CDMA communications over wireless infrared channels

Dhomeja, Sheyam Lal (1999) CDMA communications over wireless infrared channels. University of Southampton, Doctoral Thesis.

Abstract

The application of code division multiple access (CDMA) to the transmission of data over wireless infrared channels in office environments is addressed. We begin by devising a recursive algorithm for computing the impulse response of non-directed wireless infrared channels in an empty office room for non-directed line-of-sight (LOS) and diffuse wireless infrared channels. Partitions, glass windows, and a person represented by parallelepiped, are introduced into the office, and the environment is referred to as the partitioned office environment (POE). Ray tracing techniques are employed in the computation of the channel impulse response in the POE. We determine that the diffuse wireless infrared channel is more immune to the effects of shadowing, partitions and glass windows compared to non-directed LOS communications. The received signal level decreases by some 15 dB and 5 dB for non-directed line-of-sight and diffuse channels, compared to an empty room respectively.

In order to accommodate multiple users in office environments, optical CDMA is proposed and the up-link transmission performance evaluated for a single office cell. Optical orthogonal codes (OOC) are used in the CDMA process, and the probability of bit error (p_e) is both calculated and simulated for different numbers of users and different code lengths in the presence of receiver noise and shot noise. The theoretical and simulation results are in close agreement. As an example of up-link performance, for 17 users, and SNR of 30 dB, an OOC code of 341 chips having five logical 1s, a detection threshold parameter of 4.2, the p_e is 1.0 x 10^-4.

The capacity of the optical CDMA system is mainly limited by the interference created by other users in the same cell. A parallel interference cancellation technique mitigates the multiple access interference for wireless infrared asynchronous up-link transmission in an office cell. The expression for probability of bit error at the mth stage of interference cancellation is derived for AWGN noise at the receiver, which is in contrast to fibre optic systems where a Poisson-rate, photon counting model is employed. The analytical and simulation results are in close agreement. They suggest that by applying the interference cancellation technique at the base station, the performance of an infrared up-link can approach the single user bound. For example, at the third stage of parallel interference cancellation, the probability of bit error is 2.35 x 10^-5 for 17 users at an SNR of 12 dB.

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