Multichannel wideband digital receiver making use of multiple wideband tuners having individually selectable gains to extend overall system dynamic range

A technique for extending the instantaneous dynamic range available in a wideband digital basestation for use in a wireless communication network. The basestation processes the bandwidth allocated to it in two or more sub-bands. The sub-bands are each processed by a digital tuner section consisting of an amplifier, analog-to-digital (A/D) converter, and digital filter bank. The digital filter bank may make use of multirate digital signal recovery techniques for efficient implementation. In operation, a received signal strength indication (RSSI) is first determined for each subscriber unit requesting access to the basestation. A channel assignment controller then assigns a transmit frequency to the subscriber unit depending upon this RSSI. In particular, subscriber units exhibiting a relatively stronger RSSI are to transmit on a frequency assigned to a first digital tuner section, while subscriber units having a relatively weak RSSI are assigned to a second digital tuner section. The gains of the first and second tuner sections of each other adjusted independently, to insure that the relatively strong signals are not clipped by the first tuner section, and that the relatively weak signals may be correctly detected by the second tuner section. The net effect is to provide a much greater overall system dynamic range. The invention can be used to advantage in both existing AMPS type cellular systems as well as CDMA systems.

FIELD OF THE INVENTION 
This invention relates generally to wireless communication networks, and in 
particular to a wireless communication system basestation making use of 
two or more wideband, multichannel tuner sections, wherein each tuner 
section covers some preselected sub-portion of the entire radio frequency 
bandwidth to be serviced by the basestation, and wherein each tuner 
section has a gain control which may be adjusted independently of the gain 
control for at least one of the other tuner sections, and a receiver 
channel is allocated to a particular tuner section depending upon a 
received signal strength indication. 
BACKGROUND OF THE INVENTION 
The providers of current day multiple channel wireless communication 
services such as cellular mobile telephone (CMT) and personal 
communication systems (PCS) typically allocate receiver signal processing 
equipment for each single receiver channel. For example, each basestation 
is configured to provide communication capability for only a limited 
number of the channels in the overall frequency spectrum that is available 
to the service provider. 
A typical basestation may thus contain several racks of equipment which 
house multiple sets of receiver and transmitter signal processing 
components that service a prescribed subset of the available channels. For 
example, in an Advanced Mobile Phone Service (AMPS) cellular system, a 
typical basestation may service only selected number of channels, such as 
48, of the total number, such as 416, of the channels available to the 
service provider. 
There recently has been a suggestion that service providers would prefer to 
employ equipment that would be more flexible, both in terms of where it 
can be located, as well as in the extent of the available bandwidth 
coverage provided by a particular transceiver site. This is particularly 
true in rural areas where cellular coverage may be concentrated along a 
highway, and for which the limited capacity of a conventional 48 channel 
transceiver would be inadequate. In other instances, relatively large, 
secure, and protective structures for multiple racks of equipment are not 
necessarily available or cost effective, such as in PCS applications. 
One way to resolve this difficulty would be to implement the basestation 
transceiver apparatus using a high speed analog-to-digital (A/D) converter 
and equipment which makes use of efficient digital filters. On the 
transmit side, the basestation would also include an inverse FFT 
processing combiner which outputs a combined signal representative of the 
contents of the communication channel signals processed thereby. In this 
manner, relatively compact, lightweight, inexpensive, and reliable digital 
integrated circuits may be used to cover the entire channel capacity 
offered by the service provider, rather than only the subset of the 
available channels. For an more detailed description of such a system, 
please refer to our co-pending United States patent application entitled 
"Transceiver Apparatus Employing Wideband FFT Channelizer with Output 
Sample Timing Adjustment and Inverse FFT Combiner for a Multichannel 
Communication Network" filed Apr. 10, 1994 and which is assigned to 
Overture Systems, Inc. the assignee of this application. 
In such a configuration, on the receiver side, an antenna feeds a radio 
frequency tuner which selects an appropriately-sized bandwidth from among 
the radio frequency bandwidth available to the service provider. The 
analog tuner typically comprises one or more bandpass filters, amplifiers, 
local oscillators, and mixers to translate the selected bandwidth to a 
convenient center frequency at or near a baseband frequency. 
The translated baseband signal is then amplified and then digitized by a 
high speed A/D converter. The A/D converter is characterized by a sampling 
rate and a digital word size, or resolution. The sampling rate is selected 
according to the bandwidth to be covered by the receiver, and a minimum 
sampling rate is at least twice the bandwidth to be covered, as dictated 
by the well-known Nyquist criterion. The word size is selected depending 
upon the desired sensitivity of the receiver. The greater the number of 
bits in each digital word output by the A/D, the greater the sensitivity, 
or dynamic range. 
Current day state of the art circuit technologies typically limit the 
resolution of the A/D converter to approximately 12 bits at a sampling 
rate of 25 MegaHertz (MHz). This 12-bit converter thus provides a dynamic 
range of 72 to 80 decibels (dB) at most. Because analog bandpass filters, 
amplifier, and mixers are readily available which have a much greater 
dynamic range, wideband receivers of this type are thus considered to be 
dynamic range-limited by the A/D converter. This creates a number of 
problems. 
First, the above dynamic range specification is for a signal of a single 
frequency of predictable maximum amplitude. However, in an application 
such as a wideband basestation, many signals of different amplitudes are 
present. Thus, the gain of the amplifier stage prior to the A/D converter 
must be carefully controlled, so that channel signals having the largest 
expected magnitude will be received without distortion. If this is not 
done, the resulting "clipping" of the received signal will create many 
undesired spurious tones. These spurious tones, in turn, cannot otherwise 
be separated from the desired lower magnitude signals in adjacent 
channels. 
On the other hand, if the gain of the amplifier is reduced too much, in an 
effort to avoid the spurious sidelobe effect, this reduces the available 
dynamic range in the wideband receiver, and the smaller-amplitude signals 
may fall below the noise floor of the A/D converter, and thus may not be 
detected at all. 
The conventional wisdom is thus that one must either accept a limit on the 
dynamic range of a wideband digital receiver which makes use of a given 
A/D converter, for a given basestation bandwidth. Otherwise, one must 
reduce the bandwidth covered by the digital receiver, so that fewer 
channels may be processed by a slower-speed A/D converter having a larger 
number of bits. 
What is needed is a way to increase the dynamic range available in a 
wideband digital basestation, without reducing the bandwidth covered 
thereby. Indeed, it would be preferable if the receiver could somehow be 
designed such that this dependency of the bandwidth coverage on the 
dynamic range of the A/D converter could be eliminated. 
SUMMARY OF THE INVENTION 
Briefly, the invention is a wideband digital receiver consisting of at 
least a pair of digital tuner sections. Each digital tuner section 
consists of an A/D converter and digital filtering equipment dedicated to 
processing a portion of the entire bandwidth serviced by the receiver. 
Each radio frequency signal path leading to the input of a digital tuner 
section has an amplifier with a separately adjustable gain control input. 
The gain control inputs of the tuner sections may thus be set 
independently of each other. 
In operation, a base station channel assignment controller then assigns 
subscriber units to operate a channel frequency serviced by one particular 
tuner section or another, depending upon a received signal strength 
indication (RSSI) of the signals received from the subscriber unit. 
As one example, a first one of the digital tuner sections may be allocated 
to processing the signals from remote units with relatively strong RSSI, 
in a range, say, from zero decibels (0 dB) to -70 dB. A second digital 
tuner may be dedicated to processing the signal received from subscriber 
units having a somewhat weaker received signal strength of approximately 
-30 dB to -100 dB. In this manner, even when each of the digital tuner 
sections makes use of a 72 dB A/D converter, the overall system exhibits a 
dynamic range of approximately 100 dB, rather than the lower dynamic range 
provided by a conventional system. 
The gain of the first digital tuner section allocated to processing 
relatively stronger signals can be optimized as needed, so that spurious 
sidelobes are not created in adjacent channels. The gain of the second 
digital tuner section allocated to processing relatively weaker signals 
can also be adjusted independently of the gain of the first digital tuner 
section, so that even the weakest expected signals are correctly received. 
Other difficulties may be avoided as well. For example, a typical 
multichannel wireless system allocates certain channels for audio signal 
transmission and other channels for transmission of the control signals 
that manage access to the audio channels. The control signaling channels 
will usually exhibit a relatively strong received signal strength 
indication. Thus, according to the above criteria, those channels are 
typically assigned to the first digital tuner section which services such 
stronger signals. 
A system operating in this manner will thus exhibit a capture and hold 
effect which may or may not be desirable. This is because when the control 
channels are assigned to the tuner section servicing the stronger 
amplitude signals, a subscriber unit will not be granted access to the 
system until the control signals it transmits are strong enough to be 
within the capture range of that first tuner. The first tuner will then 
continue to process the signals transmitted by that subscriber unit until 
they become weak enough for hand-off to a second tuner section assigned to 
process relatively weak signals. The second tuner will then continue to 
process, or hold the signal from that subscriber unit, continuing to 
process this relatively weak signal until it is weak enough to require 
transfer to another basestation site. 
Thus, a given subscriber unit will not be captured by a given basestation 
until it is within a particular close range, but the subscriber unit will 
continue to be serviced by that same basestation even after it travels 
beyond that close range, to a range which is typically much greater than 
the initial capture range. 
When this capture and hold effect is not desirable, then one or more 
narrowband receivers may be used with the wideband digital tuners. The 
narrowband receivers are dedicated to processing the control channel 
signals, and the digital tuner sections are then assigned to processing 
just the voice and/or data information channel signals. This permits the 
gain of each individual narrowband receiver to be adjusted as needed, such 
as with a conventional automatic gain control circuit, so that either 
strong or weak control channel signals are correctly received, while at 
the same time providing the advantages of having the multiple digital 
tuner sections. 
In another embodiment, the multiple digital tuner sections need not each 
service the same bandwidth, but rather may each be dedicated to covering a 
different proportion of the bandwidth serviced by the basestation. 
In particular, the relative sizes of the bandwidths covered by individual 
tuners are selected depending upon the expected distribution of the 
density of relatively strong signals and relatively weak signals. In the 
typical situation, where the subscriber units are more or less evenly 
distributed geographically, there usually are many more weaker signals 
than stronger signals to be serviced. This is because the received signal 
strength varies as an inverse exponential of the distance between the 
basestation and the subscriber unit. 
Distributing the bandwidth among different tuner sections can also be 
advantageous in situations where the basestation is more than one type of 
subscriber unit. For example, certain emerging standards such as Code 
Division Multiple Access (CDMA) cellular systems, occupy the same 
bandwidth as older cellular systems such as Advanced Mobile Phone System 
(AMPS). The received signal strengths are typically much stronger for AMPS 
systems which must share the same bandwidth with CDMA systems. Thus, by 
assigning the stronger amplitude AMPS encoded signals to the first tuner 
section, and the weaker CDMA encoded signals to the second tuner section, 
the capture range for the CDMA signals can be greatly improved.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 shows a block diagram of a multichannel basestation 10 implementing 
the concept of this invention. In particular, the basestation 10 includes 
an antenna 11, a power splitter 12, a plurality of amplifier units 14-1, 
14-2, . . . , 14-K, a plurality of digital tuners 15-1, 15-2, . . . , 
15-L, a plurality of digital channelizers 16-1, 16-2, . . . , 16-L, and a 
channel assignment controller 18. 
In operation, radio frequency (RF) energy in the form of electrical signals 
from subscriber units 20 is first received at the antenna 11. The 
subscribers 20 may be mobile units, as shown, or portable units. The 
particular radio frequencies of interest depend upon the particular 
communication service to which the basestation 10 is being applied. For 
example, in a cellular mobile telephone (CMT) application operating in 
accordance with the Advanced Mobile Phone Specification (AMPS), the 
bandwidth of interest is within the range of approximately 824 MegaHertz 
(MHz) to 849 MHz. The bandwidth is typically shared by two competing 
service providers, so-called "System A" and "System B" providers, so that 
a given AMPS basestation typically has 12.5 MHz of bandwidth available to 
it. 
This approximately 12.5 MHz bandwidth is allocated as four-hundred and 
sixteen (416) channels, each having a 30 kilohertz (kHz) bandwidth. For 
the AMPS "System A" service provider, the 416 channels are transmitted 
over the air from the remote unit 20 in a pair of radio frequency bands, 
ranging from 824 to 835 MHz and from 845 to 846.5 MHz. For a "System B" 
service provider, the 416 available channels are transmitted in radio 
frequency bands ranging from approximately 835 to 845 MHz and from 846.5 
to 849 MHz. 
While the following discussion will consider the AMPS example in detail, it 
should be understood that the invention is not limited to a particular 
cellular system. Thus, other bandwidths and other types of systems such as 
Personal Communication System (PCS) can be accommodated. 
Regardless of the particular radio frequency band of interest, the power 
splitter 12 divides the signal energy from the antenna 11 among a 
plurality of signal paths 13-1, 13-2, . . . , 13-L. In the illustrated 
embodiment, the K signal paths are then each provided as an input to a 
respective one of the amplifier units 14-1, 14-2, . . . , 14-L. 
The gains of the amplifiers units 14-1, 14-2, . . . , 14-L are individually 
controlled by gain control inputs K-1, K-2, . . . , K-L to determine 
whether the associated tuner 15-1, 15-2, . . . , 15-L is to service 
relatively strong or relatively weak signals. The gains of the various 
amplifiers 14 are thus different from one another, that is the gain 
setting of the amplifier 14-1 is different from the gain setting K-2 of 
the amplifier 14-2, which is different from that of amplifier 14-L, and so 
on. 
The gain control of each individual amplifier 14 is set so that the 
collective sum of the expected signal amplitudes in the bandwidth covered 
by its respective digital tuner section 15 does not cause a respective 
analog to digital (A/D) converter used by the tuner 15 to saturate. 
More particularly, the output of each amplifier 14 is coupled to a 
respective one of the digital tuner sections 15. An exemplary digital 
tuner section 15-1, shown in FIG. 2, consists of a first downconverter 
section 150 which includes a first bandpass filter 151, amplifier 152, and 
mixer 153. A second downconverter section 155, which includes a second 
bandpass filter 156, and amplifier 157, processes the output of the first 
downconverter section 150. The downconverter sections 150 and 155 
translate radio frequency energy in the selected 12.5 MHz bandwidth of 
interest to a convenient intermediate frequency (IF) in a well known 
manner. 
A preselector bandpass filter 160 and amplifier 162 select and amplify a 
predetermined bandwidth portion from the 12.5 MHz bandwidth processed by 
the basestation 10. In the example being discussed, the preselector 
bandpass filter 160 may, for example, select only 2.5 MHz of the 
approximately 12.5 MHz being processed by the basestation 10 to be 
forwarded to the first tuner section 16-1. 
There may or may not be an overlap of the bandwidths covered by the various 
digital tuners 15-1, 15-2, . . . , 15-L. For example, it may be desirable, 
for reasons that will become clear later on, for the digital tuner 15-1 to 
cover some of the same frequencies covered by digital tuner 15-2. 
The amplifier unit 14-1 can be any of the various tuner amplifiers 152, 
157, and/or 162, and the gain control input K-1 can be used to adjust any 
or all of their respective gain inputs. What is important is that the gain 
plan of the entire digital tuner 15-1 be such that the final baseband 
signal level presented to the input of the A/D converter 163 be 
controllable. Thus, although a single amplifier unit 14-1 was shown in 
FIG. 1, it should be understood that this amplifier function may actually 
be implemented in one or more of the tuner amplifiers 152, 157 and/or 162. 
The A/D converter 163 then operates on this baseband signal to provide 
digital samples, in a manner which is well known in the prior art. 
This digital sampled signal, still having an approximately 2.5 MHz 
bandwidth, must then be further filtered to separate it into the 
individual 30 kHz channel signals. 
Returning attention to FIG. 1, the digital channelizer 16-1, being a bank 
of digital filters with each filter having a 30 kHz bandwidth, performs 
this function. The digital channelizer 16-1 may implement the filter bank 
using any of several different filter structures, and no particular 
digital filter structure is critical to the operation of the invention. 
One possible filter bank architecture is described, however, in more 
detail below. The channelizer 16-1 outputs N individual digital channel 
signals, with each of the N outputs representing information in one 
channel. 
These digital channel signals 17 are then provided to digital signal 
processors 165-1-1,165-1-2, . . . , 165-1-N, which remove any remaining 
modulation specified by the air interface standard implemented by the 
basestation 10. The digital signal processors 165 also reformat these 
baseband audio or data signals as necessary for transmission to a local 
Mobile Telephone Switching Office (MTSO). For example, the channel outputs 
may contain voice and/or data information formatted in accordance with the 
well known T1 standard format for transmission to the MTSO. These T1 
signals are then processed by the MTSO in an known fashion to ultimately 
complete a telephone call from the subscriber unit 20 to the desired 
destination, such as by using the Public Switched Telephone Network 
(PSTN). 
The channel assignment controller 18 also communicates with the MTSO via an 
MTSO control interface 19, in order to perform the control functions 
necessary to set up a call. For example, when a subscriber unit wishes to 
place a call, a subscriber unit 20 indicates this by transmitting one or 
more of the appropriate control signals on one of the radio frequency 
channels dedicated for control signaling. The basestation 10 provides 
these control signals at certain ones of the channel outputs 17-1, . . . , 
17-A, and they are in turn forwarded to the channel MTSO control interface 
19 to set up the end to end connection. 
In the AMPS system being described, the "System A" control channels are 
present in a frequency band from 834.36 to 835 MHz, and the "System B" 
control signals are present from 835 to 835.63 MHz. In the embodiment 
shown, the various preselector filters in the digital tuners 15-1, 15-2, . 
. . , 15-N insure that the control channels are processed by the first 
digital tuner section 16-1, rather than some other tuner section 16-2, . . 
. , 16-K. The control signal channels 17-1, . . . , 17-A are then provided 
to both the channel assignment controller 18 and the MTSO control 
interface 19 in order to set up the call. 
As mentioned previously, the digital channelizer 16-1 uses any suitable 
digital filtering algorithm to separate the channels. In the preferred 
embodiment, however, the digital channelizer 16-1 makes use of multirate 
digital signal processing techniques to minimize the amount of necessary 
computation. As shown in FIG. 3, the channelizer 16-1 thus preferably 
consists of a convolutional digital filter 210, a fast Fourier transform 
(FFT) unit 212, and a plurality, N, of interpolating lowpass filters 
214-1, 214-2, . . . , 214-N. In operation, the convolutional digital 
filter 210 accepts the sampled signal from the respective A/D converter 
163 (FIG. 1 ) and performs a first set of convolutional operations 
necessary to separate the wideband digital input signal into the N digital 
channel signals. The convolutional filter 20 may be a so-called overlap 
and add digital filter, or may be a polyphase digital filter. 
As a second step in the channelization process, the output of the 
convolutional filter 2 10 is passed to an N-point, complex valued FFT 
engine 212. The N FFT output taps represent N time-domain, 
bandpass-filtered signals, with each such signal representing one of the 
416 channels processed by the basestation 10. 
Before passing the individual digital signals to the DSP processors 165-1-1 
through 165-1-N, each digital channel signal output by the FFT processor 
212 may also be processed by a sample rate adjuster 214-1, . . . , 214-N 
which adjusts the effective timing of the sample values in each digital 
channel signal 213-1, 213-2, . . . , 213-N. These rate adjusters 214 make 
use of a multirate digital signal processing technique to minimize the 
rate at which samples must be provided by the FFT processor 212. Each rate 
adjuster 214 provides samples of its respective digital channel signal 213 
taken at or near a position of peak symbol amplitude. 
For a more detailed explanation of a preferred embodiment of the 
channelizer 16-1, reference should be had to a co-pending U.S. Patent 
Application entitled "Transceiver Apparatus Employing Wideband FFT 
Channelizer with Output Sample Timing Adjustment and Inverse FFT Combiner 
for a Multichannel Communication Network", filed Apr. 10, 1994, and which 
is assigned to Overture Systems, Inc., the assignee of the present 
application. 
Returning attention to FIG. 1 briefly, it has been mentioned above that the 
gain control inputs K-1, K-2, . . . , K-L are carefully chosen to provide 
increased dynamic range of the overall system 10. In particular, the gain 
K-1 of the signal path through the first digital tuner section 15-1 is, 
for example, adjusted to capture and service the channels having the 
largest expected received signal strength indication (RSSI). The signal 
path through the second digital tuner section 15-2 has a slightly larger 
gain. That second tuner 15-2 is thus meant to exclusively service channels 
having a somewhat smaller RSSI. If required, additional amplifiers 14-3, . 
. . , 14-K and tuner sections 15-3, . . . , 15-K are employed to service 
channels having still smaller RSSI's. 
Thus, in accordance with a fundamental concept of the invention, upon 
receiving a control signal indicating that a subscriber unit 20 wishes to 
place a call, the channel assignment controller first determines the RSSI 
of the signals received from the subscriber unit 20. The channel 
assignment controller 18 then assigns a transmit frequency to the 
subscriber 20 depending upon the value of the RSSI. Subscriber units 20 
having a relatively strong RSSI are assigned a transmit frequency covered 
by the first tuner 15-1, and subscriber units 20 having relatively weaker 
RSSI are assigned a transmit frequency covered by the second tuner 15-2. 
FIG. 4 is a plot illustrating how two of the digital tuner sections 15-1 
and 15-2 may be employed in this manner. The plot is of frequency spectrum 
covered by the basestation 10, with signal amplitude along the vertical 
axis and frequency along the horizontal axis. The horizontal axis 
represents the entire bandwidth covered by the basestation 10. As 
indicated above the plot, the first tuner 15-1 is dedicated to covering a 
contiguous section of the lower frequencies in the bandwidth of interest, 
while the second tuner 15-2 covers the upper frequency portion. 
Certain subscriber units, identified by the signals indicated by reference 
letters B, E, and D, have been assigned to channels 17-B, 17-E, and 17-D 
covered by the first digital tuner section 15-1 and first channelizer 
16-1. The signals from these subscribers are being received with a 
relatively strong RSSI, from 0 to -70 dB. Subscriber units providing 
relatively weaker RSSI, such as the signals indicated by letters M and O, 
are assigned to the second digital tuner 15-2 and second channelizer 16-2. 
These signals from these subscribers are being received with relatively 
weaker amplitudes. 
The horizontal lines and shading represent the amplitude coverage provided 
by each digital tuner section 15. For example, with reference to tuner 
section 15-1, the gain control K-1 is set such that the magnitudes of the 
digital samples output by its respective A/D converter 163-1 map to analog 
amplitudes of 0 dB to -70 dB. That is, the maximum magnitude digital 
sample maps to approximately -0 dB, and a minimum magnitude digital sample 
maps to -70 dB. For the second tuner 15-2, the gain of the amplifier 14-2 
is set so that the maximum and minimum amplitudes map to -30 and -100 dB, 
respectively. 
As shown in the flowchart of FIG. 5, to accomplish this result, the channel 
assignment controller 18 initially assigns a request by a subscriber unit 
20 for a new channel based upon the received signal strength indication of 
the control signal received from the subscriber unit 20. In particular, as 
shown at step 400, the RSSI of the signal from the control channel 17-1, . 
. . , 17-A assigned to the subscriber 20 is first determined. At step 410, 
if the RSSI of the control channel signal received from the subscriber 
unit 20 is above a predetermined threshold, such as at least about -30 dB, 
then it is assigned to a frequency covered by the first tuner 15-1 in step 
414. Such is the case in FIG. 4 for the unit transmitting signal B, which 
has been assigned to a channel 17-B covered by the first tuner 15-1. If 
however, in step 410, the RSSI is weaker than the threshold, such as for 
signal M in FIG. 4, then the subscriber unit 20 is assigned a channel 
frequency such as 17-M covered by the second tuner 15-2. 
The subscriber unit 20 is then enabled to transmit audio signals on the 
assigned channel, and is permitted to place the call. During progress of 
the call, as indicated by step 420 of FIG. 4, the RSSI of the channel to 
which the subscriber unit 20 has been assigned continues to be monitored 
by the channel assignment controller 18. As necessary, that is, if the 
amplitude of the RSSI changes, the subscriber unit 20 may eventually be 
re-assigned to one of the other tuners 15. 
To accomplish the assignment of a frequency to the subscriber unit 20, a 
transmitter portion of the basestation 10 (not shown) originates an 
in-band control signal in a manner which is well known and specified by 
the AMPS standard. The in-band control signal instructs the remote unit 20 
to switch to the new transmit frequency. 
This approach thus permits both relatively strong and relatively weak 
amplitude signals to be captured and processed by the same basestation 10, 
thereby increasing the overall effective dynamic range of the basestation 
10. 
While the above process has been described for a system having two tuners 
15-1 and 15-2, it should be understood that multiple comparisons against 
multiple RSSI thresholds may be done to assign the subscriber to one of 
three or even one of N channels. 
With this system, the control channels, such as channels 17-1 and 17-2, 
must typically be assigned to the first tuner 15-1, in order to prevent 
strong control signal amplitudes such as that of channel 17-1 from being 
clipped. Otherwise, undesired spurious tones would be created within the 
control channel bandwidth. 
However, using this arrangement, a capture and hold effect will be observed 
by the subscriber unit 20. In particular, as shown in FIG. 6, the 
subscriber unit 20 will not obtain access to the basestation 10 until it 
is within a particular distance, or range, R.sub.1, from the basestation 
10. Assuming an omni-directional radiation pattern, the range R.sub.1 
corresponds to a distance at which the first tuner 15-1 can detect the 
control signal transmitted by the subscriber unit 20, that is, the point 
at which the RSSI of the control signal rises above the -70 dB point (FIG. 
4). The remote unit 20 is then assigned a transmit channel with the 
bandwidth of the first tuner 15-1, such as channel 17-D (FIGS. 4 and 5). 
As the remote unit 20 continues the call while moving in the direction of 
the arrow A, the RSSI of the transmit channel 17-D continues to be 
monitored by the channel assignment controller 18. When the RSSI of 
channel 17-D becomes weak, that is, when its RSSI drops close to -70 dB at 
a point R.sub.x, the remote unit 20 is handed off to the second tuner 
15-2. 
Tuner 15-2 then continues to process the signal from remote unit 20, which 
is now assigned to the frequency 17-M. This continues until the RSSI of 
the channel 17-M is weak enough to require transferring control of remote 
unit 20 to another cell site. This point, at range R.sub.2, corresponds to 
an RSSI of about -100 dB, is the maximum extent of the range of the 
digital tuner 15-2. 
While the basestation 10 will operate under these circumstances, the 
capture and hold effect may not be desirable in some instances. If so, 
then one or more narrowband control receivers can be used together with 
the wideband digital tuner sections 15-1 and 15-2, as shown in FIG. 7. As 
for the embodiment of FIG. 1, the basestation 10 again includes an antenna 
11, a power splitter 12, IF amplifier units 14, tuners 15, channelizers 
16, and digital signal processors 165. However, the power splitter 12 also 
provides a signal 13-K to a number of narrowband control receivers 23-1, 
23-2, . . . , 23-R. 
The narrowband control signal receivers 23 each service only one of the 30 
kHz control signal channels 17-1, 17-2, . . . , 17-A. Thus, the individual 
control channel receivers 23-1, 23-2, . . . , 23-R may have separately 
adjustable gains, and indeed, may typically use known automatic gain 
control techniques to process signals having RSSI's over the entire 
desired capture range of 0 to -100 dB of the basestation 10. In this 
manner, the capture and hold effect can be eliminated. 
Also as for the FIG. 1 embodiment, the control channel signals from the 
narrowband control channel receivers 23 are in turn provided to the 
channel assignment controller 18, which, as explained above in connection 
with FIG. 4, determines how to assign an audio transmit channel 17-A, . . 
. , 17-Q, 17-M, . . . , 17-Z to a particular remote unit. 
FIG. 8, which is a plot similar to FIG. 4, illustrates how the arrangement 
of FIG. 7 provides a uniform capture effect for the control channels. As 
shown, a strong control channel signal 17-1 transmitted by a subscriber 20 
at a close range, R.sub.1, as well as a weak control channel signal 17-2 
transmitted from a distant range, R.sub.2, are both now properly received. 
For either of the embodiments of FIG. 1 or FIG. 7, the tuners 15-1, 15-2, . 
. . , 15-K need not cover equal bandwidths, and may also cover overlapping 
bandwidths. This is especially of interest when one considers that the 
RSSI of a given subscriber unit 20 varies as the inverse square function 
of its range to the basestation 10. Thus, assuming an approximately even 
geographic distribution of remote units in the area surrounding the 
basestation 10, there are typically many more signals with relatively weak 
RSSI magnitudes. 
For example, if the tuner 15-1 has an expected capture radius, R.sub.1, of 
one (1) mile, and the second tuner has a capture radius, R.sub.2, of two 
(2) miles, the second tuner 15-2 may thus typically need to process four 
times as many channels as the first tuner 15-1. Accordingly, the bandwidth 
covered by tuner 15-2 is thus arranged to be twice that covered by tuner 
15-1. 
For either of the embodiments of the invention shown in FIG. 1 or FIG. 7, 
it may also be desirable, depending upon the application of the 
basestation 10 and the desired dynamic range and frequency coverage of 
each tuner section 15, to include two or more channelizers 16 connected to 
the output of each A/D converter 163. This is indicated by the dashed line 
162 in FIGS. 1 and 7. This feature may be desirable in situations where 
the basestation 10 must provide channelization according to two different 
air interface standards. 
It may also be necessary for the basestation 10 to process relatively high 
magnitude, narrowband-encoded channel signals, as well as relatively low 
magnitude, wideband-encoded channel signals which occupy the same 
bandwidth. For example, it may be desirable for the basestation 10 to 
process AMPS channel signals each having a bandwidth of 30 kHz, while at 
the same time processing Code Division Multiple Access (CDMA) channel 
signals each having a bandwidth of 1.25 MHz, even where the AMPS and CDMA 
channels occupy the same RF bandwidth. This is accomplished by using two 
tuners 15 and two channelizers 16, with each tuner 15 and channelizer 16 
being configured to service one of the respectively different channel 
bandwidths and expected signal strengths. 
Thus, the invention provides an additional advantage for certain second 
generation cellular systems such as those making use of CDMA encoding of 
the audio channels. CDMA cellular signals typically use the same transmit 
frequencies as AMPS-type signals. However, because of the high frequency 
pseudorandom encoding specified for CDMA, a typical CDMA signal has a much 
smaller amplitude and a much wider bandwidth than a typical AMPS signal. 
The basestation for such a CDMA system, and especially a hybrid 
basestation which must process both AMPS and CDMA signals, is thus 
required to detect and correctly receive lower amplitude CDMA signals as 
well as higher amplitude AMPS signals. 
Certain ones of the tuners 15 and channelizers 16 may thus be dedicated to 
processing the higher amplitude AMPS signals, such as tuner 15-1 and 
channelizer 16-1 while another tuner 15-2 and channelizer 16-2 can be 
dedicated to processing the much lower amplitude CDMA channels. 
An alternate arrangement of the functions performed by the front end 
portion of the basestation 10, including the power splitter 12, amplifiers 
14-1, 14-2, . . . , 14-L, and digital tuner sections 15-1, 15-2, . . . , 
15-L is also possible. 14-2, . . . , 14-L, and digital tuner sections 
15-1, 15-2, . . . , 15-L is also possible. As shown in FIG. 9, the RF-IF 
downconverter 150 and IF-to-LF downconverter 155 may be shared among the 
various digital tuners 15-1, 15-2,. . . , 15-L. In other words, the 
position of the downconverters 150, 155 and the power splitter 12 may be 
reversed. However, when the front end of the basestation 10 is configured 
as shown in FIG. 9, the dynamic range sensitivities for each of the 
prefilters 160-1, 160-2, . . . , 160-L and amplifiers 162-1, 162-2, . . . 
, 162-L will be greater than for the arrangement of FIG. 1. 
In addition, rather than use the architecture of FIG. 3, the channelizers 
16 may make use of a digital integrated circuit which accepts a wideband 
digital input and provides a downconverted output, such as HSP 50016 
Digital Downconverter sold by Harris Semiconductor, Inc., of Palm Bay, 
Fla. 
While we have shown and described several embodiments in accordance with 
the present invention, it is to be understood that the invention is not 
limited thereto, but is susceptible to numerous changes and modifications 
as known to a person skilled in the art, and we therefore do not wish to 
be limited to the details shown and described herein but intend to cover 
all such changes and modifications as are obvious to one of ordinary skill 
in the art. and we therefore do not wish to be limited to the details 
shown and described herein but intend to cover all such changes and 
modifications as are obvious to one of ordinary skill in the art.