Global System for Mobile Communications GSM GSM burst

Global System for Mobile Communications (GSM)

GSM burst • The GSM burst, or transmission can fulfill a variety of functions. Some GSM bursts are used for carrying data while others are used for control information. As a result of this a number of different types of GSM burst are defined. • Normal burst uplink and downlink • Synchronization burst downlink • Frequency correction burst downlink • Random Access (Shortened Burst) uplink

Five major GSM bursts

GSM normal burst • This GSM burst is used for the standard communications between the base station and the mobile, and typically transfers the digitized voice data. • The structure of the normal GSM burst is exactly defined and follows a common format. It contains data that provides a number of different functions: • 3 tail bits: These tail bits at the start of the GSM burst give time for the transmitter to ramp up its power • 57 data bits: This block of data is used to carry information, and most often contains the digitized voice data although on occasions it may be replaced with signaling information in the form of the Fast Associated Control Channel (FACCH). The type of data is indicated by the flag that follows the data field

GSM normal burst Cont. . • 1 bit flag: This bit within the GSM burst indicates the type of data in the previous field. • 26 bits training sequence: This training sequence is used as a timing reference and for equalization. There is a total of eight different bit sequences that may be used, each 26 bits long. The same sequence is used in each GSM slot, but nearby base stations using the same radio frequency channels will use different ones, and this enables the mobile to differentiate between the various cells using the same frequency. • 1 bit flag Again this flag indicates the type of data in the data field.

GSM normal burst Cont. . • 57 data bits Again, this block of data within the GSM burst is used for carrying data. • 3 tail bits These final bits within the GSM burst are used to enable the transmitter power to ramp down. They are often called final tail bits, or just tail bits. • 8. 25 bits guard time At the end of the GSM burst there is a guard period. This is introduced to prevent transmitted bursts from different mobiles overlapping. As a result of their differing distances from the base station. GSM Normal Burst

GSM synchronization burst • The purpose of this form of GSM burst is to provide synchronization for the mobiles on the network. • 3 tail bits: Again, these tail bits at the start of the GSM burst give time for the transmitter to ramp up its power • 39 bits of information: • 64 bits of a Long Training Sequence: • 39 bits Information:

GSM synchronization burst Cont. . • 3 tail bits Again these are to enable the transmitter power to ramp down. • 8. 25 bits guard time: to act as a guard interval. GSM Synchronization Burst

GSM frequency correction burst • With the information in the burst all set to zeros, the burst essentially consists of a constant frequency carrier with no phase alteration. • 3 tail bits: Again, these tail bits at the start of the GSM burst give time for the transmitter to ramp up its power. • 142 bits all set to zero: • 3 tail bits Again these are to enable the transmitter power to ramp down.

GSM frequency correction burst Cont. . • 8. 25 bits guard time: to act as a guard interval. GSM Frequency Correction Burst

GSM random access burst • This form of GSM burst used when accessing the network and it is shortened in terms of the data carried, having a much longer guard period. This GSM burst structure is used to ensure that it fits in the time slot regardless of any severe timing problems that may exist. Once the mobile has accessed the network and timing has been aligned, then there is no requirement for the long guard period. • 7 tail bits: The increased number of tail bits is included to provide additional margin when accessing the network. • 41 training bits:

GSM random access burst Cont. . • 36 data bits: • 3 tail bits Again these are to enable the transmitter power to ramp down. • 69. 25 bits guard time: The additional guard time, filling the remaining time of the GSM burst provides for large timing differences. GSM Random Access Burst

GSM frame structure

Signal Processing in GSM

Speech Coding • The GSM speech coder is based on the Residually Excited Linear Predictive Coder (RELP), which is enhanced by including a Long. Term Predictor (LTP). • The coder provides 260 bits for each 20 ms blocks of speech, which yields a bit rate of 13 kbps. • RELP Vocoder consists of two filters a) Short Term prediction (STP) filter b) Long Term prediction (LTP) filter

Speech Coding Cont. . (STP) • Speech data inherently contain a high level of redundancy, therefore it is possible to predict a future speech sample from previous speech samples. i. e. speech samples may be approximated as the linear combination of past speech samples. • In GSM 8 past samples are used to predict the next samples. The filter which do this is called Short Term Prediction (STP) filter.

Speech Coding Cont. . (LTP) • The output from STP generally exhibits a certain amount of periodicity related to the pitch period of the original speech when it is voiced. • This periodicity represents a further level of redundancy, which can be removed using a pitch predictor or LTP. • In GSM Long Term Prediction filter operates on a 5 ms speech (40 samples)

Speech Coding Cont. . (VAD) • During a typical telephone conversation, a person generally speaks for 40% of the time. • The interference level may be reduced and the MS battery recharging cycle increased by turning the transmitter off during periods of silence. • This techniques known as discontinuous transmission (DTX) and relies on the accurate detection of periods of silence in the user’s speech.

Speech Coding Cont. . (VAD) • This is achieved using voice activity detection (VAD) where the energy in the speech signal; is computed for each speech block and a decision is made using an adaptive threshold as to whether the block contains speech or background noise. • The periods of complete silence introduced by DTX process at the receiver are annoying since they give the impression that the link has been lost. To avoid a silence descriptor (SID) frame is sent instead of speech frame. The SID is the same size as the speech frame.

Channel Coding • Since channel coding increases the data rate and hence bandwidth. For this reason, channel coding is applied in a selective manner in GSM. • RELP Vocoder delivers 260 bits for each 20 ms speech. These bits are divided into three classes depending on the impact on the received speech quality if they are received in error. • Among 260 bits, the most important 50 bits forms class Ia. Class Ib consists of 132 bits of moderate importance and the remaining 78 bits are least important belonging to class II.

Error Protection for speech signal in GSM

Channel Coding Cont. . • After decoding most important 50 Ia bits are further checked for error by using 3 bits Cyclic Redundancy Check (CRC) error detection technique to ensure their correctness. • Conventional coder is used with k=5 (memory), therefore four zeros are added after Ib bits to complete the operation of convolutional encoder.

Interleaving • In the mobile radio environment, the error in the transmitted bits tends to occur in bursts as the MS moves into and out of deep fades. • The convolution error correcting code is most effective when the errors are randomly distributed throughout the bit stream. • For this reason the coded data is interleaved before transmission. At the receiver, the de-interleaving process trends to distribute the error bursts randomly throughout the effectiveness of the subsequent channel decoding.

Interleaving Cont. . • In GSM, Interleaving is accomplished as follows: • Since each GSM frame contains 2 Blocks of 57 bits each of speech or data. Therefore 8 sub-blocks of 57 bits each are formed from the 456 bits at the output of channel coder as follows (at the nth time)

Interleaving Cont. . • Each sub-block contains half bits of time slot remaining half is occupied by sub-blocks from either the previous speech frame or the next frame. • Below blocks of 114 bits each, thus forms the data for respective time slot in 8 consecutive GSM frames.

Interleaving Cont. .

Ciphering • The GSM System has the ability to encrypt the information on the radio path to reduce the security threat posed by eavesdroppers. • Ciphering modifies the contents of the eight interleaved blocks through the use of encryption techniques known only to the particular mobile station and base transceiver station. • Security is further enhanced by the fact that the encryption algorithm is changed from call to call.

Ciphering Cont. . • Two types of ciphering algorithms, called A 3 and A 5, are used in GSM to prevent unauthorized network access and privacy for the radio transmission respectively. • The A 3 algorithm is used to authenticate each mobile by verifying the users passcode within the SIM with the cryptographic key at the MSC. • The A 5 algorithm provides the scrambling for the 114 coded data bits sent in each TS.

Burst Formatting • Burst Formatting adds binary data to the ciphering blocks, in order to help synchronization and equalization of the received signal.

Modulation • The modulation scheme used by GSM is 0. 3 GMSK, where 0. 3 describes the 3 DB bandwidth of the Gaussian pulse shaping filter with relation to the bit rate (e. g. , BT 0. 3). • GMSK is a special type of digital FM modulation. Ones and zeroes are represented by shifting the RF carrier by plus or minus 67. 708 k. Hz. • Modulation techniques that use two frequencies to represent ones and zeroes are called frequency shift keying (FSK). In the case of GSM, the data rate of 270. 833 kbps is chosen to be exactly four times the RF frequency shift.

Modulation Cont. . • This has the effect of minimizing the modulation spectrum and improving channel efficiency. • FSK modulation where the bit rate is exactly four times the frequency shift is called minimum shift keying (MSK). • In GSM, the modulation spectrum is further reduced by applying a Gaussian pre-modulation filter. This slows down the rapid frequency transitions, which would otherwise spread energy into adjacent channels.

Frequency Hopping • Under normal conditions, each data burst belonging to a particular physical channel is transmitted using the same carrier frequency. • However, if users in a particular cell have severe multipath problems, the cell may be defined as hopping cell by the network operators, in which case SLOW FREQUENCY HOPPING may be implemented to combat the multipath or interference effects in that cell.

Frequency Hopping Cont. . • Frequency Hopping is carried out on a frame-by -frame basis, thus hopping occurs at a maximum rate of 217. 6 hops per second. • As many as 64 different channels may be used before a hopping sequence is repeated. • Frequency hopping is completely specified by the service provider.

Equalization • Equalization is performed at the receiver with the help of the training sequences transmitted in the midamble of every slot. • The type of equalizer for GSM is not specified and is left up to the manufacturer.

Demodulation • The portion of the transmitted forward channel signal which is of interest to a particular user is determined by the assigned TS and ARFCN. • The appropriate TS is demodulated with the aid of synchronization data provide by the burst formatting. • After demodulation, the binary information is deciphered, de-interleaved, channel decoded, and speech decoded.

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