Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-01-18
2002-10-29
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S130000, C375S213000, C370S471000
Reexamination Certificate
active
06473449
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to wireless local-area networks, and more particularly to wireless local-area networks for use in high-data-rate applications subject to multipath interference.
BACKGROUND OF THE INVENTION
Computer communications networks for allowing computers to communicate data to and from other computers have become common. For example, a user of a first computer can send and receive files and real-time data to and from a second computer. A local-area network (LAN) is a computer communications network which provides computer communications among a plurality of computers situated within a common locale. For example, a LAN is typically used to interconnect personal computers or workstations within an office or school building, or to interconnect computers situated in several buildings of a campus or office park. The computers connected to the LAN typically communicate among one another, and usually also communicate with one or more centralized or specialized computers, such as a host computer, with an output device, such as a printer, and with a mass data storage device, such as a file server.
A computer communications network, such as a LAN, employs a transmission medium to communicate data signals among the plurality of data devices in the network. Usually, the transmission medium is a network of wires. Wires can be cumbersome in that they can present routing problems, occupy space, require installation time, and inhibit the mobility of the computers connected to the network.
To overcome the problems associated with using a system of wires as the transmission medium, a plurality of radio transceivers can be used to communicate radio signals for carrying data messages among the computers in the computer communications network. Use of radio transceivers has gained little acceptance so far due to low data transmission rates and/or unreliability. Typically, if the data transmission rate is lowered, the reliability can be improved. Alternatively, high data transmission rates can be achieved, albeit with reduced reliability.
The principle barrier to high data rate communications between computers in a wireless local-area network is an interference phenomenon called “multipath”. A radio signal commonly traverses many paths as it travels towards a receiver. Multiple propagation paths can be caused by reflections from surfaces in the environment, for example. Some of these paths are longer than others. Therefore, since each version of the signal travels at the same speed, some versions of the signal will arrive after other versions of the signal. Sometimes the delayed signals will interfere with more prompt signals as the delayed signals arrive at the receiver, causing signal degradation.
Multipath time-delay spread is the time that elapses between the moment that the earliest version of a transmitted signal arrives at a receiver, and the moment that the latest version of the signal arrives at the receiver.
To understand multipath effects and the instant invention, it is helpful to discuss the term “symbol”. One or more symbols can be combined to form a message that conveys meaning. Each symbol must be uniquely recognizable, and is selected from a set of possible symbols, referred to as a symbol alphabet. The number of symbols in the symbol alphabet is referred to as the “order” of the symbol alphabet. For example, the letters “a”, “b”, and “c” are symbols from the English alphabet, where the order of the English alphabet is 26. The numbers “0” and “1” are symbols of the binary number system, which is of order 2.
It is possible to represent a sequence of symbols from a first alphabet with a symbol from a second alphabet, such as representing the binary symbol sequence “
101
” by the symbol “a”. This binary symbol sequence consists of three binary symbols. Since each binary symbol can be either one of two possible symbols, in a sequence of three binary symbols, there are eight possible unique binary symbol sequences. Thus, an alphabet of order eight is required to represent the eight possible unique binary symbol sequences of three symbols each. In general, an alphabet of order M=2
N
is required to represent the M=2
N
possible unique binary symbol sequences of N symbols each.
Just as binary signalling can be referred to as 2-ary signalling, a signalling system that represents three-element binary symbol sequences using a symbol alphabet of order eight is referred to as 8-ary signalling. In the terminology of communications system design, an 8-ary symbolic representation is said to represent each symbol using “3 bits per symbol”.
In general, a signaling system that represents an N-element binary symbol sequence using a symbol alphabet of order M=2
N
is referred to as M-ary signalling. In M-ary signalling, the equivalent binary data rate R is the symbol rate S multiplied by the number of bits per symbol N, i.e., R=S*N. The number of bits per symbol N is log
2
M. Thus, for 8-ary signalling, N=3, and therefore the equivalent binary data rate is three times the symbol rate (assuming no error correction coding and no overhead bits).
In binary signalling, the equivalent binary data rate is equal to the symbol rate, i.e, R=S, because when M=2, the number of bits per symbol N is one. Consequently, “bit” and “symbol” are often used interchangeably in discussions of binary signalling.
In radio communications, a transmitter includes a modulator that provides a transmitted signal representative of information presented to the modulator. Conversely, a receiver includes a demodulator that receives the transmitted signal and ideally provides the original information represented by the transmitted signal. Commonly, the information presented to the modulator includes a plurality of symbols, where each symbol is selected from a finite set of symbols. For each symbol presented to the modulator, the modulator generates a corresponding symbol waveform selected from a set of discrete symbol waveforms, the symbol waveform then being transmitted over a communications channel to be received by at least one receiver.
Each symbol waveform that is transmitted is subject to distortion and noise, thereby making each received symbol waveform differ from the corresponding original transmitted symbol waveform, and become more similar to other symbol waveforms that were not actually transmitted. Consequently, it is necessary to decide which symbol of the discrete set of known symbols was most likely transmitted. This decision is performed in the demodulator of the receiver, the output of the demodulator being a sequence of symbols, selected from the known set of symbols, that represents the best estimation of the transmitted symbol sequence.
To decide which symbol sequence has been transmitted, for each transmitted symbol, the demodulator processes the corresponding received symbol waveform for a period of time called a coherent integration interval. It is essential that each coherent integration interval be coincident with each received symbol waveform, thereby providing correct synchronization. In the absence of correct synchronization, the symbol content of the received waveform will be misinterpreted.
To further clarify the concept of multipath interference, consider the case of a message transmitted as a binary data modulation waveform, wherein each message symbol consists of a single bit. When the multipath time-delay spread is longer than the duration of a symbol waveform, symbol waveforms of the first version of the received signal overlap non-corresponding symbol waveforms of the excessively delayed versions of the received signal. This phenomena is called intersymbol interference (ISI).
For example, in a typical indoor or campus radio network environment, the time-delay spread can be greater than 500 nanoseconds (ns). Since in binary data modulation, data rate is the multiplicative inverse (reciprocal) of symbol duration, a time delay spread of 500 ns implies that data rates even much less than two million bits per s
Cafarella John H.
Fischer Jeffrey H.
Al-Beshrawi Tony
McDermott & Will & Emery
Pham Chi
Proxim Inc.
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