Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-05-01
2003-04-22
Le, Thanh Cong (Department: 2749)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S063300
Reexamination Certificate
active
06553234
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to fixed wireless communication networks and more particularly to a method of frequency reuse in such a communications network.
2. Description of the Related Art
As depicted in
FIG. 1
, a Local Multi-point Distribution System (LMDS)
8
is a fixed access wireless system comprised of a collection of base transceiver stations (BTS)
10
which broadcast and receive signals from a large number of terminal radio stations (TS)
12
. The terminal stations are fixed and equipped with directional antennas (not shown), each of which are oriented toward a serving BTS. The base transceiver stations
10
can accommodate omni-directional transmission in which case they are equipped with omni-directional antennas, or they can be sectorized in which case they are equipped with directional sector antennas. A specified base transceiver station
10
and its associated terminal radio stations
12
comprise a “cell”
14
the basic building block in a wireless network. The cells
14
are theoretically considered to be circular in shape. The base transceiver stations
10
are each connected to a backbone network
16
, which may be a computer network, a cable television network, a public telephone network or the like. Connection to backbone network
16
may be accomplished using copper wire, fibre optics, wireless transmission or a combination of these communication means.
An LMDS
8
typically operates in the frequency range of 24.0 GHz to 42.0 GHz. In general, LMDS system architectures generally differ in terms of cell size, modulation format and BTS antenna type. Other system design parameters include antenna patterns, antenna heights, antenna pointing, cell spacing, frequency reuse plan, polarization reuse plan and link budget. For example, a two-way multiple access LMDS system described by Texas Instruments (TI) to the FCC Negotiated Rulemaking Committee (NRMC) on the LMDS/FSS 28 GHz band, July-September 1994, utilizes 52 Mbps Quadrature Phase Shift Keying (QPSK) and four directional sector antennas at each BTS to provide omni-directional cell coverage with a nominal cell radius of 5 km. Typically multiple TSs within a cell are serviced by their associated BTS in accordance with time and frequency division multiplexing techniques. In operation, a given BTS broadcasts on a number of assigned frequencies with each TS antenna programmed to receive on one designated frequency.
A broadband fixed wireless network as described above provides the physical infrastructure to provide wireless access to services ranging from one-way video distribution and telephony to fully-interactive switched broadband multimedia applications. Various techniques are employed to maximize the amount of traffic which can be carried in the network. As noted above, efficient modulation schemes from QPSK to Quadrature Amplitude Modulation (e.g. 64 QAM) are used to increase the frequency efficiency of the system. Additionally, due to the limited frequency range available for operation of an LMDS, frequency reuse is employed for re-assigning available frequencies between cells in a fixed wireless network. Typically, a network manager utilizing a programmed general purpose computer would determine the manner in which frequencies will be allotted between the cells
14
in an LMDS
8
.
Frequency reuse is a method of optimizing spectrum usage, enhancing channel capacity, and reducing interference. It will be appreciated by those in the art, interference can also be reduced using other techniques such as space separation of transmitting equipment, time separation of broadcasts and signal polarization. Frequency reuse involves channel numbering, channel grouping into subsets, and assigning particular channels to particular cells. A plurality of cells are then associated together into clusters and utilize all of the assigned frequency channels in a prescribed manner. Groups of clusters are then used to provide coverage over a defined geographic area and the frequency channels allocated to one cluster are reused in other clusters. The scheme for recycling or reassigning the frequency channels throughout the coverage area is referred to as a reuse plan. The distance between a first cell within a cluster using a particular frequency channel and a second cell using the same frequency channel is further known as a reuse distance. The principal objective of such a reuse scheme is to ensure adequate channel isolation to reduce channel interference while maintaining a high channel capacity.
It will be understood by those skilled in the art that the term decibel which will be used throughout the description, is a common unit used in relation to radio frequency transmission to denote relative differences in signal strength and is expressed as the base 10 logarithm of the ratio of the powers of two signals i.e. dB=10 log (P
1
/P
2
). Logarithms are useful as the unit if measure because signal power tends to span several orders of magnitude.
As explained above, the reuse of the same frequency channels by a number of different cells implies that cells may suffer from co-channel interferences. Depending on the operating frequency band, the terminal station directional antenna can have a high gain in the range of 25 to 45 dbi. Therefore, the directional antenna will reject most of the interfering radio signals from nearby BTSs, except those signals arriving at the TS from distant BTSs lying in the same direction as the BTS serving the TS. It is generally desirable for the received strength of the serving carrier (C) within each cell to be higher than the total co-channel interference level (I). As a result, the higher the carrier to interference (C/I) ratio, the better the data transmission quality. A higher C/I value is obtained partly by controlling the channel reuse distance. The larger the reuse distance between adjacent cells utilizing the same frequency channels, the lesser the co-channel interferences created between those cells. Since the C/I ratio is normally dictated by, among other things, the equipment used in the wireless network (i.e. its ability to discriminate a useful signal), in order to maximize frequency reuse, the minimum acceptable reuse distance is identified for a stated C/I and the available frequencies are distributed between cells accordingly. A number of other physical factors can also affect C/I in wireless networks e.g. buildings, geography, antenna radiation patterns, and transmitting power.
It will also be appreciated by those in the art that there is a trade-off between modulation schemes and frequency reuse plans. The higher and more efficient the modulation scheme, the higher the minimum required carrier to interference (C/I) level, which forces a reduced frequency reuse factor (the ratio between the amount of bandwidth used at each cell and the total frequency bandwidth available). For example, if 16 QAM is used instead of QPSK, the frequency planner would have to consider BTSs which are further away since a higher C/I ratio is required to meet the QAM specification (i.e. in theory 4 bits/s/Hz). The C/I ratio is related to the frequency reuse plan (N/F) where N indicates the number of cells included within a single cluster and F indicates the number of frequency groups. For example, the C/I ratio is directly related to the following equation:
D
R
=(3
×F
)
½
×R
where: D
R
is the reuse distance; F is the number of frequency groups; R is the radius of a cell.
Accordingly, the larger the F value, the greater the reuse distance. However, it is not always possible to use a larger F value to increase the C/I ratio. Since the total number of available frequency channels (T) is generally fixed within a wireless network, if there are F groups, then each group will contain T/F channels. As a result, a higher number of frequency groups (F) would result in fewer channels per cell and lesser transmission capacity.
In a mobile cellular radio system, capacity is not a major issue when the system initially goes int
Alcatel Canada Inc.
Cong Le Thanh
Pennie & Edmonds LLP
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