GSM Transmit Frequencies

 GSM-like systems have been specified for the 900 MHz (GSM), 1800 MHz (DCS-1800) and 1900 MHz (PCS-1900, USA) radio bands. In GSM, mobiles transmit in a 890-915 MHz band and receive in a 935-960 MHz band. The duplex spacing is 45 MHz. These bands are divided into 125 bands with 200 kHz spacing. The Absolute Radio Frequency Channel Number (ARFCN) n (n = 0,1, ... 124) identifies the uplink carrier frequency

890.0 MHz + n 0.2 MHz.

The corresponding downlink is 45 kHz higher. Extensions (E-GSM) are channel 974 to 1023, which are allocated just below the original GSM bands. DCS-1800 uses channel 512 to 885, i.e., carrier frequencies of

1,710.0 MHz + (n-511) 0.2 MHz.

and a duplex spacing of 95 MHz. PCS-1900 operates in channels numbered from 512 to 810, with frequencies of

1,850.0 MHz + (n-512) 0.2 MHz.

and a duplex spacing of 80 MHz.

Multiple Access

Each GSM channel uses a TDMA method to support simultaneous calls from multiple subscribers. Mobile terminals transmit in bursts of 577 microseconds. One burst fits into one time slot. Through feedback signals, the system can ensure that the signals from all users arrive exactly in their assigned time slot, even if propagation distances to and from different users vary over time. The terminal has 28 microseconds of guard time to power up or switch off the RF signal.

It has been noted that this pulsed transmission has may cause some interference the electronic equipment if it is not properly protected. Particularly airbag ignition circuits have suffered interference from GSM handset transmissions.

A set of 8 TDMA slots is called a frame. In GSM, the terminal transmits and receives during different times slots. This avoids the expensive duplex filter that otherwise would have been needed to avoid that strong transmit signals leak into the receiver.

Burst Structure

A normal burst contains Random access bursts carry much less data but have long guard spacings to avoid that poorly aligned burst transmissions interfere in other slots. The frequency correction burst and synchronization bursts allow proper operation of the terminal.

Frame Structure

The GSM standard distinguishes between physical channels and logical channels. Logical channels carry user speech or data. Data or signalling messages can travel over different physical channels. Logical channels can be The base station can accommodate these logical channels onto radio carriers in many different ways.

Traffic Channel Frame Structure

This frame structure uses 26 frames to build a multi frame. Frames 0 to 11 and 13 to 24 carry speech or user data. Frame 12 is used as SACCH, Frame 25 is idle, allowing the base station to measure field strengths from mobiles in other cells, when needed for handovers. 26 frames last 120 msec.

Signaling Frame Structure

The Signaling Frame Structure is a multiframe of 51 frames to accommodate control channels (FCCH, SCH, BCCH, CCCH) in the downlink and random access (RACH) in the uplink. The 51-multiframe lasts for 235.38 msec.

Superframes

One superframe contains 1326 TDMA frames (6.12 seconds) can either carry 51 of 26-Multiframes or 26 of 51-Multiframes. One hyperframe contains 2048 superframes which is equivalent to 2,715,648 TDMA frames.

Channel Coding

Speech or user data bits are protected using two concatenated codes. GSM has blocks of 260 bits, containing 50 class Ia bits, 132 class Ib bits and 78 Class II bits. The first step involves block coding. Three parity bits are added to allow error detection for the first 50 class Ia bits. 4 bits are added to the Class Ib bits. The 189 Class Ia and Ib bits are then convolutionally encoded at rate 1/2, i.e., the generate 378 code bits. The coder has constraint length K = 5. Five consecutive bits are used to create the transmit bits. Every time one bit is fed into the encoder, two bit appear at the output. The first output bit is the exor of the input bit with the third and fourth preceding bits. The second output bit is the exor of the input bit with its immediate preceding input bit and the third and fourth preceding bits.

Figure: Convolutional encoder for transmission in GSM. Each input bit generates two output bits, determined by the input bit and four previous bits. The encoder contains four delay elements (squares in the Figure).

The class II bits remain unprotected. Together with 378 coded class I bits, each block contains 456 bits. These 456 bits are split into 4 bursts of 114 bits each, or 8 subblocks. If the speech codec receives the information that bits are unreliable it discards the block of 260 bits and interpolates the speech.

Interleaving

A transmission burst may easily be lost because of channel fading. Therefore portions of each block of speech data are spread over several transmission bursts.

Data at 9.6 kbit/s

For reliable communication at a user bit rate of 9.6 kbit/s, the data terminal initially encodes the raw data, giving a bit rate of 12 kbit/s. That is, 12 kbit/s is the bit rate over the interface between the GSM handset and the data source (PC, fax or whatever). The GSM handset then provides for additional error protection, using convolutional encoding. For data transmission the error control and interleaving parameters differ essentially from the case of speech transmission. The CRC parity bit are not used for data. The convolutional encoding still involves rate 1/2 and K=5. The 244 data bits are encoded into 488 bits. This exceed the 456 bits that can be accommodated in one block by 32 bits. So 32 bits redundant bits are left out. This is called "puncturing". The interleaving works over 22 frames, to ensure that bursts of errors are effecitively spread over a longer a period of time.

Modulation

 GSM uses Gaussian Minimum Shift Keying (GMSK).



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