Abstract:
GSM (Global System for Mobile communication) is a popular mobile communication
system especially for its roaming facility and handset changing flexibility. Due to
increasing demand it is now essential to increase the cell capacity. In GSM release 99,
AMR (adaptive multi rate) speech coding rates are specified which can ensure toll grade
voice quality even for HR (Half Rate) traffic channel. This makes the cell capacity
doubled. In this thesis a new technique has been presented which can extend the cell
capacity further by introducing a QR (Quarter Rate) traffic channel in combination with
the QPSK and 8PSK modulation scheme. Using the proposed technique it is also possible
to provide high quality voice using high AMR speech coding rates.
A link adaptation algorithm has been developed for selecting the link condition such as
speech coding rate, traffic channel rate and modulation scheme for the call set up by a
mobile terminal. If the carrier to noise ratio (CIN) experienced by the mobile terminal is
more than the minimum allowable CIN ratio of a certain link condition, the link condition
is assumed to be feasible for the mobile terminal. This minimum allowable CIN has been
determined by running a MATLAB simulation for the proposed coding scheme. The CIN
ratio for an acceptable voice quality (i.e., 10.5 BER) is considered as the minimum
allowable CIN ratio. The formation of voice blocks from the raw voice packet 'and
mapping of this voice block to TDMA frames has also been presented. The framework of
signalling for the proposed QR traffic has been presented with the analysis of required
CIN for acceptable bit error rate ..The proposed technique is devised in such a way that it
can be applied in the GSM system without the change of installed hardware. The
performance of the proposed scheme has been compared with that using Grameen
Phone's coverage prediction model, by running the developed algorithm for link
adaptation. It is observed that the cell capacity is increased by 63.5% on the average. It
has also been found that for FR and QR, QPSK modulation shows better performance
than GMSK and 8PSK. Whereas GMSK performs better in HR. 8PSK can only be used
for few higher speech coding rates which are not feasible for GMSK and QPSK.
