GSM Physical Layer


This article describes GSM physical layer i.e. layer-1 which sits below GSM layer-2(LAPDm) We will discuss physical layer with respect to mobile station transmitter in detail.

This tutorial section on GSM basics covers following sub topics:
Main Page  Frame  Burst types  PHY  gsm stack  speech processing  ARFCN  Channel types  GSM Security  Network Entry  Mobile basics 

As shown in the block diagram, gsm physical layer composed of two main parts Baseband and RF. Baseband part consists of FEC(forward Error Correction),ciphering,burst formation and modulation. Informations are passed through this layer before it is pumped into the air from Mobile device. Information are mainly of two types, traffic and control signals. Traffic is divided into speech and data. Control signals are mainly from upper layers and are used for establishing,maintaining and terminating connection of mobile station with the GSM network. All these three types of informations are treated differently by physical layer in GSM.

gsm physical layer transmitter

Let us understand GSM Physical layer transmitter modules as depicted in figure.

1. Source Encoding:
Speech Encoding uses 13kbps RELP (Residually Excited Linear Predictive coder). For speech channel processing(TCH/FS and TCH/HS), 260 bits are occupied on 20 ms block. This source encoding block exist only for speech channel processing. TCH stands for traffic channel. FS stands for full rate speech and HS for half rate speech. For data channel (TCH) processing, 240 bits are sent in 20 ms blocks. For control channel processing, maximum of 184 bits(23 octets) are transmitted. These speech/data as well as signaling information bits are inputted to the next block(FEC).

Based on what information is fed as input to GSM layer 1 i.e. gsm physical layer, cyclic encoder configuration, convolution encoder (coding rate, polynomials) and interleving is done. This information can be speech, data or control signal.

2. Forward Error Correction coding:
Let us see how these informations go through the convolution coding block first.
2a. Channel coding for Speech TCH/FS:
260 bit block is divided using splitter block and passed as mentioned below:
For TCH/FS cyclic encoder of (n,k)=(53,50) is used.
Step 1. 50 bits, pass through CRC and 3 bits are added, gives 53 bits
Step 2. 132 bits
Step 3. 78 bits
Output of Step 1, i.e. 53 bits are added with 132 bits and 4 tail bits are added,which will give 189 bits. This 189 bits are fed as input to rate 1/2 convolution encoder(C.E.). This gives 378 bits. 378 bits are added with 78 bits, this gives 456 bits in 20ms. These bits of 456 are mapped to bursts using interleaving module.
2b. Channel coding for Data:
Here 240 bits are input to the C.E.module after 4 tail bits are added. This gives 488 bits, outof which 32 bits are punctured which produces 456 bits in 20 ms. Passed to GSM interleaving module.
2c. Channel coding for Signaling or control channel:
Here signaling information of about 184 bits are fire coded using block encoder(cyclic encoder of (n,k)=(224,185) is used) which gives 224 bits(after 40 parity bits are added) Four zero bits are added, which produces 228 bits. This 228 bits are given as input to rate 1/2 C.E. which produces 456 bits, this is passed to interleaving block. For RACH, n=14 and k=8 is used. For SCH n=35,k=25 is used.

3. Interleaving:
In GSM physical layer, interleavers are of three types based on control,speech or data channel. The function of interleaver is interleaving of information bits on to bursts. More than one bursts carry data for one channel. The same is explained below for different types of channels.

Interleaving for GSM control channel
456 bits are divided into eight blocks of 57 bits each. Different blocks here carry different bit positions. for example,
1st block contains bit numbers [0,8,16,...448],
2nd block contains bit numbers [1,9,17,...449],
3rd block contains bit numbers [2,10,18,...450],
4th block contains bit numbers [3,11,19,...451],
5th block contains bit numbers [4,12,20,...452],
6th block contains bit numbers [5,13,21,...453],
7th block contains bit numbers [6,14,22,...454],
8th block contains bit numbers [7,15,23,...455],
The first 4 blocks(1st to 4th) are mapped to even number bits of 4 bursts. The last 4 blocks(5th to 8th) are mapped to odd number bits of the same 4 bursts. Hence the new control channel data repeats after 4 bursts, hence the interleaving depth is 4. This type of interleaver is called as block rectangular interleaver.
Interleaving is not applied to RACH,FCCH and SCH.
Interleaving for GSM Speech channel
Here eight sub blocks of 57 bits are mapped to 8 bursts. The first blocks of 57 bits are mapped to even number bits of four bursts consecutively. The other four sub blocks of 57 bits are mapped to odd number bits of next four fresh bursts and not the one used to map first four blocks. Hence new data starts after 8 bursts which is the interleaving depth. This GSM interleaver is also named as block diagonal interleaver.

Interleaving for GSM data channel
The data block of 456 bits are divided as follows.
1st, 22nd bursts will contain 6 bits.
2nd , 21st bursts will contain 12 bits.
3rd,20th bursts will contain 18 bits.
4th to 19th bursts(total 16 bursts) will contain 24 bits each.
This information just splitted is carried over 22 bursts. This GSM interleaver is called as diagonal interleaver.

4. Ciphering:
Ciphering block in gsm physical layer, uses A3 and A5 encryption algorithms. Encryption is changed call by call to enhance privacy or secrecy. Ciphering or encryption is not applied FCCH,SCH,BCCH,PCH,AGCH,CBCH as these frames are meant for all Mobiles. Also these frames are very useful to be decoded by all the mobile subscribers to establish and maintain GSM connection.

5. Burst formation:
Burst formation block frames the burst as required by GSM frame structure. For more on this read GSM tutorial in tutorials section.

6. Differential encoding and modulation:
The data then is passed through differential encoder and modulation. Modulation block minimizes the occupied BW using GMSK modulation with BT of 0.3.

7. RF Transmitter:
The modulated baseband information is upconverted and amplified before being transmitted over the air.

Traffic Channel, Control Channel and Packet Switched Channel Processing

As we know different channels are handled in different ways in gsm physical layer. For detailed depictive view of the Traffic Channel, Control Channel and Packet Switched Channel Processing is covered in the following links:

GSM Traffic Channel processing through physical layer
GSM Control Channel processing through physical layer
GSM Packet Switched Channel processing through physical layer

Useful Links

GSM tutorial covering GSM basics,network architecture,network elements,system specifications,applications,GSM burst types,GSM frame structure or frame hierarchy, logical channels,physical channels, GSM physical layer or speech processing and GSM mobile network entry or call setup procedure, Read more.
GSM Protocol stack , covers layer 1,2 and 3 of GSM network elements MS,BTS,BSC and MSC.READ MORE.

GSM Standard References

3GPP TS 45.002 describes burst building and burst multiplexing
3GPP TS 45.003 describes coding and interleaving
3GPP TS 45.004 describes differential encoding and modulation
3GPP TS 45.005 describes transmitter,receiver and antenna part
3GPP TS 43.020 & 23.221 describes encryption/ciphering part


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