Multiplex communications – Generalized orthogonal or special mathematical techniques – Particular set of orthogonal functions
Reexamination Certificate
1999-10-06
2004-09-28
Olms, Douglas (Department: 2732)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Particular set of orthogonal functions
C375S148000, C375S349000
Reexamination Certificate
active
06798737
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for interference cancellation in CDMA systems, particularly in conjunction with such system having plural mobile stations.
2. Brief Description of the Prior Art
Code division multiple access (CDMA) communication systems are communication systems wherein data is transmitted in accordance with a particular code whereas in time division and frequency division systems the transmission is based upon allocated time slots for transmission and reception and difference in frequency band of the communication, respectively. In CDMA, all frequencies and all time slots are available whereby communication with a particular receiver is provided by matching a code applied to the message being sent with the code of the selected receiver. The selected receiver receives only those communication wherein transmitted code and receiver code match. In such CDMA systems, the codes of all receivers in the system must be orthogonal to the other receivers in the system in order to avoid distortion due to cross talk and the like.
A problem arises during transmission from the base station to the mobile receivers in that, though the signals being transmitted to different mobile receivers in the system may initially be orthogonal to each other as required, the transmitted signal may be reflected and/or refracted one or more times, such as from buildings in the transmission path and/or other reflecting and/or refracting media. These reflections and/or refractions cause the originally orthogonal signals to shift so that they are no longer completely orthogonal and are received by other receivers with varying degrees of magnitude, thereby causing interference in the other receivers. The quality of the received signal is a function of the amplitude of the transmitted signal divided by a function of the noise plus the interference. Since the noise is generally not changeable in established equipment, minimization of the interference is desirable and generally essential for signal quality improvement and is therefore the apparent approach required to improve received signal quality. Current commercial CDMA systems do not have a mechanism for cancellation of such types of interference.
Interference cancellation in CDMA systems has been studied extensively as a way to increase capacity, decrease the requirements for power control or improve the bit error rate at the receiver and is of particular interest with reference to wideband code division multiple access (WCDMA) systems. One type of interference cancellation is multiuser detection where the receiver demodulates the information from many different transmitters. This cancellation usually occurs at the base station which must receive signals from all of the mobile units within its cell, but such cancellation can also be provided at the mobile unit as well. The optimal interference cancellation method is presented by S. Verdu, “Minimum probability of error for asynchronous Gaussian multiple-access channels,” IEEE Transactions on Information Theory, vol. IT-32, No. 1, pp. 85-96, January 1986. This procedure can be used to find an upper bound on the performance gain that can be achieved through interference cancellation. The optimal interference cancellation has been shown to involve maximum likelihood sequence estimation (MLSE), however, MLSE has a complexity which is exponential with the number of users, making this solution uneconomical and unimplementable in a cost and power-efficient manner. Interference cancellation can be used at the base station to cancel the effect of other mobile units in a single cell and it can be used at the mobile station to cancel out the interfering signals from the current base station or other base stations.
It should be understood that the term “receiver” as used herein refers both to an instrument capable of reception only as well as an instrument capable of transmission and reception, the “receiver” generally being mobile.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a multi-stage system for cancellation of the above-described interference which is generally simple and relatively inexpensive. The invention described herein is used at the mobile station to cancel interference from the base station caused by multipath, the case wherein the received signal arrives as transmitted as well as in reflected and/or refracted state. The approach taken relates to multi-user detection (MUD), which is a type of interference cancellation in which the signals on many channels are demodulated at the receiver. In this context, MUD refers to the demodulation of many channels transmitted by one base station.
Briefly, there is provided a mobile station with multiple RAKE fingers for receiving a signal having a Walsh code. Each RAKE finger is a correlator which demodulates one multipath component of the received signal. (The basis vectors of the Hadamard transform can be generated by sampling Walsh functions which take only binary values ±1 and form a complete orthogonal basis for square integrable functions. The Hadamard transform is also known as the Walsh-Hadamard transform as explained in Jain, A. K., Fundamentals of Digital Image Processing, Prentice-Hall (1989).) After performing the standard procedure of removing the long code used by CDMA systems (the long code is a well known pseudo-random sequence taking binary values ±1 which modulates the transmitted signal to make it appear as Gaussian noise to other users in the system), the mobile station applies this signal to an inverse Walsh transform circuit (IFWT) to remove the appropriate Walsh code from each channel. This permits all of the channels transmitted by a base station to be demodulated simultaneously and provides a separate output for each channel. For each RAKE finger, a single channel response is estimated from the pilot symbols (training symbols which are known at the receiver and can be used for such purposes as synchronization, channel estimation and signal-to-interference ratio estimation) in each of the channels. The channel estimate averages the received pilot symbols to estimate the amplitude and phase of the channel. The output of each IFWT is multiplied by the complex conjugate of the channel response, and after maximal-ratio combining, each of the outputs is transferred to data decision blocks (estimate which symbol was transmitted by finding the closest constellation point to the received signal. For example, with QPSK modulation and a constellation of {1+j, 1−j, −1+j, −1−j}, if the received symbol is 0.8+1.1j, then the data decision block will decide 1+j was sent) which make symbol decisions for each of the channels and are estimates of the transmitted data.
Interference cancellation is performed by transferring the estimated data for each channel to a Walsh transform circuit where the estimated data is respread into orthogonal channels using the Walsh code for each channel. This signal is multiplied by the downlink long code. For each RAKE finger, this regenerated signal is multiplied by the estimate of the channel response and is delayed by the channel delay of each finger. For each RAKE finger, the regenerated signal from all of the other fingers is subtracted from the original signal received by that finger. The original signal minus the regenerated interference is then fed back to the RAKE fingers where the signal is again recirculated through the above described circuitry as many times as desired, each recirculation removing additional interference. After the desired number of iterations, the estimated data from the last iteration for the channel(s) assigned to the mobile device is taken as the output of the receiver.
A standard implementation of the despreading and respreading operations at the receiver for M channels, each channel with a spreading factor of N, would require MN operations. The despreading and respreading operations are the most computationally intensiv
Dabak Anand G.
Hosur Srinath
Schmidl Timothy M.
Brady W. James
Hoel Carlton H.
Nguyen Van
Olms Douglas
Telecky , Jr. Frederick J.
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