Method and apparatus for acquiring a pilot signal in a CDMA receiver

A searcher receiver (114) includes a sample buffer (202) which stores signal samples loaded using a real time clock. A real time linear sequence generator (RT LSG) (206) stores an initial state and is clocked using the real time clock. The contents of the RT LSG are loaded into a non-real time linear sequence generator (NRT LSG) (208) when sample processing begins. Samples are correlated using a non-real time clock to allow signal processing to be uncoupled from the chip rate. The analog front end (108) may be powered down or tuned to another frequency during non-real time processing.

FIELD OF THE INVENTION 
The present invention relates generally to digital communication. More 
particularly, the present invention relates to a method and apparatus for 
pilot channel acquisition in a spread spectrum communication system such 
as a code division multiple access (CDMA) cellular telephone system. 
BACKGROUND OF THE INVENTION 
Direct sequence code division multiple access (DS-CDMA) communication 
systems have been proposed for use in cellular telephone systems with 
traffic channels located at 800 MHz and in the personal communication 
system (PCS) frequency band at 1800 MHz. In a DS-CDMA system, all base 
stations in all cells may use the same radio frequency for communication. 
One known DS-CDMA system is defined in Telecommunications Industry 
Association/Electronic Industry Association (TIA/EIA) Interim Standard 
IS-95, "Mobile Station-Base Station Compatibility Standard for Dual-Mode 
Wideband Spread Spectrum Cellular System" (IS-95). 
In addition to traffic channels, each base station broadcasts a pilot 
channel, a synchronization channel, and a paging channel. The pilot 
channel or pilot signal is a pseudorandom noise or PN code. The pilot 
channel is commonly received by all mobile stations within range and is 
used by the mobile station for identifying the presence of a CDMA system, 
initial system acquisition, idle mode hand-off, identification of initial 
and delayed rays of communicating and interfering base stations, and for 
coherent demodulation of the synchronization, paging, and traffic 
channels. 
The pilot signal transmitted by each base station in the system uses the 
same PN code but with a different phase offset. The base stations are 
uniquely identified by using a unique starting phase or starting time for 
the PN sequences. For example, in IS-95, the sequences are of length 
2.sup.15 chips and are produced at a chip rate of 1.2288 Mega-chips per 
second and thus repeat every 262/3 milliseconds. The minimum time 
separations are 64 chips in length allowing a total of 512 different PN 
code phase assignments for the base stations. 
At the mobile station, the received RF signals include pilot, 
synchronization, paging, and traffic channels from all nearby base 
stations. The mobile station must identify all the pilot signals that are 
receivable including the pilot signal from the base station with the 
strongest pilot channel. In prior art mobile stations, a correlator has 
been used as a receiver pilot searching element to serially search for the 
PN phases of the receivable pilots. The received PN phase is correlated 
with system PN codes generated in the mobile station. Knowledge of the 
correct PN phases of the base site(s) with which the mobile station 
communicates allows the coherent detection of all the other channels 
transmitted by the base station. Incorrect PN phases will produce a 
minimal output from the correlator. 
Because the PN sequence phase space is large, the prior art serial, real 
time, correlation technique has taken a prohibitively long time to 
correctly locate pilot signal energy. At a minimum, with strong signals, 
system acquisition upon powering up the mobile station can take up to 2.5 
seconds or more. With no receivable pilots present, the mobile station 
will continue to search the entire phase space of the PN sequences until a 
system time out occurs, which may be 15 seconds. Then the mobile station 
moves to another RF frequency and again attempts to acquire the CDMA 
system. The searching process is repeated on subsequent frequencies until 
a pilot signal is found. 
The long time delay in system acquisition is inconvenient and undesirable 
for most users. A user turning on a radiotelephone expects to be able to 
use the radiotelephone immediately, with minimal delay. A delay of even 
2.5 seconds is too long for many users and longer delays could have 
serious consequences, for example, for emergency "911" calls. 
The prior art pilot channel searching method creates further limitations 
for all of the other uses of the pilot channel after initial system 
acquisition. Typical DS-CDMA mobile station receivers utilize a rake 
receiver having three or more independently controlled fingers which are 
time aligned to the correct PN sequence phases as determined by the 
receiver pilot phase searching element. The rake fingers are normally 
assigned to the strongest rays received from all communicating base 
stations as determined by the receiver pilot phase searching element. Ray 
assignments are updated in a maintenance process using the pilot phase 
searching element information. 
If the pilot phase searching element is slow, resulting in slow maintenance 
of the assignment of the strongest rays to the rake fingers, the receiving 
performance of the mobile station is degraded under fading conditions. 
Under certain conditions called "rapid PN," there is a high percentage of 
dropped calls. The rapid PN problem occurs because the available PN pilot 
signals are changing so fast that prior art searching elements cannot keep 
up. 
Idle hand-off is the process of attaching to and listening to the paging 
channel of the base station with the strongest pilot as identified by the 
pilot searching element. When the mobile station receives a page or 
accesses the system to place a call, it is important that the mobile 
station is listening to the page from or tries to access the base station 
associated with the strongest received pilot. This requires a fast pilot 
phase searching element, particularly when the mobile station is in 
motion. 
The poor performance of the prior art searching mechanism also affects the 
soft handoff performance of the mobile station. When in a call on a 
traffic channel, the pilot searching element is used to maintain the 
proper rake finger assignments for optimum demodulation of the traffic 
channel and to identify interfering base sites. If an interfering base 
site is found, it is reported by the mobile station to the base site as a 
candidate for soft hand-off. Soft hand-off is a DS-CDMA system condition 
where a mobile station is communicating with more than one base site 
simultaneously. Pilot signals from adjacent base stations need not be 
closely located in the pilot phase space. Thus, in addition to speed, the 
searching element needs to be nimble, that is, able to look across the 
entire phase space as well as looking only at specific PN offsets. 
New requirements for mobile stations will require Mobile Assisted Hard 
Handoff, or MAHHO, capabilities. In MAHHO, the mobile station changes the 
frequency of the radio link as it is handed off from one base station to 
another. Due to the full duplex nature of the CDMA air interface, this 
requires breaking the radio link, going to another frequency, looking for 
pilot signals, returning to the original frequency and reacquiring the 
pilot to reestablish the link. The prior art searching element which 
requires 2.5 seconds to acquire a pilot is unsuitable for MAHHO purposes. 
Another limitation of the prior art involves slotted mode operation. For 
battery powered portable mobile stations it is also very important to 
conserve battery charge when waiting for pages. IS-95 provides a slotted 
mode that allows portable stations to power down except for the periods 
when their assigned paging slot information is transmitted by the base 
stations. The paging slot interval can be as short as 1.28 seconds, and 
periods of 1.28 seconds multiplied by powers of two can be used for more 
battery savings. During these intervals, the mobile station only needs to 
monitor the paging channel for up to 160 ms and "sleeps" in a low power 
mode the remainder of the time. 
When operating in slotted mode, a portable station may have to search the 
phase space of as many as twenty base stations every time it wakes up. To 
reliably receive the paging slot after waking up, the portable station 
must be listening to the base station which is providing adequate signal 
strength. When the mobile station is in motion, the correct base station 
to decode can easily change from one paging interval to the next paging 
interval. Therefore it is very important to have a fast pilot searching 
mechanism to identify the correct base station pilot before the start of 
the assigned paging slot. Using the prior art pilot searching mechanism 
requires the portable station to wake up well before the paging slot to 
allow sufficient time to sequentially search the PN sequence phase space. 
This negates a substantial part of the potential battery savings afforded 
by slotted mode. 
Accordingly there is a need for a fast and accurate pilot searching 
mechanism that will improve mobile station performance in the areas of 
DS-CDMA system identification (service detection), initial system 
acquisition, idle mode hand-off, soft hand-off, slotted mode operation, 
and identification of initial and delayed rays of communicating and 
interfering base stations for the purposes of coherent demodulation of the 
synchronization, paging, and traffic channels.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring now to FIG. 1, a communication system 100 includes a plurality of 
base stations such as base station 102 configured for radio communication 
with one or more mobile stations such as radiotelephone 104. The 
radiotelephone 104 is configured to receive and transmit direct sequence 
code division multiple access (DS-CDMA) signals to communicate with the 
plurality of base stations, including base station 102. In the illustrated 
embodiment, the communication system 100 operates according to TIA/EIA 
Interim Standard IS-95, "Mobile Station-Base Station Compatibility 
Standard for Dual-Mode Wideband Spread Spectrum Cellular System," 
operating at 800 MHz. Alternatively, the communication system 100 could 
operate in accordance with other DS-CDMA systems including PCS systems at 
1800 MHz or with any other suitable DS-CDMA system. 
The base station 102 transmits spread spectrum signals to the 
radiotelephone 104. The symbols on the traffic channel are spread using a 
Walsh code in a process known as Walsh covering. Each mobile station such 
as the radiotelephone 104 is assigned a unique Walsh code by the base 
station 102 so that the traffic channel transmission to each mobile 
station is orthogonal to traffic channel transmissions to every other 
mobile station. 
In addition to traffic channels, the base station 102 broadcasts a pilot 
channel, a synchronization channel and a paging channel. The pilot channel 
is formed using an all-zero data sequence that is covered by Walsh code 0, 
which consists of all zeros. The pilot channel is commonly received by all 
mobile stations within range and is used by the radiotelephone 104 for 
identifying the presence of a CDMA system, initial system acquisition, 
idle mode hand-off, identification of initial and delayed rays of 
communicating and interfering base stations, and for coherent demodulation 
of the synchronization, paging, and traffic channels. The synchronization 
channel is used for synchronizing mobile station timing to base station 
timing. The paging channel is used for sending paging information from the 
base station 102 to mobile stations including the radiotelephone 104. 
In addition to the Walsh covering, all channels transmitted by the base 
station are spread using a pseudorandom noise (PN) sequence, also referred 
to as the pilot sequence. The base station 102 and all base stations in 
the communication system 100 are uniquely identified by using a unique 
starting phase, also referred to as a starting time or phase shift, for 
the pilot channel sequence. The sequences are of length 2.sup.15 chips and 
are produced at a chip rate of 1.2288 Mega-chips per second and thus 
repeat every 262/3 milliseconds. The minimum permitted time separation is 
64 chips, allowing a total of 512 different PN code phase assignments. The 
spread pilot channel modulates a radio frequency (RF) carrier and is 
transmitted to all mobile stations including the radiotelephone 104 in a 
geographic area served by the base station 102. The PN sequence is complex 
in nature, comprising both in-phase (I) and quadrature (Q) components. It 
will be recognized by those ordinarily skilled in the art that all 
processing of the pilot signal described herein involves both I and Q 
components. 
The radiotelephone 104 comprises an antenna 106, an analog front end 108, a 
receive path including an analog to digital converter (ADC) 110, a rake 
receiver 112 and a searcher receiver 114, a controller 116, and a transmit 
path including a transmission path circuit 118 and a digital to analog 
converter 120. The antenna 106 receives RF signals from the base station 
102 and from other base stations in the vicinity. Some of the received RF 
signals are directly transmitted, line of sight rays transmitted by the 
base station. Other received RF signals are reflected or multipath rays 
and are delayed in time. 
Received RF signals are converted to electrical signals by the antenna 106 
and provided to the analog front end 108. The analog front end 108 filters 
the signals and provides conversion to baseband signals. The analog 
baseband signals are provided to the ADC 110, which converts them to 
streams of digital data for further processing. 
The rake receiver 112 includes a plurality of receiver fingers, including 
receiver finger 122, receiver finger 124 and receiver finger 126. In the 
illustrated embodiment, the rake receiver 112 includes three receiver 
fingers. However, any suitable number of receiver fingers could be used. 
The receiver fingers are of conventional design. Each receiver finger has 
a finger linear sequence generator (LSG) 128 used in detection of pilot 
signals in the receiver finger. 
The controller 116 includes a clock 134. The clock 134 controls timing of 
the radiotelephone 104. The controller 116 is coupled to other elements of 
the radiotelephone 104. Such interconnections are not shown in FIG. 1 so 
as to not unduly complicate the drawing figure. 
The searcher receiver 114 detects pilot signals received by the 
radiotelephone 104 from the plurality of base stations including the base 
station 102. The searcher receiver 114 despreads pilot signals using a 
correlator with PN codes generated in the radiotelephone 104 using local 
reference timing. After this despreading, the signal values for each chip 
period are accumulated over a preselected interval of time. This provides 
a coherent sum of chip values. This sum is compared against a threshold 
level. Sums exceeding the threshold level generally indicate that 
appropriate pilot signal timing has been determined. Structure and 
operation of the searcher receiver 114 will be discussed in detail below 
in conjunction with FIG. 2. 
Referring now to FIG. 2, the searcher receiver 114 includes a sample buffer 
202, a correlator 204, and a PN generator 205. The PN generator 205 
includes a real-time linear sequence generator (RT LSG) 206, a non-real 
time linear sequence generator (NRT LSG) 208, a mask circuit 210, a mask 
register 212, a register 214, a slew controller 216, a slew counter 217, a 
clock controller 218 and a clock divider 220. 
The searcher receiver 114 detects pilot signals to acquire system timing 
for the radiotelephone 104. In accordance with the present invention, the 
searcher receiver 114 samples the received signal at a first rate, storing 
a plurality of signal samples. The searcher receiver 114 then processes 
the plurality of signal samples at a second rate, the second rate being 
greater than the first rate, and identifies one or more signals based on 
the plurality of pilot signal samples. 
The sample buffer 202 collects a predetermined number of signal samples. 
The sample buffer 202 has an input 226 coupled to the ADC 110 and an 
output 224 coupled to the correlator 204. The ADC receives an analog 
signal s(t) from the analog front end 108 and converts the analog signal 
to digital samples. The ADC has a clock input 228 coupled to the clock 
controller 218 and produces one digital sample in response to each 
received clock signal. 
The clock controller 218 has an input 232 coupled to a clock input, a first 
output 233 coupled to the ADC 110, a second output 234 coupled to the 
clock divider 220, and a third output 236 coupled to the NRT LSG 208. The 
clock controller 218 produces clock signals at the first output 233 to 
provide a real time sample clock to the ADC 110. The clock controller 218 
produces clock signals at the second output 234 to provide a real time 
chip clock to the RT LSG 206. The real time chip clock increments the RT 
LSG 206 as samples are stored in the sample buffer 202. The clock 
controller 218 produces clock signals at the third output 236 to provide a 
non-real time chip clock. The clock input 230 receives clocking signals 
from any suitable source, such as the clock 134 of the controller 116. In 
the illustrated embodiment, the clock controller 218 provides the real 
time sample clock to the ADC 110 at a rate twice the chip rate of 1.2288 
Mega-chips per second. Other suitable sampling rates may be selected. 
As a result, during each chip time, two samples are stored in the sample 
buffer 202. The samples are stored sequentially, in first in, first out 
fashion. A read/write pointer 222 indicates the location in the sample 
buffer for reading and writing data. A total of 2N samples are stored, 
where N is the span of the sample buffer in chip intervals. Stated 
alternately, N is the correlation length and 2N is the buffer size. One 
example for the dimension of the sample buffer is 512. 
The samples stored in the sample buffer 202 represent the signal received 
at the radiotelephone 104 from any nearby base station, such as base 
station 102 (FIG. 1). The signal may contain a directly-received pilot 
signal or a multipath ray. The sample buffer 202 thus provides a buffer 
for storing a plurality of samples of a received signal. 
The RT LSG 206 is a conventional linear sequence generator which produces a 
pseudo-random sequence from a given starting point in response to a clock 
signal received at an input 240. The RT LSG 206 receives clock signals 
from the clock controller 218. These clock signals are thus real-time 
clock signals and the RT LSG generates a sequence of values in response to 
the real time clock signal. 
The NRT LSG 208 is a conventional LSG which produces a sequence identical 
to the sequence produced by the RT LSG 206 when loaded with the same state 
and clocked via input 242. In accordance with the present invention, the 
searcher receiver 114 loads the state of the RT LSG 206 into the NRT LSG 
208 at a particular point in time relative to storing the predetermined 
number of samples in the sample buffer 202. At substantially the same 
time, the contents of the RT LSG 206 are transferred into the register 214 
for subsequent use. The operation of loading the NRT LSG state from the RT 
LSG state at a specific point in time relative to filling the buffer 
provides a timing reference. From this timing reference, outputs from 
non-real time circuits can be mapped to real-time timing adjustments using 
the slew counter 217. The register 214 thus stores the initial state of 
the NRT LSG 208 to allow the NRT LSG to be reset to its initial reference 
value. 
The clock input 242 of the NRT LSG 208 is coupled to the second clock 
output 236 of the clock controller 218. In accordance with the present 
invention, the NRT LSG 208 is clocked at a rate different from and 
substantially faster than the RT LSG 206. Thus, the NRT LSG 208 increments 
in response to a non-real time clock signal. 
The mask circuit 210 employs a predetermined mask that, when Exclusive-ORed 
with the contents of the NRT LSG 208, yields the correct state of the PN 
generator 205 at a predetermined time in the future. The mask circuit 210 
is loaded with any mask stored in the mask register 212, such as mask 1, 
mask 2, . . . mask M. The masks correspond to individual phases of the 
phase space of the pilot signals in the communication system 100 (FIG. 1). 
The correlator 204 correlates the plurality of samples in the sample buffer 
202 and the sequence of values from the NRT LSG and produces a correlation 
result. In the illustrated embodiment, the correlator 204 includes a first 
correlator, including multiplier 250 and summer 252, and a second 
correlator, including a summer 256 and multiplier 258. The correlator 204 
also includes logic 254. The multiplier 250 and summer 252 produce a first 
correlation result based on even-numbered samples from the sample buffer 
202 and provide the first correlation result to the logic 254. The 
multiplier 258 and summer 256 produce a second correlation result based on 
odd-numbered samples from the sample buffer 202 and provide the second 
correlation result to the logic 254. In the illustrated embodiment, the 
second correlator, including multiplier 258, receives samples from the 
sample buffer 202 which are one sample (one-half chip time) later than 
samples received by the first correlator including multiplier 250 
It will be recognized that any number of sample phases could be processed 
in correlator 204 by varying the number of correlators and associated 
logic. Reducing from two phases to one phase by sampling once every chip 
time would reduce the hardware required by eliminating one correlator. On 
the other hand, increasing the number of phases would provide better time 
resolution for the correlation. 
The logic 254 compares the correlation result to a predetermined threshold 
and discards correlation results that do not exceed the threshold. 
Correlation results which at least exceed the threshold are stored as 
corresponding to possible correct pilot phases. Thus, the logic 254 
includes some memory for storing data. The stored correlation results are 
sorted to provide an indication of relative pilot phase correlation. 
The slew controller 216 controls slewing of the NRT LSG 208 to permit 
proper alignment of the NRT LSG with the RT LSG. Each time the NRT LSG 208 
is incremented relative to the RT LSG, the slew counter 217 is 
incremented. At the moment the RT LSG 206 state is loaded into the NRT LSG 
208, the two sequence generators are synchronized and the slew counter 217 
is initialized. As will be described below, they will subsequently become 
unsynchronized during searching operations. However, all that is necessary 
to reference back to real time is a count of the number of samples the NRT 
LSG has been shifted relative to the synchronization point. The slew 
counter 217 provides this count. The RT LSG 206 serves as a timing 
reference for maintaining a real time reference and is clocked 
continuously, at the chip rate. 
FIG. 3 is a flow chart illustrating a method for operating the 
radiotelephone 104 of FIG. 1 for acquiring a pilot signal in a CDMA 
receiver. The method begins at step 302. At step 304, the real time (RT) 
clock is enabled. The clock controller 218 provides a clock signal at the 
second output 234 (FIG. 2) at a rate twice the chip rate of 1.2288 
Mega-chips per second. This is the real time clock signal for the ADC 110. 
This clock signal is divided by the clock divider 220 to provide the real 
time clock signal for the RT LSG 206. At step 306 the RT LSG 206 is loaded 
with an initial value, initializing the timing reference. 
At step 308, an acquisition mask is loaded from the mask register 212. The 
acquisition mask is a mask suitable for initial acquisition of a pilot 
signal and is, for example, a zero shift mask which does not shift the 
contents of the NRT LSG 208. At step 310, an integration length and a 
window size are loaded. The window size, W, is the number of delays, in 
chip intervals, to process. In IS-95, the window size value is received by 
the radiotelephone 104 from the base station 102. A typical value for the 
window size is 60 chip intervals. 
The integration length is the number of samples summed by the summer 252. 
The integration length in the illustrated embodiment is equal to N, half 
the number of samples in the sample buffer 202, but may be any suitable 
value. In some instances, it is preferable to integrate over fewer than N 
samples. For example, if the analog front end 108 is not adequately tuned 
to the transmission frequency of the base station 102, there is a 
decorrelating effect by integrating or correlating over a large number of 
samples. In such an instance, the decorrelating effects are reduced by 
integrating over a smaller number of samples, such as N/2, N/4, etc. A 
first integration is performed, integrating over, for example the first 
N/2 samples, then followed by a second integration over the second N/2 
samples. These correlations can be performed without having to power up 
the RF components or having to again collect samples, since all samples 
are initially collected in the sample buffer 202. 
In FIG. 3, step 312 and step 324 are illustrated in dashed lines to 
particularly indicate that they are optional steps. In step 312, the 
radiotelephone 104 powers up a predetermined portion of the CDMA receiver. 
In the illustrated embodiment, power is provided to the radio frequency 
(RF) components of the radiotelephone 104 (FIG. 1). RF components include 
the analog front end 108 and the ADC 110. In step 324, after sample 
collection steps (step 314-step 322), the RF components are powered down. 
This feature allows the RF components, which consume relatively large 
amounts of power from the battery which powers the radiotelephone 104, to 
be energized only when they are needed, during sample collection, thus 
conserving battery charge. Steps 312 and 324 are optional in that they may 
not be used during all sequences through the flow diagram of FIG. 3. 
Additionally, while in a call, the radiotelephone can briefly tune to 
another frequency, collect a buffer of samples, retune to the original 
frequency and search for pilot energy on the collected samples. 
At step 314, a first sample is collected in the sample buffer 202. Clock 
signals are provided at twice the chip rate to the ADC 110 and two samples 
(corresponding to one chip) are sequentially loaded into the sample buffer 
202. At the time when the first sample is stored in the sample buffer 202, 
at step 316, the contents of the RT LSG 206 are loaded into the NRT LSG 
208. In step 318, additional samples are collected in the sample buffer 
202 by storing a pilot signal sample in the sample buffer 202 and the RT 
LSG 206 is clocked at step 320. At step 322, the sample buffer 202 is 
checked for a full condition. Control remains in the loop formed by step 
318, step 320 and step 322 until the condition is met. Alternatively, 
another condition is checked, such as collecting a predetermined number of 
samples or any other suitable condition. At step 324, power to RF 
components is optionally reduced or the RF is retuned. 
At step 326, a non-real time clock is enabled. The clock controller 218 
provides the non-real time clock to the NRT LSG 208. The non-real time 
clock rate may be any available clock rate or multiple thereof, but is 
preferably substantially faster than the real time clock rate used for 
clocking samples into the sample buffer 202. For example, in an IS-95 
system where the real-time clock is related to the chip rate of 1.2288 
Mega-chips per second, the non-real time clock rate might be 80 MHz. At 
step 328, the acquisition mask is loaded to the mask circuit 210. At step 
330, the samples in the sample buffer 202 are processed, a method which is 
illustrated in more detail in FIG. 4. The method of FIG. 3 ends at step 
332. 
Referring now to FIG. 4, a method for operating the radiotelephone 104 of 
FIG. 1 to process stored pilot signal samples is illustrated. The method 
begins at step 402. 
At step 404, the correlator 204 correlates stored samples in the sample 
buffer 202 and contents of the non-real time linear sequence generator 
(NRT LSG) 208. The correlation result from the summer 252 is provided to 
the logic 254 which determines if the correlation result exceeds a 
threshold, step 406. If not, control continues at step 412. If the result 
does exceed the threshold, the result is stored. In addition, the slew 
counter value contained in the slew counter 217 is stored, step 410. The 
slew counter value corresponds to the number of times the NRT LSG 208 has 
been incremented. At step 412, the NRT LSG 208 is incremented, 
establishing an NRT LSG alignment value for each correlation. Also, the 
slew counter 217 is incremented and a window size is decremented. At 414, 
the window size is checked and if the exit condition is not met, the 
method remains in the loop which includes step 404-step 414. The loop 
repetitively performs the correlation of stored samples and contents of 
the linear sequence generator, NRT LSG 208. 
In one embodiment, the invention provides an "early dump" capability. In 
this embodiment, the correlator 204 correlates less than a full buffer of 
samples, for example, N/2 samples. The result of this correlation is 
compared to a threshold. If the correlation exceeds the threshold, the 
remainder of the samples in the sample buffer are correlated and operation 
continues as described above. In a two phase correlator, as is shown in 
FIG. 2, if either of the two correlation values exceeds the threshold, 
processing continues as above. However, if both correlation results are 
less than the threshold, the correlation is aborted, the NRT LSG 208 is 
incremented and the slew counter 217 is incremented and processing 
continues. Early dump capability improves the performance of the searcher 
receiver by allowing PN phases which contain little or no energy to be 
quickly discarded without performance of a full correlation. 
At step 416, the logic 254 selects the set of best correlations for 
assigning at least one receiver finger of the rake receiver to the 
detected pilot signals. The set of best correlations may have one or a 
plurality of correlations, depending on the correlation results and the 
number of rake receiver fingers to be assigned. Based on the correlation 
results, the logic 254 selects a number of optimum pilot signals 
corresponding to receiver fingers in the rake receiver 112 to be assigned. 
If a single ray has been located, either a directly-received ray from a 
base station or a multipath ray, a single receiver finger of the rake 
receiver 112 (FIG. 1) will be assigned, step 418. If multiple rays have 
been located from different base stations (with different pilot signal 
phases), the multiple rays will be assigned to multiple fingers of the 
rake receiver 112. Similarly, if all fingers of the rake receiver 112 have 
been previously assigned, as part of the maintenance process, the logic 
254 will determine if a finger should be reassigned to a different ray 
based on the correlation results. Thus, step 418 includes assigning 
receiver fingers based on the correlation results. 
The process of assigning fingers includes the slewing of the finger LSGs to 
bring them into alignment with the pilots and multipath components of 
interest. The slew counter value stored in step 410 for a pilot or path 
which exceeded threshold in step 405 provides the time difference in 1/2 
chips between the mobile's timing and that of the pilot or path of 
interest. At step 420, the slew counter value stored by the logic 254 is 
provided to the linear sequence generator 128 of the receiver finger which 
is being assigned to the detected pilot signals. Thus, the searcher 
receiver 114 provides an NRT LSG alignment value which corresponds to one 
of the set of best correlations to the finger linear sequence generator 
associated with the at least one receiver finger. The at least one 
receiver finger uses the slew counter value to align its finger LSG to the 
timing of the detected pilot signal and begins detecting the pilot signal. 
The method for processing samples ends at step 422. 
Referring now to FIG. 5, a method for operating the radiotelephone 104 of 
FIG. 1 for maintaining finger assignments is illustrated. The method 
begins at step 502. At step 504, the mask for a pilot of interest is 
loaded from the mask register 212 to the mask circuit 210. Also, 
integration length and window size are loaded. At step 506, RF components 
are powered up if necessary. If a search or another frequency is required, 
the radio can be tuned to the new frequency. 
At step 508, a number of sample pairs equal to one-half the window size 
(W/2) are collected in the sample buffer 202. Sampling is done using the 
real time clock. Sample pairs are collected because, as noted above, the 
pilot signal is sampled at twice the chip rate. Each sample pair 
corresponds to one chip. Other numbers of chips or samples are collected 
in the sample buffer 202 depending on the particular implementation. 
At step 510, the contents of the RT LSG 206 are loaded into the NRT LSG 
208. By storing W/2 pairs of samples prior to loading the state of the RT 
LSG, the NRT LSG is effectively advanced by half the window size in chips 
with respect to the first sample. Now, if W correlations are performed 
sequentially, beginning with the initial state and incrementing one chip 
per correlation, the search will span -W/2 to +W/2. Once the NRT LSG has 
been loaded in step 510, the remaining N-(W/2) samples must be collected, 
step 511. After the samples are collected, RF components are optionally 
powered down in step 512 or retuned to the original frequency. 
At step 514, the NRT clock rate is selected and applied to the NRT LSG 208 
for processing the samples. The mask of interest is applied to the 
contents of the NRT LSG 208 at step 516 and at step 518, the samples are 
processed. During step 518, steps corresponding to step 402-step 422 in 
FIG. 4 are performed. After the buffer full of samples is processed, at 
step 520 it is determined if there are more pilot signals of interest. For 
example, after waking from a slotted mode sleep time, the searcher 
receiver 114 has a list of active pilots, a list of candidate pilots and a 
list of neighbor pilots to scan for pilot signal energy to locate suitable 
pilot signals for finger assignment. If there are more pilots of interest, 
at step 522, the initial state of the NRT LSG 208 which was stored in the 
register 214 is loaded into the NRT LSG 208, resetting the NRT LSG 208 to 
an initial condition and a new mask is loaded in the mask circuit 210, 
shifting the NRT LSG to a next state. The next state of the NRT LSG 
corresponds to to a next pilot of interest. Other suitable ways of 
shifting the NRT LSG state include calculating the next state of the NRT 
LSG and incrementing or decrementing the NRT LSG to produce the next state 
of the NRT LSG. Also, at step 522, the read/write pointer 222 of sample 
buffer 202 is reset to 0, and the slew counter 217 is reset. This 
corresponds to resetting the NRT LSG to an initial condition using the 
timing reference value. The mask for the next pilot of interest is loaded 
at step 516. Step 516-step 522 are repeated until all pilots of interest 
have been processed. The method ends at step 524. 
As can be seen from the foregoing, the present invention provides a method 
and apparatus for rapidly acquiring pilot signals in a CDMA receiver. 
Since multiple samples are collected in a buffer, signal processing can be 
uncoupled from the chip rate and pilot signal acquisition decisions can be 
made on a much faster basis, using a non-real time clock. Since the 
operation of the receiver searcher is faster, delays in pilot channel 
acquisition are substantially eliminated, also eliminating problems such 
as rapid PN. In slotted mode operation, the radiotelephone need only 
awaken long enough before its assigned slot for the rapid pilot 
acquisition to occur. Pilot channel maintenance is also more rapid, 
improving the reliability of idle handoff and soft handoff. Since samples 
are buffered, after collection of samples, the analog front end is free to 
tune to another frequency during Mobile Assisted Hard Hand Off (MAHHO). 
While a particular embodiment of the present invention has been shown and 
described, modifications may be made. For example, method steps may be 
rearranged, substituted and deleted as appropriate. 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.