We study the effect of the HPA non-linearity on the symbol error rate in OFDM transmission. The conventional analysis (Gaussian distribution of the distortion terms) is revisited and it is shown that it fails to give accurate results in the lower probability region. A new approach is presented providing upper bounds on the assumption that the OFDM signal can be modeled as a stationary Gaussian random process

This work describes a method to optimally transmit multimedia data over wireless fading channels via Orthogonal Frequency Division Multiplexing (OFDM). It is well known that multimedia data is sensitive to fading effects because each data bit varies in importance. Hence, reliable broadband wireless media communication systems maintain a number of very specific needs such as unequal error protection and time-varying bandwidth requirements, etc. Up to now, most OFDM systems allocate bits in a uniform manner across the subcarrier distribution. This type of methodology is fine for text data transmission with a performance measure in terms of the bit error rate (BER), but not sufficient to maintain high end-to-end visual quality for multimedia data transmission. To improve the overall system performance, we focus on optimal bit allocation in a rate-distortion sense. Our algorithm examines the data content and its associated performance and allocates the most important bits to the best channels in order to minimize the overall distortion. The proposed scheme demands channel state information that specifies estimates on the subcarrier quality distribution. It is demonstrated by experimental results that a significant gain in multimedia QoS is achieved with our approach with a layered video codec i.e. Motion JPEG2000.

The problem of PN code acquisition at base stations in asynchronous direct-sequence code-division multiple access (DS-CDMA) systems equipped with antenna arrays is investigated in this work. Both the fading and the near-far interference effects of the communication channel are considered. The antenna system in the base station is a one-dimensional (1D) linear beamforming array. After formulating the acquisition problem, we propose several methods for its solution. They include: the least squares method, the constrained optimization method, the weighted eigenvectors method and the generalized likelihood ratio test method. The system parameters such as the fading gain, the direction of arrival (DOA) and transmitted data symbols from each active mobile user are assumed to be unknown but deterministic in our model. The only knowledge is the spreading code of the desired user. It is demonstrated by simulation results that our proposed solutions are resistant to the near-far effect and robust to the change of the fading environment.

Future-generation wireless communication systems, designed to support very-high-rate data services, will require ultra-wide frequency bands. However, the frequency spectrum is a very scarce resource and has (for the most part) already been allocated to different entities in small blocks. No one entity owns a contiguous ultra-wide bandwidth. Hence, any company seeking to create ultra-wide bandwidth communications will need to construct a system that operates over non-contiguous frequency bands. In this paper, we extend the MC-CDMA and CI/MC-CDMA architectures such that they operate over non-adjacent frequency bands. In this way, the MC-CDMA and CI/MC-CDMA systems support ultra-wideband communications by operating over a large set of non-contiguous frequency bands, where each band may be either unlicenced or appropriately licensed. Furthermore, we ensure that these proposed systems (1) exploit the full diversity benefits available over non-adjacent bands, maximizing performance, and (2) exploit the total bandwidth to maximize network capacity, as measured by number of users (per cell). Simulation results over typical multi-path fading channels confirm that these proposed systems not only support ultra-wideband communications over non-contiguous frequency bands, but outperforms similar systems over contiguous frequency bands (as a direct result of larger frequency diversity available).

In this paper, we propose a novel FSK scheme that enhances performance and throughput relative to traditional FSK. Specifically we propose a novel BFSK implementation where, rather than frequency f₀ representing bit '0' and f₁ representing bit '1', we transmit multiple, orthogonal, in-phase carriers around f₀ or f₁ to represent binary information. We demonstrate the high performance of this FSK system in frequency selective channels, where a simple coherent reception technique enables diversity gain (resulting in significant performance improvement relative to traditional BFSK). We also introduce the concept of dense pulse packaging (DPP) that ensures a doubling of the data rate with minimal cost in performance, by pseudo-orthogonal positioning of pulse shapes at the transmitter.

We present a simplified Fourier-transform process, called the orthogonal frequency Fourier transform (OFFT). Conventional divide-and-conquer techniques, such as the fast Fourier transform (FFT), reduce the number of operations in a Fourier transform and simplify at least some of the complex-valued terms (i.e. twiddle factors). The FFT reduces the number of multipliers, which account for much of the chip area and power consumption in digital VLSI design. The OFFT and inverse OFFT exploit orthogonal frequency relationships to replace multiplications with simpler sampling and adding operations. Specifically, the OFFT replaces twiddle factors with step functions, which are superpositions of harmonic sinusoids. The resulting transform is adapted to add samples that are selected relative to at least one periodic step function, thus eliminating all complex multiplications. In phase and quadrature phase OFFT processing may be performed. OFFTs can be combined with pass-band sampling to simultaneously perform filtering, down conversion, and demodulation. Inverse OFFTs combined with pass-band filters can be used to provide up conversion of multi-carrier signals. Since OFFTs are substantially less complex than FFTs, OFFT processing is applicable to digital radio systems where there are considerable constraints on power consumption and chip size. The OFFT is particularly useful for processing multi-carrier transmission protocols in wireless communications, such as Carrier Interferometry, Orthogonal Frequency Division Multiplexing, and Multi-carrier Code Division Multiple Access, which are quickly gaining favor over single-carrier protocols. OFFT algorithms can process a greater number of carriers and provide lower complexity compared to FFTs.

We study the optimal transmission strategy of a single-user multiple-input multiple-output (MIMO) communication system with covariance feedback. We consider the situation with correlated receive and correlated transmit antennas in Rayleigh flat fading. Furthermore, we assume that the receiver has perfect channel state information (CSI) while the transmitter knows only the transmit correlation matrix and the receive correlation matrix. We show that transmitting in direction of the eigenvectors of the transmit correlation matrix is the optimal transmission strategy. In addition to this, the optimal power allocation is studied and a necessary and sufficient condition for optimality of beamforming is derived. The impact of the receive correlation on the achievable capacity and on the capacity-range of beamforming is analyzed. All theoretical results are illustrated by numerical simulations.

An embedded differential space-time coding method is proposed for wireless broadcast application in this work. Our previous work investigated embedded space-time codes for layered media broadcast. In such a system, a transmitter sends out multi-layer source signals by encoding different layers with different space-time codes. Then, the receiver can retrieve a different amount of information depending on the number of antennas it has as well as the level of receiving power. The receiving terminals with one antenna can decode only the base layer information with a low complexity while terminals with more antennas can retrieve more layers of information. In this research, we further consider the case when the channel state information (CSI) is unknwon. The differential coding technique is adopted here. Transmitted symbols are encoded as a certain combination of previous symbols so that the receiver can retrieve the information without CSI. We explore the embedded design with differential STC to enable layered media transmission. Differential detection with Kalman filtering is also investigated to help improve performance.

Mobile location is one of the fastest growing areas for the development of new technologies, services and applications. This paper describes the channel models that were developed as a basis of discussion to assist the Technical Subcommittee T1P1.5 in its consideration of various mobile location technologies for emergency applications (1997 - 1998) for presentation to the U.S. Federal Communication Commission (FCC). It also presents the PCS 1900 extension to this model, which is based on the COST-231 extended Hata model and review of the original Okumura graphical interpretation of signal propagation characteristics in different environments. Based on a wide array of published (and non-publicly disclosed) empirical data, the signal propagation models described in this paper were all obtained by consensus of a group of inter-company participants in order to facilitate the direct comparison between simulations of different handset-based and network-based location methods prior to their standardization for emergency E-911 applications by the FCC. Since that time, this model has become a de-facto standard for assessing the positioning accuracy of different location technologies using GSM mobile terminals. In this paper, the radio environment is described to the level of detail that is necessary to replicate it in a software environment.

The upcoming Ultra-wide-band (UWB) radio technology holds great promise for revolutionizing wireless communications. UWB radios transmit using precise, very short (e.g. picosecond) impulses spread over a very large bandwidth (up to a few Ghz). The significant advantages of this technology are low-power operation, mitigated multi-path fading effects, high bit-rates and unique precise position/timing location ability. However, one of the drawbacks of this technology, in its current state, is the high channel acquisition time, i.e. the time for a transmitter and receiver to achieve bit synchronization. This tends to be quite high, of the order of a few milli-seconds. Hence, it is important for current medium access control (MAC) protocol design to consider the impact of acquisition time. In this paper, we study the performance of two standard MAC protocols - the distributed CSMA/CA protocol and the centralized TDM protocol in the context of UWB wireless local area networks. We study effects of varying packet frame sizes and packet arrival rates and present a quantification of the impact of acquisition time on overall performance.

We propose and analyze a MAC protocol aptly designed for WLAN, based on OFDM modulation. The goal of the analysis is to provide an efficient tool to deliver to voice and data terminals acceptable QoS connections, overcoming the problem of large number of retransmissions, high delays and low reliability. Simulation results show that, in a real environment, the proposed algorithm is able to achieve good performance in terms of throughput and message delivery delay.

A novel adaptive mapping from physical measurements in a non-stationary wireless environment to a variable length Markov chain (VLMC) model is proposed in this research. The proposed scheme consists of two main components: the estimation of channel signal-to-noise ratio (SNR) distribution and discrete VLMC modeling. To obtain the channel SNR distribution, a kernel density stimation algorithm is used to track local hanges of channel statistics resulting from varying mobile environments. With the estimated channel SNR distribution, an iterative partitioning mechanism is performed to construct the VLMC model, which yields a much larger and structurally richer class of models than ordinary higher order Markov chains. Application of this model is presented, which is the computation of fading parameters such as the fading duration and the level crossing rate. The accuracy of the proposed VLMC scheme and the performance of its applications are demonstrated via simulation in a micro-cell non-stationary wireless environment.

IMT-2000 has attracted strong attention as the Next Generation Mobile Communication System. That can provide high-bit-rate data communication service, so that a great number of packets conveyed by TCP will be transmitted over a wireless link. W-CDMA standardized in 3GPP (standardizing IMT-2000) allows dynamic allocation of transmission rates to flows over a wireless link in response to changing link condition for each flow, which is thought to be essential for next generation mobile communication system. Therefore, in this paper, we study the characteristics of the dynamic allocation scheme when TCP flows share a wireless link, in particular by focusing on the throughput performance of those TCP flows. First, we examine the effectiveness of the dynamic allocation of down link transmission rate for TCP flows in response to changing FER by means of simulations. Through simulation results, we will show how it can improve the total throughput performance of TCP flows. Furthermore, we can obtain an effective way to allocate transmission rates to flows with different FERs in order to achieve high total throughput. Finally, we will deal with a case of multiple flows from a fixed host to a mobile host. In actual networks, it will very often happen. In this case, we will show that the total throughput of TCP flows less degrades than in the above single flow case even when FER becomes high.

The paper addresses the problem of constructing signature sequences or quasi-synchronous CDMA systems. The major feature of QS-CDMA is hat the duration of any signature waveform is significantly longer than the maximal signal delay with respect to a common clock. In contrast to completely asynchronous CDMA systems for which all aperiodic correlations must be taken into consideration, in case of quasi-synchronous CDMA, it is sufficient to focus on a small window around the zero shift, the so-called interference window. This implifies sequence design and allows us to reduce both the inter-symbol interference and multiple access interference drastically, thereby enhancing the system performance tremendously. This is all the more important since construction of sequences for completely asynchronous CDMA systems is a notoriously difficult problem and no such sequence sets are known. Consequently, random sequences must be used that are known to induce a significant capacity loss. In this paper, we present two possible methods for obtaining sequences with small aperiodic correlation sidelobes in the vicinity of the zero shift. We discuss advantages and disadvantages of QS-CDMA and W-CDMA, the major air interface for third generation mobile communications systems. Some simulation results are presented.

This paper gives the complete analysis of SINR when using OVSF/truncated PN codes in downlink channel for multi-rate W-CDMA systems in a multi-user and multi-path fading environment when RAKE receiver is used at the mobile end. We point out those terms ingored under the situation that the autocorrelation property of PN sequence holds in previous work. Then we give simulations to show the reasons why these terms can not be omitted, and give appropriate mathematical representation to model these terms. At last, we give the comparison of the analytical result of SINR formula and numerical result in different cases. The simulations have shown that by taking these terms into consideration, we can have better fit to the numerical result.

The bit error probability (BEP) of a multistage linear parallel interference canceller (LPIC) is a long-code code division multiple access (CDMA) s ys tem with chip-synchronized asynchronous transmission is analyzed in this work. By assuming the decision statistic to be a Gaussian random variable, the BEP can be obtained by plugging the conditional mean and variance of the decision statistic into the Q function. We formulate the effective correlation matrix D(i) in the asynchronous environment, and show that the conditional mean and the variance of the decision statistic can be expressed as functions of moments of D(i). The computation of moments of D(i) can be divided into two parts. One is the expectation of cosine terms, which can be solved by the method introduced in our previous work of a synchronous system [1]. The ot her is the expectation of cross-correlation terms, which is much more complex than the case in [1]. Besides tools in [1], we develop an evolutionary forest to deal with manipulations of D(i) and show that the computation of the expectation of cross-correlation terms is equivalent to solving a list coloring problem.

In this paper, performance of different MultiUser Detection (MUD) receivers are investigated for a MultiCarrier (MC)-Code Division Multiple Access (CDMA) system in an Indoor Wireless Local Area Network (WLAN). Simulation results show that benefits of MC-CDMA schemes in terms of capacity, throughput and Bit Error Rate (BER), are enhanced by means of Least Mean Square Error (LMSE) and Selective Parallel Interference Cancellation (S-PIC) receivers, at the expense of a moderate increase in overall system complexity. In particular, the S-PIC receiver is able to outperform other techniques in all the conditions examined. Moreover, we have investigated robustness of our proposed scheme with respect to phase and/or amplitude estimates errors, to prove that performance are not highly affected if errors are not large enough.

The technique of joint blind channel estimation and multiple access interference (MAI) suppression for an asynchronous code-division multiple-access (CDMA) system is investigated in this research. To identify and track dispersive time-varying fading channels and to avoid the phase ambiguity that come with the second-order statistic approaches, a sliding-window scheme using the expectation maximization (EM) algorithm is proposed. The complexity of joint channel equalization and symbol detection for all users increases exponentially with system loading and the channel memory. The situation is exacerbated if strong inter-symbol interference (ISI) exists. To reduce the complexity and the number of samples required for channel estimation, a blind multiuser detector is developed. Together with multi-stage interference cancellation using soft outputs provided by this detector, our algorithm can track fading channels with no phase ambiguity even when channel gains attenuate close to zero.