Portable radio telephone and methods of operation

Details Portable radio telephone and methods of operation Portable radio telephone and methods of operation

1999-01-19

2001-10-30

Rao, Seema S. (Department: 2661)

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

C370S332000, C370S337000, C455S450000

Reexamination Certificate

active

06310867

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to portable radio telephones and in particular to such telephones for communication with base stations in a digital cellular radio telephone system employing transmission by a plurality of carrier frequencies in frames, such as TDMA frames, each consisting of a predetermined number of time slots. The invention also relates to a method of operation of such radio telephones, frequently called handsets. The invention is concerned with the allocation of a channel (that is a combination of a carrier frequency and time slot) to a portable radio telephone when a connection is first required (“call set-up”) or when a change in channel is required (“handover”) during a call to maintain call quality. Handover can either be to a different channel at the same base station (intracell) or to a different channel at a different base station (intercell).
Most of the currently manufactured DECT base station equipment only contains a single transceiver, and is therefore unable to open more than a single communication channel on different frequency carriers at the same instant. This restricts single transceiver base stations to only using a single time slot at any one time, effectively excluding the other carriers on this time slot and thereby turning them “blind”. The DECT standards have foreseen this and include the blind slot information message to inform the handset of time slots it should avoid using. However, this information is not totally reliable, particularly on intercell handover when it is very difficult to obtain the blind slot information of any base station other than the one that the handset is connected to. In addition to slots “blinded” by the base station, a handset will be unable to switch communication channels to a slot immediately adjacent to that which it is using.
This invention aims to provide a portable radio telephone, and a method of operation, employing a dynamic channel assignment algorithm that gives very robust performance, regardless of the availability and accuracy of the blind slot information. In addition to this, if an identical algorithm is deployed on every portable radio telephone in the system, further improvements in call blocking and call quality are likely.
SUMMARY OF THE INVENTION
According to the invention there is provided a portable radio telephone for communication with base stations in a digital radio telephone system employing transmission by a plurality of carrier frequencies in frames each consisting of a predetermined number of time slots, wherein the portable radio telephone is operative to select for the radio telephone a communication channel, that is a combination of carrier frequency and time slot, by storing information concerning slots excluded by virtue of transmission by the radio telephone or transmission by a communicating base station, and checking the slots in a predetermined sequence, which preferably corresponds to the temporal sequence of the slots or to the reverse of the temporal sequence of the slots, until an available non-excluded slot is found and a channel therein is selected.
According to another aspect the invention provides a method of communicating between a portable radio telephone and base stations in a digital cellular radio telephone system employing transmission by carrier frequencies in frames each consisting of a predetermined number of time slots, comprising storing information concerning slots excluded by virtue of transmission by the radio telephone or transmission by a communicating base station, and checking the slots in a predetermined sequence, which preferably corresponds to the temporal sequence of the slots or to the reverse of the temporal sequence of the slots, until an available non-excluded slot is found and a channel therein is selected.


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