Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1999-07-16
2002-06-25
Pham, Chi (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S240020, C375S240110, C375S240180, C375S350000, C348S395100, C348S398100, C348S404100, C370S319000, C370S335000, C370S342000, C370S465000, C370S479000
Reexamination Certificate
active
06411653
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to telecommunications in general, and, more particularly, to a technique for using cascaded polyphase DFT-filter bank in software-defined radios to support wireless telecommunications.
BACKGROUND OF THE INVENTION
FIG. 1
depicts a schematic diagram of a portion of a typical wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals
101
-
1
through
101
-
4
) that are situated within a geographic region. The heart of a typical wireless telecommunications system is Wireless Switching Center (“WSC”)
120
, which may also be known as a Mobile Switching Center (“MSC”) or Mobile Telephone Switching Office (“MTSO”). Typically, Wireless Switching Center
120
is connected to a plurality of base stations (e.g., base stations
103
-
1
through
103
-
5
) that are dispersed throughout the geographic area serviced by the system and to the local and long-distance telephone and data networks (e.g., local-office
130
, local-office
138
and toll-office
140
). Wireless Switching Center
120
is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal (e.g., wireline terminal
150
), which is connected to the system via the local and/or long-distance networks.
The geographic region serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called “cells.” As depicted in
FIG. 1
, each cell is schematically represented by a hexagon; in practice, however, each cell usually has an irregular shape that depends on the topography of the terrain serviced by the system. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Wireless Switching Center
120
.
For example, when wireless terminal
101
-
1
desires to communicate with wireless terminal
101
-
2
, wireless terminal
101
-
1
transmits the desired information to base station
103
-
1
, which relays the information to Wireless Switching Center
120
over wireline
102
-
1
. Upon receipt of the information, and with the knowledge that it is intended for wireless terminal
101
-
2
, Wireless Switching Center
120
then returns the information back to base station
103
-
1
over wireline
102
-
1
, which relays the information, via radio, to wireless terminal
101
-
2
.
A base station will typically receive numerous communications from a number of wireless terminals that are located in the cell serviced by the base station. These numerous communications are received as an analog wide-band radio frequency (RF) signal at the base station. As used herein, the term “wide-band” refers to a band or range of radio spectrum that contains multiple narrow-bands. As used herein, the term “narrow-band” refers to a carrier band, which has a specified bandwidth for modulation and demodulation. Such carrier bands or specified bandwidths are specific to different communications standards. For example, a narrow-band is defined as 30 kHz for TDMA (IS-136), and a signal of 15 MHz would be a wide-band signal because it would have 500 narrow-bands for the TDMA system (500=15 MHz/30 kHz).
The analog wide-band RF signal is then typically separated by frequency into narrow-band channels at the base station. Individual communications contained in the narrow-band channels are then further processed within the telecommunications system.
One technique in the prior art for processing the analog wide-band RF signal is through the use of a software-defined receiver at the base station. In this prior art technique, the software-defined receiver will often contain, among other things, an analog-to-digital converter for converting an analog signal into a digital signal and a polyphase filter bank for separating the digital signal into narrow-band channels. Each narrow-band channel comprises a “pass-band” (representing a frequency band containing information associated with the narrow-band channel), a “stop-band” (representing a frequency band that does not contain such information) and a “transition-band” (representing a frequency band between the pass-band and the stop-band). The purpose of the polyphase filter bank is to organize information contained in the digital signal into appropriate “pass-bands” of the narrow-band channels.
A schematic diagram of a polyphase filter bank is shown in FIG.
2
. The digital signal is divided into a number, M, of branches by decimating the digital signal on a time basis. Decimating a digital signal decreases the sampling rate of such signal typically through a process of filtering and downsampling. If a digital signal has a sampling rate of R, a decimator will decrease the sampling rate by a factor, D, to produce a new sampling rate of R/D. For example, when a signal has a sampling rate of 9 and is decimated by a factor of three, the decimator will form a new signal with a sampling rate of 3. In this example, a decimator performs integer decimation because the D factor is an integer. Fractional decimation is also possible and is typically achieved through a combination of decimation and interpolation, which will be described below.
Each branch contains a Finite Impulse Response Filter (FIR) through which the decimated digital signals are filtered. The decimated digital signals are stored in locations or “taps” within the FIR filters. The Crochiere and Rabiner equation provides the number, N, of FIR taps required for filtering such decimated digital signals.
[1]
N
≅
D
∞
(
δ
p
,
δ
s
)
Δ
F
/
F
,
where:
&dgr;
p
is the “ripple” or mean amplitude of the signal in the pass-band,
&dgr;
s
is the “ripple” or mean amplitude of the signal in the stop-band,
D
∞
(&dgr;
p
,&dgr;
s
)=log
10
&dgr;
s
*[0.005309*(log
10
&dgr;
p
)
2
+0.07114*log
10
&dgr;
p
−0.4761]−[0.00266*(log
10
&dgr;
p
)
2
+0.5941*log
10
&dgr;
p
+0.4278],
“*” indicates multiplication,
&Dgr;F is the bandwidth of the transition-band in Hz, and
F is the sampling rate of a FIR filter in Hz.
The output digital signals from the FIR filters enter a Discrete Fourier Transform (DFT), such as a Fast Fourier Transform (FFT), where the separate digital signals are organized into M channels. Such an arrangement of FIR filters followed by a FFT transform is called a polyphase filter bank.
As illustrated in
FIG. 3
, the polyphase filter bank can be cascaded where polyphase filter banks are repeated for several stages, forming a cascaded polyphase DFT-filter bank, to transform a wide-band digital signal into a large number of narrow-band channels. A large number of narrow-band channels are not typically formed within a single polyphase filter because the size of that polyphase filter bank would become too large to effectively process the numerous communications.
Similarly, a polyphase filter bank or a cascaded polyphase DFT-filter bank can be used in a software-defined transmitter. As shown in
FIG. 4
, M narrow-band channels are combined into a single digital signal through use of an inverse Fast Fourier Transform (IFFT) or an inverse Discrete Fourier Transform (IDFT) methods and interpolating the digital signal on a time basis, in well-known fashion. Interpolating a digital signal increases the sampling rate of such signal typically through a process of upsampling and filtering. If a digital signal has a sampling rate of R, an interpolator will increase the sampling rate by a factor, L, to produce a new sampling rate of R*L. For example, when a signal has a sampling rate of 9 and is interpolated by a factor of three, the interpolator will form a new signal with a sampling rate of 27. In this example, the interpolator performs integer interpolation becaus
Arunachalam Sridhar
Mardani Reza
Breyer Wayne S.
Lucent Technologies - Inc.
Pham Chi
Tran Khanh C
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