GSM:

GSM stands for Global System for Mobile Communication. It is the most successful second generation digital mobile radio system. GSM became popular very quickly because it provided improved speech quality and through a uniform international standard, made it possible to use a single telephone number and mobile unit around the world. The European Telecommunications Standardization Institute (ETSI) adopted the GSM standard in 1991, and GSM is now used in 135 countries.

GSM also stands out from other technologies with its wide range of services i.e.

• Telephony, asynchronous and synchronous data services (2.4/ 4.8/ 9.6 kbit/s), access to packet data network (X.25), telematic services (SMS, fax, videotext etc.), many value added features (call forwarding, caller ID, voice mailbox) and, e-mail and internet connections.

Speech signal processing of GSM is shown below:

Speech coder:

In transmitter section, first of all, data in the form of speech supplied to speech coder where, speech coder convert that raw data or speech firstly into digitized form and then  encoded data using source encoder technique. The speech coder is based on a type of linear predictive coding (LPC) called residual pulse excited linear predictive coding (RPE-LPC). It delivers 260 bits in each 20 msec time interval, hence a bit rate of 13 kbps as shown in fig.

Channel encoder: After speech encoder, coded data or information entered to the channel encoder device where channel encoding takes place so that coded data or information reached their destination safely without added any corrupted data.

Bit interleaving: After channel encoding, data entered into bit interleaving device. Bit interleaving is a technique for making forward error correction more robust with respect to burst errors. We know that the significant bits are encoded by a rate ½, constraint length, L=5 convolutional encoder, and coded and un-coded bits are blocked interleaved to produce data at a rate of 22.8 kbps.

Encryption and Burst assembly: After error correction, the coded bit stream is encrypted in encryption device and then organised for burst transmission in time slots that carry 114 coded bits and some overhead bits.

Modulation: The modulation used to transmit the bits in each time slot is GMSK with BT=0.3 which is a signal pulse. The output of GMSK modulator is translated in frequency to the desired carrier frequency, which is hopped to a different frequency in each frame.

At the receiver end:  the received signal from transmitter end is de-hopped and translated to baseband, thus creating in phase (I) and quadrature (Q) signal components, which are sampled and buffered. The 26 known transmitted bits are used to measure the channel characteristics and, thus, to specify the matched filter to the channel corrupted signal. The data bits are passed through the matched filter, and then output is processed by a channel equalizer, which may be realized either as a decision feedback equalizer or an ML sequence detector that is efficiently implemented via Viterbi algorithm.

The bits in a burst at the output of equalizer are de-assembled, de-encrypted, de-interleaved and passed to the channel decoder. The decoded bits are used to synthesize the speech signal that was encoded via RPE-LPC