Cell identification based on distance

Details Cell identification based on distance Cell identification based on distance

1998-11-20

2001-09-04

Le, Thanh Cong (Department: 2749)

Telecommunications

Radiotelephone system

Zoned or cellular telephone system

C455S436000, C455S423000, C455S426100, C455S517000, C455S446000

Reexamination Certificate

active

06285874

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to the problem of determining the identity of a cell for e.g. cell handoff or automatic frequency planning, and more particularly to the problem of determining the true identity of a cell when only the transmission frequency and a non-unique identity code is known.
RELATED ART
In a cellular mobile communications system as shown in
FIG. 1
, a user of a mobile station, e.g. MS
1
-MS
4
, communicates with the system through a radio interface while moving around the geographic area covered by the system. The radio interface between the mobile station and system is implemented by providing base stations, e.g. BS
1
-BS
10
, spread throughout the geographical area of the system. Each base station is capable of radio communication with the mobile stations operating within the system.
In a typical mobile system each base station of the system controls communications within a certain geographic coverage area ideally represented by a hexagonal shape called a cell. A mobile station, e.g. MS
1
, which is located within this cell communicates with the base station controlling that cell, e.g. BS
1
. When a call is initiated by the user of this mobile station MS
1
, or received at the system and destined for this mobile station MS
1
, the call is set up on radio channels assigned to the base station BS
1
controlling the cell A in which the mobile station MS
1
is located. There are only 10 cells A-J shown in FIG.
1
. Obviously mobile systems are not limited to such a small size and 10 cells are shown merely for illustration. Similarly only 4 mobile stations MS
1
-MS
4
are shown while there can be numerous mobile stations operating within each cell. In addition to the base stations there is also a Mobile Switching Center MSC that controls communications between the system, which is itself in contact with other systems, and the base stations. The MSC is normally connected by several lines, not shown here, to each base station.
If a mobile station moves away from the original cell in which the call was set up, and the signal strength on the radio channels of the original cell weakens, the system will affect a transfer of the call to radio channels of a base station controlling a neighbor cell into which the mobile station moves. For example, cell A will normally have as its neighbors cells BG. As the mobile station moves through the system, control of the call may be transferred from the neighboring cell to another cell. This transfer of the call from cell to cell is termed handoff.
Handoff can only be effective if the call is transferred to radio channels that provide adequate signal strength for two-way communications. This requires sufficient signal strength at both the receiver of the mobile station and receiver of the base station to which the handoff is made. The signals must also be sufficiently strong in relation to any noise or interference in the network.
To perform a handoff it is necessary that some sort of signal strength or interference level measurement process be used to determine which of the neighboring cells is to be selected for handoff. Various measurement processes are known in the art and are not the subject of the present invention. In current systems this is done by either making measurements at the receivers of neighboring base stations on signals transmitted from the mobile station, by making measurements at the receiver of the mobile station on signals transmitted from neighboring base stations, or by making measurements at both mobile stations and base stations.
For example, in the analog cellular system based on the EIA/TIA—533 Mobile Station—Land Station Compatibility Specification (AMPS), handoff measurements are done solely at neighboring base stations. When a measurement at the base station, e.g. BS
6
, covering the cell, e.g. cell F, in which the mobile, e.g. MS
2
, is currently operating indicates that the signal strength from that mobile has dropped below a certain level, the base station BS
6
informs the mobile switching center MSC in control of the system. The MSC then initiates the handoff measurement process by ordering neighboring base stations, e.g. BS
1
BS
5
BS
7
-
10
, to monitor the signal transmitted by the mobile station MS
2
on the current radio channel assigned to the call, and measure the strength of the signal. After receiving measurement results, the MSC will then initiate the handoff of the call from the current cell F to the cell, A, E, H, I, J, or G, containing the base station reporting the highest received signal strength.
The MSC uses a “neighbor cell list” that is associated with the current cell to determine which base stations receive the measurement order from the MSC. This neighbor cell list is created and set manually by the system operator and remains fixed until the operator later manually modifies the list. Because this type of handoff measurement process consumes a large amount of signalling capacity, the number of cells on the neighbor cell list is limited. The neighbor cell list usually contains the cells bordering the current cell. If hexagonal cell shapes, as in
FIG. 1
, are used, this yields six bordering cells.
As contrasted with analog systems, digital systems usually allow the handoff measurement process to be performed at the mobile station. An example of this is the IS-54 standard using time division multiple access (TDMA) signal transmission. In TDMA, communications between a base station and a particular mobile station are transmitted on radio channels that may also be used for communications between the same base station and several other mobile stations.
Voice transmissions are digitized and transmitted as bursts in timeslots that are time multiplexed on the radio channels. Each mobile station in communication with a base station is assigned a timeslot on both the reverse channel and forward channels. The assigned timeslots are unique for each mobile station so communications between different mobiles do not interfere with each other.
In IS-54B the handoff method is called Mobile Assisted HandOff (MAHO). Handoff measurement is done at the mobile station when the mobile is neither transmitting on the reverse channel timeslot nor receiving on the forward channel timeslot. During these times between bursts the mobile station periodically monitors radio channels of each base station located in close proximity. The control channel of each neighboring base station is typically used as the measurement channel.
For each call the measurement channels are contained in the neighbor cell list of the cell in which the call is proceeding. In addition to measuring over these measurement channels, the mobile also measures the received signal strength on the current channel on which the call is proceeding. The mobile transmits these measurement results to the current base station which then forwards them to the MSC. If the received signal strength on the current channel falls below the strength on a measurement channel of a neighboring cell, the MSC initiates handoff to that neighboring cell.
Since a MAHO is performed primarily within the mobile station, resources for performing it are limited. Because only a limited number of measurements can be performed each second, it is necessary to average measurements to provide a reliable signal strength value. Thus, it is necessary to limit the number of cells that comprise the neighbor cell list for MAHO purposes. IS-54B standards are limited to 12 cells, while IS136 sets a size limit of 24 cells.
When the system operator creates a neighbor cell list for a cell, he wishes to ensure that calls in the cell can be handed over to a second cell, no matter what type of movements take place. A difficulty with this is that the actual coverage area of a cell is difficult to predict. The size and shape of a cell's coverage area may vary due to different effects such as base stations being located in different positions or shadowing effects on radio coverage caused by obstacles, e.g. buildings.
The best cell for a handoff

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