Frequency-multiplexed cellular telephone cell site base station and method of operating the same

The present invention is directed to a cellular telephone cell site base station (10) having a single radio transmitter (40) and radio receiver (14) for transmitting and receiving, respectively, multiplexed frequency-modulated (FM) signals. A filter bank synthesizer (36), associated with the transmitter, multiplexes individual signals for transmission by the transmitter by exponentially modulating the signals to higher frequency bands followed by low-pass filtering and interpolation using discrete Fourier transform techniques. A filter bank analyzer (18), associated with the receiver, demultiplexes multiplexed FM signals received by the receiver by exponentially modulating each signal to its original spectral position followed by low-pass filtering and decimation, using discrete Fourier transform techniques.

TECHNICAL FIELD 
This invention relates to a cellular telephone cell site base station for 
transmitting and receiving frequency-multiplexed signals using a single 
transmitter and receiver, respectively. 
BACKGROUND OF THE INVENTION 
Cellular telephony is based on the concept of dividing a geographical area 
into a plurality of individual sub-areas or "cells." Situated within each 
cell is a base station containing at least one receiver and transmitter 
tuned to a particular frequency (channel) for communicating with a mobile 
transceiver (i.e., terminal), also tuned to receive and transmit on that 
channel. In actuality, the receive and transmit portions of the channel 
are spaced 45 MHz apart. Typically, the mobile terminal is carried in a 
vehicle in transit within that cell. As the vehicle containing the mobile 
terminal travels from one cell into another, the call between the mobile 
terminal and the cell site base station is transferred to a base station 
of the new cell. Typically, the new cell site base station has its 
receiver and transmitter tuned to a different channel, necessitating that 
the mobile terminal change its reception and transmission channel as well. 
The purpose in assigning a different channel to adjacent cells is to allow 
distant cells to use the same channel, thereby increasing the effective 
number of channels. 
To facilitate more than a single call within each cell, a cell site base 
station must have the ability to receive and transmit calls on different 
channels. At the present time, such a capability is obtained by providing 
the cell site with multiple transmitter-receiver pairs, each tuned to a 
particular channel. The disadvantage of this approach is that the cost of 
providing such equipment rises in direct proportion to the number of 
channels to be offered. Moreover, each receiver-transmitter pair occupies 
a certain amount of physical space which is often at a premium within the 
cell site itself. 
Thus, there is a need for a cellular telephone system which can transmit 
and receive multiple channel using a single transmitter and receiver pair. 
SUMMARY OF THE INVENTION 
Briefly, in accordance with a preferred embodiment of the invention, there 
is provided a cellular telephone cell site base station for transmitting 
and receiving frequency-multiplexed signals. The cell site base station 
has a receiver section that includes a single receiver capable of 
receiving a frequency-multiplexed signal, the signal having K signal 
components (channels), where K is an integer, each channel typically being 
transmitted by a separate mobile terminal. Signal processing means are 
coupled to the receiver for first converting the analog signal into 
digital samples and then converting the digitized, frequency-multiplexed 
signal samples into K individual base-band channels by digitally, 
exponentially modulating the individual base-band signal components 
followed by low-pass filtering and decimation. Each of the K individual 
base-band frequency signal channels is then demodulated, in accordance 
with the manner in which the channel had been originally modulated, to 
yield a separate voice signal for transmission to a land-based telephone 
network. 
The cell site base station also has a transmitter section for transmitting 
a frequency-multiplexed signal containing K base-band channels, each 
intended for reception by a separate mobile terminal. The transmitter 
section includes K individual modulators, each serving to modulate a voice 
signal in a particular format (FM, TDMA, etc.). The outputs of the 
individual modulators are input to a second digital signal-processing 
means which serves to digitally, exponentially modulate the individually 
modulated channels from the modulators, then low-pass filter, interpolate 
and sum the channels to yield a frequency-multiplexed signal having K 
separate channels. The frequency-multiplexed signal is then input to a 
transmitter which has the capability of transmitting such a signal for 
reception by separate mobile terminals.

DETAILED DESCRIPTION 
FIG. 1 is a block schematic diagram of a cellular telephone cell site base 
station 10 for transmitting and receiving a frequency-multiplexed 
base-band signal in accordance with the invention. However, as will be 
appreciated from the following discussion, the invention may be equally 
useful for other types of radio transceiver systems. The base station 10 
is comprised of a receiver section 12 for receiving a 
frequency-multiplexed signal comprised of K individual base-band signals 
(channels) where K is an integer. Typically, each individual base-band 
channel within the multiplexed signal received by the receiver section 12 
is transmitted by a separate one of a plurality of mobile cellular 
telephone terminals (not shown). In practice, the channels in the 
frequency-multiplexed signal are spaced 30 kHz apart although other 
spacings could be used. The receiver section 12 comprises a radio receiver 
14 which has the capability of receiving the K-channel, 
frequency-multiplexed signal. 
The radio receiver 14 receives the frequency-multiplexed signal at Radio 
Frequency (RF) and, in response, generates an analog frequency-multiplexed 
signal at a baseband frequency. The output signal from the receiver 14 is 
supplied to an analog-to-digital conversion device 16 which converts the 
analog signal from the receiver 14 into a digital signal that is input to 
a filter bank analyzer 18. As will be described with respect to FIG. 2, 
the frequency bank analyzer 18 exponentially modulates the digital signal, 
then low-pass filters and decimates such signals to yield K complex 
signals, each corresponding to a particular channel transmitted by an 
individual mobile terminal. 
Each of the K complex signals produced by the frequency bank analyzer 18 is 
supplied to a separate one of K demodulation channels 1,2 . . . K. Each 
demodulation channel includes a demodulator 20 for demodulating the signal 
supplied to it in accordance with the manner in which the signal was 
modulated. When the received signal is frequency modulated, each of the K 
channels of the frequency-multiplexed signal received at the demodulator 
20 is constructed to demodulate such an FM signal by computing the angle 
difference (tan.sup.-1 (I/Q)), where I and Q are the angles of the 
in-phase and quadrature phase components of the channelized signal, 
respectively. Alternatively, the originally received frequency-modulated 
signal may be time-division modulated (TDMA), in which case each 
demodulator 20 is configured to demodulate such TDMA signals. 
Still referring to FIG. 1, each of the K demodulation channels also 
includes a data and speech circuit for processing the output signal 
produced by the demodulator 20 to enhance the voice (or data) portion of 
the signal. Typically, the data and speech circuit 22 includes a digital 
signal processor (not shown). The manner in which the data and speech 
circuit 22 processes the output signal of the demodulator depends on the 
manner in which the demodulator 22 demodulates the signal input to it. 
In the case where the demodulator 22 receives an FM signal, the data and 
speech circuit 22 processes the demodulator output signal by performing 
the following operations thereon: (1) high pass filtering, (2) decimation 
with low-pass filtering, (3) expansion, and (4) de-emphasis. Such 
processing steps are further described in my U.S. Pat. No. 5,001,742, 
issued Mar. 19, 1991, and assigned to AT&T Bell Laboratories, such patent 
herein incorporated by reference. 
In practice, each separate one of the K channels of the K channels of the 
originally received, frequency-multiplexed signal has a Supervisory Audio 
Tone (SAT) signal imposed thereon by a separate mobile terminal for the 
purpose of establishing whether the terminal is in contact with the cell 
site base station 10. For this reason, each data and speech circuit 22 is 
also provided with the capability of processing the SAT signal 
superimposed on incoming channel in the manner disclosed in my 
aforementioned U.S. Pat. No. 5,001,742. The output signal of the data and 
speech circuit 22 is input to a cell site trunk interface circuit 24, as 
are known in the art, which serves to interface the data and speech 
circuit output signal (which is either a voice or data signal) for input 
to an outgoing telephone trunk line 25. 
In the case where the signal input to the demodulator 20 is TDMA modulated, 
the data and speech circuit 22 serves to perform the steps of channel 
decoding and then speech decoding in a well-known manner. 
In addition to the receiver section 12 thus described, the cell site base 
station 10 of the present invention also includes a transmitter section 26 
which, as will be described, operates to frequency-multiplex K individual 
voice signals and to transmit such a frequency-multiplexed signal 
containing the K individual channels to one or more mobile terminals. The 
transmitter section 26 comprises K separate modulation channels 1,2. . . 
K, each including a cell site interface circuit 28, similar to the circuit 
24, for interfacing the modulation channel to an incoming telephone trunk 
line 30 which carries an incoming voice or data signal. The interface 
circuit 28 of each modulation channel is coupled to a data and speech 
circuit 32, typically comprised of a digital signal processor (not shown). 
Each data and speech circuit 32 serves to process the incoming voice or 
data signal from the cell site interface circuit 28 depending on the 
manner in which the signal is to be modulated for ultimate transmission. 
In the case where signals are to be frequency modulated, then, the data 
and speech circuit 28 processes the incoming voice or data signal by 
performing the following steps: (1) high-pass filtering, (2) compression, 
(3) pre-emphasis, (4) limiting the signal, and (5) interpolating and 
low-pass filtering the signal. For a further description of the manner in 
which such steps are carried out, reference should be had to my 
aforementioned U.S. Pat. No. 5,001,742, herein incorporated by reference. 
As indicated earlier, it is desirable for each mobile terminal in 
communication with the cell site 10 base station to superimpose an SAT 
signal on the voice or data signal it transmits. By the same token, it is 
desirable for the cell site base station 10 to superimpose an SAT signal 
on each channel transmitted to a particular mobile terminal. Accordingly, 
each data and speech circuit 32 is provided with the capability of 
superimposing an SAT signal on the voice or data signal processed thereby 
in the manner described in my aforementioned U.S. Pat. No. 5,001,742. 
If the signal ultimately to be transmitted is to be modulated in a 
different manner, then the manner in which the data and speech circuit 32 
processes the incoming voice or data signal will be different. When the 
signal input to the data and speech circuit 32 is to be subsequently TDMA 
modulated, then the data and speech circuit first codes the incoming voice 
(or data) signal and thereafter codes the channel in a well-known manner. 
The output of the data and speech circuit 32 of each modulation channel is 
input to a modulator 34 designed to modulate the signal in a particular 
format, say FM or TDMA, depending on the manner in which the signal 
supplied from the data and speech circuit had been processed. The signals 
from the modulators 34 of the modulation channels are input to a filter 
bank synthesizer 36. As will be better described with respect to FIG. 3, 
the filter bank synthesizer 36 serves to exponentially modulate the 
individual signals from the modulators 34, followed by low-pass filtering, 
interpolation and summing, to yield a K-channel frequency-multiplexed 
signal having digitized in-phase and quadrature phase components. A 
digital-to-analog conversion device 38 (typically comprised of a pair of 
D/A converters) serves to convert the digital in-phase and quadrature 
phase components of the frequency bank synthesizer 36 output signal into a 
corresponding pair of analog signals which are input to a radio 
transmitter 40. The transmitter 40 combines the analog in-phase and 
quadrature-phase components of the synthesized K-channel 
frequency-multiplexed signal and then transmits the combined signals for 
reception by one or more mobile terminals (not shown). 
Referring to FIG. 2, there is shown a block schematic diagram of a symbolic 
representation of the filter bank analyzer 18 of FIG. 1. As shown in FIG. 
2, the filter bank analyzer 18 is comprised of K separate channels 1,2 . . 
. K. Each channel includes a complex modulator 42 which serves to 
exponentially modulate the real and quadrature phase signal components 
supplied to the filter bank analyzer by the function e.sup.-j.omega. 
k.sup.n where: 
EQU .omega..sub.k =2.pi.k/K, k=1,2 . . . K-1 (1) 
By exponentially modulating the incoming signal in this fashion, each 
complex modulator 42 serves to shift a separate one of the K channels of 
originally received signal to its original spectral position. 
Each of the K channels of the filter bank analyzer 36 of FIG. 2 also 
includes a low-pass filter 44 for low-pass filtering the output signal of 
the modulator 42. A decimator 46 serves to decimate the output signal of 
the low-pass filter 44 by a factor of M, where M is a constant, so that 
the output signal of the decimator, representing a separate one of the K 
channels of the original frequency-multiplexed signal, is returned to its 
original sampling rate. 
In practice, the frequency bank analyzer 18 is typically not implemented by 
means of separate discrete components, but rather, the analyzer is 
comprised of a digital signal processor (not shown) which is programmed to 
carry out the steps of (a) exponential modulation, (b) low-pass filtering, 
and (c) decimation using fast Fourier transform techniques as are 
described at pages 289-310 of the text Multirate Digital Signal Processing 
by Ronald E. Crochiere and Lawrence R. Rabiner (Prentice Hall, 1983), 
herein incorporated by reference. To compensate for the fact that the 
signals input to the filter bank analyzer 18 were originally obtained from 
analog signals of a continuous nature, the analyzer typically weights the 
signals, using an empirically established weighting factor, prior to 
processing. 
Referring now to FIG. 3, there is shown a block schematic diagram of a 
symbolic representation of the filter bank synthesizer 36 of FIG. 1. As 
seen in FIG. 3, the filter bank synthesizer is comprised of K individual 
channels 1,2,3 . . . K. Each channel includes an interpolator 48 for 
interpolating a signal received from a corresponding modulator 34 of FIG. 
1 by the factor of M. The output signal of the interpolator 48 of the 
channel is low-pass filtered by a low-pass filter 50 before being input to 
a complex modulator 52 which serves modulate the low-pass filtered signal 
by e.sup.-j.omega. k.sup.n where .omega..sub.k is given in eq. 1. The 
output signal of the complex modulator 52 of each channel is summed by a 
summing amplifier 54 to yield a frequency-multiplexed signal comprised of 
K individual channels. 
As with the frequency bank analyzer 18 of FIG. 2, the frequency bank 
synthesizer 36 of FIG. 3 is implemented in terms of a digital signal 
processor (not shown) programmed to perform the steps of (1) 
interpolation, (2) low-pass filtering, and (3) complex modulation using 
inverse fast Fourier transform techniques. For a further description of 
how such steps may be carried out using inverse fast Fourier techniques, 
reference should be had to the aforementioned text Multirate Digital 
Signal Processing by Ronald E. Crochiere and Lawrence R. Rabiner 
incorporated by reference herein. 
The foregoing describes a cellular telephone cell site base station 10 
which advantageously utilizes a single transmitter 40 and a single 
receiver 14 for transmitting and receiving, respectively, a 
frequency-multiplexed signal containing K separate channels, each channel 
representing a signal associated with a separate mobile terminal. By 
frequency multiplexing and de-multiplexing K separate channels using the 
filter bank synthesizer 36 and filter bank analyzer 18, respectively, the 
need for multiple transmitter-receiver pairs is obviated, thereby 
achieving a savings in terms of both cost and space. 
It is to be understood that the above-described embodiments are merely 
illustrative of the principles of the invention. Various modifications and 
changes may be made thereto by those skilled in the art which will embody 
the principles of the invention and fall within the spirit and scope 
thereof.