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To: foundation who wrote (88231)	11/25/2000 12:41:01 PM
From: S100	Respond to of 152472

re Qualcomm sage Klein Gilhousen studied a range of spectrum Presentation is here cdg.org Optimum Bandwidth for CDMA Adapted from a presentation by Klein S. Gilhousen at the International Conference on Personal, Mobile Radio, and Spread Spectrum Communications Beijing, China, October 12-14, 1994 The effects of multipath propagation on the CDMA signal are quite different from those of narrowband signals. While increasing spreading bandwidth leads to an asymptotic improvement in Erlang capacity of CDMA per megahertz, multipath propagation phenomena have increasingly detrimental effects. The direct sequence modulation and Rake receiver do mitigate the multipath that leads to narrowband Rayleigh fading. However the complexity of the Rake increases with bandwidth. So what is the "optimum" spreading bandwidth? Why did the designers of IS-95A choose 1.25 MHz? This question can never be answered precisely because of the varied environments that may be served by the systems in question, and because of the constantly changing technology. It is quite possible that, faced with the same design problem today, that some of the design choices might be different. Some entrepreneurs have, in fact, proposed SSMA schemes with radically larger bandwidths, claiming superior performance. These pages discuss the effect on system performance, complexity and co-existence with other systems of the choice of spreading bandwidth in a CDMA spread spectrum system. The choice of 1.25 MHz is at least highly plausible, if not provably optimum. CDMA Capacity and the Law of Large Numbers In reality, all CDMA systems are also, at least to a certain extent, Frequency Division Multiple Access (FDMA) systems. Usually, a given spectral allocation will be available for provision of a communications service. The system designer has the choice of filling the entire spectral allocation with a single spread spectrum waveform or of dividing the spectral allocation into a number of sub-bands, each of which is filled with a narrower bandwidth spread spectrum waveform. In this latter case, the resulting system is considered to be a hybrid of FDMA and CDMA. IS-95A is such a hybrid system because a number of 1.25 MHz bandwidth channels can be defined and used simultaneously. The radio channel capacity of a CDMA cellular system is defined as the number of subscribers that can access a particular base station of the system simultaneously. Calculation of the number, N, of equal power, fixed rate interfering users in an isolated cell of a CDMA system is generally accepted to be given by snip due to many equations, tables, graphs, etc. snip The problem this causes CDMA systems is apparent. The A' segment will contain at most a single 1.25 MHz bandwidth CDMA channel; the B' segment will contain at most two CDMA channels. Even that is possible only if the two CDMA channels are overlapped slightly, entailing some loss of total capacity. So, if all portions of the cellular spectrum are to be able to be utilized for CDMA, a 1.25 MHz bandwidth is the maximum possible bandwidth for a non-overlay approach. While the situation in the United States is somewhat unique, it is not unusual around the world to find similar situations where very wide bandwidths simply cannot be given exclusive frequency allocations. Typically, existing systems are not very tolerant to a new system sharing the spectrum with them because they were not designed to do so. Hence it is desirable to use relatively narrow bandwidth CDMA in order to minimize the possibility of interference to existing services and to minimize the amount of unusable spectrum allocation. In the United States there is also a problem of transition. Most cellular carriers will add CDMA service to existing AMPS systems. With IS-95A, the approach to transition is to convert the spectrum in 1.25 MHz segments. The operator would turn off the AMPS radios in about 1.8 MHz (the 1.25 MHz bandwidth of a single CDMA channel, plus guard bands). The CDMA system would then begin operating in this cleared segment. The reduction in capacity of the AMPS system could be handled by increasing the number of AMPS base stations to offset the capacity loss. However if heavy users of the AMPS system can be provided with dual-mode subscriber units before cutover, then the AMPS build-out can be avoided. Distribution of the dual mode units can take place over a prolonged period of time. Initially the dual mode units will operate on the AMPS system. Subsequent to cutover they will seek out the CDMA system, where available, thus relieving the AMPS system of sufficient load to more than compensate for the small reduction in its capacity. Complexity It should be clear that increasing spread bandwidth can only result in increased complexity of the systems. For example, increasing the spread bandwidth a factor of 8 from 1.25 MHz to 10 MHz will require the receiver correlators to operate 8 times faster. This increased speed will require both more silicon area and greater power consumption. In addition, many more correlators are required as shown above in order to adequately process the multipath signals