Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1999-06-25
2003-11-18
Cumming, William (Department: 2683)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C455S067150, C370S331000, C375S131000
Reexamination Certificate
active
06650630
ABSTRACT:
BACKGROUND
This invention relates to radio communication and particularly to cellular radio telephone systems that use time division duplex communication.
Applications of mobile and cordless radio telephony are becoming increasingly widespread, and cellular radio telephone systems are well known and have reached a high level of maturity. Cellular systems typically consist of a backbone network of a plurality of radio base stations located at strategic positions. Each base station covers a respective geographic area called a cell, and since adjacent cells partly overlap, a portable device like a mobile telephone can move from one cell to another without losing contact with the backbone network. As a portable device moves during a communication session, the connection is handed off from one radio base station to another according to a process that depends on, among other factors, the location of the portable relative to the base stations.
All over the world, cellular systems continue to be deployed to offer nation-wide public telephony. Current examples of wide-area mobile telephone systems are the Global System for Mobile communications (GSM), the Digital Advanced Mobile Phone System (D-AMPS), the IS-95 (CDMA) system, and the Personal Digital Cellular (PDC) system. These systems are run by operators that offer various public services using portions of the radio spectrum that typically are licensed by national regulatory bodies.
In addition to these licensed-spectrum cellular communication systems, new kinds of cellular system are now entering the market that are deployed in restricted areas like indoor environments (e.g., offices, houses, exhibition halls, etc.) and local areas (e.g., school campuses, office parks, etc.). These new systems are privately owned and typically use unlicensed portions of the radio spectrum like the globally available industrial, scientific, and medical (ISM) bands at 900 MHz, 2400 MHz, and 5700 MHz. Examples of such local-area, unlicensed-spectrum mobile communication systems are the Digital European Cordless Telephone (DECT) system, the Personal Handyphone System (PHS), and wireless local area computer networks (WLANs).
Adroit resource allocation in a cellular system is critically important. To connect a mobile or cordless terminal to the backbone network, both an access point to the network (e.g., a radio base station) must be available and a radio channel must be available to connect the terminal to the access point. Both the access point and the radio channel can be considered allocable system resources. When a connection has to be made, the access point and/or the terminal has to select a radio channel, but radio resources are scarce. The cellular system concept is a way to support a large number of terminals with a limited radio spectrum by organizing the spectrum into channels that can be used simultaneously for different connections, provided the geographical distance between users participating in different connections is large enough that their mutual interference is small relative to their intended received signals.
In most cellular systems, the access point closest to a remote terminal seeking a connection is allocated to that terminal since that access point usually provides the lowest propagation loss to that terminal. The remote terminals regularly scan the spectrum for control or beacon signals broadcast by the access points on predetermined radio channels, and each terminal locks, or synchronizes, itself to the strongest control or beacon channel it receives.
In some mobile systems, a terminal does not by default lock to the strongest access point but chooses an access point based on other criteria, e.g., whether a base station has radio channels available and/or whether the interference on any available radio channels is sufficiently low. Indeed, it is not the channel having the highest carrier power that is important but the channel having the highest carrier-to-interference (C/I) ratio. An exemplary communication system in which base station and channel selection are based on the C/I ration is described in the U.S. Pat. No. 5,491,837 to Haartsen for “Method and System for Channel Allocation Using Power Control and Mobile-Assisted Handover Measurements”, which is expressly incorporated here by reference.
In general, a “channel” can be a carrier frequency, a time slot, a code, or a hybrid of these, according to the particular access technique used by the communication system. In a frequency division multiple access (FDMA) system, a radio channel is a radio frequency (RF) carrier signal for transmitting and an RF carrier signal for receiving that are usually allocated for the duration of a communication session. (Separating the transmit and receive carriers, which are usually selected from respective dedicated bands, permits simultaneous transmission and reception and is called Frequency Division Duplex (FDD).) The Advanced Mobile Phone System (AMPS) and the Nordic Mobile Telephone (NMT) system are examples of simple FDMA systems that use carrier frequency modulation. In a time division multiple access (TDMA) system like GSM, each carrier signal is time-shared by up to eight users, i.e., each carrier signal transports successive frames of eight time slots each, and one or more time slots in each frame are allocated to the session. In a direct-sequence code division multiple access (CDMA) system, an information bit stream to be transmitted is effectively superimposed on a much-higher-rate bit stream that may consist of successive repetitions of a unique code sequence, and the superimposed bit streams may then be scrambled by multiplication by another, usually pseudo-noise, bit stream, with the result transmitted as a modulation of an RF carrier signal.
First-generation cellular systems like AMPS and the NMT system are analog, which is to say that an analog (temporally continuous) information signal to be transmitted modulates the frequency of the carrier signal. The primary use of analog systems is voice service, although low-rate digital data transmission is possible by using analog modems. In second-generation systems like GSM and D-AMPS, the information signal to be transmitted is digital (binary bits), which enables the information to be compressed, error-correction coded, organized into packets, and transmitted in bursts or packets. Thus, a carrier signal does not have to be in use all the time for one connection; instead, the carrier can be divided into slots, and different slots can be allocated to different users as in TDMA. In current second-generation TDMA systems, as well as in CDMA systems, the spectrum is still divided into bands of carrier frequencies, so such systems still have FDMA elements. If each carrier is divided into time slots, this results in a hybrid FDMA/TDMA system, and if each user is separated by a respective code, this results in a hybrid FDMA/CDMA system. Hybrid FDMA/TDMA/CDMA systems have also been described.
Another benefit of time-slotted systems is that downlink (base-station-to-remote-terminal) transmission and uplink (remote-terminal-to-base-station) transmission do not have to occur simultaneously, which is to say that FDD is not necessary. Instead, downlink and uplink transmissions can happen in different time slots on the same carrier, which may be called time division duplex (TDD). Full duplex operation is obtained by alternating between transmission and reception. Communication systems like cellular systems that are used for wide-area services still use FDD, which is preferable when access points are placed at elevated positions because FDD helps prevent interference between access points. Indoor communication systems and other high-data-rate systems preferably use TDD, in which the spectrum is not split into dedicated downlink and uplink bands. This enables the cost of the radio transceivers to be reduced because the transmission and reception processes occur sequentially with the same hardware, avoiding the costly duplexer that is otherwise required to provide sufficient isolation between
Burns Doane Swecker & Mathis L.L.P.
Cumming William
Telefonaktiebolaget LM Ericsson (publ)
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