Frederik Vanhaverbeke

Extending the capacity of multiple access channels

Multiple access techniques which allow a communication medium to be shared between different users represent one of the most challenging topics in digital communications. In terms of the number of users that can be accommodated on a given channel, there are two distinct classes of multiple access techniques. The first class includes the well-known FDMA, TDMA, and OCDMA. On a channel whose bandwidth is N times the bandwidth of the individual user signals, these techniques can accommodate N users without any mutual interference, but not a single additional user can be supported beyond this limiting number. The second class includes CDMA with pseudo-noise spreading sequences (which we refer to as PN-CDMA) and some other related schemes. PN-CDMA does not have a hard limit on the number of users that can be accommodated, but is subject to multi-user interference which grows linearly with the number of users. In this article, after reviewing the capacity limits of existing multiple access techniques, we describe some newly introduced concepts which allow us to accommodate N users without any interference while also accommodating additional users at the expense of some SNR penalty.

DS/CDMA with two sets of orthogonal spreading sequences and iterative detection

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 N. Each of the available orthogonal spreading sequences of length N is assigned to one of the first N users which employ a common pseudonoise (PN) scrambling sequence. When the number of users K exceeds N, say K=N+M with M<N, the M additional users reuse M 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 K>N. The proposed technique thus accommodates N users without any mutual interference and a number of additional users at the expense of a small signal-to-noise ratio penalty.

Increasing CDMA capacity using multiple orthogonal spreading sequence sets and successive interference cancellation

In this paper, we present a code-division multiple access (CDMA) system with a spreading factor of N that can accommodate up to mN users, where m/spl ges/2. The m sets of orthogonal spreading sequences are constructed by overlaying the same N orthogonal Walsh-Hadamard sequences with a set-specific pseudo-noise (PN) sequence. Interference between user signals from different sets is cancelled by means of soft-decision iterative interference cancellation. We show that this technique, which is referred to as m-OCDMA, can accommodate more users and is computationally less complex than conventional PN-CDMA with the same type of interference cancellation. Assuming BPSK transmission over an additive white Gaussian noise (AWGN) channel and perfect power control, a degradation of 0.4 dB at the BER of 10/sup -5/ in m-OCDMA is obtained with 2.56N users in the uplink or 3.03N users in the downlink. In PN-CDMA, this figure is 2.16N for both the uplink and the downlink.

Binary signature sets for increased user capacity on the downlink of CDMA systems

We compare the maximum achievable common signal to interference and noise ratio (SINR) target level Kmax in the downlink of various code division multiple access (CDMA) systems with binary sequence sets. We derive an upper bound to Kmax for binary systems with symbol-aligned users, and we compare this upper bound to the value of Kmax for quasi-orthogonal sequences (QOS), Grassmannian signatures and binary Pados-Karystinos signatures. Secondly, we turn our attention to systems where the (binary) signatures of the users are displaced with respect to each other. In this context, we show that the value of Kmax of improved random O(rthogonal)CDMA/OCDMA (O/O) can further be increased as compared to the original improved. O/O system (with square-root cosine rolloff pulses) by an appropriate choice of the chip pulse. In addition to this, improved O/O systems with fixed signature set can achieve values of K max that are as high as those in the improved random O/O system, while not having the drawback of a spread on Kmax. Applying a steepest descent search to such a fixed improved O/O system, can result in a system with a binary signature set that has even higher values for Kmax well beyond the upper bound on Kmax for systems with aligned chip pulses

Sum capacity of equal-power users in overloaded channels

We consider the sum capacity of code-division multiple-access (CDMA) signature-sequence sets in overloaded channels that contain an orthogonal subset of dimension N. We introduce a family of signature-sequence sets that consists of a hierarchy of orthogonal subsets (HOS), and show that various signature-sequence sets designed for channel overloading belong to this family. Assuming an equal average energy (EAE) constraint on the user signals, an expression for the sum capacity of the HOS family is derived. We point out that the maximum sum capacity over the HOS family is achieved by the multiple-orthogonal (m-O) CDMA sequence sets. The sum capacity of m-O is only slightly less than the upper bound imposed by the Gaussian multiple-access channel. Moreover, m-O is superior to a system with N orthogonal sequences and (K-N) pseudonoise (PN) sequences (PN/O), which, in turn, outperforms a system with K PN sequences. Both for PN/O and sequence sets over HOS, there is no additional loss in sum capacity if we require the sequences to consist of chips that are binary or quaternary valued.

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.

Some Novel Concepts in Multiplexing and Multiple Access

This paper introduces some novel multiplexing and multiple access concepts which increase the number of users that can be accommodated on a given channel. Specifically, N users are accommodated without any interference on a channel whose bandwidth is N times the bandwidth of the individual user signals, and additional users are accommodated at the expense of some signal-to-noise ratio (SNR) penalty. This breaks the hard capacity limit of N users that is specific to orthogonal-waveform multiple access (OWMA) schemes, and the multiuser interference (MUI) that is specific to CDMA with pseudo-noise spreading sequences (PN-CDMA) does not appear in the proposed multiple access techniques until the number of users exceeds N.

Sum capacity of the OCDMA/OCDMA signature sequence set

The sum capacity of the OCDMA/OCDMA (O/O) signature sequence set is investigated under the assumption of equal average-input-energy constraints and perfect synchronization of the users. It is shown that the spectral efficiency of the O/O signature sequence set is only dependent on the SNR and load, but independent of the choice of the orthogonal bases. Moreover, this sequence set is found to be only slightly less efficient in terms of sum capacity than the Welch-bound equality sequences.

Optimal signature sets for oversaturated quasi-scalable direct-sequence spread-spectrum systems

Signature sets that maximize the sum capacity of an oversaturated code-division multiple-access (CDMA) system are derived, under the constraint that the signature sets have to be quasi-scalable. It turns out that the optimal signature set is composed of orthogonal signature sets, with at most one incomplete orthogonal set. As compared to the Gaussian multiple-access channel (GMAC), the loss in spectral efficiency for this multiple-orthogonal CDMA (OCDMA) system remains very low.

An improved OCDMA/OCDMA scheme based on displaced orthogonal user sets

We introduce a new type of OCDMA/OCDMA for oversaturated channels, by displacing in time the orthogonal signature sets of the users. A displacement by an integer multiple of a chip period considerably improves the performance of iterative detection of the user data, as compared to quasi-orthogonal sequences (QOS) and conventional random O/O sequences. An additional displacement by half a chip period reduces the variance of the cross correlation between the users of the two sets by up to 50%, and results in an additional performance improvement for square root cosine rolloff chip pulses. This improved O/O system can accommodate a number of users equal to twice the spreading gain N, when N/spl ges/32. For a practical rolloff of 25% and N=128, the acceptable channel overload can almost be tripled with improved O/O as compared to conventional O/O.