In this paper, the impact of chip duty factor (DF) on direct sequence (DS), time hopping (TH) and hybrid DS-TH ultra wideband (UWB) systems is investigated in realistic environments. Rake receivers are designed to perform energy capture (EC) on received UWB signals over multipath and multi-user environment in the presence of narrowband interference. It is found that by applying lower DF in the signal design, multipath resolvability can be increased and system performance can be improved. However, in contrary to the common belief, lower DF does not always contribute to performance improvement. On the other hand, it is observed that at extremely low DF, EC capability may be compromised, causing performance degradation. The optimum DF values for respective systems are determined and discussed in this paper. Additionally, the strength and tradeoff for DS, TH and DS-TH UWB systems employing varying DF are investigated and compared over multipath and multi-user environment. In a multipath environment, a selective Rake receiver with less than 10 fingers is found to be sufficient for energy capture. In a single user environment, DS-UWB system has the most superior performance, followed by DS-TH-UWB and TH-UWB systems. And in a multi-user environment, DS-TH-UWB is found to outperform the rest, followed by DS-UWB and TH-UWB systems.

This paper investigates the impact of chip duty factor (DF) in DS-UWB system with Rake receiver over AWGN and UWB indoor multipath environment corresponding to system parameters such as spreading bandwidth and chip length. Manipulating DF in DS-UWB system offers several advantages over multipath channel and thus, capable of improving system performance for better quality of communication. Although employing lower DF generally improves performance, in some exceptional cases on the other hand, degradation can be observed despite decreasing DF. Therefore, the objective of this paper is to clarify the relationship between DF and DS-UWB system performance. We discovered that with constant processing gain and spreading bandwidth, performance improvement can be observed at DF lower than 0.17. Additionally, with spreading bandwidth as tradeoff parameter, significant performance improvement can only be observed below DF of 0.85.

A simple exact error rate analysis is presented for random binary direct sequence code division multiple access (DS-CDMA) considering a general pulse shape and flat Nakagami fading channel. First of all, a simple model is developed for the multiple access interference (MAI). Based on this, a simple exact expression of the characteristic function (CF) of MAI is developed in a straight forward manner. Finally, an exact expression of error rate is obtained following the CF method of error rate analysis. The exact error rate so obtained can be much easily evaluated as compared to the only reliable approximate error rate expression currently available, which is based on the Improved Gaussian Approximation (IGA).

This paper investigates the error rate performance of parallel combinatorial spread spectrum (PC/SS) communicaion systems that use coherent and differential multiphase modulation: multiphase parallel combinatorial spread spectrum (MPC/SS) communication systems. The PC/SS systems are multicode SS systems based on orthogonal pseudo-noise (PN) sequences. Data is transmitted by delivering a combination of multiple PN sequences among a set of pre-assigned PN sequences. In the MPC/SS systems, every PN sequence on transmission is modulated by q-ary coherent or differential phase shift keying (PSK). Symbol error rate (SER) and average bit error rate (BER) in coherent and differential MPC/SS systems are investigated. The BER comparison between the MPC/SS systems and simple multicode SS systems with q-ary coherent and differential PSK is also presented. Numerical results show that the MPC/SS systems are superior to the conventional q-ary PSK systems, if they have equal spectral efficiency.

Transmit diversity, a key technique derived against multi-path mitigation in wireless communication system, is examined and discussed. Especially, we present an approach to investigate perfect/imperfect channel detection when the maximal ratio receiver combined scheme (MRRC) and a simple transmit diversity scheme (STD) are used in the wireless systems, which provide remarkable schemes for diversity transmission over Rayleigh-fading channels using multiple antennas. In order to effectively make use of the transmit diversity techniques, the same approach is extended to process the situation of one transmit antennas and N receive antennas in MRRC scheme (1 N MRRC) and two transmit antennas and N receive antennas in STD scheme (2 N STD). The effects of perfect/imperfect channel detection and the diversity reception with independent and correlated Rayleigh-fading signals are evaluated and compared carefully.

The demand for wireless mobile communications has grown at a very high rate, recently. In order to solve the non-uniform traffic rates, the use of cell splits is unavoidable for balancing the traffic rate and maximizing total system capacity. For cell planning, a DS-CDMA cellular system can be comprise of different cell sizes because of different demands and population density of the service area. In this paper, we develop a general model to study the forward link capacity and outage probability of a DS-CDMA cellular system with mixed cell sizes. The analysis of outage probability is carried out using the log-normal approximation. When a macrocell is split into the three microcells, as an example, we calculate the multi-cross interferences between macrocells and microcells, and the forward link capacities for the microcells and the neighboring macrocells. The maximum allowable capacity plane for macrocell and microcell is also investigated. The numerical results and discussions with previous published results of reverse link are summarized.

In this paper, the impact of timing jitter in direct sequence ultra wideband (DS-UWB) system is investigated over multipath fading channel. Also, a novel hybrid direct sequence multiband UWB (DS-MB-UWB) system is proposed to mitigate the impact of timing jitter. We analyze and compare the system performance for conventional DS-UWB and hybrid DS-MB-UWB with Rake receiver in the presence of timing jitter over additive white Gaussian noise (AWGN) and multipath channel. Theoretical framework is developed to calculate the amount of average energy captured in the multipath profiles and symbol error rate (SER) considering the presence of timing jitter. It is found that DS-MB-UWB system, which employs multiple sub-bands is more jitter-robust than conventional DS-UWB systems. Besides, timing jitter is found to have different impacts on DS-UWB and DS-MB-UWB systems corresponding to different parameters such as number of sub-bands employed, pulse shape, center frequency, bandwidth, number of combined paths in Rake receiver and channel power delay profile (PDP). These different impacts are analyzed and discussed in the paper.

In this paper, we propose the soft boundary concept achieved by dynamic tilted antenna to solve the issue of traffic congestion occurred in cellular wireless systems. The tilted antenna array can provide the merit of traffic balance and also achieve the optimization of the signal-to-interference ratio (SIR) at receivers by automatically tilting the antenna and implementing the soft boundary among cells, corresponding to the variation of traffic. According to our results, it is shown that power ratio control do not necessarily improved system performance when there is a large variation in traffic because it only control power levels. Also the properly chosen angle of tilt antenna can relieve the traffic congestion and perform the system performance optimization.

In this paper, the performance of a low duty factor (DF) hybrid direct sequence (DS) multiband (MB)-pulsed ultra wideband (UWB) system is evaluated over realistic propagation channels to highlight its capability of interference mitigation. The interference mitigation techniques incorporated in the DS-MB-UWB system is a novel design that includes the utilization of the frequency-agile multiple sub-band configuration and the coexistence-friendly low DF signaling. The system design consists of a Rake type receiver over multipath and multi-user channel in the presence of a coexisting narrowband interferer. The propagation channels are modeled based on actual measurement data. Firstly, by suppressing the power in the particular sub-band coexisting with the narrowband signal, performance degradation due to narrowband interference can be improved. It is observed that by fully suppressing the sub-band affected by the narrowband signal, a typical 1-digit performance improvement (e.g. BER improves from 10-3 to 10-4) can be achieved. Secondly, by employing lower DF signaling, self interference (SI) and multi-user interference (MUI) can be mitigated. It is found that a typical 3 dB improvement is achieved by reducing the DF from 0.5 to 0.04. Together, the sub-band power suppression and low DF signaling are shown to be effective mitigation techniques against environment with the presence of SI, MUI and narrowband interference.

Multiple antenna systems are promising architectures for overcoming the effects of multi-path interference and increasing the spectrum efficiency. In order to be able to investigate these systems, in this article, we derive generalized spatial correlation equations of a circular antenna array for two typical angular energy distributions: a Gaussian angle distribution and uniform angular distribution. The generalized spatial correlation equations are investigated carefully by exact and approximate analyses.

In this paper, an online SNR estimator is proposed for parallel combinatorial SS (PC/SS) systems in Nakagami fading channels. The PC/SS systems are called as partial-code-parallel multicode DS/SS systems, which have the higher-speed data transmission capability comparing with conventional multicode DS/SS systems referred to as all-code-parallel systems. We propose an SNR estimator based on a statistical ratio of correlator outputs at the receiver. The SNR at the correlator output is estimated through a simple polynomial from the statistical ratio. We investigate the SNR estimation accuracy in Nakagami fading channels through computer simulations. In addition, we apply it to the convolutional coded PC/SS systems with iterative demodulation and decoding to evaluate the estimation performance from the viewpoint of error rate. Numerical results show that the PC/SS systems with the proposed SNR estimator have superior estimation performance to conventional DS/SS systems. It is also shown that the bit error rate performance using our SNR estimation method is close to the performance with perfect knowledge of channel state information in Nakagami fading channels and correlated Rayleigh fading channels.

In this paper, we derive spatial correlation functions of linear and circular antenna arrays for three types of angular energy distributions: a Gaussian angle distribution, the angular energy distribution arising from a Gaussian spatial distribution, and uniform angular distribution. The spatial correlation functions are investigated carefully. The spatial correlation is a function of antenna spacing, array geometry and the angular energy distribution. In order to emphasize the research and their applications in diversity reception, as an example, performance of the antenna arrays with MRC in correlated Nakagami fading channels is investigated, in which analytical formulas of average BER for the spatial correlation are obtained.

We propose a novel adaptive filtering scheme named metric-combining normalized least mean square (MC-NLMS). The proposed scheme is based on iterative metric projections with a metric designed by combining multiple metric-matrices convexly in an adaptive manner, thereby taking advantages of the metrics which rely on multiple pieces of information. We compare the improved PNLMS (IPNLMS) algorithm with the natural proportionate NLMS (NPNLMS) algorithm, which is a special case of MC-NLMS, and it is shown that the performance of NPNLMS is controllable with the combination coefficient as opposed to IPNLMS. We also present an application to an acoustic echo cancellation problem and show the efficacy of the proposed scheme.

In this paper, ultra-wideband (UWB) multiple access systems are introduced by using direct-sequence (DS) and hybrid direct-sequence time-hopping (DS/TH) code division multiple access (CDMA) that use arbitrary chip-duty of the spreading sequences. The bit error probabilities are presented. First of all, the variances of the multiple access interference are developed by investigating the collision properties of the signals. Afterward, various approximations are applied. The standard Gaussian approximation (SGA) for the DS system is shown to become extremely optimistic as the chip-duty becomes low. Though the hybrid system performs better, the SGA still remains optimistic. To obtain accurate results, Holtzman's simplified improved Gaussian approximation (SIGA) and Morrow and Lehnert's improved Gaussian approximation (IGA) are used. A shortcoming of the SIGA is rediscovered that renders it unusable for low-duty DS systems, especially, at high signal-to-noise ratio. However, for the hybrid system, the SIGA works as an excellent tool. The IGA is used to get accurate results for the low-duty DS systems. It is shown that lowering of chip-duty by keeping chip rate and chip length unchanged improves performance for asynchronous DS and both asynchronous and synchronous hybrid systems. However, under the same processing gain, a high-duty system performs better than a low-duty system. Performance of synchronous DS system remains independent of chip-duty.

This paper proposes iterative demodulation/decoding for parallel combinatorial spread spectrum (PC/SS) systems. A PC/SS system conveys information data by a combination of pre-assigned orthogonal spreading sequences with polarity. In this paper, convolutional coding with a uniform random interleaver is implemented in channel coding, just like as a serial concatenated coding. A 'soft-in/soft-out' PC/SS demodulator based on a posteriori probability algorithm is proposed to perform the iterative demodulation and decoding. Simulation results demonstrate that the proposed iterative demodulation/decoding scheme bring significant improvement in bit error rate performance. This proposed decoding scheme achieves high-speed transmission by two approaches. One is a puncturing operation, and the other is to increase the number of transmitting sequences. In the latter approach, lower error rate performance is achieved comparing with that the punctured convolutional code is used to increase the information bit rate.

This paper proposes a hybrid multiband (MB) ultra wideband (UWB) system with direct sequence (DS) spreading. The theoretical error analysis for the DS-MB-UWB multiple access system with Rake receiver in the presence of multipath and narrowband interference is developed. The developed theoretical framework models the multiple access interference (MAI), multipath interference (MI) and narrowband interference for the designed UWB system. It is shown that the system error performance corresponding to the combining effects of these interference can be accurately modeled and calculated. Monte Carlo simulation results are provided to validate the accuracy of the model. Additionally, it is found that narrowband interference can be mitigated effectively in the multiband UWB system by suppressing the particular UWB sub-band co-existing with the interfering narrowband signal. A typical improvement of 5 dB can be achieved with 75% sub-band power suppression. On the other hand, suppression of UWB sub-band is also found to decrease frequency diversity, thus facilitating the increase of MAI. In this paper, the developed model is utilized to determine the parameters that optimize the UWB system performance by minimizing the effective interference.

Performance of Rake reception of Ultra Wideband (UWB) signals is evaluated from energy capture perspective. In addition to ordinary all Rake (ARake) and selective Rake (SRake) receivers which are considered in conventional spread spectrum communications, we introduce optimum ARake and SRake receivers which include the estimation of delay of the combining multipaths. Impact of pulse-width is discussed on their performances considering the relationship between pulse-width and fading. Time hopping M-ary pulse position modulation (TH-MPPM) and binary phase shift keying (TH-BPSK) are considered as modulation schemes. Extensive simulation results are presented showing the performances of the Rakes introduced using IEEE 802.15.3a UWB channel models (CM1 to CM3). Performance of MPPM is shown for various values of M and modulation parameters. The impact of pulse-width is illustrated mainly using BPSK. It is shown that the total energy capture (i.e. by ARake) strongly depends on the pulse-width, and the shorter the pulse-width the more is the amount. The energy capture also varies a lot for employing either optimum or ordinary Raking method. Energy capture by SRake additionally strongly depends on the number of combined paths until the number is 20 for optimum SRake and 10 for ordinary SRake; however, afterwards saturating effects are seen. Several aspects regarding the performance versus complexity issue of Rake receivers are also discussed.

Upper bounds on the bit error rate (BER) for maximum likelihood (ML) decoding are derived in convolutional coded parallel combinatorial spread spectrum (PC/SS) systems over additive white Gaussian noise (AWGN) channels. PC/SS systems can achieve higher data transmission than conventional multicode SS systems. To make the derivation tractable, we put a uniform interleaver between a convolutional encoder and a PC/SS transmitter. Since the PC/SS transmitter is employed as the "inner encoder," the bounds are obtained in a similar manner of the derivation in serially concatenated codes through a uniform interleaver. Two different error patterns in the PC/SS system are considered in the performance analysis. Numerical results show that the derived BER bounds are sufficiently accurate. It is found that the coded PC/SS systems outperform coded all-code-parallel DS/SS systems under the same data rate conditions if the number of pre-assigned PN codes increases.