Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-11-15
2002-07-02
Le, Thanh Cong (Department: 2684)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C455S436000, C455S442000
Reexamination Certificate
active
06414947
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a communication network and a method of allocating resource therefor, and in particularly to a CDMA cellular radio communication network and a method therefor.
BACKGROUND OF THE INVENTION
In a cellular communication system each of the user terminals (typically mobile stations) communicates with typically a fixed base station (base station transceiver—BTS). Communication from the user terminal to the base station is known as uplink and communication from the base station to the user terminal is known as downlink. The total coverage area of the system is divided into a number of separate cells, each predominantly covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells.
FIG. 1
illustrates a cellular communication system
100
. In the system, a base station
101
communicates with a number of user terminals
103
over radio channels
105
. In the cellular system, the base station
101
covers users within a certain geographical area
107
, whereas other geographical areas
109
,
111
are covered by other base stations
113
,
115
.
As a user terminal moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the user terminal and the base station of the first cell, to being between the user terminal and the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.
All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. A call from a user terminal is routed through the fixed network to the destination specific for this call. If the call is between two user terminals of the same communication system the call will be routed through the fixed network to the base station of the cell in which the other user terminal currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a user terminal and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
A cellular mobile communication system is allocated a frequency spectrum for the radio communication between the user terminals and the base stations. This spectrum must be shared between all user terminals simultaneously using the system.
One method of sharing this spectrum is by a technique known as Code Division Multiple Access (CDMA). In a Direct Sequence CDMA (DS-CDMA) communication system, the signals are, prior to being transmitted, multiplied by a high rate code whereby the signal is spread over a larger frequency spectrum. A narrowband signal is thus spread and transmitted as a wideband signal. At the receiver-the original narrowband signal is regenerated by multiplication of the received signal with the same code. A signal spread by use of a different code will at the receiver not be de-spread but will remain a wide band signal. In the receiver the majority of interference caused by interfering signals received in the same frequency spectrum as the wanted signal can thus be removed by filtering. Consequently a plurality of user terminals can be accommodated in the same wideband spectrum by allocating different codes for different user terminals. Codes are chosen to minimise the interference caused between user terminals typically by choosing orthogonal codes when possible. A further description of CDMA communication systems can be found in ‘Spread Spectrum CDMA Systems for Wireless Communications’, Glisic & Vucetic, Artech house Publishers, 1997, ISBN 0-89006-858-5. Examples of CDMA cellular communication systems are IS 95 standardised in North America and the Universal Mobile Telecommunication System (UMTS) currently under standardisation in Europe.
Typically, CDMA communication systems use a handover technique known as soft hand over. In a soft handover, a user terminal is simultaneously served by more than one base station.
FIG. 2
illustrates the situation where a user terminal
201
is in soft handover with two base stations
203
,
205
each covering a cell
207
,
209
. In the uplink the signal received at each base station
203
,
205
is communicated to a controller
211
where the signals are combined and the received data is recovered. In the downlink the data to be transmitted to the user terminal is communicated from the controller to both base stations
203
,
205
and both base stations simultaneously transmit the data to the user terminal
201
. The user terminal
201
combines the two signals and demodulates the signal.
The quality of the radio communication between the user terminal and the base station is determined by the signal to noise level of the signals where the noise includes both thermal noise and interference noise. Other base stations and user terminals generate interference, which increases the noise level and thus reduces the quality. In order to attain an acceptable quality level the interference must thus be kept sufficiently low. A major technique for interference reduction in CDMA system is use of power control whereby the transmitted power of each user terminal and base station is maintained at the minimum level required for the signal to be received at an acceptable quality. Uplink power control can be implemented by the base station measuring the received signal quality and transmitting power up information to the user terminal when the signal quality is below an acceptable level, and power down information when the signal quality is above this level. Similarly, downlink power control can be implemented by the user terminal transmitting power up or power down information depending on the signal quality of the signal received at the user terminal.
In a communication system such as cellular communication systems, efficient resource control is essential for achieving the highest performance of the communication system. The communication thus comprises means for controlling the transmission of data services, allocating resources to different users, scheduling the transmission of data, providing error checks, determining serving base station etc.
The procedures and routines required for controlling data transmission in communication systems are typically considered in the frame of a layered logical structure where lower layers provide functionality to higher layers. The lowest layer is the physical layer which is responsible for communicating data bits over the radio communication links between base stations and user terminals. The physical layer thus provides a number of bit communication services to the next layer which is the MAC (Medium Access Control) layer. The MAC layer provides functionality for allocation of available resource to different users and for scheduling data transmissions to meet required quality of service. Above the MAC, the RLC (Radio Link Control) protocol allows retransmissions of errored data across the radio link. The Radio Resource Control sits above the RLC and covers functions including call admission, configuration of other entities (like the MAC and physical layer) and management of radio resources across multiple cells.
The resource allocation and data scheduling is a very complicated process where allocation in one cell may impact the performance in a different cell. This is specifically problematic for a communication network such as a CDMA network operating in soft handover where a specific user terminal may be served by several parallel communication links from different base stations. Hence, ideally, a resource allocation should consider conditions and resource requirements in all cells and perform a global optimisat
Barrett Stephen John
Legg Peter Jonathon
Cong Le Thanh
Motorola Inc.
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