1 omf000001 um interface and radio channels issue2.1

8/3/2019 1 OMF000001 Um Interface and Radio Channels ISSUE2.1

1/43

1

www.huawei.com

Copyright 2008 Huawei Technologies Co., Ltd. All rights reserved.

Um Interface andRadio Channels

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.com
http://www.docudesk.com/http://www.docudesk.com/


2/43

2

Page2Copyright 2008 Huawei Technologies Co., Ltd. All rights reserved.

Contents

1. Overview

2. Processing of Voice Signal

3. Radio Channel



3/43

3


Radio Interface

Another MSC

HLR/AUC

SMC

PSTN

ISDN

OMC

MS

Um

MS

A-bis

BSC

A

BTS

MSC/VLR

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 are 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

interface between the client terminal and the network in ISDN , in which the m means

mobile.



4/43

4


Communication management (CM)Communication management (CM)

Radio resources management (RR)Radio resources management (RR)

Mobility and security management

(MM)

Mobility and security management

(MM)

Integrated managementIntegrated management

TCH0 TCH1 TCH2SACCHTCH23 IDLTCH0 TCH1 TCH2SACCHTCH23 IDL

Multiframe

Physical link layer (L1)

Data link layer (L2)

Network application layer (L3)

Hierarchical Structure of Um Interface

RACH BCCH AGCH/PCH SDCCH SACCH TCH FACCH

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:

CC (call control service), SS (supplementary service) and SMS (short message service).



5/43

5


Radio Access Technology

Frequ

ency

Time

Power

Frequ

ency

Time

Power

FrequencyTime

Power

FDMAFDMA

TDMATDMA

CDMACDMA

User

User

User

User

User

User

User

User

TimeFDMA

Frequency

TDMATime

Frequency

CDMA

Frequency

Time

Code

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:

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.



6/43

6


Radio Access Technology

Frequ

ency

Time

Power

Frequ

ency

Time

Power

Frequ

ency

Time

Power

FDMAFDMA

TDMATDMA

CDMACDMA

User

User

User

User

User

User

User

User

TimeFDMA

Frequency

TDMATime

Frequency

CDMA

Frequency

Time

Code

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.



7/43

7


Contents

1. Overview


3. Radio Channel



8/43

8


Voice Signal Processing

((A/DA/D))

2020 msms

22.8kbit/s22.8kbit/s

13kbit/s13kbit/s8KHz,13bit8KHz,13bit


SegmentationSegmentation Speech codingSpeech coding Channel codingChannel coding

InterleavingInterleaving EncryptionEncryption)) Burst formatting ModulationModulation

Transmission

Voice

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, 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 125s 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.



9/43

9


Analog-Digital Conversion



10/43

10


Speech Coding

The EnhanceEnhanceFull RateFull Ratecoding mode is called CELPCELP(Code Excited LinearCode Excited Linear

Predictive CodingPredictive Coding)

The Full RateFull Ratecoding mode is called RPERPE--LTPLTP(Regular Pulse ExcitedRegular Pulse Excited--LongLong

Term Prediction).Term Prediction).

The HalfHalfRateRatecoding mode is called VCELPVCELP((VectorVector--Sum Excited LinearSum Excited Linear

PredictionPrediction))

13kbit/s13kbit/s260260 bits(FRbits(FR))

112112 bits(HRbits(HR))

244244 bits(EFRbits(EFR)) 12.2kbit/s12.2kbit/s

5.6kbit/s5.6kbit/s

((A/DA/D))

2020 msms

8KHz,13bit8KHz,13bit

SegmentationSegmentation Speech codingSpeech coding

Voice

The voice compression coding technique is widely used in the modern digital

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.

With 20ms as the unit, the voice coder outputs 260bits after compressed 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

advanced voice coder in the future can further reduce the rate to 6.5kbps (half-rate voice

coding).



11/43

11


Channel Coding

13kbit/s13kbit/s260260 bits(FRbits(FR))

112112 bits(HRbits(HR))

244244 bits(EFRbits(EFR)) 12.2kbit/s12.2kbit/s

5.6kbit/s5.6kbit/s

((A/DA/D))

2020 msms

8KHz,13bit8KHz,13bit

SegmentationSegmentation Speech codingSpeech coding

Voice

Channel codingChannel coding

22.8kbit/s22.8kbit/s456bits(FR/EFR)456bits(FR/EFR)

228bits(HR)228bits(HR) 11.4kbit/s11.4kbit/s

Block

Coder 1:2

Convolutional

Coder

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 beclassified 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.



12/43

12


1 2 3 4 5 6 7 8 ... ... 452 453 454 455 4561 2 3 4 5 6 7 8 ... ... 452 453 454 455 456

B

8

16

.

.

.

456

8

16

.

.

.

456

2

10

.

.

.

450

2

10

.

.

.

450

6

14

.

.

.

454

6

14

.

.

.

454

1

9

.

.

.

449

1

9

.

.

.

449

4

12

.

.

.

452

4

12

.

.

.

452

7

15

.

.

.

455

7

15

.

.

.

455

3

11

.

.

.

451

3

11

.

.

.

451

5

13

.

.

.

453

5

13

.

.

.

453

.... ....

B0 B1 B2 B3 B4 B5 B6 B7

{A4,B0} {A5,B1} {A6,B2} {A7,B3} {B4,C0} {B5,C1} {B6,C2} {B7,C3}{A4,B0} {A5,B1} {A6,B2} {A7,B3} {B4,C0} {B5,C1} {B6,C2} {B7,C3}

First interleaving:

Second interleaving:

Interleaving

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, sometechnique 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.



13/43

13


1 2 3 4 5 6 7 8 ... ... 452 453 454 455 4561 2 3 4 5 6 7 8 ... ... 452 453 454 455 456

B

8

16

.

.

.

456

8

16

.

.

.

456

2

10

.

.

.

450

2

10

.

.

.

450

6

14

.

.

.

454

6

14

.

.

.

454

1

9

.

.

.

449

1

9

.

.

.

449

4

12

.

.

.

452

4

12

.

.

.

452

7

15

.

.

.

455

7

15

.

.

.

455

3

11

.

.

.

451

3

11

.

.

.

451

5

13

.

.

.

453

5

13

.

.

.

453

.... ....

B0 B1 B2 B3 B4 B5 B6 B7

{A4,B0} {A5,B1} {A6,B2} {A7,B3} {B4,C0} {B5,C1} {B6,C2} {B7,C3}{A4,B0} {A5,B1} {A6,B2} {A7,B3} {B4,C0} {B5,C1} {B6,C2} {B7,C3}

First interleaving:

Second interleaving:

Interleaving

After channel coding, the extracted 456 bits are distributed into 8 groups with each

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 ofthe 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).



14/43

14


Voice Burst

TailTail

bitbit

TailTail

bitbit

GuardGuard

periodperiodDatDat

aa

DataData Training sequenceTraining sequence

57 encrypted bits57 encrypted bits33

bitsbits8.25 bits8.25 bits26 bits26 bits1

33

bitsbits57 encrypted bits57 encrypted bits 1

((A/DA/D))

2020 msms


13kbit/s13kbit/s8KHz,13bit8KHz,13bit


SegmentationSegmentation Speech codingSpeech coding Channel codingChannel coding

InterleavingInterleaving EncryptionEncryption)) Burst formatting ModulationModulation

Transmission

Voice

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.



15/43

15


Voice Signal Processing

33.8 kbit/s

22.8 kbit/s

13 kbit/s

8 kHz13 bits

Transmittingpart

Receiving part

Speaker

ReceiverDemodulator

AdaptiveEqualization

De-ciphering

De-interleaving

Channel decoding

Speech decoding

D/A-conversion

TransmitterModulator

Burst formatting

Ciphering

Interleaving

Channel coding

Speech codingSegmentation

A/D-conversion

Microphone

13 kbit/s

Transmittingpart

Receiving part

TransmitterModulator

Burst formatting

Ciphering

Interleaving

Channel coding

D/D-conversion

8 kHz8 bits64 kbit/s PCM

ReceiverDemodulator

AdaptiveEqualization

De-ciphering

De-interleaving

Channel Decoding

GSM Network

Mobile equipment



16/43

16


Transmission delay t


TA

The mobile phone should

send the signal in advance!!

Timing Advance (TA)

Transmission delay is unavoidable in the radio interface. If the mobile station movesaway from the base station during a call, the further distance the more delay. The uplink is as

the same.

If the delay is too high, the timeslots of the signal from a certain mobile station and thatof 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 measurementreport sent from the mobile station to the base station carries a delay value. Moreover, thebase station should monitor the time when the call arrives and send an instruction to themobile station via the downlink channel every 480ms so as to inform the mobile station thetime of advance transmission. This time is the TA (timing advance), which ranges between0~63 (0~233s ). The TA value is limited by the timing advance code 0~63bit of the GSMsystem. Therefore, the maximum coverage distance of the GSM is 35km. Its calculation is asfollows:

1/2*3.7s /bit*63bit*c=35km

{In the formula, 3.7s /bit is the duration per bit (156/577); 63bit is the maximum bitnumber of the time adjustment; c is the light speed (transmission speed of the signal); and 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 mayreach up to about 3bit (1.6km).



17/43

17


Timing Advance (TA)



TA

The mobile phone should

send the signal in advance!!

When the MS is in idle mode, the time sequence within the MS can be adjusted via the

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 100s slower than that of the base station. When the mobile phone sends its

first RACH signal, it is already 100s later. For there is still another 100s of transmission

delay, when the signal reaches the base station, the total delay is 200

s . It is very possiblethat 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 200s in

advance.



18/43

18


Frequency Hopping

Frequency

f 0

Frame

f 1

f 2

f 3

f 4

Time

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 hoppingtechnology:

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 theinterference sources, the better the system performance.



19/43

19


Frequency Hopping

Frequency

f 0

Frame

f 1

f 2

f 3

f 4

Time

The radio interface of the GSM system is designed with the slow frequency hopping (SFH)

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.



20/43

20


Prolong battery life andreduce interference

DTX

DTX: Discontinuous Transmission

Shut off the transmission at voice intervals;

Only transmit SID frames

The transcoder at the RX terminal produces comfortable noise.

VAD: Voice Activity Detection

Implemented by the transcoder.

Actually, during the communication process, the mobile subscriber talks only 40% of the

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 alsosave 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

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

transmitted in 8 continuous bursts. In other time, there is no message transmitted.



21/43

21


DTX

DTX: Discontinuous Transmission

Shut off the transmission at voice intervals;

Only transmit SID frames

The transcoder at the RX terminal produces comfortable noise.

VAD: Voice Activity Detection

Implemented by the transcoder.

Prolong battery life andreduce interference

The DTX mode is optional. However, the transmission quality will be reduced a bit in the

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 voicesignal 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.



22/43

22


Power Control

Prolong battery life

Reduce network interference

Include both uplink power control and downlink power control

Level and quality are taken into account

BSC or BTS is the final adjudicator

BCCH Carrier is not

involved in power control.

Time

Signal level

Target level value:

e.g. -85 dm

During the process of radio transmission of signals, to reduce the interference, to increase

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 TXterminal 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.



23/43

23


Contents

1. Overview


3. Radio Channel



24/43

24


Frequency

200kHz

15/26ms

BP

Slit

Time

Frame and Channel

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.

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

moment and frequency, i.e. the specified slot. Generally, the time of slots in a channel is

discontinuous.



25/43

25


32107654321

Physical Channel of Logical Channel

0

differentmessage

types

Logical channel

Physical Channel

Logical channel

Logical channel

Logical channel

Logical channel

TDMA FRAME

200K

577 s

The physical channel is the medium over which the information iscarried.

The logical channel consists of the information carried over thephysical channels.

A Physical Channel (a TS, defined by a fixed position (0-7) on a given TDMA frame) may be used tobroadcast messages containing different kinds of information:

traffic messages for speech and data,

signaling messages for different procedures and supplementary services,

synchronization messages for temporal and logical synchronization between the mobile stations andthe BTS,

measurements messages for uplink report of the downlink measurements,

control messages to manage the access to the network.

All these kinds of messages are classified in Logical Channels. Depending on the quantity of

information to transmit and on their consistency, several logical channels may be mapped onto onephysical channel, in order to use its successive Time Slots as much as possible (optimization of theresources number by maximizing the occupancy duration of each).

As a conclusion:

Physical Channel = information container

Logical Channel = specification of the information global content



26/43

26


8bit 41 synchronousbits

36 encrypted

bits3bit 68.25bit

Tail bit Tail bit Guard intervalData

Guard interval

3bit 142bit 3bit 8.25bit

Tail bit Tail bitData

3bit 39 encryptedbits

39 encrypted

bits3bit 8.25bit

Tail bit Tail bit Guard intervalDataData

Burst

64 synchronous bits

Access burst (AB): Used in MS initial access

Frequency correction burst (FB): Used in frequency synchronization

between MS and BTS

Synchronous burst (SB): Used in timing synchronization betweenMS and BTS

The Logical Channel is used in time multiplex in a physical channel, which is categorized

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

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.



27/43

27


3bit 142 modulation bits 3bit 8.25bit

Tail bit Tail bit Guard interval

Burst

3bit 57 encrypted bits 57 encrypted bits 3bit 8.25bit

Tail bit Tail bit Guard intervalDataData

26bit1 1

Training sequence

Frame stealing flag

Normal burst (NB)

Used to carry the information of the traffic channel and the control channel

except for RACH

Dummy burst (DB)

Used in transmission of filling frames by BTS at timeslots when there is no

information delivered



28/43

28


Broadcast control channel

(BCH)Control channelCommon control channel

(CCCH)

Voice channel

(TCH)

FCCH SCH BCCH TCH/FAGCH RACH SDCCH FACCH

SACCH

TCH/HPCH

Common channel

(CCH)

Dedicated channel

(DCH)

Logical channel

Logical Channel Type

GSM900 and DCS1800 have the same logical channel category

(system information)

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 ControlChannel (CCCH) and Dedicated Control Channel (DCCH).



29/43

29


FCCHSCH

BCCH

PCH

AGCH

BCCH

CCCH

Common

Channel

SDCCH

SACCH

FACCH

TCH/F

TCH/H

DCCH

TCH

Dedicated

Channel

Downlink Logical Channel

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.



30/43

30


Downlink Logical Channel

FCCH

SCH

BCCH

PCH

AGCH

BCCH

CCCH

Common

Channel

SDCCH

SACCH

FACCH

TCH/F

TCH/H

DCCH

TCH

Dedicated

Channel

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.



31/43

31


RACH CCCH

Common

channel

SDCCH

SACCH

FACCH

TCH/F

TCH/H

DCCH

TCH

Dedicated

channel

Uplink Logical Channel

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).



32/43

32


Uplink Logical Channel

RACH CCCH

Common

channel

SDCCH

SACCH

FACCH

TCH/F

TCH/H

DCCH

TCH

Dedicated

channel

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 ofcontrol channel uses the same physical channel as that used in SDCCH.



33/43

33


Use of Logical Channels

FCCH

Allocate signaling channel

Power-on Search for frequency correction burst

Search for synchronous burst

Listen to the system information

Monitor paging message

Send access burst

Set up the call

Allocate voice channel

Conversation

Release the call

Idle mode

SCH

BCCH

PCH

RACH

AGCH

SDCCH

SDCCH

TCH

FACCH

Dedicated mode

Idle mode



34/43

34


26-frame multi-frame

TCH/F+FACCH/F+SACCH/F (full-rate TCH)

TCH/H+FACCH/H+SACCH/H (half-rate TCH)

51-frames multi-frame

FCCH+SCH+BCCH+CCCH (main BCCH)

FCCH+SCH+BCCH+CCCH+SDCCH/4+SACCH/4 (combined BCCH)

BCCH+CCCH (extended BCCH)

SDCCH/8+SACCH/8 (main SDCCH)

Physical Combination of Logical Channel

As shown above, CCCH=PCH+RACH+AGCH; downlink CCCH=PCH+AGCH; and uplink

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.



35/43

35


Structure of Main BCCH

5046-4942-45414020-3916-1912-1511106-92-510Frame

Number

ICX4CX4SFCX4CX4SFCX4BX4SFChannel

Grpup5

Group3,4

(same as

Group2)

Group2Group1Group

1 multi-frame (51TDMA Frames) 235.38ms Downlink

5049484713-461211109876543210Frame

Number

RRRRRRRRRRRRRRRRRRRChannel

1 multi-frame (51TDMA Frames) 235.38ms Uplink

F:FCCH; S:SCH; B:BCCH; C:CCCH; R:RACH; I:IDLE

The TDMA/FDMA multiplexing is used in GSM, the information needed in the

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.



36/43

36


Structure of Combined BCCH

5046-

49

42-

454140

36-

39

32-

353130

26-

29

22-

252120

16-

19

12-

1511106-92-510

Frame

Number

IA3

4

A2

4SF

D3

4

D2

4SF

D1

4

D0

4SF

C

4

C

4SF

C

4

B

4SFChannel

IA1

4

A0

4SF

D3

4

D2

4SF

D1

4

D0

4SF

C

4

C

4SF

C

4

B

4SFChannel

Grpup5Group4Group3Group2Group1Group

1 multi-frame (51TDMA Frames) 235.38ms D ownlink

47-50464541-4437-4014-3610-136-9540-3

Frame

Number

D24RRD14D04RRA14A04RRD04Channel

D24RRD14D04RRA34A24RRD34Channel


F:FCCH; S:SCH; B:BCCH; C:CCCH; D:SDCCH ;A:SACCH; R:RACH; I:IDLE

It is used in the configuration of cells of low traffic density and small capacity. The

Combined BCCH is only configured at timeslot 0.

Channel combination: FCCHSCHBCCHCCCH+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

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.



37/43

37


Structure of Logical Channel

Combination Frame-Main SDCCH

50494844-

47

40-

43

36-

39

32-

35

28-

31

24-

27

20-

23

16-

19

12-

158-114-70-3

Frame

Number

IIIA7

4

A6

4

A5

4

A4

4

D7

4

D6

4

D5

4

D4

4

D3

4

D2

4

D1

4

D0

4Channel

IIIA3

4A2

4A1

4A0

4D7

4D6

4D5

4D4

4D3

4D2

4D1

4D0

4Channel

1 multi-frame (51TDMA Frames) 235.38ms Downlink

47-

50

43-

46

39-

42

35-

38

31-

34

27-

30

23-

26

19-

22

15-

181413128-114-70-3

Frame

Number

A4

4

D7

4

D6

4

D5

4

D4

4

D3

4

D2

4

D1

4

D0

4III

A3

4

A2

4

A1

4Channel

A0

4

D7

4

D6

4

D5

4

D4

4

D3

4

D2

4

D1

4

D0

4III

A7

4

A6

4

A5

4Channel


D:SDCCH; A:SACCH; I:IDLE

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.



38/43

38


Structure of Logical Channel

Combination Frame-TCH

T:TCH;A:SACCH; I:IDLE

25242322212019131812611543210Frame

Number

ITTTTTTT,,,TATTTTTTTTChannel

1 multi-frame (26TDMA Frames) 120ms

25242322212019131812611543210Frame

Number

atTtTtTT,,,tATttTtTtTChannel

1 multi-frame (26TDMA Frames) 120ms

Case of one full rate TCH

Case of two half rate TCHs

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.



39/43

39


0 1 2 3 2044 2045 2046 2047

0 1 2 3 48 49 5047

0 1 24 25

0 1 24 25 1 49 500

0 1 4 5 762 3

TB3

TB3

GP8.25

TB: Tail bitTB3

TB3

GP8.25

GP: Guard periodTB

3

TB

3

GP8.25

TB3

TB3

GP 68.25

58 information bits26 trainingsequences 58 information bits

Constant bit 142

Information bit 39 Extended training sequence 64 Information bit 39

Synchronous sequence 41 Information bit 36

Normal burst (NB)

Frequency correction burst (FB)

Synchronous burst (SB)

Access burst (AB)

1 hyper frame=2048 super-frames=2715648TDMA frames (3 hours, 28 minutes, 53 s econds and 760 mill iseconds)

1 super-frame=1326TDMA frames (6.12 seconds)

1 multiframe=26TDMA frames (120ms) 1 multiframe=51TDMA frames (3060/13ms)

1TDMA frame=8 timeslots (120/26=4.615ms)

1 timeslot=156.25 bit duration (15/26=0.577ms)(1 bit duration: 48/13=3.68us)

BCCHCCCHSDCCH

TCHSACCH/T

FACCH

Frame

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 thecomplete 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 60/134.615ms altogether.

Every timeslot contains 156.25 bit duration, which seize is 15/260.557ms. 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 3060/13235.385ms, which is

used especially in the control channel.



40/43

40


0 1 2 3 2044 2045 2046 2047

0 1 2 3 48 49 5047

0 1 24 25

0 1 24 25 1 49 500

0 1 4 5 762 3

TB3

TB3

GP8.25

TB: Tail bitTB3

TB3

GP8.25

GP: Guard periodTB

3

TB

3

GP

8.25TB3

TB3

GP 68.25

58 information bits26 trainingsequences 58 information bits

Constant bit 142

Information bit 39

Extended training

sequence 64 Information bit 39

Synchronous sequence 41 Information bit 36

Normal burst (NB)

Frequency correction burst (FB)

Synchronous burst (SB)

Access burst (AB)

1 hyper frame=2048 super-frames=2715648TDMA frames (3 hours, 28 minutes, 53 s econds and 760 milli seconds)

1 super-frame=1326TDMA frames (6.12 seconds)

1 multiframe=26TDMA frames (120ms) 1 multiframe=51TDMA frames (3060/13ms)

1TDMA frame=8 timeslots (120/26=4.615ms)

1 timeslot=156.25 bit duration (15/26=0.577ms)(1 bit duration: 48/13=3.68us)

BCCHCCCHSDCCH

TCHSACCH/T

FACCH

Frame

Multiple multi-frames constitute a Super frame, which is a coherent 5126TDMA 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

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.



41/43

41


PCH RACHAGCH

Downlink CCCH Uplink CCCH

How to determine the total CCCH resources of the cell? How to

allocate AGCH and PCH reasonably?

Configuration of Common Control Channel

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.



42/43

42


Summary

In this course, we have learned:

Processing of Voice Signal

Radio Channel in Um Interface

Key Technical in Um Interface



43/43

Thank youwww.huawei.com

1 omf000001 um interface and radio channels issue2.1

Documents