Recently, a lot of attention has been devoted to Multi-carrier CDMA (MC-CDMA) as it potentially offers the advantages of both CDMA and OFDM. Several ways of uniting CDMA and OFDM have been reported [1,2]. Typically, data symbols are first spread by orthogonal codes, and then mapped into the sub-carriers of an OFDM modem in such a way that the spreading operation is performed in either time or frequency domain, or both. In this manner, multi-user access and time or frequency diversity gain are provided by the CDMA portion, while the OFDM portion offers resilience to frequency-selective fading. Most of the previous studies are mainly concerned with the features and performance of MC-CDMA schemes, how they compare to conventional DS-CDMA and OFDM, and detection strategies [1,2] (reference numerals in [ ] refer to the references listed at the end of the text).
It is known that a major benefit of MC-CDMA is that the frequency domain spreading process distributes a symbol over a number of sub-carriers, and since in a frequency-selective channel the sub-carrier channel functions have different amplitude and phase, the spreading takes advantage of the frequency diversity inherently present. Another feature of MC-CDMA is the easy multi-user access, orthogonal codes may be used to separate the symbols from different users. De-spreading at the receiver with the appropriate code readily recovers the data for a specific user.
The fact that under a frequency-selective fading channel the sub-channel frequency response functions are non-uniform has been well exploited in the past in OFDM systems. Specifically the topic of adaptive modulation, or bit loading, has been studied to make the most of the frequency diversity available [3,4]. The basic idea is simple—if a sub-carrier is in good condition it is loaded up with high order modulation symbols with little or no coding to improve the throughput; conversely if it is poor a low order, more robust modulation is used to improve the error performance. Typically a sub-channel's condition is assessed by its local SNR and loaded with a modulated symbol appropriate for the SNR. It has been shown that this technique can significantly improve the performance of an OFDM modem [3,4].
A similar idea may, in principle, be applied to a MC-CDMA system. However, very little has been done in this area. Almost all the reported MC-CDMA systems employ the same modulation (and coding) scheme across all the sub-carriers. While the uniform spreading process achieves some frequency diversity gain, further improvement may be obtained by adaptively loading up the sub-carriers according to their conditions. One problem in a multi-user scenario is that since different users have different channel conditions, a sub-carrier allocation that works for one user does not necessarily work, or even makes sense, for another user.
In a multi-user environment, the channel conditions are generally different from one user to another, and hence the sub-carrier bit allocations will also be different between the users. This creates a synchronization problem when the users' spread chips are summed at the transmitter, since the chips from the users' modulated symbols are no longer assigned to the same set of sub-carriers. Once the synchronization is lost, the receiver will no longer be able to recover the user symbols.
Little prior art concerning adaptive modulation or related topics for frequency domain spreading MC-CDMA systems is available. McCormick and Al-Susa [2] mentioned a scheme that uses short spreading codes, assembles the users in small groups and allocates each user its best selection of sub-carriers. Kim, Georghiades and Huang [5] presented a sub-band selection scheme to choose suitable sub-carriers for individual users to transmit data. This may be regarded as a simple adaptive scheme that only turns the sub-carriers on or off. Lok [6] achieved adaptation by varying a user's assigned spreading code according to noise and interference level. Zhu and Bar-Ness [7] proposed an algorithm to allocate power to the sub-carriers, the algorithm adapts the power only but not the modulation order, i.e. the number of bits loaded into each sub-carrier are not adapted. Vishwanath, Jafar and Goldsmith [8] suggested an iterative water-filling algorithm to maximize the sum of throughputs of the users when only one sub-carrier is allocated to a user. They also formulated the problem for the more general case when more than one sub-carriers are allocated to a user but no exact solution was given.
There is thus a need in the art to develop new adaptive schemes for multi-user downlink MC-CDMA systems that employ frequency domain spreading.