ECG Acquisition in a Computer - ECG Acquisition and Automated Remote Processing - page 84

The half-speed transmission of symbols or signatures or bands has many
inherent advantages of the communication system, the most effective one of which
is to reduce the inter-symbol interference (ISI) due to multipart propagation effect
in wireless applications. The overall signal-to-noise ratio is better in reduced speed
of communication. The reduced speed also consumes less transmitting power, thus
increasing the power spectral density.
In general, telephone application, the voice (analog) signal from microphone is
digitized and fed to the QPSK modulation for its transmission. In the developed
prototype, ECG signal is not directly fed to the digitizer unit because of the DC
like nature of some part of ECG signal. Instead, ECG is digitized first and this
digital bit stream is modulated using bi-phase modulation scheme in order to
reduce the long-percentage of 'zero' or 'one' bits of digital ECG data. This bi-
phase signal (as discuss in Sect. 4.3.1 ) is fed to speaker input of telephone receiver
to transmit it using QPSK modulation.
4.3.5 Spread Spectrum Technique
Spreading the transmission over a wide band makes the transmission look like
noise to a traditional narrow band receiver. The spread spectrum (SS) uses
mathematical functions to diffuse the signal power over a large range of fre-
quencies. The use of this SS technologies is a must for unlicensed ISM band
devices which is imposed by Federal Communications Commission (FCC). Thus,
when the SS receiver performs the inverse operation, the smeared out signal is
reconstructed as a narrowband signal, and at the same time, any narrow band noise
is smeared out so the signal shines through clearly. There are three different
techniques for spreading the entire 2.4 GHz ISM band, viz., Frequency Hopping
(FH), Direct Sequence (DS) and Orthogonal FDM (OFDM).
In FHSS, the system carrier frequency jumps from one channel to another in a
random pattern by transmitting a short burst at each channel with a predefined
'dwell' time. In DSSS, the system spread the power output over a wide frequency
band using some mathematical formulations. This system requires more sophis-
ticated signal processing than FHSS process. OFDM divides the available band
into several sub bands and encodes a portion of the signal across each sub band in
parallel.
The transceiver used in the developed experimentation has adopted DSSS
technique to exploit the advantages of SS over cordless communication. In case of
wireless communication, the bi-phase encoding was customized to suit the
transceivers used. Instead of a simple bi-phase modulation, a modified bi-phase
modulation technique was implemented with bit widths 300 and 1,200 ls for bit
'1' and '0', respectively. Hence, the frequency of bit '1' was put as four times the
same for bit '0'. Figure 4.12 a, b shows the CRO screenshots for encoded and
decoded bit patterns for ECG signal at the encoding frequencies.