Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-08-25
2002-02-19
Patel, Ajit (Department: 2662)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S337000
Reexamination Certificate
active
06349094
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the field of two-way wireless communication systems and more specifically to methods and apparatus for communication with mobile telephone users (cellular and personal communication systems), basic exchange telecommunications radio, wireless data communications, two-way paging and other wireless systems.
Conventional Cellular Systems
Present day cellular mobile telephone systems developed due to a large demand for mobile services that could not be satisfied by earlier systems. Cellular systems “reuse ” frequency within a group of cells to provide wireless two-way radio frequency (RF) communication to large numbers of users. Each cell covers a small geographic area and collectively a group of adjacent cells covers a larger geographic region. Each cell has a fraction of the total amount of RF spectrum available to support cellular users. Cells are of different sizes (for example, macro-cell or micro-cell) and are generally fixed in capacity. The actual shapes and sizes of cells are complex functions of the terrain, the man-made environment, the quality of communication and the user capacity required. Cells are connected to each other via land lines or microwave links and to the public-switched telephone network (PSTN) through telephone switches that are adapted for mobile communication. The switches provide for the hand-off of users from cell to cell and thus typically from frequency to frequency as mobile users move between cells.
In conventional cellular systems, each cell has a base station with RF transmitters and RF receivers co-sited for transmitting and receiving communications to and from cellular users in the cell. The base station employs forward RF frequency bands (carriers) to transmit forward channel communications to users and employs reverse RF carriers to receive reverse channel communications from users in the cell.
The forward and reverse channel communications use separate frequency bands so that simultaneous transmissions in both directions are possible. This operation is referred to as frequency division duplex (FDD) signaling. In time division duplex (TDD) signaling, the forward and reverse channels take turns using the same frequency band.
The base station in addition to providing RF connectivity to users also provides connectivity to a Mobile Telephone Switching Office (MTSO). In a typical cellular system, one or more MTSO's will be used over the covered region. Each MTSO can service a number of base stations and associated cells in the cellular system and supports switching operations for routing calls between other systems (such as the PSTN) and the cellular system or for routing calls within the cellular system.
Base stations are typically controlled from the MTSO by means of a Base Station Controller (BSC). The BSC assigns RF carriers to support calls, coordinates the handoff of mobile users between base stations, and monitors and reports on the status of base stations. The number of base stations controlled by a single MTSO depends upon the traffic at each base station, the cost of interconnection between the MTSO and the base stations, the topology of the service area and other similar factors.
A handoff between base stations occurs, for example, when a mobile user travels from a first cell to an adjacent second cell. Handoffs also occur to relieve the load on a base station that has exhausted its traffic-carrying capacity or where poor quality communication is occurring. The handoff is a communication transfer for a particular user from the base station for the first cell to the base station for the second cell. During the handoff in conventional cellular systems, there may be a transfer period of time during which the forward and reverse communications to the mobile user are severed with the base station for the first cell and are not established with the second cell.
Conventional cellular implementations employ one of several techniques to reuse RF bandwidth from cell to cell over the cellular domain. The power received from a radio signal diminishes as the distance between transmitter and receiver increases. Conventional frequency reuse techniques rely upon power fading to implement reuse plans. In a frequency division multiple access (FDMA) system, a communications channel consists of an assigned particular frequency and bandwidth (carrier) for continuous transmission. If a carrier is in use in a given cell, it can only be reused in cells sufficiently separated from the given cell so that the reuse site signals do not significantly interfere with the carrier in the given cell. The determination of how far away reuse sites must be and of what constitutes significant interference are implementation-specific details.
TDMA Conventional Cellular Architectures
In TDMA systems, time is divided into time slots of a specified duration. Time slots are grouped into frames, and the homologous time slots in each frame are assigned to the same channel. It is common practice to refer to the set of homologous time slots over all frames as a time slot. Each logical channel is assigned a time slot or slots on a common carrier band. The radio transmissions carrying the communications over each logical channel are thus discontinuous. The radio transmitter is off during the time slots not allocated to it.
Each separate radio transmission, which should occupy a single time slot, is called a burst. Each TDMA implementation defines one or more burst structures. Typically, there are at least two burst structures, namely, a first one for the initial access and synchronization of a user to the system, and a second one for routine communications once a user has been synchronized. Strict timing must be maintained in TDMA systems to prevent the bursts comprising one logical channel from interfering with the bursts comprising other logical channels in the adjacent time slots.
Space Diversity
Space diversity is a method for improving signal quality by the use of multiple spaced-apart transmitting and receiving antennas to send forward channel signals or receive reverse channel signals from a single receiver/transmitter. On the forward link, signals from multiple spaced-apart transmit antennas are received by a single receiver. On the reverse link, multiple spaced-apart receiving antennas receive signals from a single transmitter. Micro-diversity is one form of space diversity that exists when the multiple transmitting or receiving antennas are located in close proximity to each other (within a distance of several meters for example). Micro-diversity is effective against Rayleigh or Rician fading or similar disturbances. The terminology micro-diverse locations means, therefore, the locations of antennas that are close together and that are only separated enough to be effective against Rayleigh or Rician fading or similar disturbances. The signal processing for micro-diverse locations can occur at a single physical location and micro-diversity processing need not adversely impact reverse channel bandwidth requirements. Macro-diversity is another form of space diversity that exists when two or more transmitting or receiving antennas are located far apart from each other (at a distance much greater than several meters, for example, ten kilometers). In macro-diversity systems, on the forward channel the transmitted signals from the multiple transmitter antennas are received by the single receiver and processed to form an improved quality resultant signal at that single receiver. On the reverse channel, the received signals from the single transmitter are processed and combined to form an improved quality resultant signal from that single transmitter. The terminology macro-diversity means that the antennas are far enough apart to have decorrelation at the receivers between the mean signal levels. On the forward channel, the decorrelation is between the mean signal levels for the multiple transmitted signals received by the single receiver. On the reverse channel, the decorrelation is between the mean signal levels f
Ghoshtagore Ujjal Kumar
Uyehara Lance Kazumi
Vastano John Andrew
Lovejoy David E.
mDiversity Inc.
Nguyen Hanh
Patel Ajit
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