Ultra-wideband impulse radio is a promising radio technology for networks delivering extremely high data rates at short ranges. The use of extremely short duration pulses, however, makes the synchronization task more difficult. In this chapter a two-stage acquisition with serial search noncoherent correlator for time-hopping impulse radio is proposed. The proposed two-stage acquisition scheme gets chip timing synchronization, and aligns the phase of the local time-hopping code in two successive stages. With the aid of the flow-graph approach, analytical expressions are presented for the mean acquisition time and the probability of acquisition. Numerical results in a slow fading channel show that the proposed two-stage acquisition method can offer a much shorter mean acquisition time or much higher probability of acquisition than that delivered by conventional acquisition.

Introduction

As explained in Section 1.1, one of the design challenges provided by the wide bandwidth property of IR-UWB signals is timing acquisition, so a rapid acquisition algorithm is very important in IR-UWB communications. A few papers have focused on acquisition for TH IR-UWB signals. In [1] the authors analyze the acquisition performance of the IR-UWB signal. In [2] a generalized analysis of various serial search strategies is presented for reducing the mean acquisition time for IR-UWB signals in a dense multipath environment, and finally in [3] hybrid schemes for IR-UWB signal acquisition are proposed to trade off the speed of parallel schemes with the simplicity of serial search schemes.

In this chapter, TD performance is studied by assuming that the ideal channel state information is available at the receiver side. Under a frequency-selective Rayleigh fading environment, the performance of EGC and GSC 2D-Rake receivers is analyzed for either mutually independent or mutually correlated pairs of channels. The spatial diversity gain provided by TD-STBC is compared with that of a system deploying only one transmit antenna.

Introduction

It is assumed that the ideal channel state information (CSI) is available at the receiver, in other words, the channel estimation is perfect. The performance of the TD-STBC scheme is investigated in terms of the BER (bit error rate). In the next chapter, a common pilot signal transmission is employed to assist receivers in estimating the channel fading coefficients; hence, the impact of imperfect channel estimation on system performance can be investigated through comparing the results obtained in Chapters 5 and 6. The effect of correlation between the pair of channels from two transmit antennas to receive antenna is studied. In order to emphasize the spatial diversity gain from using TD-STBC, the performance of the corresponding Rake receiver of the conventional CDMA system with only one transmit antenna is evaluated and compared.

The rest of this chapter is organized as follows. In Section 5.2, the transmitter, channel and receiver models are described. The performance of coherent reception for downlink of the CDMA system with and without TD-STBC is analyzed in Section 5.3, and the closed-form expressions of BER are obtained.

In this chapter, a common pilot signal transmission scheme is utilized to assist receivers to estimate the channel fading coefficients; hence, the effect of imperfect channel estimation on the TD-STBC system performance is investigated for the independent pair of channels. The power ratio of pilot to data channels and the lowpass filter used to improve the channel estimation are addressed.

Introduction

In this chapter, the TD-STBC in the DS-CDMA system with an imperfect channel estimation scheme based on the 3GPP standard [1] is studied. In the downlink of the WCDMA system, two common pilot channel signals are transmitted simultaneously from two antennas, which are employed to assist mobile stations to estimate the channel fading coefficients. In this chapter, however, it is merely assumed that the pair of channels corresponding to two transmit antennas are independent from each other. The impact of imperfect channel estimation on the system performance can be investigated through comparing the results obtained in Chapters 5 and 6. In terms of the resultant BER and system capacity, the effect of some important parameters on the system performance is also evaluated.

The rest of this chapter is organized as follows. In Section 6.2, the transmitter, channel and receiver models are described. The performance of coherent reception in the downlink of the CDMA system with and without TD-STBC are analyzed in Section 6.3. In Section 6.4, the numerical results of the system with various parameters and consequent discussions are presented. Finally, Section 6.5 summarizes and draws some conclusions.

The performances of optimum and per sub-carrier MMSE (or pcMMSE) detectors for OFCDM systems are compared in this chapter. In OFCDM systems, the existence of MCI in the frequency domain due to a frequency selective channel on different sub-carriers dramatically degrades the system performance. To suppress MCI, an optimum or MMSE detector is employed in this chapter. A quasi-analytical BER expression is derived in the presence of imperfect channel estimation. Numerical results show that with a linear computation complexity, the MMSE detector can improve the system performance by suppressing MCI, although it cannot perform as well as the optimum detector. Thus, in systems with a small number of code channels, the optimum detector can be employed to achieve better performance, whereas the MMSE detector is more suitable for systems with a large number of code channels. The MMSE detector is also more robust to different configurations of system parameters than the optimum detector. Moreover, it is found that pilot channel power should be carefully determined by making trade-off between the channel estimation quality and received SNR for each data channel.

Introduction

OFCDM is proposed for future broadband wireless communications. Inherited from OFDM, in OFCDM systems a high-speed data stream is divided into several parallel low-speed substreams, whose bandwidth is far smaller than the channel bandwidth, so each substream can be regarded as passing through a frequency nonselective (flat) fading. As a result, the multipath interference in frequency selective fading channels is effectively avoided.

After the wireless terminal has successfully obtained network access at the link layer, the next step is to obtain an IP address, last hop router address, and other parameters that allow the terminal to obtain routing service at the network layer. In turn, the last hop router uses address resolution to map the IP address of the wireless terminal to its link layer address so packets can be delivered from the Internet to the wireless terminal. Local IP subnet configuration and address resolution have a separate set of security issues that are independent from network access authentication. Even if a terminal is authenticated as a legitimate user and is authorized for service at the link layer by network access control, a rogue terminal can launch attacks on the local IP subnet configuration and address resolution processes of other terminals if these processes are not adequately secured.

In this chapter, we discuss the security of local IP subnet configuration and address resolution. After a short look at the impact of the Internet routing and addressing architecture on mobility and how that relates to local IP subnet configuration and address resolution, we briefly review the protocols for local IP subnet configuration and address resolution in IP networks, both for IPv4 and IPv6. We then discuss threats to the local IP subnet configuration and address resolution processes. We develop a functional architecture for IP subnet configuration and address resolution security based on the threat analysis and the existing protocols.

Wireless communications services are penetrating into our society at an explosive growth rate, and demands for a variety of high-speed wireless multimedia services continue to increase. It is everyone's wish that wireless could act like a wired connection with the same quality as fixed networks. To realize true high-speed wireless systems, sustained technical innovation on many fronts will be required. The physical limitations on and problems with wireless channels (bandwidth and power constraints, multipath fading, noise and interference) present a fundamental technical challenge to reliable high-speed wireless communications. This book is an ideal reference for graduate students and practitioners in the wireless industry.

The text of this book has been developed through years of research by the author and his graduate students. The aim of this book is to provide an R&D perspective on the field of high-speed wireless multimedia communications by describing the recent research developments in this area and also by identifying key areas in which further research will be needed.

The book is organized into four parts: introduction, ultra-wideband (UWB) communications, evolved 3G mobile communications and 4G mobile communications, with twelve chapters.

For high-speed OFDM MIMO multiplexing, a new coded layered space-time-frequency (LSTF) architecture (i.e. LSTF-c) with iterative signal processing at the receiver is proposed, where multiple encoders/decoders are designed and each independent codeword is threaded in the three-dimensional (3-D) space-time-frequency (STF) transmission resource array. The iterative receiver structure is adopted consisting of a joint MMSE-SIC detector and the maximum a posteriori (MAP) convolutional decoders. Simulation results show that the proposed LSTF architecture can achieve almost the same performance as the LSTF (i.e. LSTF-a) where single coding is applied across the whole information stream. However, due to its structure of multiple parallel lower speed encoders/decoders with shorter codeword length, the proposed LSTF architecture can be more easily implemented than the LSTF-a.

Introduction

The challenge of the detection of MIMO multiplexing is to design a low complexity detector, which can efficiently suppress multi-antenna interference and approach the interference-free performance. In this chapter, an iterative processing technique for joint detection and decoding is used in the coded MIMO multiplexing. The iterative receiver contains an MMSE-SIC detector [1], the complexity of which is much lower than that of the MAP detector especially when the number of transmit antennas Nt and modulation level are large. As a constituent code, the convolutional code is used due to the computationally efficient SISO MAP decoding. The performances of the joint iterative detection/decoding schemes for both turbo code and convolutional code are quite similar for different numbers of antennas [2], [3].

Once a wireless terminal has cleared network access control, obtained an IP address on the local subnet, and has routing service for IP packets between the terminal and the network, the terminal has access to the higher-level services available on the global Internet – Web pages, IP telephony, streaming video and the like. From the point of view of routing and packet delivery service, a wireless terminal is no different than a wired terminal. A desktop PC connected to the Internet through DSL must go through a similar process to get Internet access as a wireless terminal and the resulting routing and packet delivery service is basically the same. Unlike the user of a desktop PC, however, the user of a wireless terminal is free to move the terminal to a new location. Such a movement may cross an invisible line in the access network topology between a geographical area where the current IP address continues to provide packet delivery service and where the address stops functioning. In other words, the terminal moves from one IP subnet to another causing IP handover to occur.

If the user's mobility patterns conform to the nomadic usage model discussed in Chapter 4, then starting network access control and local IP subnet configuration from the beginning are adequate for initiating routing and packet delivery service in the new subnet.

Wireless communications and internet services have been penetrating into our society and affecting our everyday life profoundly during the last decade far beyond any earlier expectations. In addition, the demand for wireless communications is still growing rapidly and wireless systems that support voice communications have already been deployed with great success. Further wireless mobile and personal communication systems are expected to support a variety of high-speed multimedia services, such as high-speed internet access, high-quality video transmission and so on. To meet the demand for high data rate services in broadband wireless systems, various systems and/or technologies have been proposed, such as the ultra-wideband (UWB) system, and evolved third generation (3G) and fourth generation (4G) mobile communications systems.

UWB communications

In the foreseeable future, the development of low-power, short-range and high-speed transmission systems is going to play a significant role in the area of wireless communication, due to a blooming growth in demand for information sharing and data distribution tools to be used in hot-spot layer and personal network layer communications. At the same time, the radio frequency (RF) spectrum suitable for wireless links is limited, so efficient spectrum utilization is a challenging problem in physical-layer communication engineering [1]. All these have motivated the exploration of the UWB transmission system.

Recently, there has been a growing interest in the research and development of novel technologies aimed at allowing new services to use the radio spectrum already allocated to established services, but without causing noticeable interference to existing users.