Introduction of carrier interference to spread spectrum multiple access

This paper introduces a new scheme for spread spectrum multiple access. Like MC-CDMA, this scheme accomplishes spectral spreading by transmission of identical data over N carriers simultaneously. However, unlike any existing CDMA technique to date, this method supports user orthogonality not through the use of spreading codes (based on PN sequences), but rather through multiple carrier interference. Specifically, in this novel method, aptly named CIMA (carrier interference multiple access), the interference of multiple carriers enables user orthogonality or pseudo orthogonality based on user positioning in time. It is shown that CIMA supports simplified receiver structures for AWGN channels, and offers performance benefits in AWGN and fading environment

Generation of correlated Rayleigh fading channels for accurate simulationof promising wireless communication systems

In this paper, a generalized method is proposed for the accurate simulation of equal/ unequal power correlated Rayleigh fading channels to overcome the shortcomings of existing methods. Spatial and spectral correlations are also considered in this technique for different transmission conditions. It employs successive coloring for the inphase and quadrature components of successive signals using real correlation vector of successive signal envelopes rather than complex covariance matrix of the Gaussian signals which is utilized in conventional methods. Any number of fading signals with any desired correlations of successive envelope pairs in the interval [0, 1] can be generated with high accuracy. Moreover, factorization of the desired covariance matrix is avoided to overcome the shortcomings and high computational complexity of conventional methods. Extensive simulations of different representative scenarios demonstrate the effectiveness of the proposedtechnique. The simplicity and accuracy of this method will help the researchers to study and simulate the impact of fading correlation on the performance evaluation of various multi-antenna and multicarrier communication systems. Moreover, it enables the engineers for efficient design and deployment of new schemes for feasible wireless application

A Generalized Algorithm for the Generation of Correlated Rayleigh Fading Envelopes

Although generation of correlated Rayleigh fading envelopes has been intensively considered in the literature, all conventional methods have their own shortcomings, which seriously impede their applicability. In this paper, a very general, straightforward algorithm for generation of an arbitrary number of Rayleigh envelopes with any desired, equal or unequal power, in wireless channels either with or without Doppler frequency shifts, is proposed. The proposed algorithm can be applied in case of spatial correlation, such as with antenna arrays in Multiple Input Multiple Output (MIMO) systems, or spectral correlation between the random processes like in Orthogonal Frequency Division Multiplexing (OFDM) systems. It can also be used for generating correlated Rayleigh fading envelopes in either discrete-time instants or a real-time scenario. Besides being more generalized, our proposed algorithm is more precise, while overcoming all shortcomings of the conventional methods

MIMO-OFCDM systems with joint iterative detection and optimal power allocation

This paper investigates the orthogonal frequency and code division multiplexing (OFCDM) systems with multiple input multiple output multiplexing (MIMO-OFCDM) and multicode transmission. Combining the iterative detection in the space domain and the hybrid multi-code interference (MDI) cancellation and minimum mean square error (MMSE) detection in the frequency domain, a joint iterative detection is proposed, which enables space and frequency diversity gains to be jointly exploited. Moreover, using a two-dimensional (2-D) averaging channel estimation algorithm, a close form expression is derived for the optimal power allocation between the pilot and all data channels that achieves the best system performance. It is shown that the optimal power ratio mainly depends on the channel estimation algorithm, the number of transmit antennas as well as the number of pilot and data symbols in a packet, but is not sensitive to the changes in signal-to-noise ratio (SNR) and diversity gains. Simulations are conducted to verify the derived optimal power ratio and study the performance of the proposed joint detection algorithm. It is shown that considerable improvement can be obtained when the number of loops in the joint iterative detection increases. Moreover, the system performance is enhanced significantly when the frequency domain spreading factor, N/sub F/, increases. © 2006 IEEE.published_or_final_versio

Spatial Characterization of Multi-element Antennas

The overall goal of the current thesis is to establish some bases for understanding and characterization of multi-port antennas in a rich multipath environment. Multi-port antennas are the inevitable keystone of multiple input and multiple output wireless communication systems. Due to multidisciplinary nature of the applications of multi-element antennas, they are the subjects of many research groups worldwide resulting in inconsistent nomenclature among them. In this thesis, much effort is expended to look upon this realm of engineering in a unifying approach, with the major stress on electromagnetic aspects of this area. The thesis throws light upon different significant parameters as the key-gauges for characterization of multi-port antennas. We illustrate the dependency of precise measurement of diversity gains upon total number of independent measured samples. Two closed-form formulas are devised rendering diversity gains of two-port antennas in a rich scattering environment. Some examples are presented to verify the accuracy of these formulas. Moreover, received signals at different ports of a multi-element antenna in a multipath environment are the major sources for its assessment. There are certain functions governing the relation between the received signals at different ports of a radiation system and an arbitrary incident electromagnetic wave. The precise derivation of these formulas for different cases of interest are presented in this thesis. Furthermore, it has been frequently acknowledged that in a uniform rich multipath environment the beam-forming technique does not prove beneficial. Here for the first time, we stress that in a uniform non-rich scattering environment, beam-forming technique bestows considerable gain. We demonstrate that the achieved gain in respect to a similar radiation system reduces as the multipath environment becomes richer. In addition to that, it is known that reverberation chambers are the contemporary measurement tools for antenna systems to be used in scattering environments. The shape of these measurement tools plays a significant role in their ultimate performance. We show that rectangular reverberation chambers prove to be more advantageous compared with their cylindrical and spherical counterparts

Transmitter precoding for multi-antenna multi-user communications

Emerging wireless sensor networks and existing wireless cellular and ad hoc networks motivate the design of low-power receivers. Multi-user interference drastically reduces the energy efficiency of wireless multi-user communications by introducing errors in the bits being detected at the receiver. Interference rejection algorithms and multiple antenna techniques can significantly reduce the bit-error-rate at the receiver. Unfortunately, while interference rejection algorithms burden the receiver with heavy signal processing functionalities, thereby increasing the power consumption at the receiver, the small size of receivers, specifically in sensor networks and in downlink cellular communications, prohibits the use of multiple receive antennas. In a broadcast channel, where a central transmitter is transmitting independent streams to decentralized receivers, it is possible for the transmitter to have a priori knowledge of the interference. Multiple antennas can be used at the transmitter to enhance energy efficiency. In some systems, the transmitter has access to virtually an infinite source of power. A typical example would be the base station transmitter for the downlink of a cellular system. The power consumption at receivers can be reduced if some of the signal processing functionality of the receiver is moved to the transmitter.;In this thesis, we consider a wireless broadcast channel with a transmitter equipped with multiple antennas and having a priori knowledge of interference. Our objective is to minimize the receiver complexity by adding extra signal processing functions to the transmitter. We need to determine the optimal signal that should be transmitted so that interference is completely eliminated, and the benefits that can be obtained by using multiple transmit antennas can be maximized. We investigate the use of linear precoders, linear transformations made on the signal before transmission, for this purpose