Abstract:
The objective of this project was to implement a Time Division Multiple Access
modem on ADSP 21535 BLACKFIN Digital Signal Processor. This modem eventually
lead to a system for wireless communications between different earth stations exchanging
information on a time division basis. This required a thorough study, design and
simulation of the TDMA protocol and various modulation schemes before a final
implementation on the DSP. Quadrature Phase shift keying (QPSK) was used for
modulation/demodulation of the digital data. A Phase Locked Loop (PLL) and a Delay
Locked Loop (DLL) were used for carrier recovery and symbol timing recovery
respectively. The carriers were generated at the transmitter and receiver using a Direct
Digital Frequency Synthesis (DDFS) operating on the Cordic algorithm. The digital data
(baseband signal) is first processed by the TDMA logic and is forwarded to the QPSK
modulator for transmission. On the other side, the digital bit stream recovered by the
QPSK receiver is passed onto the TDMA logic at the receiver before being passed onto
the subscriber channel/user. The whole algorithm for the processing of data was
implemented in the Blackfin DSP's assembly in fixed point before its simulation in
MATLAB. The TDMA modem was designed to handle data rates of 9.6Kbps.
