Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-01-25
2004-03-09
Trinh, Sonny (Department: 2685)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S450000, C455S561000, C320S118000, C320S118000
Reexamination Certificate
active
06704572
ABSTRACT:
This invention relates to cellular communications systems and particularly to means for sharing resources between communications service operators.
In a cellular communications system, a plurality of base stations provides a radio telecommunications service to a plurality of remote subscriber units. Each base station defines a particular geographical area or “cell” proximate to the base station to produce coverage areas. The communications link from the base station to a subscriber unit is referred to as the down link. Conversely, the communications link from a subscriber unit to the base station is referred to as the uplink.
Multiple access techniques permit the simultaneous transmissions from several subscriber units to a single base station.
One type of multiple access techniques is known as code division multiple access (CDMA), employs spread spectrum signaling. Individual users in the CDMA communications network use the same carrier frequency, but are separated by the use of individual spreading codes. Hence, multiple communications channels are allocated using a plurality of spreading codes within the portion of radio spectrum, each code being uniquely assigned to a subscriber unit.
Another type of multiple access technique is the time division multiple access (TDMA) technique. Further schemes involve combinations of both TDMA and CDMA.
In a TDMA system, a communications channel consists of a time slot in a periodic train of time intervals over the same carrier frequency. Each period of time slots is called a frame. A given signal's energy is confined to one of these slots.
One particular type of TDMA scheme to which the invention is applicable is that known as a time division duplex (TDD) in which the same carrier frequency is used for both uplink and downlink. The repeating time frame is in this case divided into an interval with time slots used in the uplink direction and another interval with time slots used in the downlink direction. The allocation of time slots to either uplink or downlink transmission is controlled by the base station, taking into account the number of subscriber units it is required to support at any given time and this time frame structure is broadcast to each subscriber unit requiring communications services.
FIG. 1
illustrates the concept of this repeating time frame structure. Two repeating frames
1
and
2
are divided into a plurality of time slots
3
. During a portion
4
of each frame, the base station is in transmit mode and each mobile subscriber unit is in receive mode. During a portion
5
of each frame, the base station is in receive mode and each subscriber unit is in transmit mode. The point at which the mode of operation changes is called the switching point
6
.
In the example of
FIG. 1
, time slots
7
,
8
,
9
are allocated for communication with a first subscriber unit and time slots
10
,
11
,
12
are allocated for communications with a second subscriber unit. More subscriber units can be supported on the same single carrier by utilising the remaining time slots and still further, by employing the CDMA technique previously mentioned.
There is, of course, a finite limit to the number of subscriber units that a single base station can support at any given time and sometimes, the demand can exceed this limit.
Often, similar communications services throughout any given geographical area are provided by more than one communications services operator. An individual user may then choose to subscribe to one of the available operator's services and his subscriber unit is accordingly enabled to communicate only with those base stations under the control of his chosen operator.
As the radio spectrum is finite, each operator is allocated a portion of radio frequency spectrum (allocation often being controlled by some regulatory body).
Say for example, that a first operator has been allocated frequency f
1
for use in a TDD communications system and a second operator has been allocated frequency f
2
also for use in a TDD communications system. The first and second operators both have base stations installed to support their subscribers in the same geographical area, i.e. their respective cells could be adjacent or even overlap one another. It may be that at any given time, the first operator's base station is fully loaded, whereas the second operator's base station operating in the same area, is under utilised i.e. has some time slots available. In such a case, a subscriber unit subscribing to the first operator and attempting access to the fully loaded base station would be refused a service. It would be beneficial to this subscriber unit if it could use some of the unused resource of the second operator's base station on frequency f
2
. However, as the subscriber unit in question is not a subscriber to the second operator, he is unable to communicate with the second operator's base station on frequency f
2
.
This invention aims to provide a means for enabling an under-used resource to be reallocated in order to supplement an overloaded one.
Accordingly, the invention comprises:
A first base station for providing a communications service to at least one subscriber unit, wherein the first base station includes;
Means for receiving information relating to an assigned carrier frequency and an assigned time frame structure comprising a plurality of time slots of a communications resource provided by a second base station;
Means for identifying unused time slots comprising the communications resource; and
Means for establishing communication with a subscriber unit during the unused time slots and at the assigned carrier frequency.
Hence, by virtue of the invention, an overloaded base station can utilise the under-used resource of neighbouring base stations which are controlled by other operators. If the first base station identifies an under-utilised second base station, it can use the unoccupied time slots to serve one of its own subscribers. On receiving information concerning the carrier frequency and the time frame structure of the under-utilised resource, it can calculate a new time frame structure including a new switching point, which it can then transmit to the waiting subscriber unit along with the value of the carrier frequency it is to use. Communication is then established between the subscriber unit and the first base station at the assigned carrier frequency associated with the second base station. The first base station also continues to operate on its own previously assigned frequency with its own previously assigned time frame.
The invention has the advantage that it does not require two independent operators to closely synchronise their base stations with each other.
The means for receiving information relating to an assigned carrier frequency and an assigned time frame structure may be adapted to monitor a broadcast channel of the second base station.
The first base station may further include means for determining whether or not a resource from the second base station is available. This may take one of several forms. For example, the first base station may be adapted to receive a dedicated transmission from the second base station notifying it that the second base station is currently under-used. This dedicated transmission may be a radio transmission or one via a hard wired link between the two base stations, for example. Alternatively, the first base station may be pre-programmed to default to a condition whereby if it receives no dedicated transmission from the second base station, then it assumes that a resource is currently available.
Optionally, the first base station may include further means for receiving a message from the second base station when a previously available resource no longer becomes available for its own use.
Preferably, the first base station is adapted to constantly monitor the activities of the second base station, for example, by monitoring the broadcast channel, so that it can detect when communications with additional subscriber units are set up by the sec
Boscovic Dragan
Robinson William
Whinnett Nicholas
Motorola Inc.
Trinh Sonny
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