Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-09-28
2004-05-11
Hsu, Alpus H. (Department: 2665)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S342000, C370S350000, C370S441000, C370S447000, C375S142000, C375S145000, C375S343000, C455S450000, C455S509000, C455S522000
Reexamination Certificate
active
06735188
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to communication systems. Specifically, the invention relates to a method and apparatus for facilitating the reliable transfer of short, bursty messages over a wireless communications link.
2. Description of the Related Art
The use of wireless communication systems for the transmission of digital data is becoming more and more pervasive. In a wireless system, the most precious resource in terms of cost and availability is typically the wireless link itself. Therefore, one major design goal in designing a communication system comprising a wireless link is to efficiently use the available capacity of the wireless link. In addition, it is also desirable to reduce the delay associated with use of the link.
In a digital data system, remote units tend to generate bursty data to a hub station. The bursty data is characterized in that it has a high peak-to-average traffic ratio, meaning that blocks of data are transferred during short periods of time interposed between significantly longer periods of idleness.
In a time division multiple-access (TDMA) communication system, a separate time slot channel is assigned or dedicated to each remote unit. The remote unit uses the assigned time slot channel to transmit data to a hub station. By limiting transmissions to fall within the assigned time slot, the remote units are able to share the communication resources provided by the hub station. A TDMA system is effectively utilized when the transmission times and time slots of the remote units are all properly synchronized with each other.
In a TDMA system in which units have a pattern of use that includes bursty data, dedication of an individual time slot channel to each active remote unit does not result in efficient use of system capacity. This is because during those times when a remote unit is not utilizing the system, the time slot channel remains idle.
In a communication system with a plurality of remote units and a hub station, the hub station may detect when a remote unit has sent a burst to the hub station, determine the data content of the burst, and generate commands, such as timing synchronization signals, to feed back to the remote unit.
In present communication systems, the burst from a remote unit to the hub station typically includes a preamble which is used by the hub station to detect the burst. The added bits of the preamble increases the length of the burst and increases the duration of the time slot required by each remote unit. The more remote units in a system, the longer it takes the hub station to detect and process data bursts with their preambles. Furthermore, if the data bits in the burst are short, the preamble may be larger than the data bits. This results in an inefficient use of valuable system resources.
Thus, there is a need for a multiple-access system which provides advantageous use of system resources.
SUMMARY
The present invention relates to a method and apparatus for facilitating the reliable transfer of short, bursty messages over a wireless communications link or channel. Specifically, the invention relates to a communication system in which a plurality of remote units encode data bursts using a predetermined codeword set and transmit the encoded data bursts to a hub station over a multiple-access channel. When the hub station receives a data burst from one of the remote units, the hub station resamples the data burst at a plurality of different timing offsets and/or carrier frequency offsets. The hub station correlates the received data burst with each codeword within the predetermined codeword set to determine which codeword has the maximum correlation. The hub station then correlates the codeword (with the maximum correlation) with the plurality of different timing and/or carrier frequency offset samples and derives a timing synchronization signal and/or a carrier frequency adjustment signal to be sent back to the remote unit. The signals provide information to the remote unit to synchronize its timing or adjust its carrier frequency for transmitting subsequent data bursts. The hub station may also use the results of the correlations to estimate a signal-to-noise ratio.
There are several features or advantages of the present invention. First, in one embodiment related to encoding the data bursts, a QPSK codeword set is advantageously created such that each codeword has the same distance configuration from neighboring codewords, i.e., each codeword has the same number of codewords at a given distance as all of the other codewords. This advantageously gives each codeword equal noise immunity. The entire codeword data set is also advantageously created to perform well in low signal-to-noise environments for low probability of transmission error.
Second, the remote units preferably encode data bursts with a non-coherent code. Use of a non-coherent code advantageously does not require the determination of a carrier phase associated with the encoded burst during demodulation (at the hub station).
Third, in one embodiment, each burst is relatively short and does not include a preamble. This reduces the amount of data sent via the channel and the resources required to code and decode any preamble bits. A short burst also allows the channel to accommodate a large number of frequent transmissions or opportunities to transmit via the channel from a large number of remote units.
Fourth, in one embodiment, the channel demodulation employs a correlation-based scheme that can advantageously estimate the signal in the presence of noise. This ability allows the channel to be less susceptible to corruption by noise and to operate in a relatively low signal-to-noise ratio environment.
Fifth, in one embodiment, the channel demodulation comprises a substantially uncomplicated correlation-based architecture. This feature facilitates a highly vectorized and pipelined implementation that may be executed rapidly on a current microprocessor-based system. Thus, no tradeoff in the speed of demodulation is required to optimally and accurately demodulate the signal received via the reservation channel.
Sixth, in one embodiment, the hub station derives a timing adjustment signal to be sent back to the remote unit after a burst or number of bursts have been received and processed by the hub station. There is preferably no adjustment to the transmission timing of the remote unit while the hub station is receiving and processing a burst from that particular remote unit. This reduces the time for the hub station to receive and process each burst. This allows a large number of remote units to access the channel and the hub station to process the messages received via the channel quickly and efficiently.
Seventh, in one embodiment, the hub station derives a carrier frequency adjustment signal to be sent back to the remote unit after a burst or number of bursts have been received and processed by the hub station. There is preferably no adjustment to the carrier frequency of the remote unit while the hub station is receiving and processing a burst from that particular remote unit. This reduces the time for the hub station to receive and process each burst. This allows a large number of remote units to access the channel and the hub station to process the messages received via the channel quickly and efficiently.
One aspect of the invention relates to a method of communicating in which a plurality of remote units transmit data to a hub station. The method comprises receiving a data burst from a remote unit where the data burst is encoded using a predetermined codeword set. The method further comprises resampling the data burst received by the hub station at a plurality of different timing offsets and correlating the data burst received by the hub station with the codeword set to find the codeword with the maximum correlation. The method further comprises correlating the codeword (with the maximum correlation) with the plurality of different timing offset samples and deriving a timing synchroniza
Becker Donald W.
Leigh William E. L.
Hsu Alpus H.
Knobbe Martens Olson & Bear LLP
Tachyon, Inc.
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