Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1999-10-22
2001-01-23
Vo, Don N. (Department: 2734)
Pulse or digital communications
Receivers
Interference or noise reduction
C455S296000
Reexamination Certificate
active
06178211
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates in general to a digital signal processing radio receiver wherein an intermediate frequency signal is digitally processed, and more specifically, to filtering the intermediate frequency signal with a filter bandwidth which is adaptively controlled in response to the strength of adjacent or alternate channel interferers relative to the strength of the desired channel.
Commercial AM and FM broadcast bands include a plurality of evenly spaced channels. A particular broadcast station is allocated a unique channel to conduct broadcasting within an assigned frequency range.
The power spectrum of a broadcast transmission consists of the strength of RF signals irradiating a receiving antenna over a range of frequencies. Even though most energy in a transmission can be limited to its assigned channel, efficient use of all channels implies that some radiated energy will be at frequencies outside their assigned channels. Assignment of frequency channels to individual transmitters is determined according to geographic location and other factors to minimize interference between transmissions in adjacent channels and alternate channels (an alternate channel is a channel two channels away from the desired channel of interest). Since demand in populous areas is high for the limited number of channels available, adjacent and/or alternate channel interferers are often present. Therefore, radio receivers must be able to perform adequately in situations where a strong signal on an adjacent or alternate channel creates signal components in the desired channel which interfere with reception of the desired signal. In addition, channels even farther away than an alternate channel and even sources not associated with the broadcast band can become significant interferers.
Prior art radio receivers are known which detect the presence of adjacent channel or other interference in various ways and which attempt to reduce effects of the interference by narrowing the receiver bandwidth (or by shifting the receiver passband for the desired channel). Separate filters or returning of the receiver are typical methods for detecting presence of an interfering channel. The prior art passband switching introduces transient effects which can be heard as noise. Furthermore, analog filtering that has been used does not provide sharp band edges (i.e., they have slow roll-off), so that not all of an interferer is removed.
With the advent of high speed digital signal processing (DSP) components, radio receivers are being introduced using DSP processors to implement demodulation and various other functions in a radio receiver. In particular, it is becoming possible to digitally process the intermediate frequency (IF) signal or even the radio frequency (RF) signal, thus avoiding a large number of analog circuits and components with their associated costs and space requirements. In order to keep costs down for a particular radio receiver, DSP performance in terms of 1) chip area required for processing, and 2) execution time need to be minimized.
SUMMARY OF THE INVENTION
The present invention has the advantage of adaptively filtering an intermediate frequency signal in response to the presence of adjacent channel interference while using digital signal processing in an efficient manner.
In one aspect of the invention, a radio reception circuit uses digital signal processing to digitally process an intermediate frequency (IF) signal. Circuit means provides a carrier-based signal (which in a preferred embodiment includes an analog tuner and an analog-to-digital converter generating a digitized IF signal). The circuit means applies automatic gain control to the carrier-based signal (either prior to or after A/D conversion) in order to provide a predetermined signal level. A complex mixer is coupled to the circuit means and generates a complex I & Q signal in response to the carrier-based signal. A filter processor generates a filtered I & Q signal in response to the complex I & Q signal, the filter processor having an impulse response determined by filter coefficients loaded into the filter processor. The filter coefficients are switchable between a first bank of filter coefficients for a first bandwidth and a second bank of filter coefficients for a second bandwidth narrower than said first bandwidth. A post-filter level detector is responsive to the filtered I & Q signal to generate a first level signal. A compare block is coupled to the post-filter level detector and the filter processor for selecting one of the banks of filter coefficients in response to a comparison of the first signal level and a predetermined threshold.
REFERENCES:
patent: 5036527 (1991-07-01), Halim et al.
patent: 5339455 (1994-08-01), Vogt et al.
patent: 5777911 (1998-07-01), Sherry et al.
Simon Haykin, “Introduction to Analog and Digital Communications”, 1989, pp. 281-283.
Difiore Nicholas Lawrence
Hagan Christopher John
Wagner John William
Wargnier James Alfred
Whikehart J. William
Mollon Mark L.
Phu Phuong
Visteon Global Technologies Inc.
Vo Don N.
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