Evaluate Ber in Gsm and Wcdma Systems

Evaluate BER in GSM and WCDMA Systems

1 BER in GSM System :

1.1 Approach and Conceptual Transceiver Structure :

The simulation described consists a baseband parts only. The parts included are illustrated in figure 1.1, where a conceptual block diagram of a GSM transmitter and receiver system is sketched. Only the highlighted blocks are implemented.

The voice interfaces - including microphone, speech encoder/decoder, and loudspeaker - are not intended to be included in the toolbox. Instead, to supply the input signal to the channel encoder/interleaver random bits are generated, as Figure 3.3 displays. By comparing this random input sequence with the reconstructed sequence delivered by the channel decoder/de-interleaver block the BER (Bit Error Rate) performance of the system is estimated.

1.2 Overall Transmitter Structure

The overall structure of the implemented transmitter is illustrated in Figure 3.4. The transmitter is, as illustrated, made up of four distinct functional blocks.

To provide an input data stream to the channel encoder/interleaver a sequence of random data bits is generated by the random bit generator. The input sequence of Channel encoder block is o 260 bit long vector. The Channel encoder splits the incoming 260 information bits into three different classes, i.e. class Ia, class Ib, class II depending on the importance of the bits. The Channel encoding scheme utilized in GSM is illustrated in Figure 3.5. This sequence is - after processing - then accepted by the MUX which splits the incoming sequence to form a GSM normal burst. As this burst type requires that a training sequence is included this also must be supplied. This is in Figure 1.3 illustrated by the TRAINING parameter. The term TRAINING is also used throughout the simulation implementations to represent the training sequence. Upon having generated the prescribed GSM normal burst data structure the MUX returns this to the GMSKmodulator, where GMSK is short for Gaussian Minimum Shift Keying. The GMSK-modulator block performs a differential encoding of the incoming burst to form a NRZ (Non Return toZero) sequence. This modified sequence is then subject to the actual GMSK-modulation after which, the resulting signal is represented as a complex baseband signal using the corresponding I and Q signals.

Figure 3.5 : Channel encoding in GSM.

1.3 Overall Receiver Structure

The overall structure of the implemented data receiver is illustrated in Figure 3.6. Here three functional blocks are designed in order to implement the data receiver.

The demodulator accepts a GSM burst, r, using a complex baseband representation. Based on this data sequence, information concerning the oversampling rate OSR, the training sequence TRAINING, and the desired length of the receiving filter, Lh, the demodulator determines the most probable bit sequence. This demodulated sequence is then used as input to the DeMUX where the bits are split in order to retrieve the actual data bits from the sequence. The remaining control bits and the training sequence are here discharged. As a final operation to retrieve the estimated transmitted bits channel decoding and de-interleaving is performed. It is important to note that the parameter values of OSR and TRAINING used in the receiver must equal those used in the transmitter.

1.4 Simulation results :

This part presents the simulation results for the GSM system at AWGN channel. The figure 3.7 show the BER vs Eb/No curves for differrent class in normal burst structure. The OSR factor is 2 and the length of impulse response is 2Tb (Tb is bit time duration). We can see from figure 3.7 that the BER of class Ia, class Ib is decrease significantly as compared with class II when it's smaller 10%.

Figure 3.7 BER vs Eb/No curves for differrent class in normal burst structure.

2 BER in WCDMA System :

This part describes the simulator designed to evaluate the Bit Error Rate (BER) at the

uplink of a Wideband CDMA (WCDMA) system. Data is transmitted in a frame by frame basis over a time varying multipath channel. Receiver design incorporates rake diversity combining. Additive White Gaussian Noise (AWGN) is added at the front end of