GSM Physical Layer (Layer-1) Explained

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

As shown in the block diagram, the GSM physical layer is composed of two main parts: Baseband and RF. The Baseband part consists of FEC (Forward Error Correction), ciphering, burst formation, and modulation.

Information passes through this layer before it’s transmitted into the air from the mobile device. This information is mainly of two types: traffic and control signals. Traffic is divided into speech and data. Control signals mainly come from upper layers and are used for establishing, maintaining, and terminating the connection of the mobile station with the GSM network. All three types of information are treated differently by the physical layer in GSM.

Image alt: gsm physical layer transmitter

Let’s understand the GSM Physical Layer transmitter modules as depicted in the 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 a 20 ms block. This source encoding block exists only for speech channel processing.

• TCH stands for traffic channel.
• FS stands for full rate speech.
• HS stands for half rate speech.

For data channel (TCH) processing, 240 bits are sent in 20 ms blocks. For control channel processing, a 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., the GSM physical layer), cyclic encoder configuration, convolution encoder (coding rate, polynomials), and interleaving are performed. This information can be speech, data, or a control signal.

2. Forward Error Correction Coding (FEC)

Let’s see how this information goes through the convolution coding block first.

2a. Channel coding for Speech TCH/FS

A 260-bit block is divided using a splitter block and passed as mentioned below: For TCH/FS, a cyclic encoder of (n,k) = (53,50) is used.

• Step 1. 50 bits pass through CRC, and 3 bits are added, giving 53 bits.
• Step 2. 132 bits.
• Step 3. 78 bits.

The output of Step 1 (i.e., 53 bits) is added with 132 bits, and 4 tail bits are added, which will give 189 bits. This 189 bits is fed as input to a rate 1/2 convolution encoder (C.E.). This gives 378 bits. 378 bits are added with 78 bits, giving 456 bits in 20ms. These 456 bits are mapped to bursts using an 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, out of which 32 bits are punctured, which produces 456 bits in 20 ms. This is then passed to the GSM interleaving module.

2c. Channel coding for Signaling or Control Channel

Here, signaling information of about 184 bits is fire coded using a 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, producing 228 bits. This 228 bits is given as input to a rate 1/2 C.E., which produces 456 bits. This is then passed to the interleaving block.

• For RACH, n=14 and k=8 is used.
• For SCH n=35, k=25 is used.

3. Interleaving

In the GSM physical layer, interleavers are of three types based on control, speech, or data channels. The function of the interleaver is to interleave information bits onto bursts. More than one burst carries 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, so the interleaving depth is 4. This type of interleaver is called a 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 the next four fresh bursts, and not the one used to map the first four blocks. Hence, new data starts after 8 bursts, which is the interleaving depth. This GSM interleaver is also named a 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 split, is carried over 22 bursts. This GSM interleaver is called a diagonal interleaver.

4. Ciphering

The ciphering block in the 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 to FCCH, SCH, BCCH, PCH, AGCH, or CBCH as these frames are meant for all Mobiles. These frames are very useful to be decoded by all the mobile subscribers to establish and maintain GSM connections.

5. Burst Formation

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

6. Differential Encoding and Modulation

The data is then passed through differential encoder and modulation. The modulation block minimizes the occupied BW using GMSK modulation with a 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 the GSM physical layer.

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 the transmitter, receiver, and antenna part
• 3GPP TS 43.020 & 23.221 describes encryption/ciphering part