Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1998-12-11
2001-09-11
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S350000, C375S232000
Reexamination Certificate
active
06289062
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to high speed wireless communication systems, and more particularly, to a method and system for enhancing the quality of a wideband wireless Personal Communication Services by utilizing an adaptive antenna array system.
2. Discussion of the Related Art
The following background is described in connection with a Time Division Multiple Access (TDMA) technology and a wideband Personal Communication Services (PCS) system, as an example. Heretofore, in this field, speedy data transfer forms the cornerstone of modern wireless communication services. Faster communication connections always result in improved system capacity and service quality. Currently, the Global System for Mobile Communication (GSM) based PCS systems operate at 1900 MHz, and support only up to a rate of 9.6 Kbps for data transfer. Higher rate wideband applications are constantly being sought after to meet the ever growing demand of wireless communication services. Accordingly, High Speed Circuit Switched Data (HSCSD) and General Packet Radio Services (GPRS) are being standardized to accommodate this grave need. It is foreseeable that a wideband PCS system, such as one with a 1.6 MHz carrier bandwidth will eventually replace the lower speed system such as the GSM based PCS 1900 system. With a higher speed data service such as a wideband PCS system, however, frequency selective fading is a serious technical hurdle to be overcome. Moreover, since the high rate data transfer requires more information being transmitted through a multi-path radio propagation system, Inter-symbol Interference (ISI) also becomes another major drawback. A current solution for reducing the effect of ISI in a PCS 1900 system is to integrate an equalizer into a receiving system.
FIG. 1
illustrates a portion of a typical PCS communication system with the integration of an equalizer. In this part of the PCS communication system, an input signal
10
goes through a receiver filter
12
, and is then picked up by a sampler
14
and processed by an equalizer
16
. Thereafter, a decision device
18
produces a final output
19
. However, this type of communication system using an equalizer will not be feasible in a wideband PCS system due to a unacceptably large path delay difference. In other words, the time delay from the time a first signal will arrive at a receiving device and the time a last signal will arrive through a different path is intolerably big in comparison to the bit duration of a signal. For instance, a typical bit duration in an high rate communication system is on the order of a micro second or tenth of micro second, while a path delay could be on the order of up to 5 micro seconds. Furthermore, Co-channel Interference (CCI) is another technical obstacle for a wideband PCS. In a cellular system, frequencies are reused to increase the capacity, i.e., a frequency band is used in two different cells belonging to different clusters, sufficiently separated so that they don't interfere significantly with each other. In reality, signals using the same radio frequency channel may still unexpectedly infringe upon and weaken each other.
In a TDMA system for wireless services, a single frequency channel is divided into a number of time slots, with each communication signal using one of these slots. With TDMA, an audio signal is digitized, that is, divided into a number of digital packets, each on the order of milliseconds in length. These packets will occupy different time slots. Thus, a TDMA system allows a number of users to share a single radio frequency channel by uniquely allocating the time slots in the channel for each user. For instance, a number of cellular phone users can carry on their conversations simultaneously using the same radio frequency channel to transfer digital packets. This is in contrast to having one user occupy the entire channel for a conversation, to the exclusion of all others. By dividing the radio frequency channel into time slots, a TDMA system increases the capacity of cellular frequencies, but the problem of CCI and ISI become more acute.
Referring now to
FIG. 2
, a general overview of an adaptive array system
20
for receiving and processing transmitted signals is shown. The adaptive array system
20
typically consists of a plurality of receiving devices
22
such as antennas to detect transmitted signals. A plurality of signal converting devices
24
provide for down converting or demodulating the received signals, and furnishing analog-to-digital conversion. A weight controller
26
is provided for generating a plurality of weights to be integrated into the received signals. Lastly, a combiner
28
is provided for constructing a final output signal
30
. Adaptive antenna arrays have been studied in connection with mobile wireless networks to suppress both CCI and ISI. A plurality of weights are generated and integrated into the received signals by minimizing a certain criterion to maximize the signal-to-noise ratio (SNR). Conventionally, a least-mean-square algorithm has been used to update the weights of adaptive arrays, but its slow convergence presents a tracking problem. This could be even more of a problem in a high speed system such as a wideband PCS system. An alternative method such as a direct matrix inversion algorithm has been proposed just for its fast convergence feature. The direct matrix inversion algorithm is not commercially feasible, however, because of its computational complexity. Another algorithm known as constant modulus adaptive algorithm has been considered also. However, the algorithm is problematic in that a PCS system using this algorithm will capture excessive interfering signals.
It would thus be desirable to provide a method and apparatus to combat both CCI and ISI in a wideband TDMA PCS or other similar system.
SUMMARY OF THE INVENTION
With the demand for higher rate data communication systems, a wideband Personal Communication Services system is forthcoming. A method and system of the present disclosure advantageously provide means for suppressing co-channel interference and reducing inter-symbol interference generated during the transfer of a data packet through a selected radio frequency channel. Such a high rate data communication system can include a TDMA or other wireless communication system. The embodiments of the present disclosure propose the use of an adaptive array system with the integration of a weight controller utilizing recursive least squares algorithm to generate a plurality of appropriate weights to be integrated into the received signals in order to maximize signal-to-noise ratio. The system further includes a combiner for constructively combining the weighted signals, a detector for producing a binary output, an error processor for minimizing unnecessary errors generated in the system after the signals are received, and a switching means for providing a training sequence or the binary output to the error processor. A detected signal or a training sequence is intelligently switched in as a reference sequence in order to generate a plurality of appropriate weights.
In accordance with one embodiment, to further enhance such an adaptive array system and to take advantage of the multi-path radio frequency communication environment, two multi-path diversity schemes can be integrated into the adaptive array system. A system with tapped delay line processors delays the signal propagation and correspondingly generates appropriate weights to modify the delayed signals in order to obtain an optimal SNR. An adaptive array system with parallel array processors divides the signal received from each antenna element and regroups the divided signals along with different weights in order to extract and identify signals traveled through different paths. Computer simulation graphs demonstrate the significant improvement of using the proposed systems over an one-element antenna system.
REFERENCES:
patent: 4328585 (1982-05-01), Monsen
patent: 4733402 (1988-03-01), Mons
Scheving Henrik
Wang Yiping
Bocure Tesfaldet
Haynes and Boone LLP
Nortel Networks Limited
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