Pulse or digital communications – Systems using alternating or pulsating current – Antinoise or distortion
Reexamination Certificate
1997-09-12
2001-08-28
Chin, Stephen (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
Antinoise or distortion
C375S296000, C375S297000, C455S126000, C381S001000
Reexamination Certificate
active
06282247
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a digital signal processing method and apparatus for a wireless communication system. More particularly, the present invention relates to a method and apparatus for digitally compensating for radio distortion over a wide range of temperatures.
BACKGROUND OF THE INVENTION
In any wireless communication system, various distortions are generated during signal transmission and reception. Such distortions may be caused by various components in the reception and transmission paths or by the radio-air interface. These distortions can significantly degrade communication system performance if not properly compensated.
FIG. 1
illustrates a typical wireless communication system. A signal to be transmitted is encoded in a Source Encoder
100
and a Channel Encoder
200
, then modulated in a Digital Modulator
300
. The encoded signal can be modulated according to any known modulation technique. For example, in the Personal Wireless Telecommunications Interoperability Standard (PWT), as described in Part 2: Physical Layer, TIA/EIA 662-2, a signal is &pgr;/4 Differential Quadrature Phase Shift Keyed (DQPSK) modulated. The Digital Modulator
300
is typically implemented in a digital circuitry. The modulated signal is then passed through a Transmission Channel
400
before being transmitted through the air via an Antenna
500
. A series of bandpass filters are typically employed in the Transmission Channel
400
to assure that the signal to be transmitted is confined within a pre-defined frequency band with appropriate transmit characteristics.
The transmitted signal is received at an Antenna
600
, processed through a Receiving Channel
700
, which has similar circuitry as the Transmission Channel
400
, demodulated in a Digital Demodulator
800
, and decoded in a Channel Decoder
900
and a Source Decoder
1000
. Ideally, the output from the Source Decoder
1000
is the same as the input to the Source Encoder
100
.
FIG. 2
illustrates a detailed block diagram of the Transmission Channel
400
. As shown in
FIG. 2
, the Transmission Channel
400
includes a D/A Intermediate Frequency (IF) Bandpass Filters
420
and
440
, Mixers
430
and
450
, and a Radio Frequency (RF) Front End
460
. The IF Filters
420
and
440
confine the signal to a particular frequency band, the Mixers
430
and
450
up convert the baseband modulated signal to an intermediate frequency, and the RF Front End
460
converts the up converted signal to a radio frequency. The IF Filter
420
is typically an interstage filter that is centered, for example, at 11.25 MHZ, and the IF Filter
440
is typically a SAW filter that is centered, for example, at 422.5 MHZ.
The IF Filters
420
and
440
are typically designed with analog components or surface acoustic wave technology. Due to their analog nature, the IF Filters
420
and
440
often produce imperfect frequency responses which cause channel distortion. The channel distortion degrades the quality of the transmitted signal.
Reducing channel distortion to a reasonable level has always been a great challenge in wireless communication system design. Traditionally, the problem of radio channel distortion has been solved by simply putting more restrictive requirements on the analog filter design in the transmission channel. However, the design of a perfect analog filter that meets radio transmission requirements can be technically difficult. This often results in more expensive components and a longer design cycle. It is often not feasible to obtain an optimal analog filter design due to cost and time constraints.
Digital compensation provides an attractive alternative. For example, the Digital Cordless Telephone (DCT) 1900 modem includes a digital compensation filter. However, this filter is primarily concerned with compensating distortion due to signal digitization.
The problem of channel distortion is aggravated by the fact that components of the transmission system, such as the IF Filters
420
and
440
, are typically temperature sensitive. That is, the frequency responses of these filters vary when the temperature in which the filters operate fluctuates. In wireless communication applications, systems are expected to perform in a wide range of temperatures. Base stations, for example, often operate in an outdoor environment or other environment in which the temperature is not controlled for personal comfort. In such an environment, the temperature variation over time can be quite substantial. For example, according to the Personal Wireless Telecommunications (PWT) Interoperability Standard, the class E2 temperature requirements for fixed parts (FP), radio fixed parts (RFP), and central control fixed parts (CCFP) is between −10° C. and 55° C. Other standards, such as IS136 and IS95, have similar requirements for system operation temperature. To assure optimal system performance, radio distortion should, preferably, be compensated over a wide range of temperatures.
Correcting temperature dependent radio distortion by analog means is simply too expensive to be a practical solution in commercial wireless communication applications. Conventional filtering techniques do not provide a practical means by which distortion can be compensated for over a wide range of temperatures.
It would be desirable to provide a digital compensation filter for a wireless communication system which compensates for radio distortion over a wide range of temperatures without requiring additional hardware.
SUMMARY OF THE INVENTION
present invention overcomes the above-described problems, and provides additional advantages, by providing a method and apparatus for digitally compensating for temperature dependent radio channel distortion over a wide range of temperatures in a wireless communication system. According to an exemplary embodiment of the present invention, digital compensation is performed in digital modem ASIC circuitry in the wireless communication system.
According to a first embodiment, actual frequency responses of components in the wireless communication system contributing to distortion at various temperatures are determined. The inverses of the actual frequency responses at the various temperatures are multiplied with a desired frequency response of the wireless communication system to produce compensation frequency responses. An appropriate compensation frequency response for the operating temperature is applied to the radio signal to compensate for radio distortion.
According to a second embodiment, the actual impulse responses of components in the wireless communication system contributing to distortion at the various temperatures are determined and convolved with a reference signal to produce actual output signals. A desired system impulse response is convolved with the same reference signal to produce a desired output signal. The actual output signals are subtracted from the desired output signal to produce error signals for the various temperatures. Adaptive algorithms are applied to the error signals to produce adaptive compensation impulse responses for the various temperatures. The adaptive compensation impulse responses are convolved with the reference signal and the actual impulse responses to produce updated actual output signals, and the updated actual output signals are subtracted from the desired output signal to produce updated error signals. The adaptive algorithms are applied to the updated error signals to form updated adaptive compensation impulse responses. The adaptive compensation impulse responses are updated, the updated adaptive compensation impulse responses are convolved with the actual impulse responses and the reference signal to produce updated output signals, and the updated actual output signals are subtracted from the desired output signal to produce the updated error signals until the adaptive compensation impulse responses converge to optimal solutions. Then, an appropriate adaptive compensation impulse response for the operating temperature is applied to the radio sig
Burns Doane Swecker & Mathis L.L.P.
Chin Stephen
Ericsson Inc.
Liu Shu Wang
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