Method and device for exploiting transmit diversity in time varying wireless communication systems

#The present invention generally relates to the field of communication systems and more particularly, to achieving spatial diversity in a communication link having at least two transmitting antennas and at least one receive antenna while mitigating Doppler induced channel variations and multipath interference.

#In a wireless communication system, a major design challenge is to maximize system capacity and performance in the presence of interference, and a time-varying multipath channel. A multipath channel is caused by the transmitted signal reflecting off objects near the transmitter and receiver and arriving at the receiver over multiple paths. Interference in a communication system can come from a variety of sources depending on the particular system deployment. If the system is in motion, then Doppler induced channel variations becomes a problem. Interference and multipath are major factors that limit the achievable performance and capacity of a communication system because both effects interfere with the ability of a communication receiver to properly decode the transmitted data.

#In a multipath propagation channel, the transmitted signal propagates to the receiver over a finite number Lpaths of propagation paths, where each path has an associated time delay and complex gain. In such a channel, the communication receiver receives the superposition of Lpaths delayed, attenuated, and phase-shifted copies of the transmitted signal. The number of paths Lpaths and their time delays and phase shifts depends on the physical location of the various scattering objects (such as buildings, automobiles, and trees) in the immediate vicinity of the transmitter and receiver. The complex attenuation (magnitude and phase) of each path depends on the length of each path, as well as the material composition of any scatterers or reflectors encountered along the path.

#FIG. 1 is an overview diagram of a preferred embodiment of a communication system in accordance with the present invention;

#FIG. 2 is a block diagram illustrating a preferred embodiment of a receiver device in accordance with the present invention;

#FIG. 3 is a block diagram illustrating a preferred embodiment of a transmitting device in accordance with the present invention;

#In most scattering environments, antenna diversity is a practical, effective and widely applied technique for reducing the effect of multipath fading. One approach is to use multiple antennas at the receiver and perform combining or selection and switching in order to improve the quality of the received signal. An adaptive antenna array is an array of antennas connected to a communications receiver and operates by combining the signals received by the antennas so as to optimize in an adaptive fashion the receive characteristics of the array. By weighting and then summing the multiple antenna signals, the adaptive antenna array can adapt its angular response, sometimes called the array pattern, as well as its frequency response while it operates in response to changes in the propagation environment. While operating, the adaptive antenna attempts to maximize the reception of the signal received from a desired transmitting device, as it simultaneously minimizes the effects of all other interfering signals and noise. With a properly designed adaptive array combining algorithm, the act of maximizing the reception of the desired signal permits receive spatial diversity to be exploited. In a communication system, an adaptive array's ability to suppress interference and its ability to exploit receive diversity offers the potential to reduce co-channel interference, compensate for Doppler-induced inter-carrier interference, improve coverage quality, and increase overall system capacity.

#A problem associated with employing both an equalizer and an adaptive antenna in a wireless communication system lies in the design of an algorithm and device having adequate ability to adapt to changes in the signal environment. For best performance, adaptive antennas that operate in a fast-fading multipath environment must adapt to the rapidly varying channel as well as to any changes in the nature of the desired and interfering signals. In a broadband system, a frequency-selective multipath channel will cause significant variations in the channel across the occupied bandwidth of the received signal. Equalization and interference-suppression algorithms, which cannot track these channel variations in both time and frequency, will suffer significant degradation in performance as measured by the Bit-Error Rate (BER) or Signal-to-Interference-plus-Noise Ratio (SINR).

#A common ingredient in many equalizers and adaptive antenna arrays is an algorithm and device that estimates the characteristics of the multipath propagation environment, channel transfer function, or channel frequency response, between the desired transmitting device and each of the at least one receiving antenna at the communication receiver. When transmit spatial diversity is being employed in a broadband system operating in a frequency-selective environment, then an adaptive antenna array generally requires an estimate of the channel frequency response between each receiving antenna and each of the transmitting devices that are simultaneously sending information to the array. The channel estimation algorithm in this case should operate to simultaneously solve for the channel responses of the multiple transmitting devices. Performing a simultaneous estimate of the channel responses of multiple transmitting devices is a difficult operation in a mobile broadband communication system operating in a multipath-rich environment with high frequency selectivity. Strong interference caused by a spatial diversity deployment, Doppler induced Inter-Carrier Interference (ICI), or a low reuse factor causes even more difficulty to the channel estimation algorithms.