We investigate the influence of different Walsh-Hadamard (WH) code allocation techniques on the performance of multi-carrier code division multiple access (CDMA) systems in the presence of high power amplifier (HPA) non-linearities. We consider two different multi-carrier CDMA schemes: multi-carrier CDMA (MC-CDMA) and multi-carrier direct sequence CDMA (MC-DS-CDMA) and analyse their performance in terms of total degradation and spectral spreading for different numbers of active users.

The combination of code division multiple access (CDMA) and multi-carrier modulation (MCM) (referred to as multi-carrier CDMA) has been proposed as a possible candidate for future generations of wireless/mobile systems. Much work has been done on the performance of such schemes in various environments, with different channel models, predominantly assuming linear channels. Accordingly, in this paper we explore the influence of the effects of non-linearities (introduced by the transmitter high power amplifier (HPA)) on multi-carrier CDMA systems using higher order modulation schemes. We consider two techniques for combining CDMA with MCM: multi-carrier CDMA (MC-CDMA) and multi-carrier direct sequence CDMA (MC-DS-CDMA) and analyse their performance in the presence of an HPA and additive white Gaussian noise (AWGN), paying particular attention to the total degradation and the spectral spreading.

To maximize reconfigurability in software radio receivers, digitization should occur as close to the antenna as possible. Bandpass sampling allows the digitization of bandpass signals at RF or intermediate frequencies without significantly increasing the sampling rate. This enables a more flexible receiver to be realized allowing for many radio functions to be defined in software. Sampling of signals at high carrier frequencies has associated problems such as the effects of aperture jitter. Jitter can limit the frequency at which digitization occurs and also degrade receiver performance. This paper investigates the effects of jitter on signal quality in bandpass sampling systems.

In this paper, we present an analytical approach to accurately assessing the performance of subcarrier multiplexed optical systems for the delivery of future mobile radio broadband services. The existence of a wide dynamic range of power levels in the return link, together with laser nonlinear distortion, represents a major problem and makes performance evaluation difficult. By modeling laser nonlinear distortion using Volterra series analysis, we show that the performance assessment of these systems becomes not only tractable, but also that they can be can be optimized based on the worst case condition. This analysis is performed here for the case of continuous phase frequency shift keying (CPFSK) modulation, for which we calculate the power spectral density (PSD) of the intermodulation products. The overall system performance is that investigated, and its dependence on laser bias and RIN is examined. As an illustrative example we consider a specific 50 channel CPFSK system occupying the bandwidth of 2 to 2.1 GHz.

A class of DC-free codes with minimum distance 4 which offer very simple encoding and decoding procedures is presented. Spectral analysis shows that in addition to being DC-free they offer strong spectral suppression at low frequencies. A comparison with similar codes in the literature shows that they provide a good compromise solution to the design of practically realizable, high-rate, DC-free error-correcting codes with strong low frequency suppression.>

Multilevel coding schemes with simultaneous error control and low-frequency spectral suppression are compared to traditional cascaded error control and line coding systems. A new coding algorithm is introduced and an assessment in terms of error performance and low frequency spectral suppression is provided. Results show that significant performance benefits can be gained by employing the combined coding technique.>

A comparison is made of two methods for the computation of the probability that the sum of a Gaussian and a nonGaussian random variable exceeds a certain threshold value. The two methods, the saddlepoint approximation (SAP) and the modified Chernoff bound, are closely related to one another. For the computation of error probabilities in digital optical communications where both methods can be applied, the modified Chernoff bound is found to lie close above the saddlepoint approximation for all cases of practical interest. The ratio of signal-dependent to additive noise has been established as the parameter dominating the bound tightness. Since the SAP has been reported to be extremely precise, this result indicates that the more versatile upper bound can be used with confidence both for performance evaluation and for optimization of optical communication systems.>

The procedure introduced by R.H. Deng and M.A. Herro (see ibid., vol.IT-34, p.786-92, 1988) for identifying DC-free codes is considered further. Certain of the detailed statements relating to disparity and runlength are corrected and/or refined. While this is considered important for a proper interpretation of the codes and encoding procedure it is emphasized that it in no way detracts from the usefulness of the general algorithmic approach for producing these codes.>

The problem of average error probability evaluation for direct detection binary optical communications in the presence of avalanche gain, intersymbol interference, and colored additive Gaussian noise is considered. Tight new upper and lower bounds, together with a modified Gaussian quadrature rule based on approximate moments, are derived and evaluated. The bounds are found to be much tighter than the Chernoff bound though only slightly more complex to evaluate, and can be used as approximations to the error probability in most cases of practical interest. Taken together the new bounds and the modified Gaussian quadrature rule form a comprehensive set of performance evaluation tools offering a judicious balance between complexity and accuracy.

A technique for optimizing the performance of directdetection optical communication receivers for binary signaling in the presence of signal dependent noise, additive noise, and intersymbol interference by minimizing a Chernoff bound on the average error probability is extended to accommodate timing jitter. Receivers are identified which provide significantly improved performance compared to conventional designs, and the performance of a jitter-optimized receiver is shown to be relatively insensitive to variations in the jitter level.

A technique is developed for optimizing communication receivers by minimizing a Chernoff bound (CB) on the Bayes cost function. Being couched in terms of moment generating functions, the procedure is well suited to optical systems. Optimal CB receivers are derived and shown to coincide exactly with known minimum cost Bayes receivers for certain special cases of both electrical and optical communications for which closed form analytic solutions are possible. This provides endorsement for the method, which is then applied to more complex practical cases of linear optical fiber receivers in which both colored Gaussian noise and intersymbol interference play a part. The CB receivers are shown to offer some improvement compared with a zero forcing raised cosine equalizing receiver such as is commonly adopted, and all the evidence suggests that CB receivers achieve performance very close to the best possible with a linear receiver.