Multi-carrier Cdma in Indoor Wireless Radio Networks

This paper examines a novel digital modulation/multiple access technique called Multi-Carrier Code Division Multiple Access (MC-CDMA) where each data symbol is transmitted at multiple narrowband subcarriers. Each subcarrier is encoded with a phase offset of 0 or π based on a spreading code. Analytical results are presented on the performance of this modulation scheme in an indoor wireless multipath radio channel. Introduction This paper examines the performance of a new spread spectrum transmission method called “MCCDMA” in an indoor wireless environment. MC-CDMA may be a suitable modulation technique in the indoor environment where the dispersive character of indoor propagation [1] allows for the exploitation of this technique. With MC-CDMA, each data symbol is transmitted over N narrowband subcarriers where each subcarrier is encoded with a 0 or π phase offset. If the number of and spacing between subcarriers is appropriately chosen, it is unlikely that all of the subcarriers will be located in a deep fade and consequently frequency diversity is achieved. As an MC-CDMA signal is composed of N narrowband subcarrier signals [2] each with a symbol duration, Tb, much larger than the delay spread, Td, an MC-CDMA signal will not experience significant intersymbol interference (ISI). Multiple access is achieved with different users transmitting at the same set of subcarriers but with spreading codes that are orthogonal to the codes of other users. Basic Principles of MC-CDMA The generation of an MC-CDMA signal can be described as follows. As shown in Fig. 1, a single data symbol is replicated into N parallel copies. Each branch of the parallel stream is multiplied by one chip of a spreading code of length N and then binary phase-shift keying (BPSK) modulated to a subcarrier spaced apart from its neighboring subcarriers by F/Tb Hz where F is an integer number. The transmitted signal consists of the sum of the outputs of these branches. For F = 1, this scheme is similar to performing Orthogonal Frequency Division Multiplexing (OFDM) [3] on a Direct-Sequence spread-spectrum signal [4]. Recently, there has been a growing interest on idea of combining OFDM and DS-CDMA [5] [6] [7] [8] [9]. Modern DSP methods make the implementation of MC-CDMA feasible and attractive. With F = 1, the transmit bandwidth is minimized. However, larger values of F may be desired to further increase the transmit bandwidth, i.e., to achieve a larger frequency diversity gain, without increasing the complexity in signal processing due to large spreading factors, N. The transmitted signal corresponding to the kth data bit of the mth user (a m [k]) is (1) sm t ( ) cm i [ ] am k [ ] 2πfct 2πi F Tb t + ( ) Tb t kTb − ( ) cos i 0 = N 1 − ∑ = cm i [ ] 1 1 , − { } ∈