GBC 002 E1 0 GSM Radio Technology-33

GBC_002_E1_0 GSM Air interface Technology

Course Objectives:

Describe GSM Voice Service Process Procedure

State GSM key technology

Grasp physical and logical channel in air interface

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Contents

1 GSM Speech Processing ............................................................................................................................ 1

1.1 GSM Speech Processing ................................................................................................................... 1

1.2 Voice encoding .................................................................................................................................. 1

1.3 Channel Encoding ............................................................................................................................. 2

1.4 Interlacing Technology ...................................................................................................................... 3

1.5 Encryption/Decryption ...................................................................................................................... 7

1.6 Modulation and Demodulation ......................................................................................................... 7

2 GSM Key Technologies .............................................................................................................................. 9

2.1 Diversity Reception........................................................................................................................... 9

2.2 Discontinuous Transmission ........................................................................................................... 10

2.3 Power Control ................................................................................................................................. 11

2.4 Timing Advance .............................................................................................................................. 14

2.5 Frequency Hopping ......................................................................................................................... 15

3 Frame Structure and Radio Channels .................................................................................................... 19

3.1 Radio Frame Structure .................................................................................................................... 19

3.2 Physical Channel ............................................................................................................................. 20

3.3 Logical Channels............................................................................................................................. 21

3.3.1 Common Channel................................................................................................................. 22

3.3.2 Dedicated Channel ............................................................................................................... 23

3.3.3 Channel Combination........................................................................................................... 23

3.4 Mapping between Logical and Physical Channels .......................................................................... 25


1

1 GSM Speech Processing

1.1 GSM Speech Processing

In the GSM system, the MS processes voice signals on wireless interfaces as shown in

Fig 1.1-1.

A/D

D/A

Voice coding

Channel coding

Interleaving Encryption

Burst pulse forming

Modulation

Voice decoding

Channel decoding Deinterleaving Decryption

Burst pulse disassemble Demodulation

260bit/20ms 456bit/20ms 33.8kbit/s 270kbit/s

Fig 1.1-1 Voice Processing in the GSM System

The process of sending voice signals is as follows: for analog voice signals, first make

A/D conversion before doing voice coding to output 13Kbit/s digital voice signals. To

control errors in the process of transmission, channel coding and interlacing processing

shall be conducted on digital voice signals, which are then encrypted according to the

input/output bit stream of 1:1. These bits are grouped into 8 1/2 burst pulse sequences

(corresponding to voice signals/20ms segment) before they are transmitted at about

270Kbit/s in the appropriate timeslots.

The process of receiving voice signals is as follows: for the wireless signals sent by

BTS, first do demodulation before decomposing and decrypting burst pulses. After

every 8 1/2 burst pulse sequences are received, they are subjected to interlacing

processing and re-assembled into 456 bit information. After that, do channel decoding

and detect and correct the errors that occur in the middle of transmission before finally

conducting voice decoding of the bit stream generated by the decoder and converting it

analog voices.

1.2 Voice encoding

Given below is a brief introduction to the voice coding process of the GSM system


GBC_002_E1_0 GSM Radio Technology

2

using full-rate voice coding as an example.

Currently, what the GSM system uses is 13kb/s voice coding scheme, known as

RPE-LTP (Rule Pulse Excitation-Long Term Prediction). The aim of this scheme is to

produce near-PSTN voice quality when no error occurs.

It first divides the voice into voice blocks by 20ms and samples it with 8kHz frequency

to get 160 sample values. Then each sample value is quantified to generate 16bit digital

voice signals. The 128Kbit/s data stream is obtained this way. As the rate is too high to

be transmitted on the wireless path, it needs to be compressed by a coder. If a full-rate

coder is used, each voice block will be compressed into 260bits to generate 13Kbit/s

source code rate in the end. The process of processing other signals such as channel

coding comes after that.

On the BTS side, BTS can recover 13Kbit/s source rate, but to generate 16Kbit/s rate so

that it can be transmitted on the Abis interface, it is necessary to add 3Kbit/s signaling so as

to control the operation of the remote TC. On the TC side, to accommodate 64Kbit/s

transmission rate of A interface, it is also necessary to conduct rate conversion between

13Kbit/s and 64Kbit/s.

1.3 Channel Encoding

Channel coding serves to improve transmission quality and overcome the negative

impact of interferences on signals.

Using specialized redundancy technology, channel coding inserts redundancy bits in

certain regularity at the transmitting end for coding while the receiving end in the

process of decoding detects error codes and corrects errors as many as possible using

these redundancy bits to recover the originally transmitted information.

The coding schemes as used in GSM are convolutional code and packet code which are

used in a combinational way in actual applications.

Convolutional code: compiles k information bits into n bits. Both k and n are very

small so that they are suitable for transmission in a serial port manner. Besides they

also show very little delay. The coded n code elements are not only related to k

information code elements of this packet, but also to information code elements in the

preceding (N-1), where N is called constraint length. The convolutional code is

generally represented as (n, k, N). The error tolerance of the convolutional codes


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increases as N increases while its error rate decreases as N increases. The convolutional

code is mainly designed for error correction. When the demodulator uses the maximum

likelihood estimation method, it can generate very effective error correction results.

Convolution code can be expressed as (n, k, N). The error-correction capability in

convolution encoding grows stronger with the rise of N, while the error probability

decreases exponentially as N rises. The convolution code is used to correct errors, and

it is effective when the decoder works in the maximum likelihood estimate mode.

Packet code: This is a kind of shortened loop code, which gets the redundancy bits by

increasing the exclusive-or algorithm of information bits and maps k input information

bits to no output binary code elements (n>k) through exclusive-or algorithm. The

packet code is designed mainly to detect and correct error codes in groups and it is

used in a mixed way with the convolutional code. The packet code is used for detecting

and correcting errors in groups. It is generally used along with the convolution code.

1.4 Interlacing Technology

The occurrence of burst error codes in wireless communication is usually caused by

fading that lasts a long time. It is not enough to detect and correct errors in the

above-mentioned channel coding scheme. To better address the issue of error codes, the

interlacing technology is introduced to the system. The interleaving technology is

adopted in channels to better solve the error problems.

Interlacing is in fact to send separately the original continuous bits of a message block

in certain regularity. In other words, the original continuous block in the middle of

transmission becomes discontinuous and creates a group of interlaced transmission

message blocks. At the receiving end, this kind of interlacing message blocks is

restored (de-interlaced) to original message blocks. To control the operations and

sessions, the TCAP are classified into two layers, CSL and TSL. The CSL is used to

manage the operations and the TSL is used to manage the transactions (sessions), as

shown in Fig 1.4-1.



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Packet

Interleaving

Packet after interleaving

Error code

11 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

11 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4

11 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

Fig 1.4-1 Interleaving Technology

After the interlacing technology is applied, if a message is lost in the middle of

transmission, it is in fact part of each message block that is lost, but the whole part of it.

The missing messages can be recovered easily with the coding technology.

In the GSM, different coding and interleaving modes are used in different types of

channels. See Table 1.4-1 for details.

Table 1.4-1 Coding and Interweaving of Circuit Logical Channels

Channel Type

Input

Rate (Kbit/s)

Input Code Block bits

Code Output

Code Block bits

Interleaving Depth Check Bit Tail Bit

Convolutional Code Rate

TCH/F

S

Ia 13 50 Parity check, 3 4 1/2

456 On eight 1/2 bursts Ib 13 132

II 13 78

TCH/HS

Ia 5.6 22 Parity

check, 3 6 1/3 228 On four 1/2 bursts

Ib 5.6 73

II 5.6 17

TCH/F9.6 TCH/H4.8

12 6

240 4

1/2, one bit is

removed every 15 bits.

456 Combine on 22 unequal bursts

TCH/F4.8 6 120 32 1/3 456 Ditto

TCH/F2.4 3.6 72 4 1/6 456 On eight 1/2 bursts

TCH/H2.4 3.6 144 8 1/3 456 Combine on 22

unequal bursts

SCH 25 Parity 4 1/2 78 Combine on one SB



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Channel Type Input Rate

(Kbit/s)

Input Code

Block bits

Code Output Code

Block bits

Interleaving Depth Check Bit Tail Bit

Convolutional Code Rate

check, 10 burst

RACH 8 Parity check, 6

4 1/2 36 Combine on one AB burst

FACCH 184 Packet coding, 40

4 1/2 456 On eight 1/2 bursts

SACCH BCCH

SDCCH AGCH

PCH

184 Packet coding, 40

4 1/2 456 On four whole bursts

The voice input rate on TCH/FS is 13 Kbit/s, that is, each speech frame lasts 20 ms and

contains 260 bits. According to the interference of different bits on voice, the 260 bits

are divided into I category (182 bits in total) and II category (78 bits in total). The I

category is further divided into Ia and Ib. The Ia bits are very important bits. If any of

them is incorrect, the subscriber will hear a loud noise in 20 ms voice interval. There

are 50 Ia bits and 132 Ib bits. That is, the 260 bits in a speech frame (20 ms) is { d (0),

d (1),, d (181), d (182), , d (259)}. The part with a single line is I category, and

that with a double-line is II category. It is similar to the TCH/HS.

Table 1.4-1 gives the coding and interleaving adopted in different types of transmission.

The first column lists the channels and the related transmission mode. The Input Code

Block column gives the size of the data block (bits) before channel coding. The Output

Code Block column gives the size of the data block (bits) after channel coding. In Code,

the parameters are listed in the same sequence as the coding sequence. The tail bit is

"0". The decoding is in the reverse order.

Following is description of channel coding and interweaving, taking voice

communication for example.

In the GSM, the voice input rate on TCH/FS is 13kb/s, that is, 260 bits are transmitted

every 20ms. The 260 bits are protected by means of segmented coding.

Among the 260 bits, 182 bits adopts 1/2 convolutional coding, and the remaining 78

bits are not protected. Among the 182 bits, 50 bits are performed with parity check and

then with 1/2 convolutional coding. Three information bits are added. Those 50 bits are

called Ia bits. The other 132 bits, called Ib bits, are performed with 1/2 convolutional



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coding directly.

Fig 1.4-2 shows the interleaving algorithm of voice signals on TCH/F. After channel

coding, 456 bits are carried in every 20ms. Those bits are divided into eight groups,

with the 57 bits in each group carried in different burst pulses (eight BPs in total). To

maximize irrelevancy between the bit sequences, the bits should be arranged as

described in Table 1.4-2.

0 1 2 3 4 5 6 78 9 10 11 12 13 14 15. . . . . . . .. . . . . . . .. . . . . . . .

1 2 3 4 5 6 7 8

456 bits

0 1 2 3 4 5 6 78 9 10 11 12 13 14 15. . . . . . . .. . . . . . . .. . . . . . . .

456 bits

0 1 2 3 4 5 6 78 9 10 11 12 13 14 15. . . . . . . .. . . . . . . .. . . . . . . .

456 bits

0 1 2 3 4 5 6 78 9 10 11 12 13 14 15. . . . . . . .. . . . . . . .. . . . . . . .

456 bits

57 1 57 1 57 1 57 1 57 1 57 1 57 1 57 1

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

A block

Odd bits

B block

Even bits116 bit 116 bit 116 bit

Fig 1.4-2 Interleaving of Cells

Table 1.4-2 Full-rate speech interleaving algorithm

No. Item Note

1 0, 8, , 448 Even bits (B block) in BP (N)

2 1, 9, , 449 Even bits (B block) of BP (N 1)

3 2, 10, , 450 Even bits (B block) of BP (N 2)

4 3, 11, , 451 Even bits (B block) of BP (N + 3)

5 4, 12, , 52 Odd bits (A block) v BP (N 4)



8 7, 15, , 455 Odd bits (A block) v BP (N + 7)

456 bits are divided into eight groups (rows). Each group has 57 bits (columns),

occupying Block A or Block B of BP (N) to BP (N+7). After interleaving, a BP carries



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114 bits of information plus 2 bits of stolen frame (116 bits in total). The 114 bits

contain 57 bits (odd bits) of information block A and 57 bits (even bits) of information

block B. The remaining two bits indicate respectively whether the first half BP (odd bit)

and the last half BP (even bit) are subscriber data or fast channel associated signaling.

1.5 Encryption/Decryption

There are encryption measures available in the GSM system. They are applicable to

voice, data and signaling. They are independent of the data type and work for normal

bursts only. Encryption is accomplished by exclusive or operation of an encryption

sequence (computed by A5 encryption algorithm via key Kc and frame number) and

114 information bits on a normal burst.

The original transmission data can be obtained by using the same sequence at the

receiving end to conduct exclusive-or operation with the encryption sequence.

1.6 Modulation and Demodulation

Modulation and demodulation are the last step in signal processing. Using GMSK

modulation mode at a rate of 270.833 k Baud, GSM usually conducts demodulation

with Viterbi algorithm (with a balanced demodulation method). Demodulation is the

reverse process of modulation.

GMSK is a special digital FM modulation mode. The modulation rate is 270.833

kilobauds. The Frequency Shift Keying (FSK) modulation with bit rate four times of

frequency offset is called MSK (Minimum Shift-frequency Keying). In GSM, the

Gaussian demodulation filter is used to further reduce the modulation spectrum. It can

cut the frequency conversion speed.

The GMSK can be expressed by a I/Q diagram. If there is no Gaussian filter, when a

series of constant 1s are sent, the MSK signal will be kept in the state that is higher

than the center frequency 67.708 kHz of the carrier. If the center frequency of the

carrier serves as the fixed phase reference, the signal 67.708 kHz will cause steady

increment of phase. The phase rotates 360 at 67,708 times per second. In a bit period

(1/270.833 kHz), the phase moves 1/4 a circle in the I/G diagram, that is, 90. The data

1 can be looked as 90 plus the phase. Two 1s makes a phase increment by 180, three

1s makes a increment by 270, and so on. The data 0 indicates the same phase change

in the reverse direction.



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The actual phase track is strictly controlled. In the GSM, digital filter and 1/Q or digital

FM modulator are used to generate correct phase track accurately. The Root Mean

Square (RMS) between the actual track and the ideal track allowed by GSM

specifications cannot exceed 5, and the peak deviation cannot exceed 20.


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2 GSM Key Technologies

2.1 Diversity Reception

The diversity reception technology is usually introduced to the GSM system to receive

on several tributaries the signals with little relativity but carrying the same information

and then output the signals after they are combined. In this way, the impact of fading

on the stability of receiving signals can be played down.

There are ways of diversity as follows: space diversity, frequency diversity, time

diversity and polarization diversity.

1. Space Diversity

Two receiving antennas are set in the space to receive independently the same

signals before combining and outputting them. In this way, the degree of fading

can be dramatically lowered. This is the so-called space diversity. The space

diversity is based on the fact that the field strength varies randomly with the

space. The longer the distance, the more variant the multi-path transmission will

be, and the less relevant the receiving filed strength will be. The relevancy refers

to the similarity between the signals. Therefore, the necessary distance must be

determined. According to the test and statistics, CCIR suggests the spacing

between two antennas should be larger than 0.6 wavelength, namely d0.6, to

achieve a satisfactory diversity result and that it should be better to near the odd

number multiplication of /4. Even if the distance between antennas is shortened

to be /4, good diversity effect can be achieved.

2. Time Diversity

Time diversity is to send the same message with some delays or part of the

message in different time within the delay range tolerable by the system. In the

GSM system, time diversity is achieved by the interlacing technology. In the

GSM, interleaving technology is adopted to implement the time diversity.

3. Frequency Diversity

Frequency diversity means more than two frequencies send a signal concurrently.

The receiving end combines the signals of different frequencies and reduces or



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eliminates the impact of fading with different paths of the wireless carrier waves

of varied frequencies. The frequency diversity is effective and requires one set

of antenna only. Frequency diversity in GSM is implemented by frequency

hopping technology.

4. Polarization Diversity

Polarization diversity is to receive signals by making two pairs of receiving

antennas with polarization direction into some angles against each other, which

can generate a good diversity result. The two sets of polarized antennae in

polarity diversity can be integrated in one set of antenna. Thus, only one

receiving antenna and one transmitter antenna are required in a cell. If duplexer

is adopted, only one transceiving antenna is required. It saves antennas greatly.

2.2 Discontinuous Transmission

There are two voice transmission modes. One is continuous voice coding (one speech

frame every 20ms) no matter whether the subscriber is talking or not. Another is

discontinuous transmission (DTX) with 13kb/s coding in voice activation period and

500b/s coding in non voice activation period. In addition, a comfort noise frame (20ms

per frame) is transmitted every 480ms, as shown in Fig 2.2-1.

There are two purposes of employing the DTX mode: one is to lower the general

interference level in the air; the second is to save the power of transmitters. However,

the DTX may slightly lower the transmission quality. Therefore, the DTX mode and

common mode are optional.

TRAU BTS

BTS MS

Comfort noise frame

Speech frame

480 ms

Fig 2.2-1 Speech Frame Transmission in DTX Mode



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2.3 Power Control

Power control means to control the actual transmitting power (keep it as low as

possible) of MS or BS in radio propagation, so as to reduce the power consumption of

MS/BS and the interference of the entire GSM network. Needless to say, the

prerequisite of power control is to ensure the good communication quality of the

ongoing calls. The power control process is simply illustrated in Fig 2.3-1.

A B

Fig 2.3-1 Power Control

As shown in Fig. 1.5-16, the MS at point A is far from the BS antenna. Because the

propagation loss of electric wave in air is in direct proportion to n power of the

distance, the MS at A needs higher transmit power to ensure good communication

quality. Comparatively, point B is closer to the BS transmission antenna, hence smaller

transmission loss; therefore, to obtain similar communication quality, a mobile phone

at point B can use lower transmission power during communication. When a mobile

phone in communication is moving from point A towards point B, the power control

can reduce its transmitting power gradually. On the contrary, if it is moving from point

B towards point A, the power control can increase its transmitting power gradually.

The power control is classified as uplink power control and downlink power control,

they function separately. By uplink power control, it means to control the MS

transmitting power, while downlink power control means to control the BS transmitting

power. No matter uplink power control or downlink power control, the uplink or

downlink interference is suppressed as the transmit power is reduced. Meanwhile the

power consumption of the MS or base station is reduced. The most obvious benefits are

the average conversation quality of the whole GSM network is greatly increased, and

the MS standby time is prolonged.

1. Power control process



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The original information used for decision making during a power control

process is obtained from the measurement data of the MS and BS and

corresponding control decision can be made after processing and analyzing of

the original data. Similar to the handover control process, the whole power

control process is shown in Fig 2.3-2.

Measurement data saving

Average measurement data processing

Power control decision making

Power control command sending

Measurement data correction

Fig 2.3-2 Power Control Process

1) Measurement data saving

The measurement data related to power control includes uplink signal level,

uplink signal quality, downlink signal level, and downlink signal quality.

2) Average measurement data processing

To reduce the influence of complex radio transmission on the measurement

values, the smooth processing of the measurement data usually adopts the

forward averaging method. That is, the average value of multiple measurement

values is used to make a power control decision. The parameter setting in

averaging calculation may vary with the types of the measurement data, i.e.,

quantity of the measurement data to be used may be different.

3) Power control decision making

In the decision making of power control, there are three parameters: a threshold,



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an N value, and a P value. Among the latest N average values, if there are P

parameters exceed the threshold, the signal level is too high or the signal quality

is good; if there are P parameters are lower than the threshold, the signal level is

too low or the signal quality is poor.

According to the condition of the signal level or quality, the mobile phone or BS

can judge how to control the transmitting power, and the increase or decrease

amplitudes are determined by the pre-configured values.

4) Power control command sending

According to the power control decision, the corresponding control command is

sent to the BS, which will then execute the command or transfer it to MS.

5) Measurement data correction

After power control, the original measurement data and average values are

useless. If the useless information is still kept, it may cause incorrect power

control decision. Therefore, it is necessary to discard the outdated data or update

it for later use.

The fastest power control can be performed once every 480 ms, which is the

highest speed that the measurement data is reported. In other words, an entire

power control process is executed once in at least 480ms.

2. High-speed power control

The control extent of the power control process recommended by ETSI is fixed

as 2dB or 4dB normally. However, in most practical cases the fixed power

control extent is unable to achieve optimal effects, for a simple example:

When an MS initiates a call at a location very near to the BS antenna, its start

transmitting power is the max. transmitting power of the MS in the system

message broadcast in the cell BCCH (MS_TXPWR_MAX_CCH). Its obvious

that at this time as the MS is quite close to the MS antenna, the power control

process is supposed to reduce its transmitting power as fast as possible. However,

it can hardly be achieved by the power control process recommended by the

ETSI specifications, because only 2dB or 4dB is decreased each time. In

addition, there is an interval between every two power control processes

(because enough new measurement data need be collected). Therefore, it takes a

long time to reduce the transmit power of the MS to a proper value. It is the



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same in the downlink direction. Obviously this is disadvantageous in terms of

reducing interference to the whole GSM network. To improve this, the power

control extent each time should be increased, which is the core idea of the

high-speed power control.

The high-speed power control can, according to the actual signal strength and

quality, work out the power control extent to be realized, without the limitation

of the fixed extent, thus solving the power control problem without much effort

when the MS makes the initial access. Of course its functions are not limited to

this situation. It can work in many cases e.g. fast moving mobile phones,

sudden interference or obstacles. Whenever large extent power control is

required, the high-speed power control process is the ideal solution.

2.4 Timing Advance

In the GSM, because TDMA is adopted in the air interface, the MS must employ the

TSs allocated to it only, and remain inactive in other time. Otherwise, it may affect the

MSs using other TSs on the same carrier.

In the GSM, the MS requires three intervals between timeslots when receiving or

transmitting signals. See Fig 2.4-1.

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1Offset

Downlink:

Uplink:

Sent by the BTS Sent by the MS

TDMA frame number

TDMA frame number

Fig 2.4-1 Uplink and Downlink Offset of TCH



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Suppose an MS occupies TS2 and moves away from the base station, the messages sent

from the base station will be delayed further and further in reaching the MS.

Meanwhile, the response returned by the MS will also be delayed further and further in

reaching the base station. If nothing is done to solve the problem, the message sent by

the MS from TS2 will eventually overlap with another calling message received by the

base station in TS3. Therefore, it is important to monitor the time when a call reaches

the base station. As the distance between the MS and the base station changes, the

system issues instructions to the MS, notifying it of the time advanced. This process is

the adjustment of timing advance.

After a specific connection is established, the BTS measures the time offset between

the pulse TSs and the received MS TSs. Based on the value measured, the BTS

calculates the timing advance and notifies the MS of it through the SACCH at a certain

frequency.

2.5 Frequency Hopping

In the digital mobile communication system, to enhance the anti-jamming capability of

the system, the spread spectrum technology is usually introduced. There are two modes:

direct spread mode and frequency hopping mode, which is used by the GSM system.

There are two reasons for why frequency hopping is used. First, based on the principle

of frequency diversity, this technique is used to counteract Raileigh fading. Rayleigh

fading refers to the short-term change in amplitude that mobile radio transmission

suffers inevitably in case of any obstacle. Different frequencies will suffer different

degrees of fading, which becomes more independent with the increase in frequency

difference. By means of FH, the BPs will not be damaged by Rayleigh fading in the

same way. Second, it is used on the basis of anti-jamming feature. In the area where

traffic is heavy, the cellular system is liable to be restricted by the interference from

frequency reuse, and the C/I may change a lot during the call. C depends on the

position of the MS relative to the BTS. I depends on whether the frequency is used in

the adjacent cell. FH enables it to scatter interference among many calls that may

interfere with the cell instead of one call.

FH refers to hopping of the carrier frequency within a wide frequency band at a certain

sequence. The control and information data are modulated into base band signals,

which are then sent into the carrier for modulation. Afterwards, the carrier frequency



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changes under the control of pseudo-random codes, the sequence of which is the FH

sequence. Finally, the signals are sent via the RF filter to the antenna for transmission.

The receiver determines the receiving frequency according to FH synchronization

signals and FH sequence, receives corresponding signals after FH for demodulation.

The basic structure of FH is illustrated in Fig 2.5-1.

Synchronization circuit

Frequency modulation sequence generator

Variable frequency synthesizer

Message modulation

Up converter

Send

Message demodulation Down converter

Receive

Fig 2.5-1 Basic Structure of FH

Features of frequency hopping technology: The frequency hopping technology can be

employed to increase the working band of the system so as to enhance the

anti-jamming and anti-jamming capability of the communication system. Frequency

hopping can help improve and protect the pulse of the effective information part from

the impact of Rayleigh fading in the communication environment. After frequency

hopping is done, the original data are recovered by means of channel decoding. The

times of frequency hopping are increased to boost frequency hopping gains so as to

enhance the anti-jamming and anti-fading capability of the system.

The frequency hopping technology is actually to avoid external interferences so that

they cannot follow the changes of frequencies, thus avoiding or markedly lowering

same-channel interference and frequency selective fading. The reason to increase the

number of hoppings is that the gain of frequency hopping system is equal to the ratio of

frequency hopping system bandwidth to N minimum frequency hopping intervals.



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Usually, the FH number should be greater than three. If frequency diversity is also

available for the FH system, and a message is transmitted by several groups of

frequency hopping simultaneously and then judged by the law of large numbers, more

subscribers can use services at the same time with least mutual interference.

The frequency hopping comprises baseband hopping and RF hopping.

The baseband hopping enables the transmit and receive frequencies of each

carrier unit to remain unchanged. At different frame number (FN) moment, the

frame unit sends data to different carrier units.

RF FH is to control the frequency synthesizer of each transceiver, making it hop

according to different schemes in different timeslots.


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3 Frame Structure and Radio Channels

GSM air interface uses TDMA based frame structure. Communication services are

obtained by transmission of information using logical channels on physical channels.

Mapping between the logical channel and physical channel is the process that arranges

the information to be sent to the suitable TDMA frames and timeslots.

3.1 Radio Frame Structure

Five levels of GSM radio frame structure are timeslot, TDMA frame, multiframe,

superframe and hyperframe.

Timeslot is the basic unit of a physical channel.

TDMA frame consists of eight timeslots. It is a basic unit occupying carrier

bandwidth. Each carrier has eight timeslots.

There are two types of multiframes:

One type of multiframe consists of 26 TDMA frames. This type of multiframe is

used in TCH, SACCH, and FACCH.

The other type of multiframe consists of 51 TDMA frames. This type of

multiframe is used in BCCH, CCCH, and SDCCH.

The superframe is a consecutive 51 x 26 TDMA frame. It consists of 51

26-multiframes or 26 51-multiframes.

The hyperframe consists of 2,048 superframes.

Fig 3.1-1 shows GSM frame structure.



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0

TDMA frame00 01

1 2 3 4 5 6 7 0 1 2 3 4 5 6 702

0 1 2 3 4 5 6 7

0 1 2 3 4 22232425

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 4748 4950

0 1 2 3 47 48 49 500 1 2524

2042 2043 204420452046 20476543210

1 26-multiframe = 26 TDMA frames (120 ms) 1 51-multiframe = 51 TDMA frames (3036/13 ms)

1 superframe = 1326 TDMA frames (6.12s)= 51 26-multiframe or 26 51-multiframes

1 hyperframe = 2048 superframes = 2715648 TDMA frames

Fig 3.1-1 GSM Frame Structure

3.2 Physical Channel

GSM adopts mixed technology of Frequency Division Multiple Access (FDMA) and

Time Division Multiple Access (TDMA). GSM features high frequency utilization.

FDMA - enables 124 carrier frequencies (carriers for short) to be assigned to the

uplink (from the MS to the BTS) 890 MHz 915 MHz or downlink (from the BTS to

the MS) 935 MHz 960 MHz in GSM900 band. Interval between carriers is 200 kHz.

Carriers in the uplink and downlink are in pairs called duplex communication mode.

Interval between duplex receiving and transmitting carrier pair is 45 MHz.

TDMA - enables each carrier of GSM900 band to be divided into eight time segments.

Each time segment is called a timeslot. See Fig 3.2-1.

This type of timeslot is called a channel or a physical channel. Eight consecutive

timeslots on a carrier constitute a TDMA frame, that is, a carrier of GSM provides

eight physical channels.



21

16/25 ms

200 kHz

Timeslot

Time

Frequency

Fig 3.2-1 Time-Frequency Structure of Physical Channel

Eight timeslots in TDMA frame are called physical channels.

3.3 Logical Channels

Each physical channel is time multiplexed with different logical channels. Logical

channels carry various signaling or traffic information based on user and network

requirements. To provide signaling traffic control, logical channels map on physical

channels.

Logical channels are classified into Common Channel and Dedicated Channel.

Enhanced Full-rate channel

Logical Channels

Fast Associated Control Channel

(FACCH)

Slow Associated Control Channel

(SACCH)

Stand-alone Dedicated Control Channel (SDCCH)

Frequency Correction

Channel (FCCH)

Common Control Channel (CCCH)

Dedicated Channel

Broadcast Channel (BCH)

Dedicated Control Channel

(DCCH)

Traffic Channel (TCH)

Broadcast Control Channel (BCCH)

Paging Channel (PCH)

Random Access Channel (RACH)

Access Grant Channel (AGCH)

Common Channel

Half-rate channel (TCH/H)

Full-rate Channel (TCH/F)

Synchroniza-tion Channel

(SCH)

Fig 3.3-1 GSM Logical Channels



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3.3.1 Common Channel

Common Channel is classified in two main types:

Broadcast Channel (BCH): BCH transmits broadcast messages from base station to

MS. Broadcast Channel is unidirectional channel from base station to MS. It is of three

types:

Frequency Correction Channel (FCCH): It carries the information used to correct

the MS frequency. MS receives frequency correction information through FCCH and

corrects its time base frequency.

Synchronization Channel (SCH): It carries frame synchronization (TDMA frame

number) information and Base Station Identity Code (BSIC) to MS.

Broadcast Control Channel (BCCH): It broadcasts general information of BTS. For

example, broadcasts the local cell and neighboring cell information, and

synchronization (time and frequency) information. MS listens to BCCH periodically

to obtain the information transmitted on it, such as the Local Area Identity, List of

Neighboring Cell, frequency table used in local cell, cell identity, power control

indication, intermittent transmission permission, access control, and CBCH description.

BCCH carrier is transmitted by base station at a fixed power, and its signal strength is

measured by all MSs.

Common Control Channel (CCCH): CCCH is point-to-multipoint bi-directional

channel. It carries signals required to set up a connection between base station and MS.

It is of three types:

Paging Channel (PCH): It broadcasts paging messages from base station to MS. It is a

downlink channel.

Random Access Channel (RACH): MS sends information to base station through this

channel when accessing the network at random. The information sent includes response

to the paging message of base station and access of mobile-originated call. MS also

applies for a Stand-alone Dedicated Control Channel (SDCCH) from base station

through this channel. RACH is an uplink channel.

Access Grant Channel: The base station sends the assigned SDCCH to the MS that

accesses the network successfully through this channel. The AGCH is a downlink

channel.



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3.3.2 Dedicated Channel

Dedicated channel is a traffic channel which carries voice and data. Some types of

dedicated channel are used for the control purpose.

Dedicated Channel is classified in two main types:

Dedicated Control Channel (DCCH): DCCH is a point-to-point bi-directional

channel between base station and MS. It is of three types:

Stand-alone Dedicated Control Channel (SDCCH): It carries signaling and

channel information between base station and MS, such as the authentication

and registration signaling messages. During the establishment of a call, SDCCH

supports bi-directional data transmission and short messages transfer.

Slow Associated Control Channel (SACCH): Through this channel, base

station sends power control message and frame adjustment message to MS, and

receives signal strength report and link quality report from MS.

Fast Associated Control Channel (FACCH): It carries inter-cell handover

signaling messages between base station and MS.

Traffic Channel (TCH): TCH carries voice and data. According to switching mode,

TCH can be divided into circuit-switched channel and data-switched channel.

According to transmission rate, TCH can be divided into full-rate channels and

half-rate channels.

Rate of the GSM full-rate channel is 13 kbps, and that of the GSM half-rate channel is

6.5 kbps. In addition, the enhanced full-rate channel has same rate as the full-rate

channel, which is 13 kbps. However, it has better compressed coding scheme than

full-rate channel. That is why enhanced full-rate channel provides better voice quality.

3.3.3 Channel Combination

In actual application, different types of logical channels are mapped on the same

physical channel. This is called channel combination.

Following are nine GSM channel combinations:

Full-rate traffic channel (TCHFull): TCH/F + FACCH/F + SACCH/TF

Half-rate traffic channel (TCHHalf): TCH/H (0, 1) + FACCH/H(0, 1) +

SACCH/TH (0, 1)



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Half-rate1 traffic channel (TCHHalf2): TCH/H (0, 0) + FACCH/H (0, 1)

+SACCH/TH (0, 1) + TCH/H (1, 1)

SDCCH: SDCCH/8 (0,, 7) + SACCH/C8 (0,, 7)

Main broadcast control channel (MainBCCH): FCCH + SCH + BCCH + CCCH

Combined broadcast control channel (BCCHCombined): FCCH + SCH +

BCCH + CCCH + SDCCH/4 (0,,3) + SACCH/C4 (0,, 3)

Broadcast channel (BCH): FCCH + SCH + BCCH

Cell broadcast channel (BCCHwithCBCH): FCCH + SCH + BCCH + CCCH +

SDCCH/4 (0,, 3) + SACCH/C4 (0,, 3) + CBCH

Slow dedicated control channel (SDCCHwithCBCH): SDCCH + SACCH +

CBCH

Among the above channel combinations, CCCH = PCH + RACH + AGCH. As a

downlink channel, only CBCH carries cell broadcast information and shares the

physical channel with SDCCH.

Each cell broadcasts FCCH and SCH. The basic combination in the downlink direction

includes FCCH, SCH, BCCH and CCCH (PCH + AGCH). It is allocated to TN0 of

BCCH carrier configured for a cell, as shown in Fig 3.3-2.

SF B C

R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R

51 frames

SF C C SF C C SF C C I

R R R R R R R R R R

D0 D1 D2 D3 D4 D5 D6 D7 A0 A1 A2 A3

SF C C

R R R R R R R R R R

III

D0 D1 D2 D3 D4 D5 D6 D7 A4 A5 A6 A7 III

A1 A2 A3 III

A5 A6 A7 III

D0 D1 D2 D3 D4 D5 D6 D7 A0

D0 D1 D2 D3 D4 D5 D6 D7 A4

SF B C SF C C SF D0D1 SF

D2

D3 ISF

A0

A1

SF B C SF C C SF D0D1 SF

D2

D3 ISF

A2

A3

D3

D3

R R

R R

A2 A3

A0 A1

D2D2

SF

SF

D0

D1

D0

D1

R R R R R R R R R R R R R R R R R R R R R R R

R R R R R R R R R R R R R R R R R R R R R R R

F: FCCH S: SCHB: BCCH C: CCCH (CCCH=PCH+AGCH+RACH)R: RACH D: SDCCHA: SACCH/C I: Idle

BCCH+CCCHDownlink

BCCH+CCCHUplink

8 SDCCH/8Downlink

8 SDCCH/8Uplink

BCCH+CCCH+4SDCCH/4

Downlink

BCCH+CCCH+4SDCCH/4Uplink

(a) FCCH+SCH+BCCH+CCCH

(b) SDCCH/8(0,...,7)+SACCH/C8(0,...,7)

(c) FCCH+SCH+CCCH+SDCCH/4(0,...,3)+SACCH/C4(0,...,3)

Fig 3.3-2 Frame Channel Structure



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For half-rate voice channel combination, each timeslot has two half-rate sub-channels

and corresponding SACCH, with 26 TDMA frames as a multiframe.

Fig 3.3-3 shows the frame structure.

H0

H0

S1

S0

H1

H0

H0

H0

H0

H0

H0

H0

H0

H0

H0

H1

H1

H1

H1

H1

H1

H1

H1

H1

H1

H1

26 frames

Fig 3.3-3 Half-Rate Voice Channel Frame Structure

3.4 Mapping between Logical and Physical Channels

Logical channels in GSM are much more than the eight physical channels that a GSM

carrier can provide. If each logical channel is configured with a physical channel, the

eight physical channels provided by a carrier are not enough.

In such case, extra carriers must be added. However, the communication in this way is

not highly effective. The way to solve this problem is to multiplex the CCCH, that is,

multiplex the CCCH on one or two physical channels.

Mapping between physical channels and logical channels in GSM is as follows:

Base station has N carriers, and each carrier has eight timeslots. Define the carriers as

f0, f1, f2 Downlink starts from timeslot 0 (TS0) of f0. TS0 is used to map with

control channel only. f0 is also called broadcast control channel (BCCH).

Fig 3.4-1 shows BCCH and CCCH on TS0 multiplexing.

012 7012 701

FS B C FS C C FS C C FS C C FS C C I

TDMA frame

BCCH+CCCHDownlink

F (FCCH): MS synchronizes its frequency through it. S (SYCH): MS reads TDMA frame number and Base Station Identity Code (BSIC) through it. B (BCCH): MS reads the general inforamtion of the cell through it. I (IDLE): Idle frame, containing no information. It serves as the end flag of the multi-frame.

Fig 3.4-1 Multiplexing of BCCH and CCCH on TSO



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BCCH and CCCH occupy total 51 TS0s. Although only the TS0 of each frame is

occupied, the total length is 51 TDMA frames in terms of time. Each time when an idle

frame appears, the multiframe ends. After that, a new multiframe starts from F and S.

Repeat like this, and TDMA multiframe is constructed.

When there is no paging or call connected, the base station always transmits on f0. This

enables MS to detect the signal strength of the base station to determine the cell to be

used.

For the uplink, the TS0 on f0 does not include the above channels. It is used for the MS

access only, that is, it is used as the RACH.

Fig 3.4-2 shows the TS0 of 51 consecutive TDMA frames.

012 7012 701

RR

TDMA frame

RACHUplink

RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR

Fig 3.4-2 Multiplexing of RACH on TSO

BCCH, FCCH, SCH, PCH, AGCH, and RACH are all mapped on TS0. RACH is

mapped on uplink, and the rest are mapped on downlink.

TS1 on downlink f0 is used to map DCCH to physical channel.

Fig 3.4-3 shows the mapping relationship.



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012 7012 701

D0 I

TDMA frame

SDCCH+ SA CCCHDownlink

D1 D2 D3 D4 D5 D6 D7 A0 A1 A2 A3 I I

D0 ID1 D2 D3 D4 D5 D6 D7 A4 A5 A6 A7 I I

Fig 3.4-3 Multiplexing of SDCCH and SACCH on TS1 (Downlink)

Since the bit rate in call setup and registration is quite low, eight dedicated control

channels can be placed on one timeslot to improve the multiplexing ratio of the

timeslot.

SDCCH and SACCH have 102 timeslots in total, that is, 102 time division

multiplexing (TDM) frames.

DX (D0, D1 ) of SDCCH is used in the early time when a call is set up. When the

MS transfers to the TCH, and the subscriber starts the conversation or the release is

triggered after registration, the DX is used by other MSs.

AX (A0, A1 ) of the SACCH transfers unimportant control information, such as

radio measurement data, that is TS0 of 51 consecutive TDMA frames.

TS1 on the uplink f0 has the same structure with the TS1 on the downlink f0. They have

an offset in time, which means bi-directional connection can be performed at the same

time for an MS.

Fig 3.4-4 shows the multiplexing of the SDCCH and SACCH on TS1 of the uplink f0.



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012 7012 701

A5

TDMA frame

SDCCH+ SACCCHUplink

A6 A7 D0 D1 D2 D3 D4 D5 D6 D7

A1 A2 A3 D0 D1 D2 D3 D4 D5 D6 D7

I I I

I I I

A0

A4

DX: same as uplink AX: Same as downlink

Fig 3.4-4 Multiplexing of SDCCH and SACCH on TS1 (Uplink)

Uplink and downlink TS0 and TS1 on f0 are used by the logical control channel, while

other six physical channels (TS2 to TS7) are used by TCH.

Fig 3.4-5 shows the mapping from TCH to physical channel.

0 1 2 7 0 12 7 0 1

T T

TDM A fram e

T CHDownlink T T T T T T T T T T AT T T T T T T T T T T T I

2

Note: There are 26 tim eslots in total.The sequence starts from the beg in ing after the idle tim eslot.

T=TCH A=SACCH I=Idle

Fig 3.4-5 TCH Multiplexing

Fig 3.4-5 shows TS2 time division multiplexing.

TCH carries voice or data. SACCH carries control commands such as the command to

change the output power.

Idle I does not contain any information but is used in measurement.

TDM is implemented on TS2 with 26 timeslots as a cycle.

The idle timeslot I serves as the beginning or end of the repeated sequence.



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Uplink TCH is of the same structure with the downlink TCH. They only have a time

offset, which is three timeslots. That is, the TS2 of the uplink and that of the downlink

do not appear simultaneously, which means that the MS does not send or receive data

at the same time.

Fig 3.4-6 shows the offset between the uplink and downlink of the TCH.

0

TDMA frame number

Uplink C0

00 01

From BTS to MS

From MS to BTS

Downlink C0

45MHz (GSM900)95MHz (DCS1800)

Offset

1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0

TDMA frame number00 01

1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Fig 3.4-6 Offset between Uplink and Downlink of the TCH

The conclusion is that on carrier f0:

TS0: a logical control channel, with repeat cycle of 51 timeslots.

TS1: a logical control channel, with repeat cycle of 102 timeslots.

TS2: a logical traffic channel, with repeat cycle of 26 timeslots.

TS3 to TS7: logical traffic channels, with repeat cycle of 26 timeslots.

The TS0 to TS7 of other f0 fN are all traffic channels.


GBC 002 E1 0 GSM Radio Technology-33

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