Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
2002-07-29
2004-03-09
Bayard, Emmanuel P (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
Reexamination Certificate
active
06704376
ABSTRACT:
STATEMENT OF GOVERNMENT INTEREST
Not Applicable.
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This present invention relates to telecommunications processing and more particularly to advanced receiver techniques in a multi-user environment.
2. Background Art
The telecommunications industry has been expanding at an unprecedented growth rate. In particular, the wireless sector, including 3G, wireless local area networks and Bluetooth devices, has grown far beyond expectations and at a much higher rate than the fixed telecommunications counterpart. The ability to access data and communicate anywhere at anytime has enormous potential and commercial value.
The content of the wireless sector is also changing, with more and more data being transmitted, including Internet connectivity and live feeds. The usage involving personal digital assistants (PDA's) and even smart appliances have created new markets utilizing wireless data communications. And, this wireless phenomenon is not limited to any geographical boundaries, as the growth is occurring around the globe.
Although Code Division Multiple Access (CDMA) or spread spectrum communications has been around for many years, there is an increasing interest in using spread spectrum systems in commercial applications to allow superior quality performance and a greater number of users within a given bandwidth. The digital format of CDMA architecture allows complex processing and high-level algorithms for transmission and reception.
Despite the advancements in wireless transmission and reception, there are still problems related to seamless connectivity, multimedia traffic, battery life, security, and mobility to name a few. In general, wireless channels are subject to well-known problems and there are continuous efforts to improve capacity and quality. One of the growing problems is being able to process multiple users in a given bandwidth.
For example, a base station that processes a number of cellular devices has to receive and transmit data within a certain frequency range. The ability to extract the correct data from a given user is a difficult task, especially when the effects of interference and multipaths are considered. The problem is further complicated when the number of users exceeds the number of dimensions, resulting in an overloaded condition.
In the past, prior art communication systems generally utilized Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods to achieve channel access. FDMA refers to a communication channel wherein a signal's transmission power is concentrated into a single radio frequency band. Interference from adjacent channels is limited by the use of band pass filters however for each channel being assigned a different frequency, system capacity is limited by the available frequencies and by limitations imposed by channel reuse.
In TDMA systems, a channel consists of a time slot or frame in a periodic train of time intervals over the same frequency, with a given signal's energy 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. The system capacity is limited by the available time slots as well as by limitations imposed by channel reuse, as each channel is assigned a different time slot.
One of the goals of FDMA and TDMA systems is to try and prevent two potentially interfering signals from occupying the same frequency at the same time. In contrast, Code Division Multiple Access (CDMA) techniques allow signals to overlap in both time and frequency. CDMA signals share the same frequency spectrum and in the frequency or time domain, the CDMA signals appear to overlap one another. The scrambled signal format of CDMA eliminates cross talk between interfering transmission and makes it more difficult to eavesdrop or monitor calls therefore providing greater security.
In a CDMA system, each signal is transmitted using spread spectrum techniques. The transmitted informational data stream is impressed upon a much higher rate data stream termed a signature sequence. The bit stream of the signature sequence data is typically binary, and can be generated using a pseudo-noise (PN) process that appears random, but can be replicated by an authorized receiver. The informational data stream and the high bit rate signature sequence stream are combined by multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or −1. This combination of the higher bit rate signal with the lower bit rate data stream is called spreading the informational data stream signal. Each informational data stream or channel is allocated a unique signature sequence.
In operation, a plurality of spread information signals, such as binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK) modulation, modulate a radio frequency (RF) carrier and are jointly received as a composite signal at the receiver. Each of the spread signals overlaps all of the other spread signals, as well as noise-related signals, in both frequency and time. The receiver correlates the composite signal with one of the unique signature sequences, and the corresponding information signal is isolated and despread.
A signature sequence is normally used to represent one bit of information. Receiving the transmitted sequence or its complement indicates whether the information bit is a +1 or −1, sometimes denoted “0” or “1”. The signature sequence usually comprises N pulses, and each pulse is called a “chip”. The entire N-chip sequence, or its complement, depending on the information bit to be conveyed, is referred to as a transmitted symbol.
The receiver correlates the received signal with the complex conjugate of the known signature sequence to produce a correlation value. When a ‘large’ positive correlation results, a “0” is detected, and when a ‘large’ negative correlation results, a “1” is detected.
It should be understood that the information bits could also be coded bits, where the code is a block or convolutional code. Also, the signature sequence can be much longer than a single transmitted symbol, in which case a subsequence of the signature sequence is used to spread the information bit.
Further descriptions of CDMA communications techniques are described in U.S. Pat. No. 5,506,861. This patent describes radiotelephone communication systems, and in particular, receivers for jointly demodulating a plurality of CDMA signals with multipath time dispersion.
The prior art systems do not properly account for the real world mobile communication signals that suffer from signal degradation such as interference and multipath problems. The systems of the prior art generally tended to make assumptions that all other interferers and multipaths were additive white Gaussian noise. However, this assumption is not accurate for co-channel interference and multipaths.
Multipath dispersion occurs when a signal proceeds to the receiver along not one but many paths so that the receiver encounters echoes having different and randomly varying delays and amplitudes. Co-channel interference refers to signals received from other users either directly or reflected. The receiver receives a composite signal of multiple versions of the transmitted symbol that have propagated along different paths, called rays, having different relative time. Each distinguishable ray has a certain relative time of arrival at a certain amplitude and phase, and as a result, the correlator outputs several smaller spikes. RAKE receivers are well known in the art and attempt to ‘rake’ together all the contributions to detect the transmitted symbol and recover the information bit.
Conventional RAKE receivers provide satisfactory performance under ideal conditions, however, the signature sequence must be uncorrelated with time shifted versions of itself as well as various shifted versions of the signature sequences of the ot
Learned Rachel E.
Mills Diane G.
BAE Systems Information and Electronic Systems Integration Inc.
Bayard Emmanuel P
Maine & Asmus
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