Telemetry problems with pacemakers in a noisy environment

Abstract

This report covers the investigation of problems relating to the telemetry of pacemakers in a noisy environment. An attempt was made to suggest schemes that improve system performance. We define system performance to be improved noise immunity with high reliability and increased message throughput.

Three aspects were considered, with implementation schemes that incur minimal protocol changes over systems in existence.

1. The present scheme is writing to the pacemaker and echoing each bit back within a 2ms frame. If any echoed bit is not in agreement with that sent, the message is re-transmitted from the start of the block. Because the return link is poor in terms of signal to noise ratio, echoing bits up this link degrades system performance, particularly as the noise power increases. An ARQ (autamatic repeat request) scheme is suggested as a solution to this problem.

2. When data is to be read from the pacemaker, reply is via two 4ms frames each of 6 bits. A CRC (cyclic redundancy check) is performed on the returned data (8 bits) which checks for three or less errors, and the redundancy bits are appended to the end of the 9 bits. If any errors are detected, a retransmission is required. Using a FEC (forward error correction) scheme to correct errors, together with a CRC to check the integrity of data, throughput can be significantly improved especially in a noisy environment. The scheme we suggest is to encode data bits plus CRC with a 23,12 Golay code, and send the data using four by 4ms frames. The Golay code suggested is a perfect code, triple error correcting.

3. The final aspect we considered is the possible system performance improvement using soft decision quantization at the programmer of received data. This system gives gains of the order of 1.75 dB over current practice.