We show that the common approach to channel and Time-of-Arrival (ToA) estimation for Impulse-Radio Ultra WideBand (IR-UWB), where the occurrence of pulse overlap is ignored, is bound to lead to considerable estimation errors in the presence of pulse overlap. We propose the use of a low-complexity extension of the sliding correlator as a Maximum Likelihood (ML) channel estimation method that can deal with sporadic pulse overlap due to multipath components with delay difference of less than the pulse duration. We also illustrate the superior performance of the improved estimator.

We compare the maximum achievable common signal to interference and noise ratio (SINR) target level Kmaxin the downlink of various code division multiple access (CDMA) systems with binary sequence sets. We derive an upper bound to Kmaxfor binary systems with symbol-aligned users, and we compare this upper bound to the value of Kmaxfor 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 Kmaxof 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 Kmaxthat 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 Kmaxwell beyond the upper bound on Kmaxfor systems with aligned chip pulses

We evaluate the symmetric capacity of the OCDMA/OCDMA (O/O) signature sets that maximize the sum capacity in quasi-scalable CDMA systems. It is found that the symmetric capacity is equal to the sum capacity for randomly scrambled O/O

We compare the sum capacity of strictly scalable and quasi-scalable CDMA systems. It is demonstrated that the requirement of strict scalability inevitably results in a capacity loss as compared to the optimal quasi-scalable OCDMA/OCDMA signature set.

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.

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.

We evaluate the sum capacity of the binary signatures of [G. N. Karystinos etal., (2001)(2003)] that achieve the lower bound on total squared correlation for binary signatures. Analytical expressions are derived for this sum capacity, and it is found that the restriction to binary-valued signatures can have a small but nonnegligible impact on the sum capacity.

We discuss some extensions to the conventional OCDMA/OCDMA (O/O) system. First of all, synchronous O/O, defined for spreading gain N = 2/sup n/ is extended to any spreading gain that is a multiple of 4. In a second step, we consider the performance of O/O systems, where the signatures of the two orthogonal sets are displaced with respect to each other. It is found that this displacement can result in a significant performance improvement.

We compare the achievable load of three types of overloaded signature sets with multistage successive interference cancellation: random spreading (PN), Welch-bound-equality (WBE) sequences and overall permuted OCDMA/OCDMA (o-O/O) or pseudonoise/OCDMA (PN/O). It is found that o-O/O outperforms PN and WBE for spreading factors N, varying from 16 to 256.

Two parallel interference cancellation (PIC) schemes, based on the probabilistic data association (PDA) approach are evaluated with respect to the allowable load. The N-dimensional PDA detector achieves an acceptable load that is more than three times higher than that of the one-dimensional detector. An improvement of these PIC schemes is proposed by replacing the a priori probabilities in every stage of the cancellation process by the a posteriori probabilities of the previous stage. This results in a critical load that is twice as high as that of the original PDA detector for the one-dimensional version of the PDA-PIC detector.

A parallel interference cancellation scheme, based on the iterated soft decision interference cancellation with optimized soft cancellation (ISDIC-OSC) procedure is improved dramatically by replacing the a priori probabilities in every stage of the cancellation process by the a posteriori probabilities of the previous stage. The introduction of a forgetting factor results in an additional gain in acceptable load. With this multiuser detector, loads close to unity can be achieved for PN-CDMA without significant loss as compared to the single user performance for a spreading factor of 256. Over a wide range of spreading factors, loads of about twice the acceptable load of the original ISDIC-OSC detector can be achieved with this refined detector.

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.

We consider a multiple access system with oversaturation of the channel by means of a combined OCDMA/OCDMA scheme with scrambled orthogonal bases. We investigate the performance and the associated maximum channel capacity (channel load) for uncoded BPSK modulation with binary valued chip sequences and optimum multiuser detection for spreading factor N = 4, 8, 16 and 32. It is found that, for scrambling sequences that change randomly from symbol-interval to symbol-interval, the allowable channel overload is zero for N = 4, 8, 16, and is only about 3% for N = 32. Major improvements on the allowed channel capacity are achieved by selecting a 'good' scrambling sequence and keeping it the same for all symbol-intervals. In this way the allowed channel overload can be increased to 25% for N = 8, to 31.25% for N = 16 and to at least 31.25% for N = 32.

The sum capacity of the OCDMA/OCDMA (O/O) signature sequence set is investigated when the users of the two orthogonal sets are allowed to have a different average-input-energy constraint and with perfect synchronization of the users. It is found that the sum capacity is dependent on the load and the respective SNR only, but independent of the choice of the orthogonal sets. For a fixed total power and when the power constraints of the two user sets are allowed to be different, it is shown that the sum capacity of the O/O system can be made arbitrarily close to the sum capacity of the Gaussian multiple access channel.

We present a new encoded signaling concept which consists of transmitting an additional data rate of 50 % to the original data rate that is determined by the channel bandwidth, the modulation scheme, and other physical-layer functions of the transmission system. This is accomplished by overlaying an encoded version (using a rate 1/2 code with Hamming distance 4) of N/2 excess symbols to each block of N primary symbols. The resulting interference between primary and excess symbols is handled using a joint symbol detection technique with low complexity for N=8 and N=10. Our analysis shows that the transmission of the excess data rate involves an asymptotic gain in SNR of 0.8 dB as compared to conventional BPSK or QPSK signaling. Our simulation results indicate that this gain is noticeable at bit error rate (BER) values below about 10/sup -7/.

We present a new signaling concept which consists of transmitting an additional data rate to the original data rate that is determined by the channel bandwidth, the modulation scheme, and other physical-layer functions of the transmission system. This is accomplished by overlaying M excess symbols to each block of N primary symbols, with the excess symbol period being equal to N times the primary symbol period. The resulting interference between primary and excess symbols is handled using a joint symbol detection technique. Our analysis shows that for BPSK or QPSK modulation and M=1 or M=3, transmission of the excess data rate does not degrade the minimum Euclidean distance when the excess symbol ratio M/N does not exceed 25%. Furthermore, our simulation results indicate that at bit error rate (BER) values in the order of 10/sup -3/ to 10/sup -5/, transmission of the additional data in these cases is achieved at the expense of a moderate to very small penalty in signal-to-noise ratio (SNR).

We present a multiple access technique which accommodates more than N users on a channel whose bandwidth is N times the bandwidth of the individual user signals. It makes use of two sets of orthogonal signal waveforms, one for the first N users, and one for the additional users. An iterative multistage detection technique is used to cancel interference between the two sets of users. At each stage of the detection process, the best estimate of the multiuser interference (MUI) is synthesized using the decisions available from the previous stages, and this interference is subtracted from the user signals of interest before entering these signals to a threshold detector. The new concept is described using a combination of time-division multiple access (TDMA) and orthogonal code-division multiple access (OCDMA), but it is equally applicable to any other sets of orthogonal signal waveforms. Using a binary phase-shift keying modulation (BPSK), it is shown that the proposed technique supports an excess user rate of 40% at the expense of a very small degradation of the signal-to-noise ratio (SNR).

This paper gives an overview of a previously introduced channel overloading concept that is applicable to both multiuser communications and single-user communications. We first describe it for multiuser communications using a hybrid multiple access scheme based on a combination of time-division multiple access (TDMA) and code-division multiple access (CDMA). Next, we extend it to full CDMA by defining two sets of orthogonal signal waveforms. Using a binary phase-shift keying (BPSK) modulation, it is shown that this multiple access concept can increase the number of users by 40% with respect to the hard limit of orthogonal waveform multiple access (OWMA) which includes TDMA, orthogonal CDMA (OCDMA), and orthogonal frequency-division multiple access (OFDMA). Then, we apply the same concept to single-user communications to transmit an excess bit rate in addition to the primary data rate. It is shown that an excess data rate of up to 25% can be transmitted at the price of a very small penalty in signal-to-noise ratio (SNR).