DS CDMA Scheme for WLANs with Errors and Erasures Decoding B.J.Wysocki, T.A.Wysocki University of Wollongong Northfields Av., NSW 2522 Australia wysocki@uow.edu.au Abstract - In the paper, we present simulation results for the 32 channel DS CDMA WLAN utilising modified Walsh-Hadamard spreading sequences. The method to obtain those spreading sequences is shown. The resultant system BER as well as the distribution of errors within frames is given. The obtained results indicate that with the application of a hybrid ARQ scheme with errors and erasures decoding, the number of frames, which would require retransmission is significantly lower than when the hard decoding is employed. Another benefit of using errors and erasures decoding is increase in the system reliability with a 2/3 drop in the number of undetected errors. 1. Introduction During the 1990s, direct sequence spread spectrum code division multiple access (DS CDMA) technology [1] has matured as a technique to provide multiple access to the radio channel for mobile communications. For example, it is used in mobile telephony compliant with IS-95 standard [2]. The major benefits of DS CDMA is that it can be effective in combating problems related to multi-path propagation, while providing good interference from other, narrowband devices operating in the same frequency band. This is, however, dramatically reduced if only a small processing gain [1] can be achieved, as in the case of WLANs. Under such conditions, in-band jammers, like other channels of the same WLAN acquired by means of CDMA cause severe multiple access interference (MAI), which may block the communication. In theory, cancellation of that type of interference is possible if each of the users utilize orthogonal signals to transmit the data [1]. If delays between transmitters and receiver are anyhow different, as is generally the case of terminal to base station (BS) transmission, the signals received by the BS cannot be regarded as orthogonal. Within the 50m-coverage area those differences may be in the order of a few spreading code symbols (chips) depending on the data rate. This effect is particularly critical for very short spreading sequences, like 32-bit Walsh functions. 2. Sequence design method Sets of spreading sequences used for DS CDMA applications can be represented by MxN matrices S MN , where M is the number of sequences in the set and N is the sequence length. The sequences are referred to as orthogonal sequences if, and only if the matrix S MN is orthogonal, i.e.