An Introduction to Wireless Technology, Second Edition

in this form then it is necessary to randomize (scramble) the data before

transmission and to descramble it on reception.

5.2.3.1 CDMA in FH Systems

Sharing the wideband channel between multiple FH systems is possible and can

be considered a form of CDMA. With two or more systems hopping over the

same bandwidth collisions do occur. When there is a collision data is corrupted

and lost.

In an FFH system (say 10-100 hops per bit) corrupted chips will have little effect

on user data. However, in an SFH system user data will be lost and higher layer

error recoveries will be needed. One way of avoiding the problem in the SFH

environment is to arrange the hopping patterns so that each system uses a

different

set of channels so that collisions cannot occur.

5.2.4 DSSS and SFH Systems Compared

There is some discussion in the industry over which system of spread spectrum

operation is the most cost effective. The technology is not at all mature yet and

researchers are still trying to settle the matter but there are some early

indications.

1. In a paper presented to the IEEE, Chen and Wu (1992) report a performance

comparison between the two systems. The study uses two kinds of

mathematical channel models and studied two speeds of operation. Systems

were compared

without

equalization or error correction techniques being

applied.

Their conclusion was that at speeds of 1 Mbps the SFH system was superior

to DSSS in almost every respect and significantly so in most. At speeds of

10 Mbps their conclusion is the opposite. That is, DSSS is better, again

under simulated conditions.

As manufacturers bring their systems to market, experience will show the

difference.

2. An assessment of manufacturing cost shows that SFH “should” cost less to

manufacture and operate at lower power than DSSS. (This all depends on

the system design.)

It should be noted that there are many ways to implement either system.

For example, you can have a DSSS system which uses only a very short

pseudo-random sequence. This saves significant cost in the adapter but

limits the potential for CDMA operation and removes much of the security

advantage.

3. An SFH system can easily avoid local sources of strong narrowband

interference. All it needs to do is to modify the hopping pattern to avoid the

particular frequency band. The ISM bands have many uses and sources of

narrowband interference are relatively common. While, in general, a

narrowband interferer will not bother DSSS, a strong local interferer (such as

a nearby microwave system) will. An SFH system can detect and avoid the

frequency bands involved.

4. There is a great variation in the ability of RF-LAN equipment available in the

market today to reject spurious interference. The laws of physics set the

limit of what can be achieved; the design and engineering of the equipment

will determine what is actually achieved. The choice of components involved

will have a strong influence on the ultimate outcome. It is possible to build a

Chapter 5. Radio Communication in LANs 105