PACKETIZING VOICE FOR MOBILE RADIO

In present mobile radio systems, conventional FM techniques are used t o transmit speech in its analog form. A number of digital schemes have been proposed in the past as an alternative. To overcome the effects of Rayleigh fades that appear in the received speech as noise bursts i n the form of ''pops'' and ''clicks'', we propose, in this paper, pack etizing speech and encoding the packets in an error-detecting code bef ore transmitting them. If the receiver detects an error, it throws awa y the packet and requests the transmitter to retransmit the same packe t. If the requested packet has not arrived in a given time period, the missed packets are replaced with zero amplitude values. The variable delays that packets are subjected to as a result of this procedure are smoothed out before packets are played back so that they all appear c ontiguous at the receiving end. The resulting improvement in the SNR a t the receiving end depends, among other things, on the maximum delay we permit and the vehicle speed, and is achieved in addition to any im provement that is possible with a specially designed coder. For exampl e, with 4 ms tong packets and 12 ms of delay, the SNR improves by abou t 13 dB at a vehicle speed of 35 mph and 17 dB at 12 mph.