Programmable matched filter searcher

Details Programmable matched filter searcher Programmable matched filter searcher

1999-03-31

2002-03-26

Ghebretinsae, Temesghen (Department: 2631)

Pulse or digital communications

Spread spectrum

Direct sequence

C375S343000, C708S314000

Reexamination Certificate

active

06363108

ABSTRACT:

BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for detecting a pilot signal with a programmable matched filter searcher.
II. Description of the Related Art
Pseudorandom noise (PN) sequences are commonly used in direct sequence spread spectrum communication systems such as that described in the IS-95 over the air interface standard and its derivatives such as IS-95-A and ANSI J-STD-008 (referred to hereafter collectively as the IS-95 standard) promulgated by the Telecommunication Industry Association (TIA) and used primarily within cellular telecommunications systems. The IS-95 standard incorporates code division multiple access (CDMA) signal modulation techniques to conduct multiple communications simultaneously over the same RF bandwidth. When combined with comprehensive power control, conducting multiple communications over the same bandwidth increases the total number of calls and other communications that can be conducted in a wireless communication system by, among other things, increasing the frequency reuse in comparison to other wireless telecommunication technologies. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both of which are assigned to the assignee of the present invention and incorporated by reference herein.
FIG. 1
provides a highly simplified illustration of a cellular telephone system configured in accordance with the use of the IS-95 standard. During operation, a set of subscriber units
10
a-d
conduct wireless communication by establishing one or more RF interfaces with one or more base stations
12
a-d
using CDMA modulated RF signals. Each RF interface between a base station
12
and a subscriber unit
10
is comprised of a forward link signal transmitted from the base station
12
, and a reverse link signal transmitted from the subscriber unit. Using these RF interfaces, a communication with another user is generally conducted by way of mobile telephone switching office (MTSO)
14
and public switch telephone network (PSTN)
16
. The links between base stations
12
, MTSO
14
and PSTN
16
are usually formed via wire line connections, although the use of additional RF or microwave links is also known.
Each subscriber unit
10
communicates with one or more base stations
12
by utilizing a rake receiver. A RAKE receiver is described in U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the present invention and incorporated herein by reference. A rake receiver is typically made up of one or more searchers for locating direct and multipath pilot from neighboring base stations, and two or more fingers for receiving and combining information signals from those base stations. Searchers are described in co-pending U.S. patent application Ser. No. 08/316,177, entitled “MULTIPATH SEARCH PROCESSOR FOR SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS”, filed Sep. 30, 1994, assigned to the assignee of the present invention and incorporated herein by reference.
Inherent in the design of direct sequence spread spectrum communication systems is the requirement that a receiver must align its PN sequences to those of the base station. In IS-95, each base station and subscriber unit uses the exact same PN sequences. A base station distinguishes itself from other base stations by inserting a unique offset in the generation of its PN sequences. In IS-95 systems, all base stations are offset by an integer multiple of 64 chips. A subscriber unit communicates with a base station by assigning at least one finger to that base station. An assigned finger must insert the appropriate offset into its PN sequence in order to communicate with that base station. It is also possible to differentiate base stations by using unique PN sequences for each rather than offsets of the same PN sequence. In this case, fingers would adjust their PN generators to produce the appropriate PN sequence for the base station to which it is assigned.
Subscriber units locate base stations by utilizing searchers.
FIG. 2
depicts a common type of serial correlator used for searching in a subscriber unit. This searcher is described in U.S. Pat. No. 5,644,591, entitled “METHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATIONS SYSTEM”, issued Jul. 1, 1997, assigned to the assignee of the present invention and incorporated herein by reference.
In
FIG. 2
, antenna
20
receives a signal comprising pilot signal transmissions from one or more base stations. The signal is downconverted and amplified in receiver
21
, which generates an in-phase (I) and quadrature (Q) component of the received signal and delivers them to despreader
22
. I and Q PN sequence generator
23
produces the proper I and Q PN sequences for a candidate offset as directed by searcher controller
27
. Despreader
22
receives the I and Q PN sequences and despreads the I and Q received signals, passing the results to coherent accumulators
24
and
25
. These accumulators integrate the amplitudes of the despread I and Q signals for a period of time specified by searcher controller
27
. Coherent accumulators
24
and
25
sum the I and Q amplitudes for a period of time in which the phase of the incoming signal is approximately constant. The results are passed to energy calculation block
26
where the I and Q coherent accumulations are squared and summed. The result is accumulated in non-coherent accumulator
28
. Non-coherent accumulator
28
is summing energies, and so the constant phase requirements of coherent accumulation do not apply. Energy is accumulated for a period of time as directed by searcher controller
27
. The result is compared in threshold compare
29
. Once the process is completed for the candidate offset programmed in I and Q PN sequence generator
23
, searcher controller
27
directs a new candidate offset to be analyzed.
The searcher as just described has the advantage of great flexibility. Any number of coherent integrations, C, (within the limits of coherence time) may be performed on a candidate offset, and any number of non-coherent accumulations, M, may be performed. Any number of hypotheses to search, L, can be searched. The overall search time for a window of L hypotheses is then given by L*C*M. The drawback of this architecture is that each candidate is calculated in a serial manner. To reduce search time for given M and N requires that duplicative hardware be added.
FIG. 3
shows an alternative searcher architecture, commonly called a matched filter searcher. For a discussion of this method, see Simon, Omura, Scholtz & Levitt, SPREAD SPECTRUM COMMUNICATIONS HANDBOOK, pp. 815-822, McGraw-Hill, Inc., New York (1994).
An incoming signal is received at antenna
30
and passed to receiver
31
for downconversion and amplification. I and Q channels are then delivered to delay chains
36
and
38
, respectively. Each delay chain contains N delay elements labeled DI
1
-DIN and DQ
1
-DQN. The output of each delay element is multiplied by a PN value loaded into tap value chains
35
and
37
. The tap values are created with I and Q PN generators and loaded or hard coded into multiplication elements labeled PNI
1
-PNIN and PNQ
1
-PNQN. Note that in the simple case, the tap values include only 1 and −1, so inverters (or negaters) take the place of actual multipliers. The associations of delay element outputs and tap values is shown in FIG.
3
. The tap values are made up of a portion of the PN sequence which is used to correlate with the incoming data. The results of all the multiplications are delivered to adders
34
and
32
, where they are summed. The results are then squared and summed

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