Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1998-07-01
2001-07-17
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C455S067150, C714S795000
Reexamination Certificate
active
06263030
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed toward digital data demodulation and, more particularly, toward demodulation of a desired digital signal in a mobile communication device where the channel conditions change with movement of the mobile communication device at varying speeds.
BACKGROUND OF THE INVENTION
Cellular communication systems presently implemented in the U.S. utilize digitized voice and data signals for communication between a mobile communication device, generally a cellular telephone, and a base station. Movement of the mobile communication device at varying speeds (for instance if the user holding the device is walking, running, riding a bike, or in a car or train, etc.) causes channel fading to occur across the communication channels; the change in the signal caused by channel fading varying with time.
Prior art equalizers, which are included in the mobile communication device, generally include an estimator which receives the transmitted digitized signal and produces an output representing a distorted version of the digital signal as actually transmitted. A channel tracker is also included within the equalizer which receives both the transmitted digitized signal and the output of the equalizer, and produces a channel estimate signal representing an approximation of the channel fading that has occurred to the transmitted digitized signal. The role of the channel tracker is to use the output of the estimator to estimate the changes in the signal caused by channel fading, and to produce a channel estimate signal representing an approximation of the changes in the signal due to channel fading. The channel estimate signal produced by the channel tracker is utilized by the estimator to produce a more accurate representation of the digital signal as actually transmitted, i.e., get rid of the changes due to channel fading.
When the mobile communication device is moving at high speeds, the rate of change of the characteristics of a radio channel used by the mobile communication device typically occurs quickly. The opposite is true when the mobile communication device is moving at low speeds. When a channel tracker is built, an assumption must be made about the speed of the mobile communication device in which it is to be utilized. The channel tracker is tuned to the assumed speed and can accurately track the rate of change of the channel's characteristics if the mobile communication device is moving at a speed which approximates the speed to which the channel tracker is tuned. As a result, a channel tracker tuned to a high speed (HS) channel is designed such that it tracks fast changes in the channel's characteristics well. On the other hand, a channel tracker tuned to a low speed (LS) channel is designed such that it tracks slow changes in the channel's characteristics well.
When an HS channel tracker is used in an LS channel, it will tend to track non-existing changes in the channel's characteristics. This can hurt performance by about 1-2 dB. When an LS channel tracker is used in an HS channel, it may not be able to keep up with the changes in the channel's characteristics and can lose the channel completely by the end of a frame. For this reason, an HS tracker is typically chosen and a performance hit is taken at lower speeds.
However, merely placing an HS channel tracker in a wireless communication device or cellular phone is not quite satisfactory. Many people use their cellular phones essentially as wireless phones, that is, at extremely low speeds, e.g., 1-2 kilometers per hour, where the channel is almost static. The same is generally true for personal communicator phones and phones used as wireless modems for portable computers. In such cases, an HS channel tracker is a mismatch.
While there have been successful attempts to actually estimate the speed of the mobile communication device from the received signal, tracking the speed on top of tracking the channel is extremely complex and may result in a computational nightmare. Nevertheless, it is helpful to boost performance by using a channel tracker that is matched to the speed of the mobile communication device.
The present invention is directed toward overcoming one or more of the above-mentioned problems.
SUMMARY OF THE INVENTION
An equalizer is provided for use in a mobile radio communication system for compensating, among other sources of distortion, for changing channel conditions caused by movement of a mobile communication device at varying speeds. The equalizer includes an estimator receiving a digital signal transmitted over a communication channel, the digital signal being adversely modified by fading during transmission caused by movement of the mobile communication device at varying speeds, and a channel estimate signal, the estimator responsively producing a decision signal representing an approximation of the transmitted digital signal with fading eliminated. First and second channel trackers are provided, each tuned to a different speed of the mobile communication device and each receiving the decision signal. A decision circuit receives the decision signal and responsively selects one of the first and second channel trackers to produce the channel estimate signal.
In one aspect of the present invention, the estimator includes a maximum likelihood sequence estimator.
In another aspect of the present invention, the decision circuit responsively selects one of the first and second channel trackers based on the accuracy of the decision signal to the digital signal actually transmitted.
In another aspect of the present invention, the decision circuit includes an error control decoder receiving the decision signal and producing a reliability signal indicative of the accuracy of the decision signal to the digital signal actually transmitted, and a switching circuit receiving the reliability signal and responsively selecting one of the first and second trackers to produce the channel estimate signal.
In another aspect of the present invention, the reliability signal has first and second states. The switching circuit selects said one of the first and second channel trackers to produce the channel estimate signal with the reliability signal in the first state, and the switching circuit selects the other of said one of the first and second channel trackers to produce the channel estimate signal with the reliability signal in the second state.
In another aspect of the present invention, the digital signal is encoded with a Cyclic Redundancy Check (CRC) code prior to transmission. The error control decoder detects CRC errors in the decision signal and produces the second state reliability signal if a CRC error is detected; otherwise the error control decoder produces the first state reliability signal.
In another aspect of the present invention, the estimator produces an estimator metric signal in addition to the decision signal. The error control decoder receives the estimator metric signal and produces the second state reliability signal if the estimator metric signal is above a threshold value; otherwise the error control decoder produces the first state reliability signal.
In another aspect of the present invention, the digital signal is encoded with a block or convolutional code prior to being transmitted. The mobile communication device further includes a decoder decoding the decision signal and producing a decoder metric signal in addition to the decoded decision signal. The error control decoder receives the decoder metric signal and produces the second state reliability signal if the decoder metric signal is above a threshold value; otherwise the error control decoder produces the first state reliability signal.
In another aspect of the present invention, the decoder includes a Viterbi decoder.
In another aspect of the present invention, the digital signal is encoded with a CRC code prior to transmission. The error control decoder detects CRC errors in the decision signal producing a CRC error signal. The estimator produces an estimator metric signal in
Ericsson Inc.
Pham Chi
Tran Khai
Wood Phillips VanSanten Clark & Mortimer
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