Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-07-09
2002-12-17
Chin, Wellington (Department: 2664)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C455S447000, C455S450000
Reexamination Certificate
active
06496490
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods for operating a packetized wireless communication network in which multiple radio-frequency channels are available for transmitting signals between user terminals such as mobile stations and the base stations that serve them. More specifically, the invention relates to methods for allocating the available channels among a group of base stations according to time-varying and spatially inhomogeneous demands for transmission capacity.
ART BACKGROUND
Packetized wireless communication networks, such as cellular telephone networks, are already in wide use for carrying voice traffic. The use of portable personal computers is also widespread. There is now a growing demand for wireless networks to carry high-speed data traffic in addition to voice traffic, so that data can be exchanged, without wired connections, among portable or mobile computers.
However, computer-data traffic differs significantly from voice traffic in several respects. Because of these differences, schemes for the use of network resources that afford both high quality and high efficiency of voice traffic do not, typically, offer the same for data traffic.
That is, whenever a constant level of channel capacity is allocated to a signal source, there will be a trade-off between delay and efficiency of channel usage. The more bursty the signal source, the more severe is this trade-off. For example, if the allocated capacity is set at the peak data rate, there will be nothing transmitted during the relatively long periods of silence between bursts. On the other hand, if the allocated capacity is set at the mean data rate, there will be relatively long delays while the buffers in the transmission path are emptying. Moreover, if the allocated channel capacity is set too low relative to the buffer capacity, there will be a danger of overflowing the buffers and thereby losing data.
Because speech is only moderately bursty, this trade-off is relatively mild. For that reason, the allocated channel capacity for typical speech signals can be set relatively high, e.g. at or near the peak data rate, without risking any drastic inefficiency in channel usage. As a consequence, transmission delays long enough to be annoying to the user engaging in a spoken conversation can generally be avoided.
By contrast, computer data representing, for example, a Web page will typically be delivered as a succession of bursts, each containing 100 kbits or more of data. Periods of transmissive activity may be separated by relatively long periods of inactivity lasting tens or even hundreds of seconds, during which a user at the receiving end is reading the transmitted information. A computer user will generally tolerate much more delay than will a user of a voice telephone. For example, a typical user at the present time would find it quite acceptable to wait several seconds between demanding a Web page (by clicking a mouse with the cursor situated at an appropriate portion of the computer screen) and receiving the Web page. However, the satisfactory delivery of Web-page data calls for bit-error rates that are significantly lower than those required for voice transmission.
Typical wireless links that are available at the present time are limited to bit rates of about 10 kbits per second or less. Bit rates of about 10 kbits per second are adequate for carrying at least some kinds of data traffic, but only with the relatively long delays described above. At the present time, however, such transmission is inefficient because the calls are charged as voice calls, even though they use only a limited portion of the relevant channel capacity at any given point in time.
Improvement is expected in the near future, when some wireless spectrum will be allocated for use by high-speed data networks. In fact, it may become possible to provide wireless links transmitting at a gross user bit rate of 1000 kbits per second in both the uplink and downlink directions.
Each individual user will need only a fraction of this gross rate, and his use of it will be intermittent. Therefore, even at a gross user bit rate smaller than 1000 kbits per second, it will be feasible for multiple users in each cell, such as ten or twenty users per cell, to simultaneously share the data-carrying wireless link.
Practitioners in the field of wireless data communication have recognized that there is a need for schemes for efficient sharing of this wireless resource, both as it exists at the present time, and as it will exist in the future.
In at least some wireless data networks, data will be transmitted on the down link using a set of some finite number of carriers (i.e., frequency channels), such as a set of 32 carriers. Each individual base station may be allocated some subset of these carriers, such as eight carriers per base station. This allocation is designed in such a way that neighboring base stations (more specifically, base stations having overlapping reception zones) do not share the same carriers.
If the demands on the network were uniform and predictable, a fixed allocation could be made that would use the available resources with high efficiency. However, the number of mobile stations within a given cell that may demand data services is highly variable. Within a given population of such mobile stations, the rate at which, e.g., requests for Web pages are being made is also highly variable. Neither the distribution of mobile stations nor the rate of data requests can be predicted with accuracy. Therefore, it will be typical for queues of packets to vary considerably in length from base station to base station (at any given time), and to vary considerably in length from one time to another at each individual base station.
Under these conditions, a fixed allocation of carriers to each base station may fail to provide optimum efficiency, or even a commercially viable level of efficiency, in the use of the data-carrying resources. That is because a fixed allocation lacks the flexibility to deal efficiently with a variable load.
For example, when the number of mobile stations served by a base station has reached the maximum that can be supported by the allocated carriers, the base station must reject further requests for service, even though other, unused carriers may be available at other points in the network.
Similarly, heavy demand in a particular cell may cause a long queue of packets to build up in the buffers of the corresponding base station. Although there may be unused carriers available at other points in the network, no additional carriers are available to relieve the queue at the particular base station. One consequence is that spectrum is used inefficiently because carriers go unused in some parts of the network while service requests are denied for lack of capacity in other parts of the network. A further consequence is that buffer capacity is used inefficiently. That is, packets are lost due to overflow of buffers in some parts of the network, while buffers are underfilled in other parts of the network.
In order to solve these problems of efficiency, among others, there is a need for methods of channel allocation in which available carriers are allocated among base stations, at least in part, according to time-varying and spatially inhomogeneous demand for transmission capacity. Such methods are said to carry out “Dynamic Channel Assignment (DCA).”
SUMMARY OF THE INVENTION
We have invented a method of DCA. Our invention pertains to wireless transmission systems that employ time or frequency multiplexing, or both time and frequency multiplexing. The invention is specifically addressed to the problem of avoiding interference in the channels of such systems. By way of example, one specific field of application for the invention is Time-Division Multiple Access (TDMA), including Orthogonal Frequency-Division Multiplexing (OFDM) (which is a variation of TDMA).
In a broad aspect, the invention involves partitioning base stations of a network into non-interfering sets. One example of such sets are reuse groups, as
Andrews Daniel Matthew
Borst Simon C.
Dominique Francis
Jelenkovic Predrag
Kumaran Krishnan
Chin Wellington
Finston Martin I.
Lucent Technologies - Inc.
Pham Brenda
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