Method and apparatus for modulating signal waveforms in a CDMA communication system

An improved system and method for transmitting information within a digital communication system is disclosed herein. A modulation system for use in a digital cellular communication system is described in which information is exchanged among a plurality of mobile users, via at least one cell-site, using a set of orthogonal binary codes. The modulation system allows the information carrying capacity of signal transmission on the mobile-to-cell communication link, i.e., the "reverse" link, of the communication system to be significantly improved. The modulation system receives an input information signal an generates an orthogonal sequence signal in response thereto. The orthogonal sequence signal corresponds to a selected one of a plurality of orthogonal binary chip sequences derived from a set of orthogonal binary codes. The orthogonal binary sequences are constructed such that a selected chip at a predefined sequence position within each of the orthogonal binary sequences is of the same binary value. The selected orthogonal sequence signal is combined with a control signal to provide a punctured sequence signal. This combination is performed such that the punctured sequence signal includes a control chip at the predefined sequence position having a value determined by the control signal. The punctured sequence signal may then be transmitted via a carrier signal. In a particular spread spectrum implementation, the punctured sequence signal is combined with a pseudo-noise (PN) signal in order to generate a carrier modulation signal.

BACKGROUND OF THE INVENTION 
I. Field of the Invention 
The present invention relates to communication systems, and, more 
particularly, to a novel and improved method and apparatus for 
communicating information in a spread spectrum communication system. 
II. Description of the Related Art 
Communication systems have been developed to allow transmission of 
information signals from a source location to a physically distinct user 
destination. Both analog and digital methods have been used to transmit 
such information signals over communication channels linking the source 
and user locations. Digital methods tend to afford several advantages 
relative to analog techniques, including, for example, improved immunity 
to channel noise and interference, increased capacity, and improved 
security of communication. 
In transmitting an information signal from a source location over a 
communication channel, the information signal is first converted into a 
form suitable for efficient transmission over the channel. Conversion, or 
modulation, of the information signal involves varying a parameter of a 
carrier wave on the basis of the information signal in such a way that the 
spectrum of the resulting modulated carrier is confined within the channel 
bandwidth. At the signal reception location the original message signal is 
reproduced from the received modulated signal. Such reproduction is 
generally achieved by using an inverse of the modulation process employed 
by the source transmitter. 
Modulation also facilitates multiple-access, i.e., the simultaneous 
transmission of several signals over a common channel. Multiple-access 
communication systems will often include a plurality of remote subscriber 
units requiring intermittent service of relatively short duration rather 
than continuous access to the communication channel. Systems designed to 
enable communication over brief periods of time with a set of subscriber 
units have been termed multiple access communication systems. 
A particular type of multiple access communication system is known as a 
spread spectrum system. In spread spectrum systems, the modulation 
technique utilized results in a spreading of the transmitted signal over a 
frequency band wider than the bandwidth of the signal being transmitted. 
One type of multiple access spread spectrum system is a code division 
multiple access (CDMA) modulation system. Other multiple access 
communication system techniques, such as time division multiple access 
(TDMA), frequency division multiple access (FDMA) and AM modulation 
schemes such as amplitude companded single sideband are known in the art. 
However, the spread spectrum modulation technique of CDMA has significant 
advantages over these modulation techniques for multiple access 
communication systems. The use of CDMA techniques in a multiple access 
communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 
13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM 
USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the 
present invention. 
In the above-referenced U.S. Pat. No. 4,901,307, a multiple access 
technique is disclosed where a large number of mobile telephone system 
users each having a transceiver communicate through satellite repeaters or 
terrestrial base stations using CDMA spread spectrum communication 
signals. In using CDMA communications, the frequency spectrum can be 
reused multiple times thus permitting an increase in system user capacity. 
The use of CDMA results in a much higher spectral efficiency than can be 
achieved using other multiple access techniques. 
More particularly, communication in a CDMA system between a pair of 
locations is achieved by spreading each transmitted signal over the 
channel bandwidth by using a unique user spreading code. Specific 
transmitted signals are extracted from the communication channel by 
despreading the composite signal energy in the communication channel with 
the user spreading code associated with the transmitted signal to be 
extracted. 
An improved method for communicating information signals in a spread 
spectrum communication system was disclosed in U.S. Pat. No. 5,103,459, 
issued Apr. 7, 1992, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL 
WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", which is also assigned to 
the assignee of the present invention, and which is herein incorporated by 
reference. The CDMA system as disclosed in U.S. Pat. No. 5,103,459 (the 
'459 patent) contemplated spreading all signals transmitted by a cell or 
one of the sectors of the cell with an "outer" pseudonoise (PN) code for 
both the in-phase (I) and quadrature-phase (Q) channels. The signals were 
also spread with an inner orthogonal code generated by using Walsh 
functions. A signal addressed to a particular user was multiplied by the 
outer PN sequences and by a particular Walsh sequence, or sequence of 
Walsh sequences, assigned by the system controller for the duration of the 
user's telephone call. The same inner code was applied to both the I and Q 
channels resulting in a modulation which is effectively bi-phase for the 
inner code. Constructing PN sequences which are orthogonal reduces mutual 
interference between users, allowing higher capacity and better link 
performance. 
It is well known in the art that a set of n orthogonal binary sequences, 
each of length n, for n any power of 2 can be constructed, see Digital 
Communications with Space Applications, S. W. Golomb et al., 
Prentice-Hall, Inc, 1964, pp. 45-64. In fact, orthogonal binary sequence 
sets are also known for most lengths which are multiples of four and less 
than two hundred. One class of such sequences that is easy to generate is 
called the Walsh functions. 
A Hadamard matrix of order n can be defined recursively as follows: 
##EQU1## 
where H' denotes the logical complement of H, and H(1)=0. Thus, 
##EQU2## 
H(8) is as follows: 
##EQU3## 
A Walsh sequence is one of the rows of a Hadamard matrix. A Hadamard 
matrix of order n contains n sequences, each of length n bits. 
A Walsh function of order n (as well as other orthogonal functions) has the 
property that the cross-correlation between all the different sequences 
within the set is zero, provided that the sequences are time aligned with 
each other. This can be seen by noting that every sequence differs from 
every other sequence in exactly half of its bits. It should also be noted 
that there is always one sequence containing all zeroes and that all the 
other sequences contain half ones and half zeroes. 
Neighboring cells and sectors can reuse the Walsh sequences because the 
outer PN codes used in neighboring cells and sectors are distinct. Because 
of the differing propagation times for signals between a particular 
mobile's location and two or more different cells, it is not possible to 
satisfy the condition of time alignment required for Walsh function 
orthogonality for both cells at all times. Thus, reliance must be placed 
on the outer PN code to provide discrimination between signals arriving at 
the mobile unit from different cells. However, all the signals transmitted 
by a cell are orthogonal to each other and thus do not contribute 
interference to each other. This eliminates the majority of the 
interference in most locations, allowing a higher capacity to be obtained. 
In the system of the '459 patent Walsh functions were also employed to 
encode the channel data signals transmitted over both the cell-to-mobile 
link (i.e., the "forward" link) and the mobile-to-cell link (i.e., the 
"reverse" link). In the exemplary forward link numerology as disclosed 
therein, a total of 64 different Walsh sequences were available with three 
of these sequences dedicated to the pilot, sync and paging channel 
functions. In the sync, paging and voice channels, input data was 
convolutionally encoded and then interleaved as is well known in the art. 
Furthermore, the convolutional encoded data was also provided with 
repetition before interleaving as is also well known in the art. 
A similar 64-ary orthogonal signaling technique using Walsh functions is 
described with reference to the reverse link of the system of the '459 
patent. The message encoding and modulation process on the reverse link 
begins with a convolutional encoder of constraint length K=9 and code rate 
r=1/3. At a nominal data rate of 9600 bits per second, the encoder 
produces 28800 binary symbols per second. These are grouped into 
characters containing 6 symbols each at a rate of 4800 characters per 
second with there being 64 possible characters. Each character is encoded 
into a length 64 Walsh sequence containing 64 binary bits or "chips." 
The encoding method described with reference to the reverse link is, 
however, less than optimal in that certain information is redundantly 
carried by each 64 chip Walsh sequence. It is therefore an object of the 
invention to provide a Walsh encoding technique which improves information 
carrying capacity by reducing such redundant information transmission. 
SUMMARY OF THE INVENTION 
The present invention provides an improved system and method for 
transmitting information within a digital communication system. In an 
exemplary embodiment, the present invention is directed to a modulation 
system for use in a digital cellular communication system in which 
information is exchanged among a plurality of mobile users, via at least 
one cell-site, using a set of orthogonal binary codes. 
The present invention contemplates improving the information carrying 
capacity of signal transmission on the mobile-to-cell communication link, 
i.e., the "reverse" link, an involves generating an orthogonal sequence 
signal based on the value of a data signal. The orthogonal sequence signal 
corresponds to a selected one of a plurality of orthogonal binary chip 
sequences derived from a set of orthogonal binary codes. In a preferred 
embodiment the orthogonal binary sequences are constructed such that a 
selected chip at a predefined sequence position within each of the 
orthogonal binary sequences is of the same binary value. 
In accordance with the invention, the orthogonal sequence signal is 
combined with a control signal to provide a punctured sequence. This 
combination is performed such that the punctured sequence includes a 
control chip at the predefined sequence position having a value determined 
by the control signal. The punctured sequence may then be transmitted via 
a carrier signal. In a particular spread spectrum implementation, the 
punctured sequence signal is combined with a pseudo-noise (PN) signal in 
order to generate a carrier modulation signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
I. System Overview 
An exemplary cellular subscriber communication system in which the present 
invention may be embodied is illustrated in FIG. 1. The system of FIG. 1 
utilizes spread spectrum modulation techniques in communication between 
users of the mobile units (e.g., mobile telephones), and the cell-sites. 
In FIG. 1, system controller and switch 10 typically includes interface 
and processing circuitry for providing system control to the cell-sites. 
When the system of FIG. 1 is configured to process telephone calls, 
controller 10 operates to route telephone calls from the public switched 
telephone network (PSTN) to the appropriate cell-site for transmission to 
the appropriate mobile unit. In this instance controller 10 also functions 
to route calls from the mobile units, via at least one cell-site, to the 
PSTN. Controller 10 may connect calls between mobile users via the 
appropriate cell-sites since the mobile units do not typically communicate 
directly with one another. 
Controller 10 may be coupled to the cell-sites by various means such as 
dedicated telephone lines, optical fiber links or microwave communication 
links. In FIG. 1, two such exemplary cell-sites 12 and 14, along with 
mobile units 16 and 18, are illustrated. Cell-sites 12 and 14 as discussed 
herein and illustrated in the drawings are considered to service an entire 
cell. However it should be understood that the cell may be geographically 
divided into sectors with each sector treated as a different coverage 
area. Accordingly, handoffs are made between sectors of a same cell as is 
described herein for multiple cells, while diversity may also be achieved 
between sectors as is for cells. 
In FIG. 1, arrowed lines 20a-20b and 22a-22b respectively define the 
possible communication links between cell-site 12 and mobile unit 16 and 
18. Similarly, arrowed lines 24a-24b and 26a-26b respectively define the 
possible communication links between cell-site 14 and mobile units 16 and 
18. Cell-sites 12 and 14 nominally transmit using equal power. 
The cell-site service areas or cells are designed in geographic shapes such 
that the mobile unit will normally be closest to one cell-site, and within 
one cell sector should the cell be divided into sectors. When the mobile 
unit is idle, i.e. no calls in progress, the mobile unit constantly 
monitors the pilot signal transmissions from each nearby cell-site, and if 
applicable from a single cell-site in which the cell is sectorized. As 
illustrated in FIG. 1, the pilot signals are respectively transmitted to 
mobile unit 16 by cell-sites 12 and 14 upon outbound or forward 
communication links 20a and 26a. Mobile unit 16 can determine which cell 
it is in by comparing signal strength in pilot signals transmitted from 
cell-sites 12 and 14. 
As is described in further detail below with reference to FIG. 3, voice 
transmission by each mobile unit is initiated by providing the mobile user 
analog voice signal to a digital vocoder. The vocoder output is then, in 
sequence, convolutional forward error correction (FEC) encoded, 64-ary 
orthogonal sequence encoded and modulated on a PN carrier signal. The 
64-ary orthogonal sequence is generated by a Walsh function encoder. The 
encoder is controlled by collecting six successive binary symbol outputs 
from the convolutional FEC encoder. The six binary symbol outputs 
collectively determine which of the 64 possible Walsh sequences will be 
transmitted. The Walsh sequence is 64 bits long. Thus, the Walsh "chip" 
rate must be 9600 * 3 * (1/6) * 64=307200 Hz for a 9600 bps data 
transmission rate. 
In the mobile-to-cell link (i.e., the "reverse" link) a common short PN 
sequence is used for all voice carriers in the system, while user address 
encoding is done using the user PN sequence generator. The user PN 
sequence is uniquely assigned to the mobile for at least the duration of 
each call. The user PN sequence is exclusive-OR'ed with the common PN 
sequences, which are length 32768 augmented maximal linear shift register 
sequences. The resulting binary signals then each bi-phase modulate a 
quadrature carrier, are summed to form a composite signal, are bandpass 
filtered, and translated to an IF frequency output. In the exemplary 
embodiment, a portion of the filtering process is actually carried out by 
a finite impulse response (FIR) digital filter operating on the binary 
sequence output. 
The modulator output is then power controlled by signals from the digital 
control processor and the analog receiver, converted to the RF frequency 
of operation by mixing with a frequency synthesizer which tunes the signal 
to proper output frequency, and then amplified to the final output level. 
The transmit signal is then passed on to a duplexer and an antenna. 
Although the manner in which the present invention may be embodied within 
the spread spectrum communication system of FIG. 1 is discussed in detail 
below in connection with FIGS. 3-5, the principles of the invention are 
first described with reference to the generalized representation of a 
digital communication system depicted in FIGS. 2A and 2B. 
II. Reverse Link Data Transmission 
FIG. 2A illustrates a preferred implementation of a mobile unit transmit 
modulator 30 in accordance with the invention. Data is provided in digital 
form from via input line 31 to an encoder 33, which is disposed to 
generate a code sequence output. In the preferred embodiment a 64-ary 
orthogonal signaling technique is utilized by the encoder to encode the 
input digital data. In 64-ary orthogonal signaling a set of 64 possible 
characters is available for data encoding, with each character being 
encoded into a length 64 sequence containing 64 binary bits or "chips". It 
is characteristic of 64-ary signaling that at least one chip of each 
orthogonal 64 chip sequence (e.g., Walsh sequence) is identical. As is 
described hereinafter, the present invention utilizes the identical chip 
of each sequence to transmit information over a "sub-channel" ancillary to 
the communication channel over which is transmitted the input digital 
data. 
Referring to FIG. 2A, the encoded data from encoder 33 is provided to one 
input of a modulator circuit 34 included within transmit modulator 35. The 
modulator circuit also receives a stream of sub-channel control data, 
which is combined with the data produced by the encoder 33, by overwriting 
the same chip within each code sequence. In accordance with the invention, 
this "puncturing" of the code sequence is performed in a manner which 
permits the control data to be transmitted without loss of information. 
More specifically, the present invention makes use of the fact that the 
first chip of each of each orthogonal Walsh sequence of order "n" is 
identical. It follows that a single bit of control data may be carried by 
each code sequence by overwriting the first chip. Modulator circuit 34, on 
the basis of timing information provided by a mobile unit microprocessor 
(not shown), replaces the first chip within each code sequence with a 
single bit of control data to be transmitted over the control sub-channel. 
In an exemplary embodiment the control sub-channel is utilized to send 
power control data to the cell-site, where such power control data is 
indicative of the level of power received by a given mobile unit. 
As is indicated by FIG. 2A, the transmit modulator 35 also includes a 
transmitter 36 coupled to the modulator circuit 34. A carrier signal 
generated within the transmitter 36 is modulated by the punctured sequence 
output by the modulator circuit. The resulting modulated carrier is then 
transmitted via antenna 37 to a cell-site station 40 (FIG. 2B). In an 
exemplary embodiment the information transmitted on the control 
sub-channel is extracted from the signal received at the cell-site in the 
manner described below with reference to FIG. 2B. 
III. Reverse Link Data Reception 
Referring to FIG. 2B, there is shown a block diagram of a cell-site 
receiver 40 operative to receive transmissions from the mobile units 
deployed within an associated cell or sector. Signals transmitted by 
mobile units and received on antenna 41 are provided to analog receiver 
42. Within receiver 42 the signals received from the antenna 41 are 
amplified, downconverted to an intermediate frequency, bandpass filtered, 
and sampled by an analog to digital converter. 
The digitized output from the receiver 42 is seen to be provided to 
demodulator/demultiplexer 44. Based on timing information provided by the 
cell-site control processor (not shown), the demodulator/demultiplexer 44 
identifies the first chip of each received code sequence and compares the 
identified value to a predefined threshold. Based on this comparison the 
demodulator/demultiplexer 44 assigns a logical value to the first chip 
within each sequence, thereby determining the values of each bit of 
control data received over the control sub-channel. As is indicated by 
FIG. 2B, the control sub-channel data extracted by 
demodulator/demultiplexer 44 is then provided as control data to the 
cell-site control processor. 
The digital code sequences generated within demodulator/demultiplexer 44 in 
response to the received signal energy are provided to a decoder 45 
operative to identify the orthogonal code sequences transmitted by a 
particular mobile unit. That is, the decoder 45 recovers the input digital 
data transmitted by the transmit modulator 30 (FIG. 2A) and provides the 
result to the cell-site control processor. 
In an alternate embodiment one or more other receiver systems (not shown) 
is employed to process the energy received over one or more other 
corresponding signal propagation paths. The recovered data output from 
each such receiver system is collectively provided to "diversity" combiner 
and decoder circuitry. Such circuitry is operative to combine the 
recovered data from each receiver based on the strength of the signal 
energy received over each signal propagation path. A detailed description 
of an exemplary spread spectrum diversity receiver system is described 
below in section V. 
IV. Reverse Link Spread Spectrum Data Transmission 
FIG. 3 illustrates a preferred implementation of a mobile unit transmit 
modulator 30. Data is provided in digital form from the user digital 
baseband circuitry to encoder 60 where in the exemplary embodiment it is 
convolutionally encoded. The output of encoder 60 is provided to 
interleaver 62 which in the exemplary embodiment is a block interleaver. 
The interleaved symbols are output from block interleaver 62 to Walsh 
encoder 64 of transmit modulator 50. Walsh encoder 64 utilizes the input 
symbols to generate a code sequence output. The Walsh sequence is provided 
to one input of multiplexer 65. As is described hereinafter, the 
multiplexer 65 also receives a stream of control data which is combined 
with the data produced by Walsh encoder 64 by overwriting selected Walsh 
chips within the output code sequence. In accordance with the invention, 
this "puncturing" of the code sequence is performed in a manner which 
permits the control data to be transmitted without loss of voice 
information. More specifically, the present invention makes use of the 
fact that the first chip of each of "n" Walsh sequence of order "n" is 
identically zero. It follows that a single bit of control data may be 
carried by each Walsh sequence by overwriting the first Walsh chip of each 
sequence. 
As noted above, in an exemplary implementation an accumulated set of six 
binary symbols determines which of the 64 possible Walsh sequences of 
length 64 Walsh chips are produced by Walsh encoder 64. FIG. 4 sets forth 
the set of code symbols corresponding to this set of 64 orthogonal Walsh 
sequences. As is evident upon inspection of FIGS. 4a-c, the first Walsh 
chip within each Walsh sequence of length 64 consists of logical zero. 
Multiplexer 65, on the basis of timing information provided by a mobile 
unit microprocessor (not shown), replaces the first Walsh chip within each 
Walsh sequence with a single bit of control data to be transmitted over 
the control sub-channel. In an exemplary embodiment, the control 
sub-channel is utilized to send power control data to the cell-site, where 
such power control data is indicative of the level of power received by a 
given mobile unit. One method of extracting the information transmitted on 
the control sub-channel of a given mobile unit from the encoded mobile 
unit voice data received at the cell-site is described below with 
reference to FIG. 5. 
It is anticipated that the control sub-channel could also be employed to 
facilitate acquisition and tracking of the PN spreading code received at 
the cell-site from the mobile unit. Specifically, if each bit of control 
data is uniformly selected to be zero, then the first Walsh chip of each 
"punctured" Walsh sequence obtained by modulating the code sequence with 
the control data will also be identically zero. This result follows since, 
as noted above, the first Walsh chip within each Walsh sequence consists 
of a logical zero. Since a set of four PN chips are used to modulate Walsh 
each chip, the four PN chips associated with the first Walsh chip of each 
Walsh code sequence will therefore be "transparent" to the cell-site. That 
is, by selecting each bit of the control data to be uniformly zero the 
values of the first 4 of the 256 PN chips utilized to modulate each 
64-chip Walsh sequence may be viewed at the cell-site. Knowledge of the 
first four PN chips associated with each Walsh sequence aids in, for 
example, PN code acquisition and/or tracking by enabling synchronization 
of the PN code generated within the cell-site with the PN code received 
from a given mobile unit. 
Turning again to FIG. 3, the punctured sequence output by multiplexer 65 is 
provided to one input of exclusive-OR gate 66. Transmit modulator 50 also 
includes PN generator 68, which receives the mobile unit address as an 
input in determining the output PN sequence. In an exemplary embodiment PN 
generator 68 generates a user PN sequence specifically identifying the 
mobile unit. A further attribute of PN generator 68, that is common to the 
PN generators of each mobile unit, is the use of a masking technique in 
generating the output user PN sequence. For example, a 42-bit mask is 
provided for that user with each bit of the 42-bit mask AND'ed with a bit 
output from each register of the shift registers that form the PN 
generator. The results of the mask and shift register bit AND operation 
are then exclusive-OR'ed together to form the PN generator output that is 
used as the user PN sequence. The output PN sequence of PN generator 68, 
the sequence PN.sub.U, is input to exclusive-OR gate 66. The punctured 
Walsh sequence representative of the mobile unit voice and control data is 
exclusive-OR'ed with the PN.sub.U sequence in exclusive-OR gate 66 and 
provided as an input to both of exclusive-OR gates 70 and 72. 
Transmit modulator 50 further includes PN generators 74 and 76 which 
respectively generate PN.sub.I and PN.sub.Q sequences. All mobile units 
use the same PN.sub.I and PN.sub.Q sequences, which are respectively 
associated with the In-Phase (I) and Quadrature-Phase (Q) communication 
channels. The other inputs of exclusive-OR gates 70 and 72 are 
respectively provided with the PN.sub.I and PN.sub.Q sequences output from 
PN generators 74 and 76. The sequences PN.sub.I and PN.sub.Q are 
exclusive-OR'ed in the respective exclusive-0R gates, with the output 
being provided to a mobile unit transmit power control network (not 
shown). 
In the exemplary embodiment, the reverse link uses rate r=1/3 convolutional 
code with constraint length K=9. The generators for the code are G1=557 
(octal), G2=663 (octal), and G3=711 (octal). Code repetition is used to 
accommodate four different data rates produced by the vocoder on a 20 msec 
frame basis. However, the repeated code symbols are not transmitted over 
the air at lower energy levels, rather only one code symbol of a 
repetition group is transmitted at the nominal power level. In conclusion, 
the code repetition in the exemplary embodiment is used merely as an 
expedient to fit the variable data rate scheme in the interleaving and 
modulation structure as it will be shown in the following paragraphs. 
A block interleaver spanning 20 msec, exactly one vocoder frame, is used on 
the reverse link. The number of code symbols in 20 msec, assuming a data 
rate of 9600 bps and a code rate r=1/3, is 576. The code symbols are 
written into the interleaver memory array by rows and read out by columns. 
As noted above, the modulation format is 64-ary orthogonal signalling. In 
other words, the interleaved code symbols are grouped into groups of six 
to select one out of 64 orthogonal waveforms. 
The data modulation time interval is equal to 208.33 sec, and is referred 
to as a Walsh symbol interval. At 9600 bps, 208.33 sec corresponds to 2 
information bits and equivalently to 6 code symbols at a code symbol rate 
equal to 28800 sps. The Walsh symbol interval is subdivided into 64 equal 
length time intervals, referred to as Walsh chips, each lasting 
208.33/64=3.25 sec. The Walsh chip rate is then 1/3.25 sec=307.2 kHz. For 
an exemplary PN spreading rate of 1.2288 MHz, there are exactly 4 PN chips 
per Walsh chip. 
V. Reverse Link Spread Spectrum Data Reception 
Referring to FIG. 5, there is shown a block diagram of a cell-site receiver 
operative to receive transmissions from the mobile units deployed within 
an associated cell or sector. Signals transmitted by mobile units and 
received on antenna 90 are provided to analog receiver 92, which is seen 
to include a downconverter 100. Downconverter 100 is comprised of RF 
amplifier 102 and mixer 104. The received signals are provided as an input 
to RF amplifier 102 where they are amplified and output to an input to 
mixer 104. Mixer 104 is provided another input, that being the output from 
frequency synthesizer 106. The amplified RF signals are translated in 
mixer 104 to an IF frequency by mixing with the frequency synthesizer 
output signal. 
The IF signals are then output from mixer 104 to bandpass filter (BPF) 108, 
typically a Surface Acoustic Wave (SAW) filter having a passband of 1.25 
MHz, where they are bandpass filtered. The filtered signals are output 
from BPF 108 to IF amplifier 110 where the signals are amplified. The 
amplified IF signals are output from IF amplifier 110 to analog to digital 
(A/D) converter 112 where they are digitized at a 9.8304 MHz clock rate 
which is exactly 8 times the PN chip rate. Although A/D converter 112 is 
illustrated as part of receiver 92, it could instead be a part of the data 
and searcher receivers. The digitized IF signals are output from (A/D) 
converter 112 to data receiver 116, optional data receiver 118 and 
searcher receiver 120. The signals output from receiver 92 are I and Q 
channel signals derived from mobile unit transmissions over the I and Q 
channels. 
Although as illustrated in FIG. 5 with A/D converter 112 being a single 
device, with later splitting of the I and Q channel signals, it is 
envisioned that channel splitting may be done prior to digitizing with two 
separate A/D converters provided for digitizing the I and Q channels. 
Schemes for the RF-IF-Baseband frequency downconversion and analog to 
digital conversion for I and Q channels are well known in the art. 
On the reverse link, the mobile unit does not transmit a pilot signal that 
can be used for coherent reference purposes in signal processing at the 
cell-site. Rather, in the 64-ary orthogonal signaling process employed on 
the reverse link the mobile unit transmitted symbols are encoded into one 
of 64 different binary sequences. The set of sequences chosen are known as 
Walsh functions. The optimum receive function for the Walsh function m-ary 
signal encoding is the Fast Hadamard Transform (FHT). 
As is indicated by FIG. 5, searcher receiver 120 receives the signals 
output from analog receiver 92. The searcher receiver 120 is used at the 
cell-site to scan the time domain about the received signal to ensure that 
the associated digital data receiver 116, and data receiver 118 if used, 
are tracking and processing the strongest available time domain signal. 
The searcher receiver 120 provides a signal to a control processor within 
controller 10, which in turn generates signals used by digital data 
receivers 116 and 118 for selecting the appropriate received signal for 
processing. In order to decode the spread spectrum signals transmitted to 
the particular cell-site receiver through which the mobile unit 
communicates, the proper PN sequences must be generated. Further details 
on the generation of the mobile unit signals are discussed later herein. 
In FIG. 5 optional digital data receiver 118 may be included for improved 
performance of the system. The structure and operation of this receiver is 
similar to that described below with reference to the data receiver 116. 
Receiver 118 may be utilized at the cell-site to obtain additional 
diversity modes. This additional data receiver alone or in combination 
with additional receivers can track and receive other possible delay paths 
of mobile unit transmitted signals. Optional additional digital data 
receivers such as receiver 118 provide additional diversity modes which 
are extremely useful in those cell-sites which are located in dense urban 
areas where many possibilities for multipath signals occur. 
As illustrated in FIG. 5, receiver 116 includes two PN generators, PN 
generators 120 and 122, which generate two different short code PN 
sequences of the same length. These two PN sequences are common to those 
of all cell-site receivers and all mobile units with respect to the outer 
code of the modulation scheme as discussed in further detail later herein. 
PN generators 120 and 122 thus respectively provide the output sequences, 
PN.sub.I and PN.sub.Q. The PN.sub.I and PN.sub.Q sequences are 
respectively referred to as the In-Phase (I) and Quadrature (Q) channel PN 
sequences. 
The two PN sequences, PN.sub.I and PN.sub.Q, are generated by different 
polynomials of degree 15, augmented to produce sequences of length 32768 
rather than 32767 which would normally be produced. For example, the 
augmentation may appear in the form of the addition of a single zero to 
the run of fourteen 0's in a row which appears one time in every 
maximal-length linear feedback shift register of degree 15. In other 
words, one state of the PN generator would be repeated in the generation 
of the sequence. Thus the modified sequence contains one run of fifteen 
1's and one run of fifteen 0's. Such a PN generator circuit is disclosed 
in U.S. Pat. No. 5,228,054, entitled "POWER OF TWO LENGTH PSEUDO-NOISE 
SEQUENCE GENERATOR WITH FAST OFFSET ADJUSTMENTS", which is assigned to the 
assignee of the present invention. 
In the exemplary embodiment receiver 116 also includes a long code PN 
generator 124 which generates a PN.sub.U sequence corresponding to a PN 
sequence generated by the mobile unit in the mobile-to-cell (i.e., the 
reverse) link. PN generator 124 can be a maximal-length linear feedback 
shift register that generates a user PN code that is very long (e.g., 
242-1), time shifted in accordance with an additional factor such as the 
mobile unit address or user ID to provide discrimination among users. Thus 
the cell-site received signal is modulated by both the long code PNU 
sequence and the short code PN.sub.I and PN.sub.Q sequences. In the 
alternative, a non-linear encryption generator, such as an encryptor using 
the data encryption standard (DES) to encrypt a 64-symbol representation 
of universal time using a user specific key, may be utilized in place of 
PN generator 124. 
The PN.sub.U sequence output from PN generator 124 is exclusive-OR'ed with 
the PN.sub.I and PN.sub.Q sequences respectively in exclusive-OR gates 126 
and 128 to provide the sequences PN.sub.I ' and PN.sub.Q '. 
The sequences PN.sub.I ' and PN.sub.Q ' are provided to PN QPSK correlator 
130 along with the I and Q channel signals output from receiver 92. 
Correlator 130 is utilized to correlate the I and Q channel data with the 
PN.sub.I ' and PN.sub.Q 'sequences. The correlated I and Q channel outputs 
of correlator 130 are respectively provided to accumulators 132 and 134 
where the symbol data is accumulated over a period of four PN chips. 
Again, in the exemplary embodiment the use of a PN spreading rate of 
1.2288 MHz results in the existence of exactly 4 PN chips per Walsh chip. 
It follows that the periodic outputs of accumulators 132 and 134 
correspond to successive Walsh chips within the received Walsh sequence. 
The outputs of accumulators 132 and 134 are provided as inputs to a 
detector/demultiplexer 135. Based on timing information provided by the 
cell-site control processor, the detector/demultiplexer 135 identifies the 
outputs from accumulators 132 and 134 corresponding to the first Walsh 
chip of each 64-chip Walsh sequence. The accumulator outputs corresponding 
to the first Walsh chip are then combined, and the result compared with a 
predefined threshold. Based on this comparison detector/demultiplexer 135 
makes a "hard" decision as to the logical value of the first Walsh chip 
within each Walsh sequence. The result of each such hard decision 
determines the value of the control data bit carried by the received Walsh 
sequences. As is indicated by FIG. 5, the control sub-channel data 
extracted by detector/demultiplexer 135 is then provided to the cell-site 
control processor. 
The outputs of accumulators 132 and 134 corresponding to the remaining 63 
Walsh chips of each Walsh sequence are forwarded by detector/demultiplexer 
135 to a Fast Hadamard Transform (FHT) processor 136. FHT processor 136 
produces a set of 64 coefficients for every 6 symbols. Each one of the 64 
coefficients is representative of the energy of a corresponding one of the 
64 Walsh functions. The output from FHT 136 is provided to diversity 
combiner and decoder circuity (not shown) operative to identify which one 
of 64 Walsh functions of length 64 has been received from a given mobile 
unit. 
One or more other receiver systems (not shown) may also be employed to 
process the received signals in a manner similar to that discussed with 
respect to the first receiver system of FIG. 5. The weighted 64 symbols 
output from receiver 116 and the one or more other receiver systems are 
collectively provided to diversity combiner and decoder circuitry. The 
decoder circuitry will preferably include an adder which adds the weighted 
64 coefficients from receiver 116 to the weighted 64 coefficients from any 
other receiver systems utilized. The resulting 64 coefficients are 
compared with one another in order to determine the largest coefficient. 
The magnitude of the comparison result, together with the identity or the 
largest of the 64 coefficients, is used to determine a set of decoder 
weights and symbols for use within a Viterbi algorithm decoder. 
The Viterbi decoder (not shown) is of a type capable of decoding data 
encoded at the mobile unit with a constraint length K=9, and of a code 
rate r=1/3. Periodically, nominally 1.25 msec, a signal quality estimate 
is obtained and transmitted as a mobile unit power adjustment command 
along with data to the mobile unit. This quality estimate is the average 
signal-to-noise ratio over the 1.25 msec interval. 
Each data receiver tracks the timing of the received signal it is 
receiving. This is accomplished by the well known technique of correlating 
the received signal by a slightly early local reference PN and correlating 
the received signal with a slightly late local reference PN. The 
difference between these two correlations will average to zero if there is 
no timing error. Conversely, if there is a timing error, then this 
difference will indicate the magnitude and sign of the error and the 
receiver's timing is adjusted accordingly. 
The previous description of the preferred embodiments is provided to enable 
any person skilled in the art to make or use the present invention. The 
various modifications to these embodiments will be readily apparent to 
those skilled in the art, and the generic principles defined herein may be 
applied to other embodiments without the use of the inventive faculty. 
Thus, the present invention is not intended to be limited to the 
embodiments shown herein but is to be accorded the widest scope consistent 
with the principles and novel features disclosed herein.