Matched filter using time-multiplexed precombinations

Details Matched filter using time-multiplexed precombinations Matched filter using time-multiplexed precombinations

1999-07-01

2003-05-20

Phu, Phuong (Department: 2631)

Pulse or digital communications

Receivers

Particular pulse demodulator or detector

C375S150000, C708S422000

Reexamination Certificate

active

06567483

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to matched filters for digitally coded signals and, more particularly, to a matched filter using time-multiplexed precombinations to reduce power consumption in radio receivers of Code Division Multiple Access (CDMA) signals.
BACKGROUND OF THE INVENTION
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in cellular systems is to change from analog to digital transmission. Equally important is the choice of an effective digital transmission scheme for implementing the next generation of cellular technology. Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs) employing low cost, pocket-size, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc. will be provided by cellular carriers using the next generation of digital cellular system infrastructure and cellular frequencies. The key feature demanded of these new systems is increased traffic capacity.
Currently, channel access is achieved using Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA) methods. In FDMA systems, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. Interference with adjacent channels is limited by the use of bandpass filters that only pass signal energy within the filters' specified frequency bands. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by channel reuse.
In TDMA systems, a channel consists of a time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal's energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, the problem of interference from different relative signal strength levels is reduced.
Capacity in a TDMA system is increased by compressing the transmission signal into a shorter time slot. As a result, the information must be transmitted at a correspondingly faster burst rate that increases the amount of occupied spectrum proportionally.
With FDMA or TDMA systems or hybrid FDMA/TDMA systems, the goal is to ensure that two potentially interfering signals do not occupy the same frequency at the same time. In contrast, CDMA systems allow signals to overlap in both time and frequency. Thus, all CDMA signals share the same frequency spectrum. In both the frequency and the time domain, the multiple access signals overlap. Various aspects of CDMA communications are described, for example, in “On the Capacity of a Cellular CDMA System,” by Gilhousen, Jacobs, Viterbi, Weaver and Wheatley,
IEEE Trans. On Vehicular Technology,
May 1991.
In a typical CDMA system, the informational data stream to be transmitted is impressed upon a much higher bit rate data stream generated by a pseudo-random noise code (PNcode) generator. The informational data stream and the higher bit rate code data stream are typically multiplied together. This combination of the lower bit rate informational data stream with the higher bit rate code data stream is called coding or spreading the informational data stream signal. Each informational data stream or channel is allocated a unique spreading code. A plurality of coded information signals are transmitted on radio frequency carrier waves and jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. By correlating the composite signal with one of the unique spreading codes, the corresponding information signal is isolated and decoded.
There are a number of advantages associated with CDMA communication techniques. The capacity limits of CDMA-based cellular systems are projected to be up to twenty times that of existing analog technology as a result of the wideband CDMA system's properties such as improved coding gain/modulation density, voice activity gating, sectorization and reuse of the same spectrum in every cell. CDMA is virtually immune to multi-path interference, and eliminates fading and static to enhance performance in urban areas. CDMA transmission of voice by a high bit rate encoder ensures superior, realistic voice quality. CDMA also provides for variable data rates allowing many different grades of voice quality to be offered. The scrambled signal format of CDMA eliminates cross-talk and makes it very difficult and costly to eavesdrop or track calls, insuring greater privacy for callers and greater immunity from air-time fraud. In communication systems following the CDMA or “spread spectrum” concept, the frequency spectrum of an informational data stream is spread using a code uncorrelated with that of the data signals. The codes are also unique to every user. This is the reason why a receiver that has knowledge about the code of the intended transmitter is capable of selecting the desired signal.
There are several different techniques to spread a signal. Two of the most popular are Direct-Sequence (DS) and Frequency-Hopping (FH), both of which are well known in the art. According to the DS technique, the data signal is multiplied by an uncorrelated pseudo-random code (i.e., the previously described PNcode). The PNcode is a sequence of chips (bits) valued at −1 and 1 (polar) or 0 and 1 (non-polar) and has noise like properties. One way to create a PNcode is by means of at least one shift register. When the length of such a shift register is N, the period, T
DS
, is given by the equation T
DS
=2
N
−1.
In a receiver in a CDMA system, the received signal is multiplied again by the same (synchronized) PNcode. Since the code consists of +1's and −1's (polar), this operation removes the code from the signal and the original data signal is left. In other words, the despreading operation is the same as the spreading operation.
Referring to
FIG. 1
, there is shown a schematic diagram of a prior art correlator
10
which is used to compute correlations between the last M signal samples received and an M-bit codeword. An M-element delay line
11
stores received signal samples and sequentially shifts them through each of the M stages. Consequently, the delay line memory elements contain the last M signal sample values received. After each new signal sample is shifted in and each old signal sample is shifted out, the M signal sample values are read out of the delay line into M sign-changers
13
, where the M signal sample values are multiplied by +1 or −1 according to the bits b
1
. . . b
M
of a predetermined code stored in code store
12
with which correlation is to be computed. The sign-changed values are then summed in adder
14
to produce a correlation result.
In general, the process of correlating an M-element vector A=(a
1
, a
2
. . . aM) with an M-element vector B =(b
1
,b
2
. . . bM) involves forming the inner product A·B=a
1
·b
1
+a
2
·b
2
+. . . . aM·bM. When the elements of one of the vectors (e.g., B) comprises only binary values (arithmetically +1 or −1), the products such as a
1
·b
1
simplify to ±a
1
, but the process of adding the M values ±a
1
,±a
2
. . . . ±aM is still a significant effort

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