Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
1997-11-17
2004-02-17
Trost, William (Department: 2683)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S561000, C455S277100, C455S277200, C455S279100
Reexamination Certificate
active
06694154
ABSTRACT:
BACKGROUND
The present invention pertains to a system and method for efficiently cancelling interference in a radio communication system using a directional antenna and one or more search beams.
FIG. 1
illustrates a conventional cellular radio communication system
100
. The radio communication system
100
includes a plurality of radio base stations
170
a-n
connected to a plurality of corresponding antennas
130
a-n
. The radio base stations
170
a-n
in conjunction with the antennas
130
a-n
communicate with a plurality of mobile terminals (e.g. terminals
120
a
,
120
b
and
120
m
) within a plurality of cells
10
a-n
. Communication from a base station to a mobile terminal is referred to as the downlink, whereas communication from a mobile terminal to the base station is referred to as the uplink.
The base stations are connected to a mobile telephone switching office (MSC)
150
. Among other tasks, the MSC coordinates the activities of the base stations, such as during the handoff of a mobile terminal from one cell to another. The MSC, in turn, can be connected to a public switched telephone network
160
, which services various communication devices
180
a
,
180
b
and
180
c.
A common problem that occurs in a cellular radio communication system is the loss of information in the uplink and downlink signals as a result of multi-path fading, which results when the transmitted signal travels along several paths between the base station and the intended receiver. When the path lengths between the base station and the mobile terminal are relatively small, the multiple signal images arrive at almost the same time. The images add either constructively or destructively, giving rise to fading, which typically has a Rayleigh distribution. When the path lengths are relatively large, the transmission medium is considered time dispersive, and the added images can be viewed as echoes of the transmitted signal, giving rise to intersymbol interference (ISI).
Fading can be mitigated by using multiple receive antennas and employing some form of diversity combining, such as selective combing, equal gain combining, or maximal-ratio combining. Diversity takes advantage of the fact that the fading on the different antennas is not the same, so that when one antenna has a faded signal, chances are the other antenna does not. ISI from multi-path time dispersion can be mitigated by some form of equalization, such as linear equalization, decision feedback equalization, or maximum likelihood sequence estimation (MLSE).
Interference can also degrade the signals transmitted between a base station and mobile terminals. For instance, a desired communication channel between a base station and a mobile terminal in a given cell can be degraded by the transmissions of other mobile terminals within the given cell or within neighboring cells. Other base stations or RF-propagating entities operating in the same frequency band can also create interference (through “co-channel” or “adjacent channel” interference).
Frequency re-use can be used to mitigate interference by locating interfering cells as far from each other as possible. Power control can also be used to reduce the interference by ensuring that transmitters communicate at minimal effective levels of power. Such power control techniques are especially prevalent in code-division multiple access systems, due to the reception of information in a single communication channel at each base station.
Interference can be reduced still further by using a plurality of directional antennas to communicate with mobile terminals within a cell. The directional antennas (also known as “sector antennas”) transmit and receive energy within a limited geographic region, and thereby reduce the interference experienced by those radio units outside such geographic region. Typically, radio communication cells are partitioned into three 120° sectors serviced by three sector antennas, or six 60° sectors serviced by six sector antennas. Even smaller antenna sectors can be achieved using a fixed-beam phased array antenna, which transmits and receives signals using a plurality of relatively narrow beams.
FIG. 2
, for instance, illustrates such an exemplary radio communication system
200
including a radio base station
220
employing a fixed-beam phased array (not shown). The phased array generates a plurality of fixed narrow beams (B
1
, B
2
, B
3
, B
4
, etc.) which radially extend from the base station
220
. Preferably, the beams overlap to create a contiguous coverage area to service a radio communication cell. Although not shown, the phased array can actually consist of three phased array sector antennas, each of which communicates with a 120° swath extending from the base station
220
.
FIG. 2
shows a mobile terminal
210
located within the coverage of one of the beams, B
1
. Communication proceeds between the base station
220
and this mobile terminal
210
using the beam B
1
, or perhaps, in addition, one or more adjacent beams. The reader will appreciate that modern radio communication environments typically include many more mobile terminals within cells. Nevertheless, even when there are plural mobile terminals within a cell, a subset of the beams may not include any mobile terminal stations within their coverage. Hence, in conventional fixed-beam phased array systems, these beams remain essentially idle until a mobile terminal enters their assigned geographic region. Such idle beams propagate needless energy into the cell, and thus can contribute to the net interference experienced by radio units within the cell as well as other cells (particularly neighboring cells). These beams also add to the processing and power load imposed on the base station
220
.
These concerns are partly ameliorated though the use of a variation of the above-discussed system, referred to as “adaptive” phased arrays. Such arrays allow for the selective transmission and reception of signals in a particular direction. For instance, as shown in
FIG. 3
, an array
300
can be used to receive a signal transmitted at an angle &kgr; (with respect to the normal of the array) from a target mobile terminal
380
, and can simultaneously cancel the unwanted signals transmitted by another mobile terminal
370
. This is accomplished by selecting weights (w
1
, w
2
, . . . w
n
) applied to each signal path (r
1
, r
2
, . . . r
3
) from the phase array antenna
300
so as to increase the sensitivity of the array in certain angular directions and reduce the sensitivity of the array in other directions (such as by steering a null toward an interference source). The desired weighting is selected by iteratively changing the weights through a feedback loop comprising beamforming unit
340
, summer
330
and controller
320
. The feedback loop functions to maximize signal-to-interference ratio at the output “x” of the beamforming unit. Application of an adaptive phased array antenna to the radio communication system shown in
FIG. 1
would result in the generation of a single beam (or small subset of beams) generally oriented in the direction of the single mobile terminal
210
. Such a system offers a substantial reduction in interference. For example, as disclosed in “Applications of CDMA in Wireless/Personal Communications” by Garg et al., Prentice Hall, 1997, an idealized eight-beam antenna could provide a threefold increase in network capacity when compared with existing schemes such as cell splitting (pp. 332-334). Interested readers are referred to the following documents for further details regarding adaptive phased arrays as well as information regarding adaptive diversity arrays: “Adaptive Arrays and MLSE Equalization” by G. E. Bottomley et al., Proc. VTC '95, Chicago, Ill., July 1995, pp. 50-54; “Signal Acquisition and Tracking with Adaptive Arrays in the Digital Mobile Radio System IS-54 with Flat Fading” by J. H. Winters, IEEE Transactions on Vehicular Technology, Vol. 42, No. 4, November 1993; “Adaptive Array Methods for Mobile Communication” by
S. Simanapalli
, Proc. 44th IEEE Veh.
Bottomley Gregory E.
Chennakeshu Sandeep
Molnar Karl J.
Coats & Bennett P.L.L.C.
Ericsson Inc.
Trost William
Zewdu Meless M
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