Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-04-19
2003-05-06
Urban, Edward F. (Department: 2685)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S203000, C370S281000, C370S206000, C370S329000, C370S336000, C370S337000
Reexamination Certificate
active
06560209
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention involves improvements to communications systems and methods in a wireless, frequency division duplex communications system.
2. Description of Related Art
Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths. They must make highly efficient use of the scarce bandwidth resource to provide good service to a large population of users. Examples of such communications systems that deal with high user demand and scarce bandwidth resources are wireless communications systems, such as cellular and personal communications systems.
Various techniques have been suggested for such systems to increase bandwidth-efficiency, the amount of information that can be transmitted within a given spectral bandwidth. Many of these techniques involve reusing the same communication resources for multiple users while maintaining the identity of each user's message. These techniques are generically referred to as multiple access protocols. Among these multiple access protocols are Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Space Division Multiple Access (SDMA), and Frequency Division Multiple Access (FDMA). The technical foundations of these multiple access protocols are discussed in the recent book by Rappaport entitled “Wireless Communications Principles and Practice”, Prentice Hall, 1996.
The Time Division Multiple Access (TDMA) protocol sends information from a multiplicity of users on one assigned frequency bandwidth by time division multiplexing the information from the various users. In this multiplexing schema particular time slots are devoted to specific users. Knowledge of the time slot during which any specific information is transmitted, permits the separation and reconstruction of each user's message at the receiving end of the communication channel.
The Code Division Multiple Access (CDMA) protocol uses a unique code to disinguish each user's data signal from other users' data signals. Knowledge of the unique code with which any specific information is by permits the separation and reconstruction of each user's message at the receiving end of the communication channel. There are four types of CDMA protocols classified by modulation: direct sequence (or pseudo-noise), frequency hopping, time hopping, and hybrid systems. The technical foundations for CDMA protocols are discussed in the recent book by Prasad entitled “CDMA for Wireless Personal Communications”, Artech House, 1996.
The Direct Sequence CDMA (DS-CDMA) protocol spreads a user's data signal over a wide portion of the frequency spectrum by modulating the data signal with a unique code signal that is of higher bandwidth than the data signal. The frequency of the code signal is chosen to be much larger than the frequency of the data signal. The data signal is directly modulated by the by the code signal and the resulting encoded data signal modulates a single, wideband carrier that continuously covers a wide frequency range. After transmission of the DS-CDMA modulated carrier signal, the receiver uses a locally generated version of the user's unique code signal to demodulate the received signal and obtain a reconstructed data signal. The receiver is thus able to extract the user's data signal from a modulated carrier that bears many other users' data signals.
The Frequency Hopping Spread Spectrum (FHSS) protocol uses a unique code to change a value of the narrowband carrier frequency for successive bursts of the user's data signal. The value of the carrier frequency varies in time over a wide range of the frequency spectrum in accordance with the unique code. The term Spread Spectrum Multiple Access (SSMA) is also used for CDMA protocols such as DS-CDMA and FHSS that use a relatively wide frequency range over which to distribute a relatively narrowband data signal.
The Time Hopping CDMA (TH-CDMA) protocol uses a single, narrow bandwidth, carrier frequency to send bursts of the user's data at intervals determined by the user's unique code. Hybrid CDMA systems include all CDMA systems that employ a combination of two or more CDMA protocols, such as direct sequence/frequency hopping (DS/FH), direct sequence/time hopping (DS/TH), frequency hopping/time hopping (FH/TH), and direct sequence/frequency hopping/time hopping (DS/FH/TH)
The Space Division Multiple Access (SDMA) transmission protocol forms directed beams of energy whose radiation patterns do not overlap spatially with each other, to communicate with users at different locations. Adaptive antenna arrays can be driven in phased patterns to simultaneously steer energy in the direction of selected receivers. With such a transmission technique, the other multiplexing schemes can be reused in each of the separately directed beams. For example, the specific codes used in CDMA can be transmitted in two different beams. Accordingly, if the beams do not overlap each other, different users can be assigned the same code as long as they do not receive the same beam.
The Frequency Division Multiple Access (FDMA) protocol services a multiplicity of users over one frequency band by devoting particular frequency slots to specific users, i.e., by frequency division multiplexing the information associated with different users. Knowledge of the frequency slot in which any specific information resides permits reconstruction of each user's information at the receiving end of the communication channel.
Orthogonal Frequency Division Multiplexing (OFDM) addresses a problem that is faced, for example, when pulsed signals are transmitted in an FDMA format. In accordance with principles well known in the communication sciences, the limited time duration of such signals inherently broadens the bandwidth of the signal in frequency space. Accordingly, different frequency channels may significantly overlap, defeating the use of frequency as a user-identifying-parameter, the principle upon which FDMA is based. However, pulsed information that is transmitted on specific frequencies can be separated, in accordance with OFDM principles, despite the fact that the frequency channels overlap due to the limited time duration of the signals. OFDM requires a specific relationship between the data rate and the carrier frequencies. Specifically, the total signal frequency band is divided into N frequency sub-channels, each of which has the same data rate 1/T. These data streams are then multiplexed onto a multiplicity of carriers that are separated in frequency by 1/T. Multiplexing signals under these constraints results in each carrier having a frequency response that has zeroes at multiples of 1/T. Therefore, there is no interference between the various carrier channels, despite the fact that the channels overlap each other because of the broadening associated with the data rate. OFDM is disclosed, for example, by Chang in Bell Sys. Tech Jour., Vol. 45, pp. 1775-1796, December 1966, and in U.S. Pat. No. 4,488,445.
Parallel Data Transmission is a technique related to FDMA. It is also referred to as Multitone Transmission (MT), Discrete Multitone Transmission (DMT) or Multi-Carrier Transmission (MCT). Parallel Data Transmission has significant calculational advantages over simple FDMA. In this technique, each user's information is divided and transmitted over different frequencies, or “tones”, rather than over a single frequency, as in standard FDMA. In an example of this technique, input data at NF bits per second are grouped into blocks of N bits at a data rate of F bits per second. N carriers or “tones” are then used to transmit these bits, each carrier transmitting F bits per second. The carriers can be spaced in accordance with the principles of OFDM.
Both the phase and the amplitude of the carrier can be varied to represent the signal in multitone transmission. Accordingly, multitone transmission can be implemented with M-ary digital modulation schemes. In an M-ary modulatio
Alamouti Siavash
Casas Eduardo F.
Hirano Michael
Hoole Elliott
Jesse Mary
AT&T Wireless Services Inc.
Milord Marceau
Perkins Coie LLP
Urban Edward F.
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