Time-shared lock indicator circuit and method for power control and traffic channel decoding in a radio receiver

A communication device (100) includes a plurality of receiver fingers (112, 114, 116) for receiving a spread spectrum communication signal. Each receiver finger includes a received signal strength indication (RSSI) circuit (400). The RSSI circuit (400) includes an energy calculator (406) and a filter (410, 412) for producing a filtered signal which indicates signal quality. A first comparator (414) produces a primary lock indication when the filtered signal exceeds a primary lock threshold. A second comparator (418) produces a secondary lock indication when the filtered signal exceeds a secondary lock threshold. The bandwidth of the RSSI circuit (400) can be varied between a first bandwidth for providing the primary lock indication for traffic channel decoder and a second bandwidth for providing the secondary lock indication for the power control bit decoder. This allows performance to be tailored to the individual requirements of the traffic channel decoding and the power control channel decoding.

FIELD OF THE INVENTION 
The present invention relates generally to spread spectrum radio 
communication. The present invention more particularly relates to a rake 
receiver apparatus and method for spread spectrum radio communication. 
BACKGROUND OF THE INVENTION 
Radio systems provide users of radio subscriber units with wireless 
communications. A particular type of radio system is a cellular 
radiotelephone system. A particular type of cellular radiotelephone system 
employs spread spectrum signalling. In such a system, a subscriber 
communication device such as a mobile station communicates with one or 
more remote base stations. Each base station provides communication in a 
fixed geographic area. As the mobile station moves among geographic areas, 
communication with the mobile station is handed off between the base 
stations. 
Spread spectrum signalling can be broadly defined as a mechanism by which 
the bandwidth occupied by a transmitted signal is much greater than the 
bandwidth required by a baseband information signal. Two categories of 
spread spectrum communications are direct sequence spread spectrum (DSSS) 
and frequency-hopping spread spectrum (FHSS). The spectrum of a signal can 
be most easily spread by multiplying it with a wideband pseudorandom 
code-generated signal. It is essential that the spreading signal be 
precisely known so that the receiver can despread the signal. For DSSS, 
the objective of the receiver is to pick out the transmitted signal from a 
wide received bandwidth in which the signal is below the background noise 
level. 
A cellular radiotelephone system using DSSS is commonly known as a Direct 
Sequence Code Division Multiple Access (DS-CDMA) system, according to 
Telecommunications Industry Association/Electronics Industries Association 
(TIA/EIA) interim standard IS-95. Individual users in the system use the 
same frequency but are separated by the use of individual spreading codes. 
Other spread spectrum systems include radiotelephone systems operating at 
1900 MHz, as specified in American National Standards Institute (ANSI) 
standard J-STD-008. Other radio and radiotelephone systems use spread 
spectrum techniques as well. 
In a spread spectrum communication system, downlink transmissions from a 
base station to a subscriber or mobile station include a pilot channel and 
a plurality of traffic channels. The pilot channel is decoded by all 
users. Each traffic channel is intended for decoding by a single user. 
Therefore, each traffic channel is encoded using a code known by both the 
base station and one mobile station. The pilot channel is encoded using a 
code known by the base station and all mobile stations. 
In addition to the pilot channel and traffic channel signals, downlink 
transmissions also include a power control indicator in the traffic 
channel. The power control indicator is periodically transmitted by remote 
base stations to the mobile station to control the transmission power of 
the mobile station. The power control indicator conventionally includes 
several bits which are not encoded in any way. The power control indicator 
is binary in nature, in that it either tells the mobile station to 
increase transmit power or decrease transmit power. In response to the 
power control indicator, the mobile station adjusts its transmission power 
to accommodate changing channel conditions, such as fading or blocking or 
the sudden absence of these. For accurate, reliable communication, rapid 
response by the mobile station to the received power control indicator is 
necessary. 
Mobile stations for use in spread spectrum communication systems commonly 
employ rake receivers. A rake receiver includes two or more receiver 
fingers which independently receive radio frequency (RF) signals. Each 
finger estimates channel gain and phase and demodulates the RF signals to 
produce traffic symbols. The traffic symbols of the receiver fingers are 
combined in a symbol combiner to produce a received signal. 
Generally, the rake receiver fingers are assigned to the strongest channel 
multipath rays. That is, a first finger is assigned to receive the 
strongest signal, a second finger is assigned to receive the next 
strongest signal, and so on. As received signal strength changes, due to 
fading and other causes, the finger assignments are changed. Also, when 
the mobile is in a condition known as soft handoff, the fingers may be 
assigned to any of the base stations involved in the handoff. In soft 
handoff, the mobile station and base stations determine which base station 
provides optimum communication with the mobile station. 
An average measure of multipath strength is employed to determine if a 
finger should be reassigned. The measure of multipath strength is the 
received signal-to-interference ratio (RSSI), also referred to as a 
received signal strength indication. The RSSI measurement is compared to 
predetermined lock and unlock thresholds. If the RSSI for a given finger 
is greater than the lock threshold, the finger is said to be locked. If 
the RSSI value is less than the unlock threshold, the finger is unlocked. 
The RSSI circuit provides a lock indication to a controller which controls 
the lock status of the individual fingers. 
Good power control performance requires a fast RSSI circuit. The RSSI 
circuit should track Rayleigh fading and unlock a finger if a received 
signal momentarily drops into a fade. A weak link whose power control bits 
are demodulated incorrectly can cause the mobile station to respond 
incorrectly to the power control indication. This can cause dropped calls 
and other undesirable conditions. Thus, for power control bit decoding, it 
is necessary that the RSSI circuit be fast enough to unlock any finger 
that drops into a deep fade for longer than, for example, 10 ms. 
However, the rapid response to fading required for power control bit 
decoding is not required for traffic channel demodulation. Accordingly, 
there is a need in the art for a rake receiver circuit and method in which 
a fast RSSI circuit provides a lock indication for power control bit 
decoding and a slow RSSI circuit provides a separate lock indication for 
traffic channel decoding. This would allow performance to be tailored to 
the individual requirements of the traffic channel decoding and the power 
control channel decoding. This, in turn, allows the performance to be 
optimized for accurate demodulation of both power control bits and traffic 
bits. The RSSI circuit is time-shared between the traffic channel and 
power control channel to minimize hardware requirements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1, it shows an operational block diagram of a 
communication device, mobile station 100. The mobile station 100 includes 
an antenna 102 and a filter circuit 106. The mobile station 100 further 
includes a receiver circuit 111 including a plurality of receiver fingers, 
including a first receiver finger 112, a second receiver finger 114, a 
third receiver finger 116, a combiner 118 coupled to each receiver finger, 
and a decoder 120. The mobile station 100 further includes a controller 
122, a user interface 124 and a transmitter 126. 
The mobile station 100 is preferably configured for use in a DS-CDMA 
cellular radiotelephone system including a plurality of remotely located 
base stations. Each base station includes a transceiver which sends and 
receives radio frequency (RF) signals to and from mobile stations, 
including mobile station 100, within a fixed geographic area. While this 
is one application for the mobile station 100, the mobile station 100 may 
be used in any suitable spread spectrum communication system. 
In the mobile station 100, the antenna 102 sends and receives RF signals to 
and from a base station. RF signals received at the antenna 102 are 
filtered, converted from analog signals to digital data and otherwise 
processed in filter circuit 106. The filter circuit 106 may also perform 
other functions such as automatic gain control and down-conversion to 
intermediate frequency (IF) for processing. 
The mobile station 100 employs a radio receiver which receives signals 
subject to fading. The receiver circuit 111 is a rake receiver including 
first receiver finger 112, second receiver finger 114 and third receiver 
finger 116 configured for receiving a spread spectrum communication signal 
over a communication channel. As will be described in further detail 
below, each receiver finger produces a traffic signal including traffic 
symbols. Each receiver finger further produces a lock indication 
indicative of a lock status of the receiver finger. The structure and 
operation of first receiver finger 112 will be discussed in greater detail 
below. Preferably, second receiver finger 114 and third receiver finger 
116 operate substantially the same as first receiver finger 112. 
As noted, the combiner 118 is coupled to the plurality of receiver fingers, 
first receiver finger 112, second receiver finger 114 and third receiver 
finger 116. The combiner 118 combines the traffic signal from each 
respective receiver finger in response to the lock indication from each 
respective receiver finger and forms a received signal. The combiner 118 
provides the received signal to the decoder 120. The decoder 120 provides 
de-interleaving and channel decoding and may be a Viterbi decoder or 
another type of convolutional decoder or any other suitable decoder. The 
decoder 120 recovers the data transmitted on the RF signals and outputs 
the data to the controller 122. 
The controller 122 formats the data into recognizable voice or information 
for use by user interface 124. The controller 122 is electrically coupled 
to other elements of the mobile station 100 for receiving control 
information and providing control signals. The control connections are not 
shown in FIG. 1 so as to not unduly complicate the drawing figure. The 
controller 122 typically includes a microprocessor and memory. The user 
interface 124 communicates the received information or voice to a user. 
Typically, the user interface 124 includes a display, a keypad, a speaker 
and a microphone. 
The individual receiver fingers 112, 114, 116 are assigned to receive 
different signals. In multipath conditions, the receiver fingers 112, 114, 
116 are assigned to receive individual multipath signals or rays. During 
soft handoff, the receiver fingers 112, 114, 116 are assigned to different 
base stations involved in the handoff. Assignment of the receiver fingers 
to respective signals is under control of the controller 122 in a manner 
to be described below. 
Upon transmission of radio frequency signals from the mobile station 100 to 
a remote base station, the user interface 124 transmits user input data to 
the controller 122. The controller 122 formats the information obtained 
from the user interface 124 and conveys it to the transmitter 126 for 
conversion into RF modulated signals. The transmitter 126 conveys the RF 
modulated signals to the antenna 102 for transmission to the base station. 
The structure and operation of each of the rake receiver fingers 112, 114, 
116 for receiving and demodulating signals will now be discussed, using 
first receiver finger 112 as an example. In accordance with the present 
invention, the mobile station 100 is configured to receive one or more 
spread spectrum communication signals, preferably direct sequence code 
division multiple access (DS-CDMA) signals. Each of the spread spectrum 
communication signals includes a pilot channel and a plurality of traffic 
channels. 
At a transmitter, such as at a base station in a cellular radiotelephone 
system, the pilot channel and traffic channels are encoded using Walsh 
codes. The pilot channel and traffic channels are encoded using a 
different Walsh code. Typically, the pilot channel is encoded using a 
Walsh(0) code, a first traffic channel is encoded using a Walsh(2) code, 
etc. After encoding, the signal spectrum is spread using a pseudorandom 
noise (PN) code. The spread spectrum signal in digital form comprises a 
series of chips whose respective values are defined by the PN code and the 
encoded data. The PN code for each base station is unique to that base 
station. Each receiver in the system, or subscriber in a cellular 
radiotelephone system, is assigned a unique Walsh code corresponding to 
the traffic channel on which it communicates with the base station for 
decoding the traffic channel. Each receiver also decodes the pilot 
channel. Each receiver knows the PN codes corresponding to base stations 
in the system. The pilot channel is used to estimate the channel phase and 
the channel gain of the communication channel. 
To obtain the best received signal, the receiver circuit 111 including the 
receiver fingers 112, 114, 116 and the combiner 118 attempt to combine 
symbols from as many fingers as possible. Each finger is individually 
assigned to a received signal, such as an individual multipath signal or a 
signal from one of the base stations involved in a soft handoff. A signal 
quality parameter, such as RSSI, is measured to determine whether a finger 
should be combined by the combiner 118. If the signal quality exceeds a 
lock threshold, the finger is "locked." If the signal quality falls below 
an unlock threshold, the finger is "unlocked." This finger lock status is 
used by the rake receiver circuit 111 to determine whether or not the 
finger should be used by the combiner 118. 
First receiver finger 112 includes a received signal strength indication 
(RSSI) circuit 130, a despreader 150, a pilot symbol decoder 151, a pilot 
channel summer 152, a filter 154, a conjugate generator 156, a traffic 
symbol decoder 158, a traffic channel summer 160, a delay element 162 and 
a demodulator 164. It will be recognized by those ordinarily skilled in 
the art that these elements may be implemented in hardware or in software, 
or in some combination of the two which enhances efficiency and 
manufacturability. 
The despreader 150 receives from the filter circuit 106 a digital 
representation of the spread spectrum communication signal received by the 
mobile station 100. The despreader 150 applies a pseudorandom noise (PN) 
code to the received signal. The despreader 150 despreads the received 
signal, producing a despread signal. The PN code is stored at the mobile 
station 100 and may be transmitted to the mobile station 100, for example 
from a base station, when the communication channel between the base 
station and the mobile station 100 is initiated. The PN code is unique to 
the base station so that the mobile station may select a base station for 
communication by selecting the corresponding PN code. 
The despread signal is provided from the despreader 150 to the pilot symbol 
decoder 151. The pilot symbol decoder 151 decodes the pilot channel signal 
and detects pilot symbols. The pilot symbol decoder applies a pilot 
channel code which is typically the Walsh code Walsh(0). The pilot symbol 
decoder 151 applies the decoded signal to the pilot channel summer 152. 
The pilot channel summer 152 includes a summer 166 and a switch 168. The 
summer 166 sums 64 consecutive chips to form a pilot symbol. After every 
sixty-fourth chip, the switch 168 closes to couple the summer 166 to the 
filter 154 to provide a received pilot symbol to the filter 154. 
The embodiment shown in FIG. 1 is suitable if a Walsh code is used for 
encoding the pilot channel. Since Walsh(0) consists of all binary ones, no 
decoding is necessary when the pilot channel is encoded using Walsh(0) and 
the pilot symbol decoder 151 may be omitted. However, if another Walsh 
code or another type of coding is used to encode the pilot channel, a 
decoder is necessary. Such a decoder applies a pilot code to the despread 
signal to produce the pilot channel signal. 
The filter 154 receives the pilot symbols from the pilot channel summer 
152. The filter 154 filters the pilot channel signal to obtain a complex 
representation of an estimated channel gain and an estimated channel phase 
for the communication channel, in a manner to be described below. 
The filter 154 is preferably a low pass filter. The input of the filter is 
the pilot symbols p(n). The output of the filter is the estimate h(n) of 
the channel coefficient. h(n) is a complex number containing both phase 
and magnitude information. The phase information corresponds to an 
estimate of channel phase. The magnitude information corresponds to an 
estimate of channel gain. One possible implementation of the filter 154 
will be described below in conjunction with FIG. 2. The conjugate 
generator 156 determines the complex conjugate of the signal h(n) produced 
by the filter 154. The filter 154, in conjunction with the conjugate 
generator 156, produces an estimate of the complex conjugate of the 
complex representation of channel gain and channel phase for the 
communication channel. The complex conjugate of the complex representation 
of the channel phase and the channel gain are provided to the demodulator 
164. 
The despread signal is also provided from the despreader 150 to the traffic 
symbol decoder 158. The traffic symbol decoder 158 produces a traffic 
signal in response to the spread spectrum communication signal received by 
the mobile station 100. The traffic symbol decoder 158 applies a user 
specific traffic code to the despread signal to produce the traffic 
channel signal. The user specific traffic code is the Walsh code Walsh(n) 
assigned to the mobile station 100. The traffic channel signal is provided 
to the traffic channel summer 160. 
The traffic channel summer 160 includes a summer 170 and a switch 172. The 
summer 170 sums 64 consecutive chips to form a traffic symbol. After every 
sixty-fourth chip, the switch 172 closes to couple the summer 170 to the 
delay element 162 to provide a received traffic symbol to the delay 
element 162. Thus the traffic channel summer 160 detects the traffic 
channel. 
The delay element 162 is preferably a FIFO, or first in, first out buffer. 
The filter 154 introduces a filter delay when estimating the channel gain 
and channel phase. The delay element 162 compensates for this filter delay 
to ensure that the estimated channel phase and estimated channel gain are 
used to demodulate the corresponding traffic symbols. The delay element 
162 delays the spread spectrum communication signal a predetermined time 
to produce a delayed signal. More specifically, the delay element 162 
delays only the traffic symbols of the traffic channel to produce the 
delayed traffic symbols. 
The delayed traffic symbols are provided to the demodulator 164. The 
demodulator 164 may be implemented as a multiplier which multiplies the 
delayed traffic symbols and the signal received from the conjugate 
generator 156, demodulating the delayed traffic symbols using the 
estimated channel phase and estimated channel gain. The result of this 
multiplication is provided to the decoder 120 for further processing. 
The RSSI circuit 130 includes a summer 132, energy calculator 134, a filter 
135 including a summer 136, a shifter 138, a summer 140 and a delay 
element 142, a comparator 144 and memory 146. The RSSI circuit 130 is 
coupled to the pilot symbol decoder 151. The RSSI circuit 130 samples the 
pilot symbols and produces a pilot sample signal. The filter 135 filters 
the pilot sample signal and produces a filtered signal. The comparator 144 
produces a lock indication at an output 149 when the filtered signal 
exceeds a lock threshold. 
The summer 132 is coupled to the pilot symbol decoder 151 and receives a 
signal in the form of chips. The summer 132 sums 512 consecutive chips to 
form a pilot symbol. The energy calculator 134 determines energy in the 
pilot signal and provides a signal to the filter 135. The filter 135 
averages the signal over an averaging time period, producing a filtered 
signal. The signal has an assumed average fade interval. The assumed 
average fade interval corresponds to an assumed average fade interval of 
the spread spectrum communication signal received by the mobile station 
100. The assumed average fade interval varies with operational conditions 
of the mobile station 100, such as multipath environment and speed of 
travel of the mobile station 100. The averaging time period is preferably 
longer than the assumed average fade interval of the signal. 
The shifter 138 shifts the signal to the right a predetermined number k of 
bit positions. In the preferred embodiment k=6. However, k may be any 
suitable value. Varying the value of k has the effect of varying the 
bandwidth of the filter 135. The spread spectrum signal received by the 
mobile station typically is subject to fading and the filter 135 has a 
variable bandwidth for filtering the effects of fading. In accordance with 
the present invention, the bandwidth of the filter 135 is reduced to 
filter the effects of fading. Expressed alternatively, the filter 135 
averages the signal over an averaging time period. Increasing the value of 
k increases the averaging time period over which the received signal is 
averaged. Preferably, the averaging time period is established in the 
range from 10 to 200 milliseconds (ms). In one embodiment, the averaging 
time period is established at substantially 30 ms. 
The comparator 144 has a first input 147 coupled to the filter for 
receiving the filtered signal. The comparator 144 has a second input 145 
coupled to the memory 146. The comparator 144 compares the filtered signal 
to a lock threshold 141 or an unlock threshold 143 stored in the memory 
146. The filtered signal corresponds to a signal quality parameter, such 
as an RSSI measurement. The comparator produces a lock indication at the 
output 149 in response to the comparison. 
The lock indication is provided to the controller 122. When the lock 
indication indicates that the RSSI measurement exceeds the lock threshold 
and the first receiver finger 112 should be locked, the first receiver 
finger 112 is locked by the controller 122, and the traffic signal or 
traffic symbols from the first receiver finger 112 are combined by the 
combiner 118. The combiner does not combine the traffic signal from the 
receiver finger when the filtered signal falls below an unlock threshold 
after a previous lock indication. Thus, the receiver circuit 111 locks the 
first receiver finger 112 when a signal quality parameter for the filtered 
signal exceeds a lock threshold and unlocks the first receiver finger 112 
when the signal quality parameter falls below the unlock threshold. The 
lock threshold may be different from the unlock threshold. Alternatively, 
the lock threshold may be substantially equal to the unlock threshold. 
The unlock threshold is set slightly above the noise floor of the first 
receiver finger 112. The noise floor corresponds to the minimum input 
signal level required to discriminate the input signal from noise. In an 
exemplary embodiment, the noise floor of the first receiver finger 112 is 
substantially -27 dB E.sub.c /I.sub.0 where E.sub.c is the total chip 
energy and I.sub.0, is the total interference including noise. Preferably, 
the unlock threshold is established in the range of -19 to -27 dB E.sub.c 
/I.sub.0. The inventors have determined that excellent results are 
obtained by establishing the unlock threshold at substantially -24 dB 
E.sub.c /I.sub.0, 
In conventional receiver circuits, the unlock threshold is established at 
approximately -18.5 dB E.sub.c /I.sub.0. This value provides time for 
determination of an accurate channel estimate to use for combining. Also, 
this value accommodates channel estimators which are inaccurate at low 
received signal strengths. With this unlock threshold, receiver finger 112 
can unlock during a deep fade. With any of the receiver fingers 112, 114, 
116 unlocked during a fade, the multipath ray is unusable and some useful 
pilot signal information from the spread spectrum communication signal is 
lost. The ray remains unusable until the finger's RSSI rises above the 
lock threshold. This can result in significant degradation in receiver 
performance in a two- or three-way soft handoff situation or any time the 
pilot signal is weak relative to the total power received from the 
corresponding base stations. The severity of the degradation increases 
during slow fading. 
In a mobile station employing a receiver circuit according to the present 
invention, the likelihood of a finger unlocking during a fade is reduced. 
If a multipath ray is in fade, the ray can still provide some benefit with 
coherent combining. Lowering the unlock threshold, for example, into the 
range -19 to -27 dB E.sub.c /I.sub.0, limits unlocking of the receiver 
finger and improves performance of the receiver circuit 111. Additional 
enhancements to receiver performance are obtained by immediately obtaining 
a pilot estimate for immediate combining of the finger, as will be 
discussed below in conjunction with FIG. 3. 
Referring now to FIG. 2, it shows a block diagram of a finite impulse 
response (FIR) filter 200 for use in the radiotelephone mobile station 100 
of FIG. 1. The filter 200 may be used for providing the low pass filtering 
function of the filter 154 in FIG. 1. The filter 200 includes delay 
elements 202, 204, 206, multipliers 208, 210, 212 and 214, and a summer 
216. 
Preferably, the filter 200 uses a total of 61 delay elements such as delay 
elements 202, 204, 206, not all of which are shown in FIG. 2 so as not to 
unduly complicate the drawing figure. The delay elements operate in 
sequential phases, shifting pilot symbols serially through the chain of 
delay elements. The delay elements are coupled in series so that, during a 
first phase, delay element 202 receives a first pilot symbol from the 
pilot channel summer 152 (FIG. 1). After a delay equal to one pilot symbol 
period, during a second phase, the first pilot symbol is conveyed from 
delay element 202 to delay element 204 and a second pilot symbol is 
conveyed from the pilot channel summer 152 to delay element 202. Again, 
after a delay equal to one pilot symbol period, during a third phase, the 
first pilot symbol is conveyed from delay element 204 to the next delay 
element series-coupled with delay element 204, the second pilot symbol is 
conveyed from delay element 202 to delay element 204, and a third pilot 
symbol is conveyed from pilot channel summer 152 to delay element 202. 
During each phase, the pilot symbols stored at each delay element are 
multiplied with a weighting coefficient by a respective multiplier 208, 
210, 212, 214. Preferably the filter 200 uses a total of 62 multipliers 
such as multipliers 208, 210, 212 and 214, not all of which are shown in 
FIG. 2. Each multiplier corresponds to one of the delay elements 202, 204, 
206. The multipliers multiply the delayed pilot symbol stored in the 
respective delay element by a weighting coefficient. Also, multiplier 208 
multiplies the incoming pilot symbol, at the input of delay element 202, 
by a weighting coefficient. 
The weighting coefficients may be estimated according to any appropriate 
method. In one simple example, all of the weighting coefficients may be 
set equal to unity. In such an implementation, the filter 200 is a low 
pass filter averaging a predetermined number (for example, 42) of pilot 
symbols without weighting. Preferably, the weighting coefficients are 
chosen so that the filter 200 has a frequency response close to an ideal 
rectangular frequency response of a low pass filter. 
In an alternative embodiment, the filter 154 (FIG. 1) could be implemented 
using a low pass infinite impulse response (IIR) filter. Such an IIR 
filter should have a near-linear phase response within its passband. 
The filter 154 is characterized by a group delay at the frequency of 
interest. For a linear phase FIR filter, such as the filter 200, the group 
delay of the filter is equal to one-half the delay or length of the 
filter. For a non-linear-phase FIR or for an IIR filter, the group delay 
is defined as 
##EQU1## 
where .phi. is the phase rotation introduced by the filter at frequency f 
and f.sub.0 is the frequency of interest. The delay introduced by the 
delay element 162 is substantially equal to the group delay of the filter 
154. 
FIG. 3 is a block diagram of a filter 300 for use in the radiotelephone 
mobile station of FIG. 1. The filter 300 includes a precombiner 302, a 
buffer 304, a summer 306, an accumulator 308, and a quantizer 310. The 
precombiner 302 is coupled to the pilot channel summer 152 (FIG. 1) and 
receives the despread pilot symbols at a predetermined rate, such as 19.2 
KHz. The precombiner 302 combines subsequently received pilot symbols to 
form combined pilot symbols. This acts to reduce the memory requirements 
of the filter 300. For example, the precombiner may add two pilot symbols, 
designated p(n) and p(n+1) together to produce a combined pilot symbol, 
which is then stored. In applications where memory requirements are not a 
concern, the precombiner may be omitted. 
The precombiner 302 shifts the combined pilot symbols sequentially into the 
buffer 304. The buffer preferably stores 21 combined pilot symbols, 
corresponding to 42 pilot symbols received from the pilot channel summer 
152. This also corresponds to a group delay of 1.1 milliseconds. 
During each combined pilot symbol period, the buffer 304 shifts a new 
combined pilot symbol into the buffer 304 and shifts the oldest combined 
pilot symbol out of the buffer 304. The summer 306 sums the contents of 
the buffer with the new combined pilot symbol provided by the precombiner 
302 to the summer 306. The sum is accumulated in the accumulator 308. The 
sum is then quantized to reduce storage requirements in the quantizer 310. 
This quantized result corresponds to the estimate of the channel phase and 
channel gain. 
As noted, the filter 300 is characterized by a group delay, preferably 
equal to 21 pilot symbols or 1.1 milliseconds. If the filter 300 is used 
to provide the filtering function of the filter 154 (FIG. 1), the delay 
introduced by the delay element 162 is substantially equal to the group 
delay of the filter 300. 
In accordance with the present invention, the filter 300 may be used as an 
averaging circuit over T symbols for generating a pilot estimate to permit 
a receiver finger such as receiver finger 112 to be immediately combined 
when the finger is assigned to a new signal. When a finger is assigned to 
a new multipath ray, the finger must first obtain an estimate of the new 
pilot before it can be coherently combined. In conventional receiver 
circuits, a finger is assigned unlocked and will lock once the finger's 
RSSI rises above the RSSI lock threshold. The conventional channel 
estimator uses a phase locked loop (PLL) to generate a gain and phase 
estimate of the new pilot. The PLL needs time to lock onto the new pilot. 
This delay causes degradation each time a finger is reassigned. In many 
applications, fingers frequently need to be reassigned. For example, the 
multipath profile corresponding to a given base station is constantly 
changing. Also, as the mobile station 100 crosses cell boundaries, the 
base stations in soft handoff with the mobile station 100 change over 
time. In these situations, delay in combining a receiver finger, and the 
attendant degradation in receiver performance, is unacceptable. 
In the filter 300, the accumulator 308 is an averaging circuit. After 
assigning the receiver finger 112 to a new signal, the receiver finger is 
immediately locked. In some implementations, the receiver circuit 111 may 
include a register or other memory element for storing a lock status of 
the receiver finger. Combining does not occur until the lock status is 
written as "locked" in the register. In such an implementation, the 
register may be written as "locked" and then the finger assigned to the 
new signal. The averaging circuit, accumulator 308, and the buffer 304 are 
also cleared or reset and the new signal is received at the first receiver 
finger 112. 
As the new signal is received, the receiver finger 112 detects pilot 
symbols in the new signal. The filter 300 averages successive pilot 
symbols to provide a weighted channel estimate. The filter 300 sums the 
pilot symbols, including the first pilot symbol and subsequent pilot 
symbols, and produces a sum in the accumulator 308. The weighted channel 
estimate is produced in response to the sum. The filter 300 may 
additionally divide the sum by a sample size, T, to produce the weighted 
channel estimate. The sample size T preferably corresponds to the size of 
the buffer, 42 pilot symbols or 21 combined pilot symbols, however other 
sample sizes may be used. 
Thus, upon assigning the first receiver finger 112 to the new signal as 
locked, the filter 300 detects a first pilot symbol and forms a pilot 
symbol sum (consisting initially of only the first pilot symbol). A first 
pilot channel estimate is generated in response to the pilot symbol sum. 
The filter 300 may additionally divide the pilot symbol sum by T, where T 
is a predetermined value such as 21 or 42, to generate the first pilot 
channel estimate. The first receiver finger 112 demodulates a first 
traffic symbol according to the first traffic channel estimate. The first 
traffic symbol is combined by the combiner 118 with traffic symbols from 
the second receiver finger 114 and the third receiver finger 116, with no 
delay. The filter 300 continues, detecting a next pilot symbol. The filter 
300 adds the next pilot symbol to the pilot symbol sum, generating a next 
channel estimate. The first receiver finger 112 demodulates a next traffic 
symbol according to the next channel estimate, detecting traffic symbols 
in the new signal. The combiner 118 combines the traffic symbols with 
traffic symbols from other receiver fingers, second receiver finger 114 
and third receiver finger 116, according to the weighted channel estimate. 
This process continues, weighting the channel estimates by the number of 
received pilot symbols, until the pilot symbol sum includes T pilot 
symbols. In this manner, the channel estimate improves as successive pilot 
symbols are received. The rough initial estimates have small magnitude 
since they are weighted by the small number of received pilot symbols. As 
a result, the inaccuracy of the estimate will not severely degrade 
performance of the receiver circuit. This implementation improves 
performance in situations where fingers are frequently re-assigned 
relative to other conventional methods, such as channel estimation using a 
PLL, in which a delay time is required to lock on to the new pilot. 
Referring now to FIG. 4, it shows a received signal strength indication 
(RSSI) circuit 400 for use in the mobile station 100 of FIG. 1. The RSSI 
circuit 400 includes an input 402 for receiving pilot samples, a summer 
404, an energy calculator 406, a switch 408, a first filter 410, a second 
filter 412, a first comparator 414, a preprocessor 416, a second 
comparator 418, a primary lock indicator output 420 and a secondary lock 
indicator output 422. It will be recognized by those ordinarily skilled in 
the art that any of the functions illustrated in FIG. 4 may be performed 
using software routines rather than the hardware elements illustrated. 
The RSSI circuit 400 measures a received signal strength of received pilot 
symbols. Pilot symbols are formed by despreading the signal received at 
the input 402. 
Good power control performance requires a fast RSSI circuit. The RSSI 
circuit should track Rayleigh fading and unlock a finger if a received 
signal momentarily drops into a fade. A weak link whose power control bits 
are demodulated incorrectly can cause the mobile station 100 to respond 
incorrectly to the power control indication. This can cause dropped calls 
and other undesirable conditions. Thus, for power control bit decoding, it 
is necessary that the RSSI circuit be fast enough to unlock any finger 
that drops into a deep fade for longer than, for example, 10 ms. 
This requirement applies whether or not the mobile station 100 is in soft 
handoff. The requirement is more significant in soft handoff. In soft 
handoff, power control bits are decoded using a voting method. If any base 
station sends a power control indicator commanding the mobile station to 
decrease power, the mobile station will decrease power. If all power 
control indicators from all base stations in soft handoff indicate that 
the mobile station should increase power, the mobile station will increase 
power. 
A locked finger contributes to both traffic channel demodulation and power 
control bit decoding. In a conventional receiver, the same lock indicator 
is used to control both the traffic channel combiner and power control bit 
demodulation. However, the rapid response to fading required for power 
control bit decoding is not required for traffic channel demodulation. 
Therefore, in accordance with the present invention, the RSSI circuit 400 
is time-shared between providing a primary or traffic channel lock 
indication at a relatively slow speed and providing a secondary or power 
control lock indication at a relatively fast speed. Thus, in accordance 
with the present invention, each receiver finger of a rake receiver has 
two independent lock statuses, a primary lock status used for traffic 
channel decoding and a secondary lock status used for power control 
channel decoding. The mobile station 100 includes a traffic channel 
combiner such as combiner 118 and a power control bit decoder such as 
decoder 120. The primary lock status is used by the combiner 118 and the 
secondary lock status is used by the power control bit decoder of decoder 
120. 
Each of the rake receiver fingers 112, 114, 116 includes a pilot signal 
despreader 150 for despreading the pilot channel signal and a received 
signal strength indicator circuit such as RSSI circuit 400. In RSSI 
circuit 400, the input 402 is configured to receive pilot samples, for 
example from the pilot signal despreader 150 (FIG. 1). Pilot samples are 
provided to the summer 404. The summer 404 coherently adds a predetermined 
number of pilot samples, such as 512 successive pilot samples, to form a 
pilot symbol. The pilot symbols are provided to the energy calculator 406 
for determining energy in the pilot signal. 
The switch 408 generally couples the second filter 412 to the energy 
calculator 406 for each received pilot symbol. Once every M pilot symbols, 
the switch 408 also couples the first filter 410 to the energy calculator 
406. M is a predetermined number of symbols, for example, eight symbols. 
Other values may be chosen for M as appropriate. Operation of the switch 
408, which may be a part of a software routine, is controlled by the 
controller 122. 
Structure and operation of the first filter 410 and the second filter 412 
are similar to structure and operation of the filter 135 described above 
in conjunction with FIG. 1. The value of k is controlled by the controller 
122. Typical values of k are integers in the range 3 to 6 inclusive. 
However, any suitable value may be used. The first filter 410 and the 
second filter 412 in the illustrated embodiment are each a one-pole 
infinite impulse response (IIR) filter. Other suitable filters may be 
used. The hardware elements which form the first filter 410 and the second 
filter 412 may be shared between the two filters. For example, two 
independent accumulators and two independent storage registers, one each 
for the primary or traffic channel lock indication and the secondary or 
power control lock indication, are preferably provided. This permits time 
sharing of other filter hardware between calculation of the two lock 
indication values or states. 
The switch 408 couples the filter to one of two threshold comparison 
circuits. Position of the switch 408 is controlled by the controller 122. 
In accordance with the present invention, the RSSI circuit 400 forms a 
time shared lock indicator circuit which measures pilot energy and 
provides both a primary lock indication and a secondary lock indication. 
The primary lock indication corresponds to a long term average of pilot 
energy, associated with a smaller bandwidth IIR filter. The secondary lock 
indication corresponds to a short term average of pilot energy, associated 
with a larger bandwidth IIR filter. 
In the illustrated embodiment, the switch 408 couples the second filter 412 
with the energy calculator 406 for each received pilot symbol. This 
ensures that the secondary or power control lock indication can respond to 
rapid fades. In contrast, the switch 408 couples the first filter 410 with 
the energy calculator 406 only once every M received pilot symbols. In one 
embodiment, M=8. This ensures that the primary or traffic channel lock 
indicator corresponds to a long term average of pilot energy. Thus, the 
RSSI circuit 400 forms a time shared lock indicator circuit which provides 
the primary lock indication at a first rate and provides the secondary 
lock indication at a second rate, the second rate being faster then the 
first rate. The switch 408 forms a means for varying bandwidth of the RSSI 
circuit 400. Bandwidth may be varied by other means, such as a shift 
register, a decimator or by providing separate filters having different 
bandwidths. 
The controller 122 controls the time shared lock indicator circuit. In an 
alternative embodiment, separate lock indicator circuits are provided for 
each of primary lock indication and the secondary lock indication. 
However, time sharing by using the switch 408 and under control of the 
controller 122 reduces the number of components required. 
Interspersed with the traffic symbols is a power control channel comprising 
power control bits. The time relationship between receipt of the traffic 
channel and the power control channel is defined by the communication 
protocol used for communication between the mobile station 100 and remote 
base stations. For example, according to IS-95, power control bits are 
received every 1.25 ms. 
The first comparator 414 compares the filtered signal with a first 
predetermined threshold. The first predetermined threshold is stored in 
the comparator 414 but is alternatively stored or calculated in any 
suitable manner. An exemplary value for the predetermined threshold is -20 
dB Ec/Io. The first comparator 414 provides the primary lock indication in 
a first state when the received signal strength exceeds the first 
predetermined threshold and in a second state when the received signal 
strength does not exceed the first predetermined threshold. The first 
comparator 414 thus generates a primary lock indication in response to a 
long-term average of the pilot signal energy. The primary lock indication 
is provided by the first comparator 414 at the primary lock indicator 
output 420. The primary lock indicator output 420 is coupled to the 
controller 122 (FIG. 1). 
The preprocessor 416 receives the filtered signal from the filter 410. The 
preprocessor 416 combines RSSI energies from multipath rays received from 
identical base stations. This allows the second comparator 418 to make its 
lock/unlock decision based on the total RSSI from a particular base 
station, rather than on the individual finger's RSSI. Combining the RSSI 
energies in this manner helps to minimize error in power control bit 
decoding. The second comparator 418 provides the secondary lock indication 
when the preprocessor 416 output exceeds a second predetermined threshold. 
The second comparator 418 periodically detects a received signal strength 
of the received pilot signal and provides the secondary lock indication in 
a first state when the received signal strength exceeds a second 
predetermined threshold, stored at the second comparator 418 or elsewhere, 
and in a second state when the received signal strength does not exceed 
the second predetermined threshold. An exemplary value for the second 
predetermined threshold is -17 dB Ec/Io. The second comparator 418 thus 
generates a secondary lock indication in response to the short-term 
average of the pilot signal energy. The secondary lock indication is 
provided by the second comparator to the secondary lock indication output 
422. The secondary lock indication output 422 is coupled to the controller 
122. 
In accordance with the present invention, the controller 122 controls 
finger lock status in response to the primary lock status and the 
secondary lock status as indicated by the primary lock indication and the 
secondary lock indication. The controller 122 includes a receiver finger 
of the plurality of receiver fingers 112, 114, 116 for contributing to the 
traffic channel combiner 118 when the primary lock indication for that 
receiver finger is produced. The controller excludes the receiver finger 
from contributing to the traffic channel combiner 118 when the primary 
lock indication is not produced. The controller 122 includes the receiver 
finger for contributing to the power control bit decoder 120 when the 
secondary lock indication is produced. The controller 122 also excludes 
the receiver finger from contributing to the traffic channel combiner 118 
when the secondary lock indication is not produced. 
Further in accordance with the present invention, the RSSI circuit 400 
forms a signal quality detection circuit having a variable bandwidth, 
including a first bandwidth for detecting the traffic channel and a second 
bandwidth for detecting the power control channel. The first bandwidth is 
maintained by operating on every 1/M received pilot symbol. The second 
bandwidth is established by operating on every received symbol. The 
bandwidth is variable in response to control signals received from the 
controller 122. 
The inventor has determined that significant improvement can be made to 
power control bit decoding performance in an IS-95 system by using a 
relatively fast RSSI circuit for the secondary lock indication. The RSSI 
circuit 400 shown in FIG. 4 can achieve this improvement by using the 
following exemplary configuration: setting M=1, k=3 in the second filter 
412 and second predetermined threshold=-17 dB Ec/Io. Other suitable values 
may be used. With these settings, the time constant of the circuit is 
substantially 3 ms, and the circuit will track rapid fading and unlock 
fingers whose Ec/Io falls below -17 dB Ec/Io. It is not important to make 
the RSSI circuit 400 any faster than 3 ms since the power control bits are 
received at the mobile station 100 only once every 1.25 ms. 
Although increasing the speed of the RSSI circuit will improve the power 
control performance, it will degrade the traffic channel performance. For 
optimal traffic channel decoding, a slow RSSI circuit with a low unlock 
threshold gives best performance. The RSSI circuit 400 in FIG. 4 can 
achieve this performance by using the following exemplary configuration: 
setting M=8, k=6 in the first filter 410 and first predetermined threshold 
=-20 dB Ec/I. Other suitable values may be used. With these settings, the 
time constant of the circuit is 200 ms. 
The controller 122 varies the bandwidth of the RSSI circuit 400 for traffic 
channel and power control channel detection. As an option, the first 
bandwidth, corresponding to traffic channel detection, may be adjusted 
using the shift register value k. For the first filter 410, k=3 
corresponds to a bandwidth of 6 Hz, k=4 corresponds to a bandwidth of 3 
Hz, k=5 corresponds to a bandwidth of 1.5 Hz and k=6 corresponds to a 
bandwidth of 0.75 Hz. For the power control channel, k=3 in the second 
filter 412 corresponds to a bandwidth of 48 Hz, k=4 corresponds to a 
bandwidth of 24 Hz, k=5 corresponds to a bandwidth of 12 Hz and k=6 
corresponds to a bandwidth of 6 Hz. 
In environments where pilot signals are changing rapidly and fingers are 
often being reassigned, the method and apparatus according to the present 
invention offers an additional feature. The bandwidth k in the first 
filter 410 can be adjustable over the range from 3 to 6. Thus, when 
fingers are assigned, the finger can be assigned with k=3 and with the 
primary lock indicator being locked. This means that the finger can 
immediately be used for traffic channel combining, with no delay between 
finger assignments. Once the receiver finger's RSSI has reached steady 
state, k should be switched back to 6 for a longer averaging period. 
As can be seen from the foregoing, the present invention provides a method 
and apparatus for improving performance of a mobile station in a spread 
spectrum communication system. Two separate lock indicators are provided, 
one for the traffic channel decoder and one for the power control channel 
decoder. The power control lock indicator is generated with the larger 
bandwidth RSSI circuit and the traffic channel lock indicator is generated 
with the smaller bandwidth RSSI circuit. This allows performance to be 
tailored to the individual requirements of the traffic channel decoding 
and the power control channel decoding. This, in turn, allows the 
performance to be optimized for accurate demodulation of both power 
control bits and traffic bits. The RSSI circuit is time-shared between the 
traffic channel and power control channel to minimize hardware 
requirements. 
While a particular embodiment of the present invention has been shown and 
described, modifications may be made. It is therefore intended in the 
appended claims to cover all such changes and modifications which fall 
within the true spirit and scope of the invention.