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Umts interface and radio channels

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     In the Public Land Mobile communication Network (PLMN), the MS is connected with
the network via the radio channel. In this way, the subscribers can access the network and
obtain communication services. To achieve the interworking between MS and BTS, a set of
standards are needed for signal transmission through the radio channel. This set of
specifications which related to the radio channel signal transmission, aim at Um interface.

      The Um interface is a kind of radio interface. It is responsible for the communication
between the mobile station and the BTS and provides the interworking link between the
mobile station and GSM system. Its physical connection is achieved via the radio waves.
The Um interface is the most important interface among all the interfaces in GSM system.
First of all, the complete and normative Um interface realizes full compatibility between MS
of different venders and different networks. That is fundamental conditions needed in global
roaming of the GSM system; second, the radio interface determines the rate of frequency
spectrum utilization of GSM system. The name “Um” is derived from the name of the
                                                                                    m
interface between the client terminal and the network in ISDN , in which the “m” means
“mobile”.




                                              3
     The first layer is the physical layer, which is marked as L1 and is the lowest layer. This
layer provides the radio link needed in transmission of bit stream. It defines the radio access
capability of the GSM system and provides the most fundamental radio channel (logical
channel) for the information transmission of higher-layer , including the traffic channel and
control channel. For detailed description of logical channel, please refer to relevant
documents.

     The second layer, marked as L2, is the data link layers and it is the middle layer. It
applies the LAPDm protocol. This layer includes various types of data transmission
structures. It controls the data transmission so as to ensure the reliable dedicated data links
which are set up between the mobile station and base station. The LAPDm protocol is based
on the D channel link access protocol (LAPD) in ISDN. For LAPDm, the radio transmission
and control characteristics are suitable to the signal transmission at the Um interface.

     The third layer is the network application layer, which is marked as L3 and is the top
layer. It includes various types of messages and programs for control and management of
the services. That is to say, in this layer, specific messages of the mobile station and the
system control processes are packed into different protocols and mapped to logical channels.
L3 includes three sub-layers: the Radio Resources management (RR), Mobility Management
(MM) and Communication Management (CM). These are the major contents of the
messages transmitted via the Um interface. The CM sub-layer includes three major parts:
    (ca co t o se ce),        (supp e e ta y se ce) a d S S (s o t essage se ce)
CC (call control service), SS (supplementary service) and SMS (short message service).




                                                4
      The GSM Um interface applies the multiple access technology. With this technology,
multiple subscribers can share the same public communication connection. Basically, there
are three modes of channelization for multiple access, the frequency, time and code division
multiple access connections respectively. They are frequency division multiple access
(FDMA), time division multiple access (TDMA) and code division multiple access (CDMA)

     FDMA-frequency division multiple access:

                                                           sometimes.        mode,
      The frequency division is also called channelization sometimes In this mode the whole
assignable frequency spectrum is divided into many single radio channels. Under the control
of the system, each subscriber can be served by any one of these channels.

     The analogue cell system, AMPS, is a typical example that uses the FDMA technology.
The digital cell system can also use the FDMA. The difference is that it only uses the
frequency division mode,but the GSM system uses the FDMA also.




                                              5
     TDMA-time division multiple access:

      The time division multiple access refers to dividing a broadband radio channel into
several timeslot, so that every subscriber seizes one of the timeslots; and the signal is
received (or transmitted) only in that specific timeslot. That is the reason why it is called time
division multiple access. This multiple access mode is used in digital cell systems and GSM
as well.

     CDMA-Code division multiple access:

      It is a multiple access mode in which the spread spectrum technique is used to form
different code sequences. It is quite different from FDMA and TDMA. In FDMA and TDMA,
the subscriber information is divided or separated based on the frequency and time, but
CDMA mode can transmit information of multiple subscribers via the same radio channel at
the same time.




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      The radio channel is quite different from the wired channel. First, the radio channel has
a distinct time-change characteristic. The radio channel is exposed to the air, so it is
vulnerable to the interferences in the air. The signal is influenced by various interferences,
multi-path fading and shadow fading, so the error bit ratio is rather high. To solve the
problems mentioned above, a series of forward and backward(uplink & downlink)
transmission techniques are applied. The original subscriber data or signaling data are
transformed before being carried by the radio waves. And at the other end of the
transmission,                                  done.
transmission a reverse transforming will be done This can provide necessary protection to
the transmitting signal. The transformation methods roughly include the channel
coding/decoding, interleaving/de-interleaving, burst formatting, encryption/decryption, and
modulation/demodulation. For the voice, to pass an analog-to-digital converter is actually a
sampling process in the rate of 8KHz,after quantification each 125μs contains 13bit of code
stream; then speech coding is performed with every 20ms as a segment and the code
transmission rate is reduced to 13Kbit/s, which becomes 22.8Kbit/s after the channel coding;
then the voice becomes a code stream at 33.8kbit/s after code interleaving, encryption and
burst formatting and is transmitted finally. The processing at the terminal is just the reverse
of the above procedures.




                                                8
The voice compression coding technique is widely used in the modern digital
                 systems.         technique,
communication systems In this technique a voice coder is used to set up a model to
simulate the voice and noise produced by human vocal organs. The parameters to form
the model will be transmitted through the TCH channels.

The voice coder is based on the residual excited linear prediction (REIP) coder. Moreover,
the long term predictor (LTP) is used to enhance the compression effect. LTP can make
the coding of residual data more advantageous by removing the vowels from the voice.
                      unit,
With 20ms as the unit the voice coder outputs 260bits after compressed coding       coding.
Therefore, the code rate is 13kbps. According to the different classes of the importance of
the information, the output bits can be classified into three categories: 50 very important
bits,132 important bits and 78 ordinary bits.

Comparing with the traditional PCM line on which the voice is coded directly and
transmitted (64kbps), the 13kbps voice rate of the GSM system is much lower. The more
                                                                  6.5kbps (half rate
advanced voice coder in the future can further reduce the rate to 6 5kbps (half-rate voice
coding).




                                             9
      To check and correct errors during the transmission, redundancy data and the
information calculated from the source data are added to the stream so as to increase the bit
rate. For the voice, the length of these codes is 456 bits every 20ms.

     The bit rate of code stream output from the voice coder is 13Kbit/s, which is divided into
many 20ms continuous segments with each segment containing 260 bits. They can be
classified as:

     50 very important bits;

     132 important bits;

     78 ordinary bits,



     Redundancy processing is conducted, as shown in the above diagram.

     The block coder is applied with 3 bits of redundancy code; while the excited coder
applies with 2 times redundancy and then adds 4 tail bits into the data stream.

     There are three channel coding modes in the GSM system: convolution coding, block
coding and parity coding. For detailed information, please refer to related documents.




                                              10
     If the voice signal is modulated and transmitted directly after channel coding, due to the
condition changes in mobile communication channel, a deep of the fading will influence a
successive string of bits and cause high bit error rate.

      If the bits of a successive string are interfered or lost, the other end of the
communication can not decode the interfered or lost bits. To solve this problem, some
technique or method to separate the successive bits are required. Thus the successive bits
in a message can be transmitted dispersedly so that the bit error should be discrete. In this
way, even if errors occur, the errors are only concerned with a single or very short bit stream,
which will not lead to that the whole burst or the whole message block cannot be decoded. In
this case, the channel coding will take effect and recover the bit errors. This method is called
interleaving technique. The interleaving method is the most effective coding method for
dispersion of bit errors.

      The key point of interleaving is to disperse some bits( suppose there are “b” bits) of the
code into some ( suppose to be “n” bursts) burst sequences so as to change the adjacent
relationship between bits. The higher the value of “n” is, the better the transmitting works.
However, the transmission delay is higher too. Therefore, a balanced consideration is
needed, the interleaving is related to the purpose of the channel. In the GSM system, the
second interleaving is applied.




                                               11
      After channel coding, the extracted 456 bits are distributed into 8 groups with each
                      bits.                interleaving,
group containing 57 bits That is the first interleaving also called internal interleaving as
shown in the above diagram. Through the first interleaving, the successive messages in the
groups are dispersed. One burst carries two segments of voice information composed of 57
bits. Obviously, if the two groups of 57 bits information from the first interleaving of a
successive 20ms voice blocks are inserted to the same burst sequence, the loss of the burst
will lead to loss 25% bits in the 20ms voice block. Therefore, one more interleaving is
needed between two voice blocks, which is called the inter-block interleaving or second
interleaving.

      Suppose that voice block B is divided into 8 groups: perform inter-block interleaving to
the first four groups (B0, B1, B2 and B3) of block B and the last four groups (A4, A5, A6 and
A7) of the previous voice block A ; thus, four bursts are constituted: (B0, A4), (B1, A5), (B2,
A6) and (B3, A7); to break the adjacency relationship between successive bits, bits of block
A occupy the even position of the burst while bits of block B occupy the odd position of the
burst. For example, B0 occupies the odd bit of the burst while A4 occupies the even bit.
Similarly, perform interleaving to the last four groups of block B and the first four groups of
the next block C. After the second interleaving, a 20ms voice block is inserted into 8 different
burst sequences respectively and then transmitted one by one. Even if a whole burst is lost
during transmission process, only 12.5% of each voice block is influenced and the errors can
be corrected through channel coding at the other end. In addition, the second interleaving for
the control information is different. The interleaving mode is (B0, B4), (B1, B5), (B2, B6) and
(B3, B7).




                                               12
      As shown in the diagram, the front and end 3 tail bits delimit the burst; the 26 bits are
training sequence bits; and the bit at both sides of the training bits are used as “bit stealing
flags”.




                                               13
     Transmission delay is unavoidable in the radio interface. If the mobile station moves
away f     the base station d i a call, th f th di t
      from th b      t ti during                            the      delay. Th uplink i as
                                    ll the further distance th more d l     The li k is
the same.
      If the delay is too high, the timeslots of the signal from a certain mobile station and that of
the next signal from another mobile station received by the base station will overlap each
other, thus causing inter-code interference. To avoid this, during a call, the measurement
report sent from the mobile station to the base station carries a delay value. Moreover, the
base station should monitor the time when the call arrives and send an instruction to the
mobile station via the downlink channel every 480ms so as to inform the mobile station the
time of advance transmission. This time is the TA (timing advance), which ranges between
0~63 (0~233μs ). The TA value is limited by the timing advance code 0~63bit of the GSM
system. Therefore, the maximum coverage distance of the GSM is 35km. Its calculation is as
follows:
     1/2*3.7 μs /bit*63bit*c=35km
      {In the formula, 3.7μs /bit is the duration per bit (156/577); 63bit is the maximum bit
     b     f the time adjustment; c i th li ht speed (t
number of th ti        dj t    t is the light         d (transmission speed of th signal); and ½
                                                               i i        d f the i    l)    d
indicates that the go and return trip of the signal.}
      According to the above description, the distance corresponding to 1bit period is 554m.
Influenced by the multi-path propagation and MS synchronization precision, the TA error may
reach up to about 3bit (1.6km).




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      When the MS is in idle mode, the time sequence within the MS can be adjusted via the
       channel. However,
SCH channel However the mobile station does not know how far it is away from the base
station. If the distance between the MS and the base station is 30km, the time sequence of
the MS will be 100μs slower than that of the base station. When the mobile phone sends its
first RACH signal, it is already 100μs later. For there is still another 100μs of transmission
delay, when the signal reaches the base station, the total delay is 200μs . It is very possible
that the signal collides with the pulse of the adjacent timeslot around the base station.
Therefore, RACH and some other channel access pulses will be shorter than other pulses.
Only after receiving the time sequence adjustment signal (TA) from the base station, MS can
send pulses of normal length. In this case, the MS needs to send signals by 200μs in
advance.




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      When the voice signal is transmitted after being processed and modulated, the
frequency hopping technique will be used too, i.e. the transmission carrier varies constantly
at different timeslots (of course, the variation should comply with the frequency planning
principles).

     The following two factors are considered in introduction of the frequency hopping
technology:

     1.                                                        band,
     1 For the fading process is related to the frequency band the application of the
frequency hopping in the system may reduce the effects of the rayleigh fading.

      2. Due to the interference diversity, in areas with dense traffic, the cell capacity is
restricted by the interference caused by the frequency multiplexing. In addition, the system is
designed to meet the demands of subscribers, the maximum capacity of the system is
calculated on the assumption that the quality of a certain number of calls is reduced distinctly
due to interference. The lower the diversity measured around the specified C/I value, the
larger the system capacity. The interference on a call is the average value of the interference
level caused by many other calls. Thus, for a specified interference intensity, the more the
interference sources, the better the system performance.




                                               16
The radio interface of the GSM system is designed with the slow frequency hopping (SFH)
technique.
technique The difference between SFH and the fast frequency hopping (FFH) is that the
frequency change of the latter is faster than the modulating frequency. During the whole
burst sequence transmission period of the GSM system, the transmitting frequency
remains unchanged. Therefore, it belongs to slow frequency hopping, as shown in the
above diagram.

The GSM system allows 64 types of different frequency hopping sequences. There are
mainly two parameters involved in description of them: mobile allocation index offset
(MAIO) and hopping sequence number (HSN). The values for MAIO can be as many as
the frequencies in a group; and there are 64 different values available for HSN.




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Actually, during the communication process, the mobile subscriber talks only 40% of the
                                                                              time.
time and there is not much useful information transmitted during rest of the time If all the
information is transmitted to the network, it will not only be a waste of the system
resources but also add more interference to the system. In order to overcome this problem,
the DTX technique is used in the GSM system, i.e. the transmission of radio signals is
prohibited when there is no voice signal being transmitted. This is to reduce the
interference level and increase the system efficiency. In addition, this mechanism can also
save the battery of the mobile station and prolong the standby time of the mobile station.
          ,
Note that, the DTX function is not used for data transmission.

There are two transmission modes for the GSM system: one is the normal mode. In this
case, the noise obtains the same transmission quality as the voice; the other is the
discontinuous transmission mode. In this case, the mobile station only transmits the voice
signals. The noise at the receiving end is artificial.

The artificial noise is used to inform the hearer that communication connection is ok when
                                 p     g                      g
none of the subscribers are speaking. And the noise is designed as a comfortable noise
which will not make the hearer uncomfortable.

The comfortable noise transmission also meets the requirements of the system
measurement. In DTX mode, only 260bit codes are transmitted per 480ms; while in
normal mode, 260bit codes are transmitted per 20ms. In the DTX mode, these 260 bits will
generate SID (Silence Descriptor) frames. These frames, like the voice frames, will be
processed via channel coding, interleaving, encryption and modulation, and then be
                            bursts.         time,                    transmitted.
transmitted in 8 continuous bursts In other time there is no message transmitted




                                            18
The DTX mode is optional. However, the transmission quality will be reduced a bit in the
      mode.                                                               subscribers,
DTX mode Especially when both ends of the communication are mobile subscribers the
influence on the transmission quality will be more severe because the DTX will be used
twice on the same path. In addition, to implement the DTX function, the system should be
able to indicate when to start the discontinuous transmission and when to stop it; and
when the DTX is active the coder should be able to detect whether the signal is a voice
signal or a noise signal. Thus, the VAD technique has to be used. The VAD algorithm
determines whether each output frame contains voice or background noise by comparing
the measured signal energy with the threshold defined for it. The principle of the
determination is that the noise energy should always be lower than the voice energy.




                                          19
During the process of radio transmission of signals, to reduce the interference, to increase
                                            spectrum,                                life,
the utilization efficiency of the frequency spectrum and to prolong the battery life the
transmission power can be adjusted, that is called power control. More specifically, the
power control refers to adjust the transmission power of the mobile station or base station
in the radio mode within a certain range. Its objective is the same as that of the DTX.
When the receiving level and quality is rather strong, the transmission power at the TX
terminal can be reduced appropriately so that the communication can be kept at a certain
level. In this way, the interference on other calls around can be reduced. The specific
process will be described in the subsequent content together with Huawei power control
algorithm.




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21
      The major basic concept concerned with the radio path transmission of the GSM system
is the burst sequence (simplified as Burst). It is a string of transmission units including more
than 100 modulation bits. The burst sequence has a restricted duration and seizes a
restricted radio frequency spectrum. They can be described as output from the time and
frequency window. This window is called Slot. In other words, within the system frequency
band, the central frequency of the slot is set every 200KHz (observed from the opinion of
FDMA); while the slot occurs cyclically as the time evolves, which seizes 15/26ms (i.e.
                0.577ms)                                               TDMA).
approximately 0 577ms) each time (observed from the opinion of TDMA) The intervals of
these slots are called Time Slots and the duration of them is called the time unit (marked as
BP, indicating the Burst Period).

      We can use the time/frequency chart to draw the slot as a small rectangle with the
length of 15/26ms and width as 200KHz, as shown in the above diagram. Similarly, we can
call the 200KHz bandwidth specified in GSM as Frequency Slot, which is equivalent to the
Radio Frequency Channel (i.e. RF channel) in the GSM specifications.

      The two terms: timeslot and burst sequence are different to a degree in actual
application. For example, the burst sequence is sometimes related to the time-frequency
“rectangular” unit and sometimes to its content. Similarly, the timeslot has the meaning of
time value or indicates that a slot in every 8 slots is used periodically.

     To use a specified channel means to transmit the burst sequence at the specified
           d frequency, i
moment and f                  h      ifi d l
                        i.e. the specified slot. G
                                                 Generally, the time of slots i a channel i
                                                       ll h i         f l     in   h    l is
discontinuous.



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    One TDMA frame includes 8 basic timeslots, and each timeslot is a basic physical
channel.

     The Physical Channel is a combination of FDMA and TDMA, which is composed of the
timeslot streams between the base station (BS) and the mobile station (MS). The positions
of these timeslots do not change in different TDMA frames. The above diagram shows the
complete structure of the TDMA frame, including the timeslot and burst sequence. It should
be remembered that the TDMA frame is the “physical” frame repeated on the radio link.

     Every TDMA frame includes 8 timeslots, which seize 60/13≈4.615ms altogether. Every
timeslot contains 156.25 bit duration, which seize is 15/26≈0.557ms. Multiple TDMA frames
constitute a Multi-frame, which has two types of structures including 26 or 51 coherent
TDMA frames respectively. These multiframes should be used when different logical
channels are mapping to one physical channel.

       The period of the multiframe containing 26 frames is 120ms, which is used in the traffic
channel and the associated control channel. In these frames, 24 bursts are used for the
traffic and the remaining two are used for the signaling.

     The period of the multiframe containing 51 frames is 3060/13≈235.385ms, which is
used especially in the control channel.




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Multiple multi-frames constitute a Super frame, which is a coherent 51×26TDMA frames.
           say,                                                      multi-frames
That is to say one super frame can contain either 51 26TDMA multi frames or 26
51TDMA multi-frames. The period of all super frames is 1326 TDMA frames, i.e. 6.12
seconds.

Furthermore, multiple super frames constitute a Hyper frame, which contains 2048 super
frames and its period is 12533.76 seconds, i.e. 3 hours, 28 minutes, 53 seconds and 760
milliseconds. The hyper frame is used in encrypted voice and data. Each period of the
                                      frames,
hyper frame contains 2715648 TDMA frames which are numbered in sequence from 0 to
2715647 successively. The frame number is transmitted in the synchronous channel,
which is also a necessary parameter in the frequency hopping algorithm.




                                          24
The Logical Channel is used in time multiplex in a physical channel, which is categorized
                                                                 channel.
according to the types of messages transmitted in the physical channel Different logical
channels are used in transmission of different types of information between BS and MS,
such as the signaling or traffic data. In GSM system, five different types of burst
sequences are specified for different logical channels, which have different time-amplitude
diagrams as shown in the above diagrams.

The training sequence helps to discriminate radio channels with same frequency so as to
                        signals. However,
help to demodulate the signals However there is no training sequence for FB and DB; for
SB and AB, the training sequence is constant, i.e. the synchronous bit; for NB, there are 8
different training sequences specified in the specifications. These 8 different training
sequences of NB are numbered from 0 to 7, which are called training sequence numbers.
By allocating training sequences with distinct differences to channels of the same
frequency used in cells that are close to and may interfere with each other, the co-
frequency interference can be avoided efficiently during modulation.




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26
      As we know, every cell has several TRX and every TRX includes 8 timeslots (i.e.
providing 8 basic physical channels). In the radio subsystem, the physical channel supports
the logical channel based on the type of message transmitted . In this way, the physical
channels are mapped as different logical channels. In the GSM system, the logical channel
is classified as the dedicated channel (DCH) and the common channel (CCH). Sometimes, it
can also be classified as the traffic channel and control channel.

      The traffic channel (TCH) carries voice or data, which are the full-rate traffic channel
(TCH/F) and half-rate traffic channel (TCH/H). These two types of channels carry information
at the rates of 13 kbit/s and 6.5 kbit/s respectively. The channel using half of the time slots of
a full-rate channel is the half-rate channel. Therefore, a carrier can provide 8 full-rate or 16
half-rate traffic channels.

     The frequency correction channel (FCCH) carries the information for frequency
correction of MS and BTS.

    The control channel (CCH) is used to transmit signaling or synchronous data. There
are mainly 3 types of control channels: Broadcast Channel (BCCH), Common Control
Channel (CCCH) and Dedicated Control Channel (DCCH).




                                                27
1. Frequency correction channel (FCCH)

It carries the information for frequency correction for the mobile station. The MS can
communicate with a cell and demodulate other information of the same cell just via FCCH.
Moreover, the MS can also know whether the carrier is a BCCH carrier via FCCH.

2. Synchronous channel (SCH)

After FCCH decoding, the MS will continue to decode the SCH channel message. This
message includes the information for MS frame synchronization and BS identification:
Base Station Identifying Code (BSIC). It seizes 6 bits, in which 3 bits are PLMN color
codes ranging between 0~7; while the remaining 3 bits are Base Station Color Codes
(BCC) ranging between 0~7.

The simplified TDMA frame number (RFN) seizes 22 bits.

3. Broadcast control channel (BCCH)

Generally, there is always a BCCH channel in every cell , which is responsible for
broadcasting system information to the mobile station. These system information enable
the MS to identify and access network at the idle mode.




                                          28
4. Paging channel (PCH)

This is a downlink channel which is used to page mobile stations. When the network is to
set up communication with a certain MS, it will send paging messages via the PCH
channel to all cells in the LAC area in which the certain MS has currently registered, and
indicates TMSI or IMSI of the certain mobile.

5. Access granted channel (AGCH)

This is a downlink channel used in answering a network access request by the mobile
station, i.e. allocation of an SDCCH or a TCH directly.




                                           29
1. Random access channel (RACH)

It is an uplink channel used for MS randomly access to network by requesting for an
SDCCH. The request includes a 3bit setup reason (call request, paging response, location
update request and short message request etc.) and a 5bit random reference number for
MS to differentiate the access granted messages.

2. Stand-Alone Dedicated Control Channel (SDCCH)

It is a bi-directional dedicated channel used in transmission of signaling messages
concerned with connection setup, location update message, short message,
authentication message, encryption command, channel allocation message and various
kinds of additional services etc. It can be divided into the Stand-Alone Dedicated Control
Channel (SD/8) and the Dedicated Control Channel in combination with CCCH (SD/4).




                                           30
    3. Slow associated control channel (SACCH)

     It is used together with the traffic channel or SDCCH. It carries some specific
information while transmitting the subscriber information. At the uplink, it mainly transmits the
measurement report ; while at the downlink, it mainly transmits some system information.
These messages include the quality of communication, LAI, CELL ID, BCCH signal strength
of the adjacent cell, NCC permit, cell option, TA and power control level etc.

    4. Fast associated control channel (FACCH)

     It is used together with TCH for providing signaling messages whose speed and
timeliness are much higher than the slow associated control channel (SACCH) for the
system during the transmission process. This channel uses frames borrowed from the traffic
channel for its connection and transmits such instruction messages as “handover”. For the
voice decoder can repeat the voice of the last 20ms, this kind of interruption due to frame
stealing will not be detected by the subscriber. Besides the three types of control channels
described above, there is a cell broadcast control channel (CBCH). It is used at the downlink
and carries the short message service cell broadcast (SMSCB) information. This kind of
control channel uses the same physical channel as that used in SDCCH.




                                               31
32
As shown above, CCCH=PCH+RACH+AGCH; downlink CCCH=PCH+AGCH; and uplink
CCCH=RACH.                   combinations,
CCCH=RACH In the above combinations combination 3 and 4 must be allocated to slot 0
of the BCCH carrier configured for the cell; while combination 5 must be allocated to
timeslots 2, 4 and 6 of the BCCH carrier. The FACCH works in the frame stealing mode,
for which no fixed time sequence will be allocated. In addition, the cyclic multiframe period
of SACCH/C4 and SACCH/C8 is 102 frames.




                                             33
The TDMA/FDMA multiplexing is used in GSM, the information needed in the
                                                  FCCH+SCH.
synchronization between MS and BTS is provided by FCCH+SCH

The MS determines the frequency of the BCCH carrier by searching for the frequency
correction Burst transmitted via FCCH; then it finds the SCH (synchronization channel)
according to the relationship between SCH and FCCH and decodes the current frame
number and BSIC for synchronization with BTS. Furthermore, it determines whether the
cell is barred or not and decodes the system information on BCCH.

In the structure diagram of extended BCCH, except that the F and S timeslots are
replaced by Idle timeslots, the rest of the structures are the same as that of the main
BCCH.




                                          34
It is used in the configuration of cells of low traffic density and small capacity. The
Combined BCCH is only configured at timeslot 00.

Channel combination: FCCH+SCH+BCCH+CCCH+SDCCH/4+SACCH/4

SDCCH/4: Stand-alone dedicated control channel. Each TDMA multiframe with 51 frames
has 4 SDCCH;

SACCH/4: Slow SDCCH/4 associated control channel;

Compared with the main BCCH channel, 4 signaling channels are added to the 51 frames.
The functions of these 4 signaling channels are the same as those of the SDCCH8
channel. Therefore, this channel combination can be taken as a combination of the
functions of the above two channels. This combination take effect on two aspects: first,
this reduced the quantity of AGCH+PCH on CCCH and only a small-capacity system is
provided; second, this combination provides a certain quantity of signaling channels in
         0. Thus,                                                    small capacity system.
timeslot 0 Thus it is unnecessary to assign SDCCH8 channels in a small-capacity system
This channel suitable for small-capacity systems. And it is also an example of the flexible
GSM network configuration.




                                            35
    Channel combination: SDCCH/8+ SACCH/C8

     SDCCH/8: Stand-alone dedicated control channel. Each TDMA multiframe with 102
frames has 8 SDCCH.

    SACCH/C8: Slow SDCCH/8 associated control channel.




                                        36
Channel combination: TCH/F + FACCH/F + SACCH/F

TCH/F: Full-rate voice channel;

FACCH/F: Full-rate fast associated control channel;

SACCH/F: Fast TCH/F associated control channel.




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     The common control channel includes PCH, AGCH and RACH, in which AGCH and
PCH are downlink while RACH is uplink. Its purpose is to send the access granted
(immediate assignment) message, paging message and random access message. Based on
the configuration of traffic channels in the cell and the traffic model of the cell, the CCCH
channel can be borne by one or more physical channels. Moreover, the CCCH can share the
same physical channel with the SDCCH channel. The combination mode for the common
channel in the cell depends on the configuration parameter of the common channel.

     As a way for load control, the MS may be distributed to several different sub-groups by
operators for access or other operation purposes. The CCCH grouping and paging grouping
are two examples.




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     The number of CCCH supported by the system depends on the parameters of
BS_CC_CHANS, CCCH type and the number of channels. These message are broadcast in
the control channel description domain of system information type 3.




                                       39
40
     Suppose that the number of frames of the same paging is 7 and the number of paging
        (9or3-BS_AG_BLKS_RES:
blocks (9or3 BS AG BLKS RES: number of access granted reserved blocks) of each 51
multi-frame is 4. Then the paging group number cycles between 0-27 (28 in total).




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      The number of paging blocks in each 51 multiframe is calculated according to the
following formula:

    If the CCCH and SDCCH are combined in one physical channel:

     Number of paging groups per super-group=3-number of access granted reserved
blocks

    in other cases, i.e. the CCCH is not combined with the SDCCH:

     Number of paging groups per super-group =9- number of access granted reserved
blocks

     Both the above two parameters: “BS_AG_BLKS_RES” and the “BS-PA-MFRAMS” are
broadcasted in system information type 3




                                          42
     For example:

      The system is configured as main BCCH+SDCCH+extended BCCH+TCH+extended
BCCH+TCH+TCH+TCH; the number of access granted reserved blocks is 7; the number of
multiframes of paging is 2; and the IMSI numbers of two subscribers are 460042709000034
and 4600427090037 respectively. Then:

     Subscriber 1 CCCH group number is: ((460042709000034 mod 1000) mod (3*(9-
7)*2))div ((9 7) 2)
7) 2))div ((9-7)*2) = 2;

      The paging group number is ((460042709000034 mod 1000) mod (3*(9-7)*2))mod ((9-
7)*2) = 2;

     Subscriber 2 CCCH group number is: ((460042709000037 mod 1000) mod (3*(9-
7)*2))div ((9-7)*2) = 0;

                                                                 (3 (9 7) 2))mod ((9
     The paging group number is: ((460042709000037 mod 1000) mod (3*(9-7)*2))mod ((9-
7)*2) = 1.

     Subscriber 1 will access randomly at timeslot 4 and keeps watch of the paging at
paging group 2; and subscriber 2 will access randomly at timeslot 0 and keeps watch of the
paging at paging group 1.




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