXQUAL during the handover, the value of the handover margin can be determined and a decision can be made whether that value is appropriate for the quality of service desired. Analysis of extracted data: The information extracted from the collected data is then analysed to compare it with the agreed benchmarks related to coverage, quality, handover success rate etc and is used to infer the cause of the deviation from given requirements and set benchmarks. It is also used to infer cause of detected problem in the network if there is any. There are special coverage requirement which are discussed in „coverage planning report‟ under „special coverage category‟ these specific coverage requirements are matched to find out whether the requirement of customer is taken care of or not. RXQUAL is also matched with the given requirement. If RXQUAL is poor and RXLEV is sufficiently good it can reasonably be deducted that the cause is interference. Generally a test frequency which has no adjacent or Co channel present in that area is used to find out if interference is because of multipath. If it is not because of multipath then spectrum analyser can be used to find out whether it is an adjacent channel interference or it can be deducted that it is Co channel interference. A handover margin on the high side will result in a handover occurring after the user has experienced some deterioration in quality. High handover margins can result in poor reception and dropped calls, while very low values of handover margin can produce “Ping-Pong” effects as a mobile switches too often between cells. With the help of collected data it can be found out weather uplink and downlink are balanced or not. If even after having good RXLEV and RXQUAL, calls are dropping or even when RXLEV and RXQUAL of serving cell is better than that of neighbour cell, handover is taking place, it indicates that the link needs to be balanced. BSIC for all the cells are also checked and verified with what is defined in the BSS. If same BSIC is defined for cells having same BCCH frequency and these cells coexist in the neighbour list then understandably lot of handovers will be unsuccessful.

Layer 2 and 3 messages can be used for analysing cause of a particular handover failure, call drop, very poor speech quality or any other abnormality in the performance of the network. Suggesting changes in the network configurations based on the analysis: After detection of the causes of the deviation from the requirement or network related problems, measures are taken to improve the performance of the network and to match customer‟s requirement. Network performance can be influenced by the network parameters. The configuration parameters can be divided into two groups hard configuration and soft configuration, depending on the type of control and action required to modify them. Hard Configuration: The hard configuration parameters are aspects of base station configuration and include antenna type, antenna gain, antenna orientation, effective height of antenna radiation centre, use of space diversity, antenna feeder loss and effective isotropic radiated power (EIRP). Changes in this configuration are made to meet the requirements and to deal with the analysed problems. For an example if certain area is affected by interference resulting in poor quality then one of the way to reduce interference level is by shrinking the coverage area. Shrinking of coverage area can be achieved by reducing EIRP that is by replacing the existing antenna with a lower gain or narrower horizontal beam width antenna system and by reducing transmitted power under limitation of not loosing the link balance. Most effective solution used to shrink coverage area is by increasing antenna downtilt and/or reducing antenna height. Similarly to improve coverage in certain areas the transmitted power of BTS can be changed, antennas with different gain or beamwidth can be used and the height of antenna system can be changed. For further specific coverage and quality requirements pico or micro cells can be installed inside the residential places, commercial buildings, stadiums and car parks etc. A pico cell is nothing but a cell with very low EIRP in comparison to a Macro Cell. Note that the neighbour list for these pico cells is defined differently than that for normal Macro cells. Micro cell has also got lesser coverage area than that of Macro cells. Repeaters can also be used for providing coverage to specific areas. There can be Channel selective or Band selective Repeaters where band selective repeaters amplifies the whole GSM band and transmit it towards the area required to be covered while channel selective repeaters receive power from selected channels of one or more than one parent cells, amplify it and direct it towards the area required to be covered. In the similar way if capacity requirement of certain area is more, then the coverage of a cell is needed to be compressed by any of the means discussed above so that it may cater to lesser number of customers. If the mentioned measures don‟t work for matching coverage and capacity requirement then relocation or addition of site can also be suggested. If interference is observed

during drive test then apart from reducing coverage area, frequency plan for the network can be redefined and reuse distances can be increased. After carefully studying the statistical data about the network performance if it is found that congestion for some particular sites are more and call successful rate is less, then more resources (TRX) can be added to improve availability of the traffic channels or additional BTS sites can also be added but this addition has a limit because of limited available frequency spectrum hence with higher number of sites or frequency used, reuse distance of the sites will reduce which will increase interference and hence the quality will go poorer. There are lots of other ways by which capacity can be increased without much affecting the speech quality. 1. Addition of Micro and pico cells. 2. Using Underlay and overlay cells. 3. Deploying frequency hopping Everytime TRXs are added in the network, frequency plan of the network or a portion of the network has to be changed which will further require to analyse the network using drive test system, to monitor the network‟s performance. It is possible that after addition of certain TRXs frequency reuse distance will decrease to such a level that it will introduce unacceptable amount of interference and deployed frequency plan will require to be redefined.

Soft Configurations: Other parts of the system can be controlled with soft parameters. These affect operation of algorithms within the system, and include categories such as common BTS parameters, cell access parameters etc. GSM defines around 150 soft parameters. For an example if it is found from the BSS statistics details that excessive handovers hence more utilisation of resources is taking place then reduction of overlap of the cell coverage areas can avoid them. Defined BSICs for the cells specially for cells transmitting same frequencies are set to be different otherwise lot of unsuccessful handovers will take place. Even then, if it is found that number of unsuccessful handover is high then redefining the neighbour list in BSS can control it. Several neighbours for a serving cell can be defined in GSM. Usually, we want a handover to be made to the strongest neighbour, but in some cases frequent handovers to this best neighbour can result in congestion in this cell, affecting the users initiating calls from that cell. The situation can also occur in reverse, when a handover required to the best neighbour can result in a rejection due to unavailability of resources, causing the handover to be attempted to the next best neighbour, which can delay the process and deteriorate the quality further. Under certain circumstances, we may need to remove a potential neighbour from the neighbour list and provide alternatives.

In the idle mode, the mobile always prefers to remain with or move to the best serving cell. The best cell is decided on the basis of uplink and downlink path balance in the cells. This balance is calculated by GSM defined C1 calculations. C1 calculations force the mobile to move to the strongest cell. In certain cases, such as macro-micro cell architecture, optimisation may require that in certain areas the mobile not remain in the best cell, but instead remain in a cell depending on traffic loading. C2 parameters provide the option of adding fixed positive or negative offsets to the C1 calculation in each cell. So, although C1 might be better for a neighbour cell, the application of C2 parameters could delay reselection. C2 parameters also allow the mobile to apply temporary offsets for a period known as penalty time, which helps reduce Ping-Pong effects. With the help of carefully done drive test these parameters like offset or penalty time for handovers can also be checked and verified. Optimisation of the network using drive test system is an iterative process thus after deploying discussed changes in the network drive test is done again and mentioned steps are repeated until required performance objective is fulfilled. Drive test is very effective part of the optimisation of the network but drive test data is not very effective to find out some of the very specific problems their cause and solutions to rectify them. For finding requirement of capacity, exact cause of handover failures and reduced call success rate one has got to be dependent on statistics obtained from the BSS. On the other hand drive test is the only medium with the help of which user‟s perspective of quality of service can be visualised hence simultaneous monitoring of the BSS statistics and drive test data gives most practical and optimum cause and the solution of a problem.

Drive test
Area: Lucknow cantonment board Components and instruments: 1. GPS antenna 2. Sony Ericsson handset connected to laptop 3. TEMS 6.1 a software for data collection for Ericsson

Purpose: There were many customer complaints in that area. Observations: Using handset a call is made to a no 7777 where all parameters are tested. the parameters such as RxQual,RxLev are determined. for RxQual 0-7 levels are considered where 0 is considered as best while 0 as worst. for RxLev the power level is measured in dBm where -50 to -60 dBm is considered sas good power level. the handoffs were also studied the handoffs such as intra cell and intercell were studied. At some places handoffs were not successful and there the call was dropped.

The GUI of the software contained following

1. 2. 3. 4. 5. 6. 7.

Map Radio Parameters C/I Serving Neighbour C/A GPS Current Channel

Introduction to Mapinfo
1. .xls file with the required data and containing the latitude and longitude columns. Make sure that no column is blank and the values of latitude and longitude are not mixed 2. Save the text.xls file as .txt file say temp.txt 3. Open Mapinfo software 4. Click on open button and select „Text Files (Tab-delimited) i.e. .txt files. Locate the test.txt file created and click on open. Select delimiter as tab and check titles on the 1st row. 5. A table will be created in Mapinfo. 6. Now from the menu select the option “Create Points”. 7. A box will open and it will ask for „X‟ and „Y‟ co-ordinates. 8. In the „X‟ co-ordinate column select the longitude column & in the „Y‟ coordinate select the latitude column. Also select the desired symbol 9. Close Mapinfo and open the “.tab” file created by it. 10. The Points given will be plotted on the map having the desired symbol which has been already selected. 11. We can add layers also using the Mapinfo tool. 12. Click on open and you can open Railways, Roadways etc. 13. Once opened you can change the layer properties by right clicking on it. 14. Hence, clutter can also be added in the similar way. We can manage different sites shown according to the .xls file which are clearly plotted on the map and frequency planning and other parameters can be managed. 15. “LABELS” can also be added by checking the labels box and clicking on the label. Then the desired label can be selected from the dropdown box. 16. Click on Table  Maintenance  Table Structure. A dialog box appears showing a list of fields with their name and type. Check the required boxes which you want to make searchable.

Comparison of SDH and PDH PDH
The reference clock is not synchronized throughout the network

SDH
The reference clock is synchronized throughout the network

Multiplexing / Demultiplexing operations have to be performed from one level to the next level step by step

The synchronous multiplexing results in simple access to SDH system has consistent frame structures throughout the hierarchy. The payload is transparent

The payload is not transparent

PDH system has different frame structures at different hierarchy levels

SDH system has consistent frame structures throughout the hierarchy.

Physical cross-connections on the same level on DDF are forced if any

Digital cross- connections are provided at different signal levels and in different ways on NMS

G.702 specifies maximum 45Mpbs & 140Mpbs & no higher order (faster) signal structure is not specified

G.707 specified the first level of SDH.That is, STM-1, Synchronous Transport Module 1st Order & higher. (STM-1,STM-4,STM-16, STM-64)

PDH system does not bear capacity to transport B-ISDN signals.

SDH network is designed to be a transport medium for B-ISDN, namely ATM structured signal.

Few services are available

It will transport variety of services.

Limited amount of extra capacity for user / management

It will transport service bandwidths Sufficient number of OHBs is available

Bit - by - bit stuff multiplexing

Byte interleaved synchronous multiplexing.

BTS & BSC Training

Introduction to Base Transceiver Station (BTS) A base transceiver station or cell site (BTS) is a piece of equipment that facilitates wireless communication between user equipment (UE) and a network. Though the term BTS can be applicable to any of the wireless communication standards, it is generally and commonly associated with mobile communication technologies like GSM and CDMA. In this regard, a BTS forms part of the base station subsystem (BSS) developments for system management. It may also have equipment for encrypting and decrypting communications, spectrum filtering tools (band pass filters) etc. antennas may also be considered as components of BTS in general sense as they facilitate the functioning of BTS. Typically a BTS will have several transceivers (TRXs) which allow it to serve several different frequencies and different sectors of the cell (in the case of sectorised base stations). A BTS is controlled by a parent base station controller via the base station control function (BCF). The BCF is implemented as a discrete unit or even incorporated in a TRX in compact base stations. The BCF provides an operations and maintenance (O&M) connection to the network management system (NMS), and manages operational states of each TRX, as well as software handling and alarm collection. The basic structure and functions of the BTS remains the same regardless of the wireless technologies.

Nokia Ultrasite EDGE BTS: This section describes the units of Nokia Ultrasite EDGE BTS. Following are the units and their explanation.  Base Operations and Interfaces Unit (BOI):The BOIx unit handles the control functions common among other units in the BTS. These functions include: 1. BTS initialization and self testing 2. Configuration 3. O & M signaling 4. Software download 5. Main clock functions 6. Timing functions 7. Collection and management of the external and internal alarms 8. Message delivery to the BSC 9. Cabinet Control Dual Band Diplex Filter Unit (DU2A): The DU2A unit combines output form DVxx or RTxx units into one antenna feeder. It is mounted on the top of the BTS cabinet. Transceiver Baseband Unit (BB2x) : The BB2x unit is a digital signal processing board, consisting of two independent baseband modules. Each module functions independently for its own TRX unit. The BB2x unit also controls frequency hopping.

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Dual Variable Gain Duplex Filter (DVxx): The DVxx unit performs duplex operation of TX and RX signals through a common antenna and filters and amplifies main and diversity receiver signals before they pass through the M2xA unit to the TRx unit.

The DVxx unit contains a variable-gain LNA for optimal amplification of the receive signal with or without the optional Masthead Amplifier (MNxx) unit. The high gain LNA is fixed and used without the optional MNxx. The low-gain LNA is variable and used only with the MNxx unit; the low-gain LNA is set according to antenna cable attenuation values. Feeder loss values are entered during HW configuration, which sets the correct DVxx unit gain accordingly at the RX band for optimal performance.

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Masthead Amplifier and Bias Tee units: Masthead Amplifier unit: Following are the functions of Masthead Amplifier 1. 33db RX gain in GSM/EDGE. 2. Low RX noise figure (improved RX sensitivity and SNR) 3. Low TX loss in a compact, low volume, lightweight, sealed enclosure. Bias Tee Unit (BPxx): The BPxx unit provides DC power to the MNxx using the RF cable.

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Receiver Multicoupler Unit (M2xA): The M2xA unit distributes RX signals to the TRx units. The 2-way unti is used in most Wideband Combiner or combining bypass configurations. One unit performs signal splitting for both main and diversity branches.

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Power Supply Unit: The PWSx unit converts AC or DC input voltage to DC voltage required for the Nokia Ultraite EDGE BTS. The PWSx unit distributes the appropriate voltage through the backpane to the units. The PWSx unit also supplies power for MNxx unit.

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Transceiver Unit (TRx) : The transceiver unit contains one transmitter, one main receiver, one diversity receiver. The TSxx unit performs RF modulation/demodulation and amplification for one RF carrier to handle the following signal i.e. uplink signals from the mobile stations to the BTS or the downlink signals from the BTS to the MS. These functional sections communicate with the Transceiver Baseband (BB2x) and Base Operations and Interfaces (BOIx) units through the backplane. The functional sections process the following signals i.e. data signals between the TSxx and BB2x units initialization and control signals from the BB2x unit to the TSxx unit status and alarm signals from the TSxx unit to the BB2x unit.
The RF section of the receiver converts the carrier frequency signal to the IF frequency. The IF sections of the receiver perform channel filtering to prevent interfering frequencies from distorting the signal. The IF sections also provide automatic gain control.

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Transmission Unit (VSxx) : The transmission unit connects the BTS to the rest of the network using wireline or radio interface. The FXC RRI unit is the radio link transmission unit for the Nokia Ultrasite EDGE BTS. The FXC RRI support two Flexbus connections (coaxial cable), 16x2 Mbit/s each. The Flexbus connects the RRi transmission units for the Nokia FlexiHopper Microwave, multiple BTS cabinets located at the same site can also be connected together using a Flexibus. The FXC RRI transmission unit operates as a repeater and interconnects Nokia Ultrasite EDGE BTS and the BSC using loop, chain, star and point-to-point network configuration.
The main features of the FC E1/T1 transmission unit are: one Abis line interface to the Mbit/s (E1) or 1.5 Mbit/s (T1) transmission line operation as the termination point in a chain or star configuration balanced interface that can be configured to E1 or T1 mode interface statistics gathered in compliance with ITU-T G.826 and ANSI T1.403 Recommendations handling of timeslot 0 at 2 Mbit/s interfaces. The 2 Mbit/s E1 frame/ multiframe structure complies with ITU-T G.704/706 Recommendations transmitting and receiving functions at the 2 Mbit/s interfaces.

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Wideband Combiner Unit (WCxA) : The WCxA unit combines two transmitter outputs into one. When sing the WCxA, the DVxx unit is required.
With 2-way Wideband Combining, the WCxA unit combines the transmit (TX) signals from two Transceiver (TSxx) units and feeds the combined signal to the antenna through the TX port of the Dual Variable Gain Duplex Filter (DVxx) unit.

Signal Flow in the BTS: -

Description:In the uplink path, the BTS receives signals from the MS; in the downlink path, the BTS sends the signals to the BTS. Uplink and downlink signals travel through the Air interface on different frequencies with the higher frequency carrying downlink signal. Uplink Signal Path: The uplink signal path involves the following actions: o The antenna picks up a signal from the air interface. o The antenna passes the signal to the Masthead Amplifier and the Bias Tee and then to the optional Dual Band Diplex filter. o The signal passes through Dual Variable Gain Duplex Filter to the Receiver Multicoupler and then to the Transceiver module. o The Transceiver Module converts the received signal to the Intermediate Frequncy levels and filters the signal. o The transceiver module then sends the signal to the Transceiver Baseband unit for digital processing. o The transceiver baseband unit then sends the processed signal to the transmission unit which passes the signal to the BSC through the Abis interface.

Downlink Signal Path: The downlink signal path involves the following actions: o The BSC receives the signal from the network and sends the signal to the transmission unit of the BTS through the Abis interface. o The transmission unit passes the signal to the transceiver baseband unit for the digital signal processing. o The transceiver baseband unit sends the processed signal to the transceiver unit. o The transceiver unit filters the signal and raises the carrier frequency and amplifies it. o Then the transceiver baseband sends the signal to the wideband combiner or directly to the dual variable gain duplex filter. o Then the signal passes through the options dual band diplex filter to the bias tee and masthead amplifier and through the antenna to the mobile station.

GSM Speech and Channel Encoding: Speech Encoding:
RPE-LPC (Regular Pulse Excited - Linear Predictive Coder): In modern land-line telephone systems, digital coding is used. The electrical variations induced into the microphone are sampled and each sample is then converted into a digital code. The voice waveform is then sampled at a rate of 8 kHz. Each sample is then converted into an 8 bit binary number representing 256 distinct values. Since we sample 8000 times per second and each sample is 8 binary bits, we have a bit rate of 8kHz X 8 bits = 64kbps. This bit rate is unrealistic to transmit across a radio network since interference will likely ruin the transmitted waveform. GSM speech encoding works to compress the speech waveform into a sample that results in a lower bit rate using RPELPC. A LPC encoder fits a given speech signal against a set of vocal characteristics. The best-fit parameters are transmitted and used by the decoder to generate synthetic speech that is similar to the original. Information from previous samples is used to predict the current sample. The coefficients of the linear combination of the previous samples, plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal. Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps. This way GSM can transmit 4 times (floor [64kbps/13kbps]) as many phone calls as a regular land-line telephone.

Channel Encoding:
Once we have a compressed digital signal, we must add a number of bits for error control to protect the signal from interference. These bits are called redundancy bits. The GSM system uses convolutional encoding to achieve this protection. The exact algorithms used differ for speech and for different data rates. The method used for speech blocks is described below. Bit Composition of the Speech Signal: The RPE-LPC Encoder produces a block of 260 bits every 20 ms. It was found (though testing) that some of the 260 bits were more important when compared to others. Below is the composition of these 260 bits. Class Ia - 50 bits (most sensitive to bit errors) Class Ib - 132 bits (moderately sensitive to bit errors) Class II - 78 bits (least sensitive to error) As a result of some bits being more important than others, GSM adds redundancy bits to each of the three Classes differently. The Class IA bits are encoded in a cyclic encoder. The Class Ib bits (together with the encoded Class IA bits) are encoded using convolutional encoding. Finally, the Class II bits are merely added to the result of the convolutional encoder. Below is the operation of each encoder as related to each Class of bits. Cyclic Encoding: The Class IA bits are encoded using a cyclic encoder to add three bits of redundancy. The resulting Class IA bits are of the form: where b0,b1,b2 are the three redundancy bits added by the cyclic encoder and m0,...,m49 are the original Class IA bits. The cyclic encoder produces 50+3=53 bits. Cyclic codes are linear codes (the sum of any two codes is also a codeword), as we have seen in class. In addition to being linear, a cyclic shift, or rotate, of a codeword produces another codeword Since the code used in GSM is a (53,50) code the generator polynomial used in the encoding is of degree 53-50 = 3. The specific polynomial used in GSM is x^3 + x + 1. The following block diagram can produce the codeword. Once the data has been completely shifted through the system, the contents of Reg0 through Reg2 will contain the three additional bits.

GSM chose to use cyclic encoding due to the ability to quickly determine if errors are present. The three redundancy bits produced by the cyclic encoder enable the receiver to

quickly determine if an error was produced. If an error was produced the current 53 bit frame is discarded and replaced by the last known "good" frame.

Convolutional Encoding: The resulting 53 bits of the cyclic encoder are added to the 132 Class Ib bits (plus a tail of 4 extra bits so that the encoder may be flushed) and encoded using the convolutional encoder. The convolutional encoder adds one redundancy bit for every bit that it sees based on the last four bits in the sequence. Below is a block diagram detailing the convolutional encoder.

The convolutional encoder retains a memory of the last four bits in the sequence (a single bit is retained in each flip-flop). These four bits are added together using a modulo-2 adder. The resulting bit is sent to the output via path 1. The encoder sends a second bit to the output via path 2. As a result, the convolutional encoder encodes one input bit into two output bits. In GSM there are 4 flip-flops, and the convolution performed is of D^4 +D^3 + 1 and D^4 +D^3 + D + 1. Once the convolutional encoder has encoded the bits, a new bit sequence of 378 (2*(53+132+4=189)=378) bits is produced. These 378 bits are directly added to the 78 Class II bits (directly added since these bits are least sensitive to error). As a result, the channel encoded bit sequence is now 378+78=456 bits long. Therefore, each 20 ms burst produces 456 bits at a bit rate of 22.8 kbps.

Interleaving:
Interleaving is the processes of rearranging the bits. Interleaving allows the error correction algorithms to correct more of the errors that could have occurred during transmission. By interleaving the code, there is less possibility that a whole chuck of code can be lost. Consider this example to see how interleaving works. We need to transmit 20 bits. Furthermore, 10 bits can be transmitted in one transmission burst, and the error correcting mechanism can correct 3 errors per 10 bits. Take a look at the following two scenarios:

With interleaving the receiver is able to get all 20 bits correctly but without interleaving we lose 1 complete burst. In GSM the interleaving is much more complicated than the simple example above. The 456 bits outputted by the convolutional encoder are divided into 57 bit blocks by selecting the 0th, 8th, 16th through 448th bits in the first block, the 1st, 9th 17th through 448th bits in the 2nd block and so on to have 8 blocks. Then the bits in the first 4 blocks are placed in the even bit positions for the total block of 456 bits, and the bits in the second set of 4 blocks are placed in the odd positions.

Ciphering:
Ciphering is used to encrypt the data so that no one can overhear the conversation of another user. In GSM the two parties involved in encrypting and decrypting the data are the Authentication Center (AuC) and the SIM card in the mobile phone. Each SIM card holds a unique secret key, which is known by the AuC. The SIM card and AuC then, follow a couple algorithms to first authenticate the user, and then encrypt and decrypt the data. For authentication, the AuC sends a 128-bit random number to the mobile phone. The SIM card uses it's secret key and the A3 algorithm to perform a function on the random number and sends back the 32-bit result. Since the AuC knows the SIM card's secret key, it performs the same function, and checks that the result obtained from the mobile phone matches the result it obtained. If it does, the mobile user is authenticated. Once authentication has been performed, the random number and the secret key are used in the A8 algorithm to obtain a 64-bit ciphering key. This ciphering key is used with the

TDMA frame number in the A5 algorithm to generate a 114 bit sequence. Note: the ciphering key is constant throughout a conversation, but the 114 bit sequence is different for every TDMA frame. The 114 bit sequence is XORed with the two 57 bit blocks in a TDMA burst. The only user that can decrypt the data is the mobile phone or the AuC since they are the only ones that have access to the secret key, which is needed to generate the ciphering key, and the 114 bit sequence. Note that the A3, A5, and A8 algorithms are not known to the public domain; however some information about A5 has been leaked. It is known that A5 has a 40-bit key length, which allows for the encryption to be broken in a matter of days, but since cellular calls have a short lifetime, the weakness of the algorithm is not an issue.

Description of BSC3i
The Nokia GSM/EDGE BSC3i is a modern fault-tolerant system for GSM networks. The BSC3i is based on modular software and hardware architecture. The distributed architecture of BSC3i is implemented with a high capacity and redundant multiprocessor system – DX200 computing platform. The system enables the distributed processing capacity of several computer units. The main function of the BSC3i is to control and manage the BSS and the radio channels.

Functionality of BSC3i: The BSC3i manages a variety of tasks ranging from channel administration to short message service. The main functionalities are below Management of terrestrial channels  Indication of blocking on A interface  Allocation of traffic channels between BSC and BTS Management of Radio Channels  Management of channel configuration i.e. how many traffic channels and signaling channels can be used in the BSS. This is in connection with the radio network configuration.  Management of Traffic Channels (TCH) and SDCCH. i.e. resource management, channel allocation, link supervision, channel release, power control.  Management of BCCH, CCCH i.e. channel management, random access, access grant, paging, management of PCCCH. Management of Frequency Hopping The BSC is in charge of the frequency hopping management which enables use of radio resource and enhanced voice quality. Handovers The frequency of the mobile is changed in connection with the handovers which are executed and controlled by the BSC. Maintenance The BSC offers the possibility for the following maintenance:  Fault localization for the BSC  Reconfiguration of the BSC  Reconfiguration support of the BSC  Updating software User Interface The BSC has a user friendly interface with plain text messages and commands, which are easy to learn and use. The user interface compiles with the recommendations of the ITU. These are known as MML commands.

Measurements and Observations The BSC measures the traffic, observes signaling events, and traces a specific call. It then forwards these results to OMC SMS The BSC forwards mobile originating and mobile terminating short messages transparently. Cell Broadcast Messages Cell broadcast provides the BSC with the short message service cell broadcast capabilities defined by GSM recommendations. It is used for broadcasting short messages to mobile stations in a specified area. BSC3i supports Full Rate traffic channels, Half Rate traffic channels, and Enhanced Full Rate traffic channels. Adaptive Multi Rate Codec AMR introduces new set of codecs and adaptive algorithm form codec changes thus can provide significantly better speech quality and more capacity on air interface. We can achieve very good speech quality in full rate mode even in low C/I conditions GPRS and EDGE BSC3i is capable of providing and is compatible with data services handling i.e. GPRS and EDGE. Dynamic Abis Allocation The Dynamic Abis allocation functionality allocates Abis transmission capacity to cells when needed instead of reserving a full fixed transmission link per TRX. It is implemented as a software feature. Quality of Service is improved by it.

Architecture of BSC3i:-

      

two Clock System Units (CLS) installed in a CLOC-B cartridge up to seven Base Station Controller Signalling Units (BCSU) with packet control units installed in CC3C-A cartridges two Marker and Cellular Management Unit (MCMU) installed in CC4C-A cartridges one Operation and Maintenance Unit (OMU) installed in CM2C-A cartridge up to 64 Exchange Terminal Plug-in Units (64 ET4/ET2 units with GSW1KB/S11.5 or 62 ET2 units with earlier configurations) four LAN switches (ESB26/ESB20-A) installed in the CC4C-A cartridges with the MCMU four Fan Trays (FTRB) for forced ventilation.

Base Station Controller and Signalling Unit (BCSU): The BCSU performs those functions of the BSC that are highly traffic dependent i.e. on the volume of the traffic. One BCSU can handle traffic from around 200 TRXs. The BCSU is housed in the cartridge of its own. It consists of two parts, which correspond to A and Abis interface. Packet control Units (PCUs) are also connected to BCSU. The A interface part of the BCSU does the following functions:  performing the distributed functions of the message Transfer Part (MTP) and Signalling Connection and Control Part (SCCP)  controlling the mobile and base station signaling (BSSAP)  performing all message handling and processing functions of the signaling channels connected to it The Abis interface part of the BCSU controls the air interface channels associated with the TRXs and the Abis signaling channels. Every speech circuit on the Abis interface is mapped one-to-one to a GSM specific speech/data channel on the air interface. The handover and the power control algorithms reside in this particular unit.

Bit Group Switch (GSWB): The Bit Group Switch conveys the traffic passing through the BSC and switches the tones to the subscribers of the exchange and to the trunk circuits. The GSWB also establishes the necessary conditions to the signaling units and the internal data transmission channels, and is responsible for the sub-multiplexing functions of the BSC. It switches on 8, 16, 32, 64 kbit/s level. The operation of GSWB is controlled and supervised by the MCMU (Marker and Cellular Management unit). It performs all the necessary connecting and releasing functions. It consists of a power supply and four plugin units each having 32 4Mbit/s interfaces. Capacity of GSWB is 256 2 Mbit/s PCMs. It is fully digital, one-staged, and non-blocking time switch with full availability and it is very simple.

Clock and Synchronization Unit (CLS): The clock and synchronization unit (CLS) distributes the timing reference signals to the functionality units in the BSC3i. It can operate plesiochronously synchronously with the timing references it receives from the digital PCM trunks. To oscillator of the CLS is normally synchronized to external source, usually MSC, through PCM line.

Upto three additional PCM inputs are provided for redundancy. The processor chooses the highest priority interface which is in order, for the phase lock form the six synchronization inputs.

Marker and Cellular Management Unit (MCMU): The MCMU controls and supervises the GSWB and performs the hunting, connecting and releasing of the switching network circuits. The range of tasks it handles makes up a combination of general marker function and radio resource management. The MCMU is connected to the other computer units of the exchange, OMU and BCSU, through the message bus. It performs the control of a switching matrix and the BSC-

specific management functions of the radio resources. Marker functions of the MCSU include the connection and release of the circuits of the switching matrix. When the MCMU performs these functions, it exchanges messages with other Call Control Computers via the Message Bus (MB). The Switch Control Interface writes the required connections into the switch control memory and reads it contents. The Cellular Management functions of the MCMU are responsible for the cells and radio channels that are controlled by the BSC. This responsibility is centralized in the MCMU. It keeps track of the radio resources requested by the MSC and the handover procedures of the BSC. It also manages the configuration of the network. One BSC3i always includes two MCMUs that are permanently connected to the duplicated pair of GSWB, the active MCMU to the active GSWB and passive MCMU to passive GSWB. The integrated LAN switch provides access to the operator‟s IP network.

Call Control Computers: In BSC3i the call control functions are executed by the microcomputers, called the Call Control Computers. The Call Control Computers have and identical Central Processing Units (CPU) which is based on the most suitable commercially available Intel microprocessors. The CPU board contains a microprocessor and a local Random Access Memory (RAM). All different plug-in units of the CPU are connected using PCI bus.

Operation and Maintenance Unit (OMU): The Operation and Maintenance Unit (OMU) is an interface between the MSC and a higher level network management system and the user. The OMU can also be used for local operations and maintenance. The OMU receives fault indications from the BSC. It can produce local alarm printouts to the user or send the fault indications. In the event of fault, the OMU automatically activates appropriate recovery and diagnostic procedures within the BSC. Recovery can also be activated by the MCMU if OMU is lost.     Traffic control functions Maintenance functions System configuration administration functions System management functions

OMU has its own Call Control Computers as well as local I/O interfaces. It has microcomputer of its own, alarm interface, message bus interface, peripheral device interface, analog X.25 interface, digital X.25 interface and Ethernet interface.

The alarm interface connects internal wired alarms to the OMU from the BSC cartridges, power supply, air conditioning equipment etc.

Exchange Terminal (ET): The ET performs the electrical synchronization and adaptation of external PCM lines. It performs the HDB3 (ET2E), or B8ZS or AMI (ET2A) coding and decoding, inserts the alarm bits in the outgoing direction and produces PCM frame structure. All 2.048 Mbit/s interfaces for the MSC, the SGSN and the BTSs are connected to the ET. The ETs adapt the external PCM circuits to the GSWB an synchronize to the system clock. In the incoming direction, the ET decodes 2.048 Mbit/s signal of the circuit to data signals. The decoder decodes the line code into binary format. In the outgoing direction, the Et receives a binary PCM signal from the switching network and generates PCM frame structure. The resulting signal is converted into a line code and transmitted further onto 2.048 Mbit/s.

Visit to BSC (Sitapur Road)
We had an introductory visit to the BSC and were shown different equipment and their configuration present on the site. Following are the notes regarding the visit. There are in all 3-earthing pits on the site. They are all connected together. Remember, the earthing wire is green in colour. A diesel generator (DG) is used for backup in case of mains supply failure. Its power rating is 40kVA and the internal DG battery is 12V. A permanent mains is provided to the site. There is a Switch Fuse Unit (SFU) connected to the servo which is nothing but a transformer for a regulated power supply. The supply then reaches the Insulated Transformer which stabilizes the voltage. Auto Mains Failure (AMF) is attached, it turns into action whenever there is a mains failure, at that time it automatically switches on the DG. MCBs are present for each and every unit i.e. DG, Insulated transformer etc. The IDU or the Indoor Unit is provided by NEC, SRAL. IF cable connects the IDU to the ODU. A cable known as Tributary cable is used for alarms. The mains power reaching the shelter is fed to the Power Plant. The Power Plant contains a rectifier module and a Low Voltage Disconnector (LVD). The rectifier module converts the input AC into DC which is generally around 54V. The Low Voltage Disconnector (LVD) disconnects the circuit in case the voltage falls below 44V. Next in the shelter is Nokia Ultrasite BTS. It contains Power Cards (2 in number) each 48V, BB2F cards (depending on the number of TRX), BOI card, E1/T1 or RRI cards (max.4), duplexer (DVDx), multicoupler (M2HA), wideband combiner and TRXs. Next is the Fiber Rack: - XDH – 300 is used for making fiber link to the MSC. Just besides it is Krone which is used for E1 patching. Because there will be losses while connecting by twisting the cable and these losses can be couple of db which is unacceptable, hence, Krone is used for easier lossless patching. Surpass 7070 MUX is used. There is a centralized temperature control system. I contains cooling fans which are included in the cabinet core mechanics, HETA which is the BTS cabinet heater. The software in the BOI unit controls the unit temperature, hence, controls the fan speed and the temperature. There is a cold sensor in HETA, if the temperature goes below -10oC, heater will start automatically.

Hands-on experience: Nokia BTS Manager & MML sessions: Login to Nokia BTS Manager:  Launch the Nokia BTS manager software  It asks to key in the BSC id and the BCF (i.e. the BTS)  Authorized Username and password has to be keyed in along-with the above mentioned parameters Once successfully authenticated, the software shows a GUI of the BTS. It shows the number of TRX, BB2F etc i.e. the hardware present on the BTS. It also shows the status of the equipment, if it is not functioning properly then it is indicated by red alarm. If any TRX is idle, i.e. no call is going on it then it is shown yellow. During TRX addition the TRX is remotely configured using BTS manager. The alarm window is also shown just next to the virtual BTS which shows the history of alarms incurred. MML Sessions:The software being used for the MML sessions is Reflection. It is actually software from where we can login to the desired BSC and check its components and for that fact we can monitor all the BTSs under the BSC. Short code of the desired BSC for e.g. BLKO1 is entered once reflection is started, then it requires authentication. Once authenticated, the command line interface is visible where the MML commands can be issued.

Activity: New TRX configuration using Nokia BTS manager, hub manager and MML commands Three steps are to be followed while performing the TRX addition 1. Signalling should be unlocked from the BSC end 2. Mapping of the E1 i.e. Abis interface using Nokia Hub Manager 3. Unlock traffic from the BSC end i.e. make TRX fully functional First of all we should know the BCF number of the site and the number of TRX to be added to the BTS and the sector. Open the Reflection software and enter the BSC short-code i.e. BLKO1. enter the authentication details then key in the following commands ZEEI : BCF = 45; The above command will give the number of TRXs working or in the configuring state. Working TRXs are represented by “WO”. The PCM number column will also be visible. This PCM number will be added to the next command. ZDTI:::PCM=512;

The corresponding PCM number is added to the above command and we get the signaling timeslots. We note the timeslot number corresponding to the TRX we are going to add. ZERO:BCF=45, TRX=10; The above command gets you the traffic timeslot number which are going to map on the Abis. Note it down. To unlock the signaling from the BSC end we need to enter the following command ZDTC:T045A:WO:; Here 045 is the BSCF number of the site and A (in hex) means TRX number i.e. 10 th TRX. Once th signaling has been unlocked from the BSC end, open the Nokia BTS manager. In the BSC parameters enter the BSC ID and the BCF. Once logged in open the BTS Hub Manager from the menu. After the Hub manager has started click on the “Traffic Manager”. It will show an E1 (i.e. Abis). First of all put the traffic channels by clicking say 3-4; select the TRX say the 10th TRX. The traffic will be loaded. Always make sure to load traffic before signaling. Now, for the signaling click on TRX Sig and then click on the desired timeslot say 25-0 and select the desired rate i.e. 16 kbps, 32kbps etc. For EDAP just double click on the EDAP part and add the TRXs Once everything has been done, click on OK and exit the application. The last step is to make the TRX working, for it, open Reflection and enter: ZERS:BTS=256,TRX=010:U; Here, 256 is the sector number. Type ZEEI command to check if TRX has started working or not.

TRX addition (Manual Connections on the Site): TRX addition activity was conducted on the following three sites: 1. LKO067 (Amarpuri Indira Nagar) :- 1 more TRX added to the existing 2 TRXs in the γ sector. 2. LKO227 (Kursi Road) :- 1 more TRX added to the existing 2 TRXs in the α sector. 3. LKO136 (Vikas Nagar) :- 2 TRX added to the existing 2 TRXs in the β sector Hardware Required: BB2F  TRX  Wideband Combiner (if more than 2 TRX in a particular sector) Following connection are to be made while adding the TRX:-


								
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