Multiplex communications – Duplex – Communication over free space
Reexamination Certificate
2000-03-31
2004-03-16
Hsu, Alpus H. (Department: 2665)
Multiplex communications
Duplex
Communication over free space
C370S281000, C370S277000, C370S329000, C370S330000, C455S446000, C455S447000
Reexamination Certificate
active
06707798
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to communication systems, and more particularly to a method and apparatus for reducing co-channel interference in a frame-synchronized wireless communication system.
2. Description of Related Art
A wireless communication system facilitates two-way communication between a plurality of subscriber radio stations or subscriber units (either fixed or portable) and a fixed network infrastructure. Exemplary systems include mobile cellular telephone systems, personal communication systems (PCS), and cordless telephones. The objective of these wireless communication systems is to provide communication channels on demand between the subscriber units and the base station in order to connect the subscriber unit user with the fixed network infrastructure (usually a wired-line system). In the wireless systems using multiple access schemes, frames of time are the basic transmission unit. Each frame is divided into a plurality of slots of time. Some time slots are used for control purposes and some time slots are used for information transfer. Information is typically transmitted during time slots in the frame where the time slots are assigned to a specific subscriber unit. Subscriber units typically communicate with the base station using a “duplexing” scheme that allows for the exchange of information in both directions of connection.
Transmissions from the base station to the subscriber unit are commonly referred to as “downlink” transmissions. Transmissions from the subscriber unit to the base station are commonly referred to as “uplink” transmissions. Depending upon the design criteria of a given system, the prior art wireless communication systems have typically used either time division duplexing (TDD) or frequency division duplexing (FDD) methods to facilitate the exchange of information between the base station and the subscriber units. Both the TDD and FDD duplexing schemes are well known in the art. Exemplary wireless communication systems using these schemes are described in more detail in the related U.S. Pat. No. 6,038,455, by Gardner et al., issued Mar. 14, 2000, entitled “Reverse Channel Reuse Scheme in a Time Shared Cellular Communication System”, which has been incorporated by reference herein for its teachings on wireless communication systems.
Some communication systems do not use time frames in communicating between the base station and their respective and associated subscriber units (or “terminal stations” in Broadband Wireless Access (BWA) communication systems). For example, BWA systems based on cable modem technologies do not use time frames when communicating on either the uplink or the downlink. Therefore, these systems do not allow for frame synchronization between base stations and disadvantageously do not permit coordination between the base stations for purposes of reducing co-channel interference. Similarly, un-synchronized TDD systems allow different communication cells within the system to be “free running”, in that different cells and sectors within the system operate on frames that are not synchronized in time.
Wireless communication systems rely upon frequency re-use because frequency allocation or bandwidth is typically limited. For example, in cellular communication systems and broadband wireless systems, geographic areas or regions are typically divided into cells that are nominally hexagonally or square shaped. As described in U.S. Pat. No. 6,038,455, each cell or sector is allocated one or more radio frequency channels. For example, in a cellular communication system utilizing frequency division multiple access (FDMA), adjacent or nearby cells are assigned separate frequencies. After all available frequencies have been allocated, it is necessary to begin reusing the frequencies. For example, if four frequencies are available, it is necessary to begin using the first frequency again starting in the fifth cell. Due to the nature of the systems described in the incorporated U.S. Pat. No. 6,038,455, and in PCS, cellular and paging systems of the prior art, frequency re-use cannot be used as aggressively as it can be used in BWA systems. For example, in PCS/cellular/paging systems, typically only a fraction of the frequency spectrum is used per cell. In contrast, in BWA, frequency re-use can be much more aggressive (for example, a frequency can be re-used at least once per cell, with multiple sectors).
FIG. 1
a
is a simplified diagram of an exemplary broadband wireless configuration showing frequency re-use. In broadband wireless communications, a plurality of base stations
1
communicate with fixed terminal stations.(i.e., “subscriber units”). As shown in
FIG. 1
a
, clusters of four sectors
4
surrounding base stations
1
(
1
a
-
1
d
) form cells
2
(
2
a
-
2
d
). The cells
2
are shown as being separated by the bold lines
30
and
32
. In BWA systems, a cell
2
typically comprises either four or six sectors
4
. In the case of four sectors
4
, the coverage area covered by the cell is square (as shown in
FIG. 1
a
). In the case of six sectors, the coverage area covered by the cell is hexagonal (as exemplified in
FIG. 6
described below).
Thus, each cell
2
has an associated and corresponding base station
1
. For example, cell
2
a
has an associated and corresponding base station
1
a
. Cell
2
b
has an associated and corresponding base station
1
b
, and so on. Each base station
1
typically includes an array of sectored antennas for communicating with the terminal stations within the cells
2
. In accordance with broadband wireless technology, a sectored antenna is typically 60 or 90 degrees in beamwidth for commnunicating with terminal stations within an entire sector. Thus, in a four-sector case, a base station
1
a
comprises at least four sectored antennas, one antenna per sector
4
(
4
a
-
4
d
). In a six-sector case, the base station comprises six sectored antennas. Each sector contains a plurality of terminal stations that communicate with the base station
1
a
on a unique radio frequency (RF) channel.
In broadband wireless systems, each terminal station utilizes a highly directional antenna (typically less than 3 degrees beamwidth) for communicating with its associated base station
1
. The highly directional antenna is fixed and pointed toward the associated base station
1
. The base station's sectored antenna receives energy from any terminal station operating on the same RF channel and is positioned on a line of sight relative to the sectored antenna. Line of sight (LoS) is defined herein as an unobstructed (first Fresnel zone clear) radio wave propagation path between a transmitting antenna and a receiving antenna. On the downlink, a base station's
1
sectored antenna transmits energy on an RF channel to a terminal station's highly directional antenna. On the uplink, a terminal station's highly directional antenna transmits energy on an RF channel to a base station's
1
sectored antenna.
In accordance with frequency re-use methodologies and techniques, a set of RF channels is allocated for use in each cell
2
(for example, cells
2
a
,
2
b
,
2
c
and
2
d
). As shown in
FIG. 1
a
, for example, each cell
2
utilizes a set of four orthogonal RF channels (A, A′, B, and B′) comprising two frequencies (A and B) wherein each frequency has two different polarizations (designated by the “non-primed” and “primed” indicators). Each sector
4
(
4
a
-
4
d
) of a cell
2
therefore utilizes a different orthogonal RF channel for communication between terminals in the sector and an associated sector base station (i.e., a terminal in sector
4
a
uses frequency A, a terminal in sector
4
b
uses frequency B′, a terminal in sector
4
c
uses frequency B, and a terminal in sector
4
d
uses frequency A′). The set of four orthogonal RF channels is then reused as shown in
FIG. 1
a
in adjacent cells
2
(for example, in cells
2
b
,
2
c
and
2
d
). As shown in
FIG. 1
a
, in each cell
2
(i.e., in cells
Gilbert Sheldon L.
Hadar Rami
Klein Israel Jay
Boling, Esq. William C.
Ensemble Communications
Hsu Alpus H.
Jaquez & Associates
Jaquez, Esq. Martin J.
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