Telecommunications – Carrier wave repeater or relay system – Directive antenna
Reexamination Certificate
1998-12-22
2001-11-20
Kincaid, Lester G. (Department: 2687)
Telecommunications
Carrier wave repeater or relay system
Directive antenna
C455S562100, C455S456500, C455S522000
Reexamination Certificate
active
06321066
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for directional radio communication in which signals between a first station and a second station may be transmitted only in certain directions. In particular, but not exclusively, the present invention is applicable to cellular communication networks using space division multiple access.
BRIEF DESCRIPTION OF RELATED DEVELOPMENT
With currently implemented cellular communication networks, a base transceiver station (BTS) is provided which transmits signals intended for a given mobile station (MS), which may be a mobile telephone, throughout a cell or cell sector served by that base transceiver station. However, space division multiple access (SDMA) systems have now been proposed. In a space division multiple access system, the base transceiver station will not transmit signals intended for a given mobile station throughout the cell but will only transmit the signal in the beam direction from which a signal from the mobile station is received. SDMA systems may also permit the base transceiver station to determine the direction from which signals from the mobile station are received.
SDMA systems may allow a number of advantages over existing systems to be achieved. In particular, as the beam which is transmitted by the BTS may only be transmitted in a particular direction and accordingly may be relatively narrow, the power of the transceiver can be concentrated into that narrow beam. It is believed that this results in a better signal to noise ratio with both the signals transmitted from the base transceiver station and the signals received by the base transceiver station. Additionally, as a result of the directionality of the base transceiver station, an improvement in the signal to interference ratio of the signal received by the base transceiver station can be achieved.
SUMMARY OF THE INVENTION
Furthermore, in the transmitting direction, the directionality of the BTS allows energy to be concentrated into a narrow beam so that the signal transmitted by the BTS can reach far away located mobile stations with lower power levels than required by a conventional BTS. This may allow mobile stations to operate successfully at greater distances from the base transceiver station which in turn means that the size of each cell or cell sector of the cellular network can be increased. As a consequence of the larger cell size, the number of base stations which are required can also be reduced leading to lower network costs. SDMA systems generally require a number of antenna elements in order to achieve the required plurality of different beam directions in which signals can be transmitted and received. The provision of a plurality of antenna elements increases the sensitivity of the BTS to received signals. This means that larger cell sizes do not adversely affect the reception of signals by the BTS from mobile stations.
SDMA systems may also increase the capacity of the system, that is the number of mobile stations which can be simultaneously supported by the system is increased. This is due to the directional nature of the communication which means that the BTS will pick up interference from mobile stations in other cells using the same frequency. The BTS will generate less interference to other mobile stations in other cells using the same frequency when communicating with a given MS in the associated cell.
Ultimately, it is believed that SDMA systems will allow the same frequency to be used simultaneously to transmit to two or even more different mobile stations which are arranged at different locations within the same cell. This can lead to a significant increase in the amount of traffic which can be carried by cellular networks.
SDMA systems can be implemented in analogue and digital cellular networks and may be incorporated in the various existing standards such as GSM, DCS 1800, TACS, AMPS and NMT. SDMA systems can also be used in conjunction with other existing multiple access techniques such as time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA) techniques.
One problem with SDMA systems is that the direction in which signals should be transmitted to a mobile station needs to be determined. In certain circumstances, a relatively narrow beam will be used to send a signal from a base transceiver station to a mobile station. Therefore, the direction of that mobile station needs to be assessed reasonably accurately. One problem with SDMA systems is that although the signals from mobile stations which are beyond a certain distance from the BTS are generally only received from one or only a few different beam directions, signals from mobile stations which are relatively close to base transceiver stations appear to come from a large number of beam directions. This is because the signals from the mobile station are reflected from, for example, nearby buildings and a relatively large number of those reflected signals will be received by the base transceiver station from a large number of different beam directions.
An additional problem is that the direction in which a signal is to be transmitted by the BTS to the mobile station is determined on the basis of the uplink signals received by the BTS from the mobile station. However, the frequencies of the downlink signals transmitted from the mobile station to the BTS are different from the frequencies used for the signals transmitted by the BTS to the mobile station. The difference in the frequencies used in the uplink and downlink signals means that the behaviour of the channel in the uplink direction may be different from the behaviour of the channel in the downlink direction. Thus the optimum direction determined for the uplink signals will not always be the optimum direction for the downlink signals.
It is therefore an aim of certain embodiments of the present invention to address difficulties caused by these problems.
According to a first aspect of the present invention, there is provided a method of directional radio communication in a mobile communication network between a first station and a second mobile station, said method comprising the steps of:
receiving at said first station a signal transmitted by said second station;
monitoring a parameter representative of the distance between the second station and the first station; and
transmitting a signal beam from said first station to said second station, wherein the angular spread of the signal beam transmitted by the first station is dependent on the distance represented by said parameter.
By varying the angular spread of the signal beam transmitted by the first station in dependence on the distance parameter between the first and second stations, the difference in behaviour resulting from the second station either being close to or far from the first station can be compensated for.
The distance parameter may be monitored at the first station. Preferably, the distance parameter is determined based on signals received by the first station from the second station. In one embodiment, the distance parameter is the angular spread of the signal received by the first station from the second station. When the first station is relatively far from the second station, the signal received by the first station will have a relatively small angular spread whilst if the second station is relatively close to the first station, the signal will be received with a relatively large angular spread. It should be appreciated that this technique will not give an absolute indication of the distance but will nevertheless provide sufficient information in order to allow the angular spread of the signal beam to be transmitted by the first station to be determined.
In an alternative embodiment, the distance parameter is determined based on the length of time taken for the signal from the second station to reach the first station. Said distance parameter may be timing advance information which is normally provided for controlling communication of data between the
Katz Marcos
Kiiski Matti
Kincaid Lester G.
Nokia Telecommunications Oy
Perman & Green LLP
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