transmission_principles by ondesk005

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									Transmission Principles                           Siemens




Transmission Principles



Contents
1         GSM Network Structure                        3
2         Duplex Transmission & Multiple Access       13
3         GSM - Fixed Network Transmission            25
4         GSM Air Interface                           31




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    Siemens   Transmission Principles




2                    TM2100EU03TM_0001
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Transmission Principles                             Siemens




1            GSM Network Structure

                          Transmission Principles




                            GSM Network Structure

Fig. 1




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    GSM: The Network Structure
    The international GSM service area covers all countries in which there is a GSM
    network.
    Networks provisioned by an operator on a national level for public mobile
    communication are called Public Land Mobile Networks PLMN. PLMNs built
    together with public fixed networks, i.e. "conventional" PSTN (Public Switched
    Telephone Network) or ISDN (Integrated Services Digital Network) networks the
    telecommunication infrastructure of a country.
    A Public Land Mobile Network is divided into mobile and fixed network components.
    They are connected via air interfaces.


    Fixed Network Components of the PLMN
    The fixed network components of a GSM-PLMN consist of:
    l   Base Station Subsystem BSS: The BSS is the fixed network part of the PLMN
        radio access (Radio SubSystem RSS). It realizes the radio transmission via the
        radio interface. Several fixed radio station, so-called Base Stations BS are co-
        ordinated by one control unit.
    l   Network Switching Subsystem NSS: The NSS forms the interface between the
        radio subsystem and the public fixed networks (PSTN, ISDN, PDN). It executes all
        signaling functions for setting up connections from and to mobile subscribers. It is
        similar to the exchanges of fixed network communication systems, but it
        furthermore fulfils important mobile communication specific functions, e.g. keeping
        track of the users / mobile stations location.


    Mobile components of the PLMN
    The Mobile Stations MSs are regarded as mobile part of the PLMN. The air or radio
    interface represents the connection between the MS and the PLMN fixed network
    components BSS and NSS. The organization of the radio interface is decisive for
    advantages and disadvantages of different mobile systems.




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                GSM Network Structure: Concept
                                                                    PLMN                        Fixed
       Mobile                    Um                            Public Land Mobile Network      network
   terminal device            Air Interface

                                                                                                   PSTN
                                                BSS                                             Public Switched
                                              Base Station                                     Telephone Network
                                               Subsystem

                                                                               NSS
                                                BSS
                                                                          Network Switching
                                                                             Subsystem
                                                                                                   ISDN
                                              Base Station                                     Integrated Services
                     MS                        Subsystem
                                                                        control/switching of     Digital Network
                    Mobile                                                mobile services
                    Station
                                                BSS
                                              Base Station                                          PDN
                                               Subsystem                                          Public Data
                                                                                                   Network


           Mobile                                            Fixed network
         components                                           components

Fig. 2




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    Mobile Components
    Mobile components are the Mobile Stations MS which transmit the users speech and
    data to the PLMN. The Mobile Station MS consist of:
    l   ME:      Mobile Equipment,
    l   SIM:     Subscriber Identification Module,
    The MS is not necessarily the termination point for the users data transmission. A
    Terminal Equipment TE, e.g. laptop, fax machine,... can be connected to the MS for
    final data handling.

    The Mobile Station MS
    An important difference between fixed network communications and mobile
    communications is the separation of equipment and subscriber identity. It is possible
    for the mobile subscriber to use various mobile terminal equipment with a personal
    identity by means of the SIM card, which includes his subscriber identity. The mobile
    station is defined as: MS = ME + SIM.
    The SIM card is allocated and activated by the provider upon completion of the
    contract. It is realized by means of a chip which contains a variety of permanent and
    temporary information for the subscriber (e.g. personal telephone register) and about
    him/her. Along with the personal (secret) ID numbers (IMSI - International Mobile
    Subscriber Identity, TMSI - Temporary Mobile Subscriber Identity) these stored
    information are for example algorithms and keys for ciphering the transmission.
    The PIN (Personal Identity Number) is important for the subscriber; it must be
    entered by the mobile subscriber before the start of the conversation in order to
    prevent fraud by unauthorized intruders. As a rule, calls cannot be made without a
    SIM card in the ME and without the PIN being entered. Emergency calls are an
    exception.




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     Mobile Components



                                     MS = ME + SIM




                                                          SIM
                                               Subscriber Identification Module



             SIM card: „the heart of MS“
            • Different equipments, one SIM (one bill)
            • Security: PIN (exception: emergency call)
            • Chip with subscriber identification,
                        security algorithms,
                        personal phone book,...



Fig. 3




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    The Cellular Network
    The breakthrough in mobile communications with regards to subscriber numbers and
    capacity was made possible by the introduction of the cellular radio system, making
    the full coverage over large areas possible and extending the concept of mobility.
    The cellular communication system was tested in various countries during the 1970s.
    Cellular networks of the first generation were introduced, e.g.:
    l   1979 in the USA: AMPS (Advanced Mobile Phone Service)
    l   1981 in Scandinavia: NMT (Nordic Mobile Telephone)
    l   1985 in Germany: C-450 (Siemens)
    l   1985 in Great Britain: TACS (Total Access Communications System)
    The successive digital systems of the second generation, and therefore GSM
    systems, are structured as cellular communication systems in the same way as the
    analogue systems.




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         Full coverage over
         extended areas                            The Cellular Network




                              l1979 in the USA: AMPS (Advanced Mobile Phone Service)
                              l1981 in Scandinavia: NMT (Nordic Mobile Telephone)
                              l1985 in Germany: C-450 (Siemens)
                              l1985 in Great Britain: TACS (Total Access Communications System)



Fig. 4




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     Roaming
     A further innovation of the cellular system was so called Roaming. This means that a
     subscriber can move freely within the PLMN and remain reachable on a single
     personal telephone number anywhere in this area. With GSM this concept of roaming
     can be expanded to the international area (international roaming). A subscriber
     whose home PLMN has a roaming agreement with other countries' GSM-PLMNs can
     also be reached in these PLMNs (Visited PLMN - VPLMN) without dialing the
     corresponding VPLMNs code; calls can also be made from that VPLMN. A
     prerequisite is of course that subscriber’s authorization for international roaming.


     Location Registration / Location Update / Location Area
     The subscriber has to be located in the respective cellular network. A procedure
     known as Location Registration or Location Update Procedure LUP carries out
     this function. It is important that the subscriber's temporary location area is recorded /
     registered with this procedure when the subscriber's mobile station is switched on
     and checked in, to forward calls to him. The temporary Location Area LA is the area
     in which the MS can move freely without having to carry out a location update. As a
     rule, the location area consists of a multiple cells and is configured by the operator
     according to the traffic or population density.


     Handover
     In cellular networks, it is not necessary for the subscriber to have his call interrupted
     when changing from one cell's service area to the area of a surrounding cell, as long
     as the cell areas overlap. This overlapping should be guaranteed with good planning.
     If the MS can receive better supply from another cell than the one currently in use
     during a call, the MS connection will be diverted to the relevant cell. This procedure
     designed for system quality maintenance ideally takes place without the user being
     able to notice and is known as handover.




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                         Roaming, Location Update
                               & Handover




                                  MS     BS


                            Handover

                    BS
                                Location Update:
                                • Location Area: most precise location information
                                                 stored in the network
                                • Location Registration: initial registration
                                • Location Update: update of registration


Fig. 5




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2            Duplex Transmission & Multiple Access


                          Transmission Principles

           UL                      DL


                       Duplex                      FDMA
                    transmission

         FDD                       TDD
                                                  Multiple
                                                  Access


                                           TDMA              CDMA


                            Duplex Transmission
                             & Multiple Access

Fig. 6




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     Duplex Transmission and Multiplex Procedure
     In a cell for access to a network two different principles have to be co-ordinated: The
     way of co-ordinating UL and DL, i.e. the Duplex Transmission, and the way of
     enabling the simultaneous access of several user to the same Base Station, i.e. the
     multiple access principle.


     Duplex Transmission: FDD & TDD
     Modern cellular mobile radio systems of the first (1G) and second generation (2G)
     enable full duplex transmission. Simultaneous communication on both sides, i.e.
     (virtually) simultaneous transmission and reception is thus possible.
     The transmission directions are designated as Uplink UL (MS to BTS) and Downlink
     DL (BTS to MS).
     There are two duplex transmission principles:
     l   Frequency Division Duplex FDD: Transmission and reception take place in
         different frequency ranges. The distance between the Uplink UL and Downlink DL
         frequency range is designated as duplex distance.
     l   Time Division Duplex TDD: Transmission and reception take place in the same
         frequency band. Uplink UL and Downlink DL transmission take place at different
         times. There is fast switching between UL and DL transmission, so that the user
         has the impression of simultaneous transmission and reception.




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                    FDD
           Frequency                                                               Uplink UL
         Division Duplex

                    Duplex distance
                                                                         Downlink DL
                                          UL / DL
                                         separated by
                                         frequency !     Base Station BS           Mobile Station MS




                                frequency f
                                                                    T
                                                                                          Same
         TDD            MS    transmit         receive   transmit        receive        frequency
      Time                      UL              DL         UL             DL
                                                                                       UL / DL
     Division                                                                         separated by
     Duplex             BS     receive        transmit    receive       transmit         time!
                                                                                    time t


Fig. 7




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     Multiplex Access: FDMA, TDMA and CDMA
     Several subscribers in one cell must be able to use the frequency range available for
     mobile communications together. Thus there must be procedures for regulating
     simultaneous access of different subscribers without disturbances. There are three
     different general procedures, partially in combination, which are used for co-
     ordinating the frequency resources:
     l   FDMA - Frequency Division Multiple Access
     l   TDMA - Time Division Multiple Access
     l   CDMA - Code Division Multiple Access


     FDMA - Frequency Division Multiple Access
     FDMA is a multiple access principle used widely in the first (analogue) generation 1G
     of mobile communications. It is however also used in the second (digital) generation
     2G of mobile communications, usually in combination with TDMA and in the third
     generation 3G together with CDMA.
     The available frequency reserves are divided into channels of the same bandwidth
     for FDMA. A certain frequency uplink and downlink is made available to an individual
     subscriber. Simultaneous calls and information transmissions of various subscribers
     thus take place on different frequencies. The transmitter and receiver must have a
     common knowledge about the channel frequencies to use.




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                                           Co-ordination
                                  of limited frequency resources
                                      for different subscribers




                                         Multiplex Access




                        FDMA                                   CDMA
                    Frequency Division                        Code Division
                      Multiple Access                         Multiple Access
                                              TDMA
                                            Time Division
                                            Multiple Access




Fig. 8




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     TDMA - Time Division Multiple Access
     The allocation of the available frequency range is made with respect to time for
     TDMA. A frequency band is not permanently available to one mobile station; it is
     used by several different mobile stations. Time is therefore split into individual time
     slots. The individual mobile stations are assigned the frequency range for the
     duration of a TDMA time slot in a periodically exclusive manner.
     A certain number of subscribers can use a certain frequency range virtually
     simultaneously with TDMA. The message information of a subscriber is taken apart
     and transmitted piece by piece to the corresponding time slots. The information
     carrying HF transmission in an individual time slot designated as a "burst".


     CDMA - Code Division Multiple Access
     In CDMA systems the users of one cell are not separated by frequency or time.
     Different to FDMA or TDMA simultaneously they take place in the same frequency
     range. The users are separated by unique Codes. The Base Station and Mobile
     Station must have common knowledge of the Codes used. The information of a
     single user is spread up from a narrowband signal to a wideband signal using a high-
     frequency code (high so-called "chiprate"). This spread information is transmitted via
     radio interface. After receiving the information, it is de-spread using the same code to
     regenerate the original information.
     The Codes in principal have orthogonal properties.




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           power                                   power
           P
                            time t
                                          FDMA     P
                                                                  time t
                                                                           TDMA


                                                                    TS 3
                                                                TS 2
                                                               TS 1

                    1   2      3

                            frequency f                         frequency f

           power                                 Multiple Access methods
           P
                            time t
                                     CDMA
                                                    Multiple    BS & MS share
                                                    method     knowledge about

                                                       FDMA      Frequency
                                                       TDMA      Time
                        3
                                                       CDMA      PN code
                        2
                        1

                            frequency f


Fig. 9




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     Transmission via GSM Radio Interface Um
     A combination of FDMA and TDMA is used for GSM. The GSM physical channels are
     defined by a pair of frequency bands (for UL and DL) and a Time Slot TS.


     FDMA in GSM
     In the GSM system, a band width of 200 kHz is defined for one frequency band.
     These HF channel widths are perfectly suited to the demands for speech
     transmission.
     Allocation to (E-) GSM900, GSM-R, GSM1800 and GSM1900 is as follows:
     l   GSM900: (880) 890 - 915 MHz; 925 (935) - 960 MHz; 124 (174) channel pairs ;
         with a duplex distance of 45 MHz
     l   GSM-R: 876 - 880 MHz; 921 - 925 MHz; 19 channel pairs; with a duplex distance
         of 45 MHz
     l   GSM1800:1710 - 1785 MHz; 1805 - 1880 MHz; 374 channel pairs; with a duplex
         distance of 95 MHz
     l   GSM1900: 1850 - 1910 MHz; 1930 - 1990 MHz; common use along with other
         standards (e.g. IS-95; D-AMPS); with a duplex distance of 80 MHz
     In GSM for DL the higher and for UL the lower frequency range is used in general.
     Remark: In co-ordination with the frequency plan regulation, there is a 200 kHz
     protective band inserted between the lower limit frequency and the first carrier of
     every sub-band, i.e. the corresponding channels are not used. This protective band
     known as the "guard band" is an accepted, virtually "unavoidable loss" for preventing
     interference between different applications in the totally filled frequency range.




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                            FDMA in GSM
           GSM900 / 1800 Frequency Allocation

      (880) 890 MHz               915 MHz             (925) 935 MHz                960 MHz           GSM900
          1710 MHz               1785 MHz                 1805 MHz                1880 MHz          GSM1800
                    UPLINK (UL)                                       DOWNLINK (DL)

                   Transmit band                                        Transmit band
               of the Mobile Station                                 of the Base Station


                      Duplex distance 45 MHz resp. 95 MHz
                    25 (35) MHz                                       25 (35) MHz
                      75 MHz                                            75 MHz


                                                  Guard band

                                         C                                                   C
          C C C                         124                    C C C                        124'
          1 2 3                        (174)                   1' 2' 3'                    (174')
                                        374                                                 374'
                       200 kHz
                                                            C - Radio Frequency Channel (RFC)



Fig. 10




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     TDMA in GSM
     Each of the 200 kHz frequency bands is further sub-divided by TDMA into 8 so called
     Time Slots TS. This produces 8 physical channels within one frequency band. In
     GSM a physical channel is thus defined by a determined frequency channel Uplink
     UL and Downlink DL and a determined time slot TS
     In the GSM system, up to 8 (with half-rate transmission even 16) calls can be
     transmitted "simultaneously" on one frequency band.
     A sequence of 8 time slots TS in one radio channel is referred to as a TDMA frame. A
     TDMA frame has a duration of 4.615 ms, an individual time slot a duration of approx.
     0.577 ms. The users data are transmitted virtually "piece by piece" on one specific
     time slot every TDMA frame.




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          GSM:                                           FDMA
    combined
   FDMA/TDMA

   1TS = 577 ms
    1TS = 577 ms      TDMA
   1 TDMA frame =
    1 TDMA frame =
   8 TS = 4.615 ms    frame
    8 TS = 4.615 ms

                                                                             1
                                                                         0
                                                                     7
                                                                 6
                                                             5
                                                         4
                                                     3       time
                                                2
                                            1
                                        0

                              200 kHz               frequency


Fig. 11




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3            GSM - Fixed Network Transmission


                       Transmission Principles

                           A/D conversion
                                               0011




    speech band 1


                                            1011      Multi-
                                                      plexer
                                                               band
                                                                3      2         1
     speech band 2
                                                                           common line
                                   1100
                                                                           PCM
                                                                      Pulse Code
                                                                      Modulation
     speech band 3


                     GSM - Fixed Network Transmission


Fig. 12




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     PCM30: Transmission in GSM fixed network part
     Information (conversations, data, signaling) is exclusively transmitted digitally via
     PCM30 lines in the GSM-PLMNs fixed network part.


     Pulse Code Modulation - PCM
     Sampling values of a speech information are transmitted using binary code words
     (digitally) in PCM.
     Due to the digital structure of the message, the PCM signals are less susceptible to
     interference than analogue signals. Regenerators reconstruct the original digital
     signal at the receiving end. Analogue signals, on the other hand, can only be
     amplified (including noise peaks).
     Amongst other things, during Pulse Code Modulation (PCM) an analogue oscillation
     is converted into a digital signal. A PCM signal can be transmitted alone or be
     embedded in a TDMA frame with other PCM signals (multiplexing).
     The conversion of an analogue telephone signal into a digital signal is carried out in
     three steps:
     1. Band limitation: A bandpass filter restricts the incoming signal to the audible
     frequencies, i.e. to 300 to 3400 Hz.
     2. Sampling: Sampling values are taken at fixed intervals from the limited telephone
     signal. The sampling frequency must be greater than twice the highest frequency
     within the analogue signal (Shannon Theorem). Internationally specified: 8000 Hz.
     3. 8-bit coding: Every amplitude value of the sampled (Pulse Amplitude Modulated -
     PAM) signal is transformed into an 8-bit word. The 8-bit word enables the analogue
     signal to be represented in 256 quantization intervals.
     Since the transmission of an 8-bit word requires only a portion of the sampling
     interval (125 ms) of the analogue signal, the 8-bit information is temporally
     multiplexed (TDMA-procedure). 8 bits are transmitted in each time slot.
     Using PCM30 transmission systems, a total of 30 digital user values can be
     transmitted in the time frame of the sampling period of an analogue value, i.e. in
     125 ms.




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     Generation of a PCM Signal


                           signal 1           1. Band limitation
                                                 (300-3400 Hz)
                                              2. Sampling (8000 Hz)
                                              3. 8-bit coding



                                  time slot      transmission of the coded
                                                 sample value of signal 1

                                         0 1 0 0 1 1 0 1


                                        coded sample value




                          signal 2


Fig. 13




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     PCM30
     PCM30 transmission systems use digital transmission lines or radio relay. A PCM30
     frame consists of 32 time multiplexed time slots.
     The 32 time slots can contain pulse code modulated message information (speech,
     data) or signaling information in the form of 8-bit words.
     The total bit rate of a PCM30 line is 2048 kbit/s
     l   Time slot 0: alternately frame identification word and service word (alarms)
     l   Time slots 1-15 and 17-31: calls or data
     l   Time slot 16: signaling channel
     The pulse frames are transmitted in a direct sequence.




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          PCM30: TDMA Principle
                                                  time
                                                   slot




                      telephone channels 1 - 15                  telephone channels 17 - 31


                    frame alignment/                        signaling channel
                    service word channel

  PCM30
                                                                                        PCM30




                       pulse frame            pulse frame              pulse frame



Fig. 14




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4            GSM Air Interface


                           Transmission Principles

                                Advantage:
                                 mobility




          Single cell systems        Cellular mobile communication systems
                                  1st generation    2nd generation  incl. satellite roaming
                                                     GSM (Ph1/2)          (GSM Ph2+)
  Limits:           cell            national       GSM service area          unlimited


                                GSM Air Interface

Fig. 15




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     Radio Interface: Advantages, Problems and Solutions
     The air or radio interface, i.e. the connection between the MS and fixed network
     components, represents the fundamental difference to a fixed network
     telecommunication system. The radio interface has its specific advantages, but also
     shows problems and disadvantages inherent to mobile communications.

     Advantage: Mobility
     The main advantage of mobile communications is the unrestricted mobility which can
     be achieved only via a radio interface. Mobility was extremely restricted, especially in
     the early years of mobile communications (one-cell systems). Mobility only reached
     as far as the radio coverage between the MS and the transmission/receiving
     installations would allow. These limits were stretched significantly by cellular mobile
     communication networks of the first generation (since the early 1980s). National
     borders and the degree of area coverage of a PLMN within a country formed the
     borders. In the GSM system, national borders no longer represented restrictions to
     mobility owing to “inter-national roaming”. It is still the case that nation-wide
     connectivity is only offered around urban areas and along main traffic routes in large
     areas of central Europe. Unlimited world-wide mobility is possible in co-operation
     between GSM and MSS such as Iridium, Globalstar and ICO.

     Problems & Solutions on the Radio Interface
     l   Cost Aspect: Problem - The need to built up a new network architecture with
         thousands of BTS. But: Compared with the costs for a fixed network ISDN / PSTN
         infrastructure, a GSM PLMN is comparable cheap, because there is no need for
         millions of lines into every private household.
     l   Capacity: The capacity of transmission via radio interface is a great problem in
         mobile communications. Optimized usage of radio resources reducing the cell
         sizes, introducing sector cells and introducing the Hierarchical Cellular Structures
         with Macro, Micro and Pico Cells solves this problem.
     l   Data Rate: GSM (Phase 1/2) offers a maximum 9.6 kbit/s, compared to the 64
         kbit/s of ISDN. Introduction of HSCSD, GPRS and EDGE enhances the GSM data
         rates significantly.
     l   Security Aspect: The radio interface can be intercepted with comparatively little
         technical expenditure. 1G could be intercepted without any problem, while the
         digital transmission of the second generation offers protective measures against
         interception; the transmission is coded.
     l   Health Aspect: The mobile radio frequencies lie near the resonance frequency of
         water (2.45 GHz). In order to keep thermal exposure to the mobile radio user as
         low as possible there are maximum power limitations for mobile phones, 2 W for
         GSM900 and 1 W for GSM1800.




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      The Air Interface Um:
      Problems of radio transmission and possible solutions

                                                   Construction of mobile
                                                  communication network
          Cost Aspect:                         cheaper than terrestrial network

                                       GSM900 / E-GSM: 124 / 174 frequency bands
                                       GSM1800: 374 frequency bands
          Capacity:                    increasing subscriber numbers, data transmission
                                       Þ Resource optimization / protection !!!



          Data Transmission Rate:      GSM Ph1/2: £ 9.6 kbit/s
                                       Ph2+: HSCSD, GPRS, EDGE > 100 kbit/s

                                                       Eavesdropping easy!
          Security Aspect:                            GSM offers encryption



                                       H2O resonance frequency (2.45 GHz)
          Health Aspect:                         Thermal load
                                        Þ Pmax = 2 / 1 W (GSM900/1800)


Fig. 16




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     Problems of Physical Transmission
     l   Screening: If there are hindrances between transmitter and receiver, the signals
         will weaken. A connection can thus become problematic or impossible. In GSM
         there is therefore the possibility of regulation of the transmitting power (Power
         Control - PC) from mobile and base stations over several orders of magnitude.
     l   Multipath Propagation: Multipath propagation through reflection and dispersion of
         radio waves leads to phase-shifted reception of signals of different paths. The
         interference can distort, amplify or erase the signal. An attempt to compensate for
         negative effects of multipath propagation is given by power control, frequency
         hopping, two antenna receivers for the base station (antenna diversity) and
         redundancy of the transmitted information.
     l   Distance MS - BTS: The distance between MS and BTS has proved to be
         problematic in several ways. The receive power sinks with increasing distance
         between transmitter and receiver theoretically with the square of the distance.
         Various physical effects such as atmospheric attenuation (weather-dependent)
         reduce the receive power even more. This attenuation depends on the frequency
         and increases with increasing frequency in mobile radio relevant frequency
         ranges. The distance furthermore causes a reception de-lay, which may lead to
         interference between neighboring time slots in TDMA. GSM responds to this delay
         by means of a regulation of the transmission time (Timing Advance TA). GSM900
         cells (GSM Phase 1/2) are limited to maximum 35 km, GSM1800 cells to
         maximum 8 km radius as a result of the distance-related problems. There is the
         possibility in GSM Phase 2+ to realize "Extended Range Cells" with a maximum
         radius of 100 km for GSM900.
     l   MS Speed: Moving mobile stations can cause transmission distortions due to
         Doppler effect. A compensation for this effect up to a maximum speed of 250 km/h
         (130 km/h), for GSM-R a more powerful compensation for speeds of up to 450
         km/h was developed.
     l   Interference with external systems: The receive quality can also be disturbed by
         electromagnetic waves from outside systems (e.g. car ignition, generators, PCs). A
         compensation is being tried out by means of the mechanisms described under
         multipath propagation.




34                                                                          TM2100EU03TM_0001
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Transmission Principles                                                                                       Siemens




            Radio Transmission: Physical Disturbances
                           •   Screening                      Þ signal attenuation (Power Control PC)
                           •   Multipath propagation          Þ interference (PC, f-hopping, diversity, regeneration)
                           •   Distance MS-BS                 Þ power loss (f-dep.); delay (PC, TA, cell size)
                           •   MS speed                       Þ Doppler effect (corrections)
                           •   External system interference   Þ quality loss (PC, f-hopping, regeneration)
      transmitted signal




                                                                                      received          signal to
                                                                                       signals          antenna




     Digital systems offer many
       error recognition and                           Mobility
      correction mechanisms
              ( ® redundancy)




Fig. 17




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     Siemens                                                           Transmission Principles




     Frequency Resources: Optimized Utilization
     In order to be able to keep up with the increasing demands on mobile
     communications despite the limited resources of the radio interface different
     approaches are being pursued.
     l   Additional Frequency Ranges: The simplest way to cope with the growing
         demand for mobile communications is to expand the available frequency range.
         This approach was pursued with E-GSM and GSM1800. Any further future
         expansion would be problematic as other frequency ranges are already reserved
         for other applications.
     l   Speech Compression: Speech compression in GSM allows a reduction of voice
         information from 64 kbit/s to 13 kbit/s in the so-called Full Rate FR speech and to
         5.6 kbit/s with the Half Rate HR speech. HR speech thus leads to a considerable
         increase in capacity. Central aspects of HR speech are described in the GSM Rec.
         06.02, 06.20 - 22, 06.41 and 06.42.
     l   Cell Size Reduction/Coverage: The most important measure for increasing the
         capacity of GSM networks lies in a reduction of the cell size. The resources of a
         radio cell are available to a small geographical area through the reduction of the
         cell radius or through the limitation of the cell coverage (sector cell). By doing so,
         the density of mobile communication subscribers and consequently the system
         capacity can be considerably increased. By halving the cell radius, its capacity is
         increased by a factor of four. Nevertheless the size of a (normal = macro) cell can
         not be reduced indiscriminately. Hierarchical Cell Concepts (Rec. 05.22) with
         macro, micro and Pico cells are significantly enhancing efficiency.
     l   OACSU (Off Air Call Set Up): Traffic channels are allocated only after a
         successful call setup, that is after the called subscriber (delayed allocation). The
         OACSU procedure thus serves to improve the frequency efficiency; it can be used
         for overload handling.
     l   Tariffs: Introduction of day- & night time tariffs can help to level down peak loads.
     l   Discontinuous Transmission DTX: For a conversation, this will mean that just
         speech phases are transmitted. Background noise, or so called comfort noise is
         transmitted with a greatly reduced bitrate (500 bit/s instead of 13 kbit/s as with
         speech phase) in phases in which a subscriber is silent. The other subscriber
         should thus not worry that connection has been broken off. In order to make
         discontinuous transmission possible, the presence of "useful" information for
         transmission must be determined by means of Voice Activity Detection VAD. DTX
         aspects are included in GSM-Rec.06.31 and 06.41, VAD aspects in Rec. 06.32
         and 06.42.




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Transmission Principles                                                                Siemens




     Frequency Resources: Expansion / Optimized Utilization
   • Extension of frequency range:
          GSM900:
          2 x 25 MHz    ®          E-GSM:
                                  2 x 35 MHz      +                   GSM1800
                                                                      2 x 75 MHz

   • Speech compression:             Digital speech information
                                                                                    HR:
                                                                        FR:
                       Fixed network: 64 kbit/s              ®       13 kbit/s
                                                                                    5.6
                                                                                   kbit/s
                                                                      Full Rate    Half Rate
                                                                       speech       speech
      • Cell size
        reduction:                35 / 8 km                         500 m
       (Radius reduction
       and sectorization)
                                                       ®
                                  omnicell
                                                                  180° / 120°
                                                                  sector cell
   • OACSU (Off Air Call Set Up)
   • Time Balance / Tariffs
   • DTX (Discontinuous Transmission) / VAD (Voice Activity Detection)

Fig. 18




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     Siemens                                                         Transmission Principles




     Advantages of Digital Transmission
     Digital transmission has many advantages over analog transmission:
     l   Network Capacity: The capacity of mobile communication networks can be
         considerably increased by the possibility of compressing digitalized speech
         information. The disadvantage of speech compression is a loss of information
         (reduction of speech quality).
     l   Service Offer: Digital data transmission simplifies the transmission of signaling
         information. This makes the introduction of a wide, quickly growing range of
         services possible in GSM beyond pure speech or data transmission.
     l   Cost Aspect: Digital equipment is less expensive to manufacture owing to better
         possibilities for use in highly integrated microelectronics. Purchase costs as well
         as operation and maintenance costs are thus less expensive and have allowed
         GSM's breakthrough onto the mass market.
     l   Miniaturization: Microelectronics used for digital information transmission allows a
         relatively simple reduction of the hardware (in comparison to analog transmission),
         especially of the mobile stations. Mobile phones have been used with GSM since
         the start; their weight has been reduced from over 500 g to some 50g within a
         couple of years.
     l   Security Aspect: Digital information can be ciphered much more easily than
         analog information. Transmission via radio interface is protected from fraud and
         unauthorized interception in GSM by the ciphering the digital user data (speech,
         data) and signaling data.




38                                                                           TM2100EU03TM_0001
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Transmission Principles                                           Siemens




      Advantages of Digital Information Transmission

            • Network capacity ® speech compression
            • Service offer ® signaling
            • Cost aspect ® manufacture, operation, maintenance
            • Miniaturization ® microelectronics
            • Security aspect ® easily coded
            • Transmission quality ® regenerability
                                   Code
                                 sequence


          Input data                                    Output data
                               ENCRYPTION
          (plain text)                                  (coded text)
                                 MODULE



Fig. 19




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     Siemens                                                        Transmission Principles




     l   Transmission Quality: Signal transmission via radio interface leads to consider-
         able distortions and weakening of the transmitted signals. Digital signals are
         fundamentally less susceptible to interference than analog signals and are better
         suited to regeneration. Analog speech connections become increasingly worse
         with increasing distance from the transmitter until they eventually disconnect.
         Digital transmissions on the other hand maintain a constant good quality over a
         long distance and then disconnect almost suddenly.




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Transmission Principles                                             Siemens




              Quality of Digital & Analog Signal Transmission

           S/N
           signal
           quality




                                  distance to transmitter       r

                      analog signal
                      digital signal


Fig. 20




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     Siemens                                                        Transmission Principles




     Reliable Transmission via Um: Channel Coding
     Various measures are taken in GSM to protect transmissions via radio interface from
     interference, distortions and loss of information. These measures are taken by means
     of channel coding.
     The transmission is protected in such a way that a certain number of transmission
     errors can be corrected by the error correction procedure, the so-called Forward
     Error Correction (FEC). By means of FEC the Bit Error Rates (BER) of the radio
     interface transmission are reduced to a rate of 10-5 to 10-6 from an unacceptable
     value of 10-3 to 10-1. Redundancy is added to the information to be transmitted in
     order to al-low recognition and correction of transmission errors.
     Channel coding of information on the transmit side comprises three steps:
     1. Adding of parity check bits and fill bits
     2. Error protection (redundancy) with convolutional coding
     3. Spreading by time: interleaving
     The same steps are carried out in reverse order at the receiving side.
     The added parity check bits serve to recognize incorrigible errors on the receiving
     side. The parity check bits are of special use in speech transmission. If incorrigible
     errors are indicated, the corresponding speech information is rejected and an attempt
     is made to interpolate the information from the preceding speech information.
     Convolutional coding serves to create redundancy. The original information (speech,
     data, signaling) is coded along with the parity bits. Important information runs through
     mathematical algorithms, where redundancy is added and the arrangement of the
     information is changed.
     Interleaving serves to temporally spread information. Information is collected up to a
     determined number of bits and is spread by time. The interweaving of the redundant
     information has the effect that information loss due to frequent short disturbances can
     be compensated by means of temporal spreading of the information.




42                                                                            TM2100EU03TM_0001
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Transmission Principles                                                       Siemens




                             Reliable Transmission via Um:
                                  Channel Coding


                                              Um
      Addition of:   Convo-        Inter-
                     lutional     leaving                       Convo-
       parity                                      De-inter-               Parity
                     coding                                     lutional
      and filler                                   leaving                 check
                                  temporal                     decoding
         bits        redundancy   spreading




          transmission side                             reception side



Fig. 21




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     Siemens                                                        Transmission Principles




     Speech Coding: FR, HR and EFR
     Speech transmission is of central importance in GSM. Speech information is handled
     especially by the radio interface for secure and resource-preserving transmission.
     Speech information is compressed and then redundancy is added (channel coding).
     There are three different speech codecs available in GSM for compression of speech
     information: the Full Rate (FR) Speech Codec was specified for GSM Phase 1, i.e.
     from the start, in Phase 2 the Half Rate (HR) Speech Codec and in Phase2+ the
     Enhanced Full Rate (EFR) Speech Codec were added.
     Full Rate FR and Enhanced Full Rate EFR Speech Codecs compress speech
     information from 64 kbit/s - used in digital line connected telephone networks such as
     ISDN - to 13 kbit/s respectively 12.2 kbit/s. So 13 kbit/s / 12.2 kbit/s are the net dat
     rate for speech transmission via the radio interface. The gross data rate after adding
     redundancy in channel coding is 22.8 kbit/s with FR and EFR.
     l   Half Rate HR Speech Codec compresses speech information from 64 kbit/s to 5.6
         kbit/s. The gross data rate after adding redundancy is 11.4 kbit/s. The connections
         of two Half Rate speech using subscribers can be realized in one physical channel
         together, with a gross data rate of 22.8 kbit/s.
     Models for speech generation are generally used for speech coding. Periodically re-
     turning elements of speech are identified as phonemata; redundancy is removed
     from the speech information. Even the attributes of hearing, especially the spectral
     covering effect, are taken into account in different ways.
     More efficient speech recognition mechanisms are of use for the HR introduced in
     GSM Phase 2 and EFR introduced in Phase 2+. The HR codec delivers a somewhat
     lower speech quality in comparison to the FR codec if transmission is undisturbed. It
     is more robust against radio specific disturbances owing to the relatively strong error
     protection. The EFR codec offers a significant increase in quality in comparison to the
     FR codec. It sounds more natural and "smoother" according to subjective test results.




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Transmission Principles                                                                            Siemens




                Speech Coding: FR, HR, EFR
          Speech coding ® models of speech and hearing
          • Removal of redundant information (periodic)
          • Transmission of central speech information
          • Reduction of speech information: 64 kbit/s ® 13 / 5.6 kbit/s (net data rate)

                                  Gross data rate via Um: 22.8 kbit/s


          Full Rate (FR) Codec                                     Redundancy (channel coding)
          GSM Ph1;
                                                                   9.8 kbit/s
          13 kbit/s
          Enhanced Full Rate (EFR) Codec                           Redundancy (channel coding)
          GSM Ph2+;
                                                                   10.6 kbit/s
          12.2 kbit/s
       Half Rate (HR)                     Redundancy                   HR & EFR:
       Codec; GSM Ph2;                                                 improved, acoustically optimized
                                          5.8 kbit/s
       5.6 kbit/s                                                      speech coding

                                                                       HR, FR almost the
             Gross data rate via Um: 11.4 kbit/s                       same quality


Fig. 22




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     Siemens   Transmission Principles




46                    TM2100EU03TM_0001
                            © 2002 Siemens AG

								
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