Fast communicationCode shift for intercarrier interference cancellation in MC-DS-CDMA☆
Introduction
A multicarrier DS-CDMA (MC-DS-CDMA) system is an orthogonal frequency-division multiplexing (OFDM) system in which a direct sequence code-division multiple access (DS-CDMA) signal is transmitted on each OFDM subcarrier [4]. So, on each subcarrier, the signal of several users is multiplexed on the basis of the CDMA codes assigned to each user. In a conventional MC-DS-CDMA system, each user has a specific spreading code and the user employs this code to spread the data on each subcarrier. If the MC-DS-CDMA signal is transmitted in a mobile wireless channel, the intercarrier interference (ICI) caused by the Doppler spread degrades the performance of the system [2], [5], [7]. Taking into account the autocorrelation properties of CDMA codes [3], the effects of ICI can be mitigated by applying a cyclic shift to the spread codes in the different subcarriers, as it will be shown.
Section snippets
Code shift for ICI cancellation
Consider an MC-DS-CDMA system with subcarriers. A sequence of data symbols of user , , is serial-to-parallel converted onto substreams. Within each substream, the data symbols are spread by means of the user-specific spreading code of length chips. Let the code assigned to user be expressed as follows:Note that each data symbol is spread over OFDM symbols and, therefore, a block of symbols is available at the receiver after OFDM symbols are
Simulation results
In this section, the bit error rate (BER) obtained with the conventional MC-DS-CDMA system and the BER obtained with the MC-DS-CDMA system proposed in this communication are compared. The ICI is due to a constant frequency offset . This value has been normalized by the subcarrier separation [7]. Also, additive white Gaussian noise corrupts the received signal. The simulated system has one active user with BPSK modulation on each subcarrier, subcarriers. The CDMA codes are Gold codes
Conclusions
In this paper, a MC-DS-CDMA transmitter/receiver system with very good resistance against ICI has been presented. The proposed system takes advantage of the autocorrelation properties of the CDMA codes and it achieves the same bit error rate (BER) performance as a system without ICI. Additionally, the complexity of the proposed system is similar to the one of the conventional MC-DS-CDMA system and, also, the same technique used to assign the users' spreading codes can be employed.
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Cited by (0)
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This work has been partially supported by Spanish MCYT (TIC2003-07819) and FEDER.