Increasing CDMA capacity using multiple orthogonal spreading sequence sets and successive interference cancellation (original) (raw)

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.

Williamson-Hadamard spreading sequences for DS-CDMA applications

Wireless Communications and Mobile Computing, 2003

Orthogonal bipolar spreading sequences are used in direct sequence code division multiple access (DS-CDMA) systems for both spectrum spreading and channel separation. The most commonly used sequences are Walsh-Hadamard sequences of lengths being an integer power of 2. A construction based on Williamson's arrays leading to sequences of lengths N:4 (mod 8) is presented in the paper. Aperiodic correlation characteristics, for example sequence sets of lengths 12-252 are presented. The correlation properties of the sequence sets are later improved using a diagonal modification technique.

Increasing the capacity of CDMA using hybrid spreading sequences and iterative multistage detection

We introduce a code-division multiple access (CDMA) concept which allows one to accommodate a higher number of users than the spreading factor N. The idea is to assign orthogonal spreading sequences to the first N users and pseudo-noise (PN) spreading sequences to all additional users. The proposed technique can thus accommodate N users without any mutual interference and some additional at the expense of some signal-to-noise ratio (SNR) penalty. This represents a significant capacity increase with respect to presently available multiple access techniques. When the number of users is larger than N, detection is performed in two separate steps, one for the set of users with orthogonal spreading sequences and one for the set of users with PN sequences. Furthermore, the process is iterated two or more times to obtain more reliable receiver decisions

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

Spreading code construction for CDMA

IEEE Communications Letters, 2003

Abstract|In this letter, we use the well known Walsh-Hardmard codes to construct larger sets of linearly dependent codes such that their matrix of correlations is band-diagnal. Optimal detection for system employing these spreading codes can be e ciently accomplished by a trellis decoder. Analysis and simulation show t h a t with a processing gain of N, u p t o 2N users can be accommodated without much performance degradation. This proposed approach is quite general and larger code set can be constructed analogously.

Large Sets of Orthogonal Sequences Suitable for Applications in CDMA Systems

IEEE Transactions on Information Theory, 2016

In this paper, we employ the so-called semi-bent functions to achieve significant improvements over currently known methods regarding the number of orthogonal sequences per cell that can be assigned to a regular tessellation of hexagonal cells, typical for certain code-division multiple-access (CDMA) systems. Our initial design method generates a large family of orthogonal sets of sequences derived from vectorial semibent functions. A modification of the original approach is proposed to avoid a hard combinatorial problem of allocating several such orthogonal sets to a single cell of a regular hexagonal network, while preserving the orthogonality to adjacent cells. This modification increases the number of users per cell by starting from shorter codewords and then extending the length of these codewords to the desired length. The specification and assignment of these orthogonal sets to a regular tessellation of hexagonal cells have been solved regardless of the parity and size of m (where 2 m is the length of the codewords). In particular, when the re-use distance is D = 4 the number of users per cell is 2 m−2 for almost all m, which is twice as many as can be obtained by the best known methods.

Interference-Resilient Block-Spreading CDMA With Minimum-MAI Sequence Design

IEEE Transactions on Communications, 2000

Code-division multiple-access (CDMA) schemes based on block spreading implement the spreading of entire data blocks rather than single symbols, thus achieving a higher robustness against the frequency selectivity of the channel and allowing the use of efficient modulation/equalization schemes operating in the frequency domain (FD). In this paper, we present a new block CDMA (B-CDMA) system where a single cyclic prefix (CP) is used at the end of each spread block. This provides a higher spectral efficiency with respect to existing schemes. By observing that complete orthogonality among users is achievable only for half-loaded systems on dispersive channels, we introduce new criteria for the design of spreading and despreading sequences, which aim at minimizing the mean-square error at the output of the despreader. For the equalization of the received signal, we propose an iterative block decision feedback equalizer, which iterates between equalization and decoding. Equalization filters are designed to minimize the mean-square error and take into account the residual interference due to the nonorthogonality of the spreading sequences. The performance of B-CDMA is evaluated in an uplink wireless scenario and compared to existing CDMA schemes.

A Novel Orthogonal Minimum Correlation Spreading Code in CDMA System

International Journal of Wireless and Microwave Technologies, 2014

Code Division Multiple Access (CDMA) is a technique in which transmission of information takes place simultaneously over the same available channel bandwidth. CDMA systems make use of spread spectrum (SS) technique for transmission of information by employing spreading codes. Each user is assigned with a unique spreading code which acts as a signature code for that individual user. The CDMA system experiences Multiple Access Interference (MAI) and Inter Symbol Interference (ISI) because of the non-orthogonality of the spreading codes. Both the MAI and ISI are the functions of auto-correlation and cross-correlation values of the spreading code respectively. In this paper, a novel orthogonal spreading code called "Orthogonal Minimum Correlation Spreading Code" (OMCSC) has been proposed which can serve a large number of users and is simultaneously expected to reduce the effect of MAI and ISI. Moreover, the Bit Error Rate (BER) performance of the proposed code has been compared with the existing codes using Additive White Gaussian Noise (AWGN) channel under multiuser scenario.

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

Linear receivers for the DS-CDMA downlink exploiting orthogonality of spreading sequences

1998

We address the problem of downlink interference rejection in a DS-CDMA system. Periodic orthogonal Walsh-Hadamard sequences spread different users' symbols followed by scrambling by a symbol aperiodic base-station specific overlay sequence. This corresponds to the downlink of the European UMTS wideband CDMA proposal. The point to point propagation channel from the cellsite to a certain mobile station is the same for all downlink signals (desired user as well as the interference). The composite channel is shorter than a symbol period for some user signals, while other users can have significant ISI owing to a faster transmission rate. In any case, orthogonality of the underlying Walsh-Hadamard sequences is destroyed by multipath propagation, resulting in multiuser interference if a coherent combiner (the RAKE receiver) is employed. We propose linear zero-forcing (ZF) and minimum mean-squared-error (MMSE) receivers which equalize for the estimated channel, thus rendering the user signals orthogonal again. A simple code matched filter subsequently suffices to cancel the multiple access interference (MAI) from intracell users.