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Spread Spectrum Wireless

August 2009

Traditionally telecommunication and data signals are sent using mostly time domain multiplexing (TDM) or frequency domain multiplexing (FDM) modulation techniques. Now there is a newer modulation technique called Code Division Multiple Access (CDMA), also referred to as Spread Spectrum transmission. The latter is more efficient in using the frequency spectrum than TDM or FDM and thus more simultaneous calls can be carried in an environment where bandwidth is a limiting factor, like mobile GSM for example. So how does all this work? It is very technical and best is to check the links below. What follows is an attempt to give you a flavour of what it is about:

TDM samples the transmitted signal in short time intervals, code the signal slices into bits and aggregate the signals together to achieve multiple channels to be transmitted on the same medium, like copper or air. To get more channels on the same medium the bit-rate needs to be increased to fit more signals (or timeslots) into the same time period. At the decoding end, the synchronisation of the bit-rate clocks is very important to extract the various signals (called de-multiplexing) from the transmitted signal. Typically DECT and 2G GSM systems use TDM.

FDM or FDMA (FDM with multiple access) uses frequency slots instead of timeslots. Thus each voice or data channel has its own frequency and at the receiving end requires the receiver channel filtering to be very accurate and stable. Channel separation and channel drift are some of the problems encountered.

CDMA on the other hand uses the full frequency spectrum for each channel and uses a coding mechanism, hence the TDM clock problems and the FDM channel crosstalk problems are not features of CDMA. In essence CDMA mixes pseudo-random codes or pseudo-noise (PN) digital sequences to each channel that needs to be transmitted. The PN bit rate is much higher than the bit rate of the original data channels. The channels are simply summed together as superposistions of the original signals. At the receiving end the various channels are mixed again with the PN identified for each channel. This has the effect that the data from other channels present in the received signal is suppressed as if it is noise and only the highly correlated signal associated with the PN appears. This is a form of statistical correlation technique.

The major advantages of CDMA are:
1. More channels for the bandwidth used than other technologies.
2. Less dropped calls as the channel is spread over a range of frequencies.
3. Less prone to jamming.
4. Very low crosstalk.
5. No clock jitter problems inherent in TDM systems.

See the following links:

Wikepedia
Introduction to CDMA
Direct Sequence Spread Spectrum DSSS
CDMA advantages

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