Increasing the capacity of CDMA using hybrid spreading sequences and iterative multistage detection (original) (raw)
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DS/CDMA with in-cell spreading sequence reuse and iterative multistage detection
2000
We introduce a code-division multiple access (CDMA) concept which makes it possible to accommodate a higher number of users than the spreading factor N. The idea is to assign orthogonal spreading sequences, overlaid with a pseudo-noise (PN) sequence, to the first N users, and to reuse these orthogonal spreading sequences, but overlaying them with another PN sequence for all additional
DS/CDMA with two sets of orthogonal spreading sequences and iterative detection
IEEE Communications Letters, 2000
In this letter we introduce a direct-sequence code-division multiple access (DS/CDMA) concept which accommodates a higher number of users than the spreading factor . Each of the available orthogonal spreading sequences of length is assigned to one of the first users which employ a common pseudonoise (PN) scrambling sequence. When the number of users exceeds , say = + with , the additional users reuse of those orthogonal spreading sequences but in combination with another PN scrambling sequence. An iterative multistage detection technique is used to cancel interference between the two sets of users when . The proposed technique thus accommodates users without any mutual interference and a number of additional users at the expense of a small signal-to-noise ratio penalty.
Iterative Multiuser Detection of Random CDMA Using Partitioned Spreading
2006
Iterative joint detection of random CDMA is considered in conjunction with error control coding, in particular using low-density parity-check (LDPC) codes. The spreading sequences are partitioned into sections, called "partitioned spreading (PS)", creating an artificial repetition code which interfaces the LDPC code with the multiple access channel. A suboptimal, but efficient cancelation processor is used at the multiple access processing node in the factor graph describing the overall system. This system is then analyzed using Gaussian density evolution and it is shown that partitioned spreading conditions the channels such that the efficiency of the code is increased substantially. A comparison between direct application of LDPC coding (without partition spreading) to a two- stage decoding procedure and a full iterative decoding schedule of PS-CDMA reveals that gains of over 100% in spectral efficiencies can be achieved with regular LDPC codes.
An Overview of Code-Spread CDMA
Code-division multiple-access (CDMA) has gained a lot of attention recently when the third generation mobile communication systems are developed. In this paper we discuss how using low-rate channel coding instead of direct sequence spreading can more efficiently use the available spectrum. This leads to large capacity improvements.
A spreading sequence allocation procedure for MC-CDMA transmission systems
A novel spreading sequence allocation procedure for multi-carrier code division multiple access (MC-CDMA) systems is proposed and investigated. This new technique, which relies on an analytical evaluation of the multiple access interference (MAI) mitigates the interference between different users by optimizing the spreading sequence selection within a given spreading sequence family. For low-loaded transmissions over different realistic frequency correlated channels, it is shown that this new selection procedure outperforms systems where no specific allocation rule is employed. Furthermore, this technique affects the emitted signals and its performance improvement is observed whatever the detection technique used at the receiver side. Thus, this technique improves the capacity of MC-CDMA systems
2003
In this contribution we propose and study a multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) scheme, which employs both time (T)-domain and frequency (F)-domain spreading. We investigate the achievable detection performance in the context of synchronous TF-domain spread MC DS-CDMA, when communicating over an additive white Gaussian noise (AWGN) channel. The performance of three different detection schemes is investigated, which include the single-user correlation based detector, the joint TF-domain MMSE multiuser detector (MUD) and the separate TF-domain TF-domain MMSE/decorrelating MUD. Our simulation results show that the separate TF-domain MMSE/decorrelating MUD scheme is capable of achieving a bit error rate (BER) performance that is similar to that of the significantly more complex joint TFdomain MMSE MUD schemes.
IJERT-Dynamic Spreading Code Allocation Strategy for A Downlink MC-CDMA System
International Journal of Engineering Research and Technology (IJERT), 2012
https://www.ijert.org/dynamic-spreading-code-allocation-strategy-for-a-downlink-mc-cdma-system https://www.ijert.org/research/dynamic-spreading-code-allocation-strategy-for-a-downlink-mc-cdma-system-IJERTV1IS5304.pdf The MC-CDMA (Multi-Carrier Code Division Multiple Access) transmission is a promising solution for the physical layer of future broadband wireless communication systems which will have to support multimedia services. By combining OFDM (Orthogonal Frequency Division Multiplexing) and CDMA, we obtain a high speed transmission capability in multipath environments and large multiple access capacity. Unlike CDMA, MCCDMA performs the spreading operation in the frequency domain, mapping each chip of the user spreading code on one subcarrier, and thus introduces frequency diversity. However when the MC-CDMA signal propagates through a frequency selective fading channel, the code orthogonality is destroyed and the resulting MAI limits the system performance. Several approaches have been proposed to mitigate MAI and to improve signal detection. The conventional single-user detection techniques, applying per subcarrier equalization as in OFDM systems and then correlation with the code of the desired user, offer poor performance. Indeed, by channel inversion, ZF (Zero Forcing) can eliminate MAI but in return-noise amplified on deeply faded subcarriers. The other techniques including MRC (Maximum Ratio Combining), EGC (Equal Gain Combining) and MMSE (Minimum Mean Square Error) cannot restore the orthogonality of codes and lead to residual MAI. Therefore more advanced methods such as MUD (Multi-User Detection) have been developed. In this paper, we investigate the impact of Walsh-Hadamard spreading code allocation on the performance of a downlink MC-CDMA system in a time varying frequency selective channel. The analysis shows that this impact is important on the multiple access interference and the inter-carrier interference power levels. We propose a code allocation strategy that minimizes the global interference power and significantly improves the performance of the MC-CDMA system.
On the Performance of Partitioned-Spreading CDMA with Multistage Demodulation
2006 40th Annual Conference on Information Sciences and Systems, 2006
The performance of partition-spreading CDMA (PS-CDMA), a recently proposed multiple-access system, is investigated. It is shown via variance evolution that the asymptotic channel throughput of PS-CDMA improves monotonically as the number of partitions increases. The fundamental limit of the PS-CDMA system is calculated by evaluating the spectral efficiency of the layered channel that is generated by multistage PS-CDMA demodulation. Capacity-approaching codes of high rate are suggested in the encoding for each PS-CDMA user, which justifies a low-complexity two-stage decoding scheme for PS-CDMA.
Spreading Codes in CDMA Detection
2003
This study deals with the main characteristics of the Maximal Length, Gold, Kasami, Walsh and variable-length orthogonal codes and their functions in code-division multiple access(CDMA) networks.