Pulse or digital communications – Receivers
Reexamination Certificate
1999-06-16
2004-01-27
Fan, Chieh M. (Department: 2734)
Pulse or digital communications
Receivers
C375S285000, C375S350000, C455S063100, C455S296000
Reexamination Certificate
active
06683919
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to methods and apparatus for reducing noise bandwidth in a wireless communication receiver. In preferred embodiments, the invention is a GSM receiver including a channel select filter, analog-to-digital converter, digital filter, and circuitry (operable during synchronization with a transmitter) to reduce the effective combined pass band of the channel select and digital filters, thereby reducing noise bandwidth during synchronization.
DESCRIPTION OF THE RELATED ART
In many contexts in which a signal is received after propagating over a transmission link, the receiver is typically implemented in one of two ways. In one such receiver implementation, the received signal is bandpass filtered, then undergoes frequency conversion followed by analog-to-digital conversion and digital filtering (including digital bandpass filtering to reduce noise), and then undergoes a sequence of further processing operations (at least one of the operations being performed on different frequency band of the filtered digitized signal than is another of the operations). In the other receiver implementation, the received signal is bandpass filtered, then undergoes analog-to-digital conversion followed by frequency conversion, decimation and digital filtering (including digital bandpass filtering to reduce noise), and then undergoes a sequence of further processing operations (at least one of the operations being performed on different frequency band of the filtered digitized signal than is another of the operations).
For example, in typical wireless communication systems a receiver performs filtering (including channel selection) on a received signal, thereby generating an intermediate signal. The intermediate signal is a modulated signal (e.g., a signal modulated by Gaussian minimum shift keying) which must undergo further demodulation to extract its information content. Typically, the intermediate signal undergoes down conversion to the baseband followed by analog-to-digital conversion. The resulting digitized signal is then digitally filtered to reduce noise (thereby generating a filtered digital signal) prior to further processing (e.g., demodulation). If the received signal (and thus the filtered digital signal) is time-division-multiplexed (its data being contained in a specific time slot relative to the start of each frame transmitted by the transmitting system), the receiver must perform an initial synchronization operation in which it processes an initial portion of the filtered digital signal so as to synchronize itself with the transmitting system. Typically, the synchronization frames of the filtered digital signal contain a tone of known frequency which the receiver must lock onto in order to perform the synchronization. After the synchronization has been completed, the receiver enters a mode in which it demodulates the normal transmitted data of the filtered digital signal.
One conventional type of TDMA (time division multiple access) wireless communication system is the GSM system, which uses both FDMA (frequency division multiple access) and TDMA. In a GSM system, each signal is transmitted in a selected frequency channel (the carriers being spaced 200 kHz apart from each other) in the range from 880-915 MHz (for transmission) to 925-960 MHz (for reception). Eight users can share each frequency channel, since eight time-domain-multiplexed channels are transmitted within each frequency channel. Each transmitted signal comprises frames of data. The users that share a single frequency channel access different non-overlapping time intervals of each frame transmitted in that frequency channel (in round-robin fashion). Thus, each receiving system includes a bandpass filter to select a frequency band, as well as synchronization circuitry (for synchronizing with the transmitter) so that the receiving system can select the proper time slot of the time-domain-multiplexed signal in the selected frequency band.
FIG. 1
a
is a block diagram of a portion of a receiver of a conventional GSM wireless communication system. In the GSM receiver of
FIG. 1
a
, the signal received by filter
1
(which has been transmitted over a wireless communication link) has a carrier frequency in the range is 925 MHz to 960 MHz. The received signal is bandpass filtered in filter
1
, amplified in low-noise amplifier
2
, and again bandpass filtered in filter
3
. The signal is then mixed (in RF mixer
6
) with an RF signal (from voltage controlled oscillator
5
) having frequency much lower than the 925-960 MHz carrier frequency, and the resulting intermediate frequency signal is bandpass filtered in channel select filter
7
. The pass band of filter
7
is centered so that filter
7
selects a particular one of the GSM carrier frequencies (which as noted above are spaced 200 kHz apart from each other), and has width A, where A is less than 200 kHz but much greater than. 67 kHz.
The output of filter
7
is amplified in IF buffer amplifier
8
, and then undergoes IF image rejection processing in mixer
10
(which receives an intermediate frequency signal from voltage controlled oscillator
9
) and bandpass filtering (for antialiasing) in bandpass filter
11
. The analog signal output from filter
11
is amplified in variable gain amplifier
12
, and then digitized in analog-to-digital converter
13
(which is typically a sigma-delta analog-to-digital converter).
The digital signal output from A-to-D converter
13
then undergoes mixing in mixers
14
and
15
, to generate an in-phase component I and a quadrature component Q (each of the components I and Q having a sample rate higher than the standard GSM data rate of 270.8 kb/sec). Mixer
14
typically mixes the output of converter
13
with a signal proportional to sin(&pgr;t/2T), where 1/T is equal to four times the second intermediate frequency, and mixer
15
typically mixes the output of converter
13
with a signal proportional to cos(&pgr;t/2T).
Mixers
14
and
15
perform digital “down conversion” (to the baseband) and generate the in-phase component (I) and the quadrature component (Q). Decimation filter
17
A performs noise filtering and downsampling on the in-phase component (I). Decimation filter
18
A performs noise filtering and downsampling on the quadrature component (Q). Typically, filter
17
A is identical to filter
18
A.
Digital filter
17
performs final channel selection filtering of the down-converted, in-phase component I (asserted at the output of filter
17
A) including by lowpass filtering it with a bandwidth of width B (where “B” is typically slightly less than above-mentioned width “A” of filter
7
's pass band but much greater than 67 kHz), to produce a digitally filtered in-phase component. Digital filter
18
performs final channel selection filtering of the down-converted, quadrature component Q (asserted at the output of filter
18
A), including by lowpass filtering it with a bandwidth of width B, to produce a digitally filtered quadrature component. Typically, filter
17
is identical to filter
18
. Digital filters such as filters
17
and
18
that are used in wireless communication receivers typically perform filtering in addition to lowpass filtering, but we will refer to them herein as digital lowpass filters. Specifically, we will refer to each of filters
17
and
18
as a digital lowpass filter, and to filters
17
and
18
collectively as digital lowpass filter
19
(indicated in
FIG. 1
a
).
FIG. 1
b
is a block diagram of a portion of another conventional receiver used in conventional GSM wireless communication systems. The GSM receiver of
FIG. 1
b
is identical to that of
FIG. 1
a
, except in that mixers
114
and
115
, analog-to digital converters
113
and
116
(of
FIG. 1
b
) replace analog-to digital converter
13
, mixers
14
and
15
, and decimation filters
17
A and
18
A (of
FIG. 1
a
). Components of the
FIG. 1
b
receiver that correspond to components of the
FIG. 1
a
receiver are identically numbered in
FIGS. 1
a
and
1
b
and the descr
Olgaard Christian Volf
Socci Gerard
Fan Chieh M.
Girard & Equitz LLP
National Semiconductor Corporation
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