Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-10-07
2001-09-25
Cumming, William (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C370S342000, C370S347000, C455S450000, C455S453000
Reexamination Certificate
active
06295453
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to cellular telephone systems, and, in particular, to a method for selection and assignment of traffic channels to a multiple full rate call.
2. Description of Related Art
Cellular telephone systems divide a large service area into a number of smaller discrete geographical areas called “cells” each typically ranging in size from about one-half to about twenty kilometers in diameter. Each cell is contiguous with multiple adjacent cells to provide continuous coverage throughout the service area. A base station including a plurality of transceivers capable of operating independently on different radio frequencies is provided for each of the cells. Via the transceivers, the base stations engage in simultaneous communications with plural mobile stations operating within the area of the associated cell. The base stations further communicate via data links (and voice trunks) with a central control station, commonly referred to as a mobile switching center, which functions to selectively connect telephone calls to and from the mobile stations through the base stations and, in general, control operation of the system.
Each cell is allocated the use of a predetermined set of frequencies, wherein each frequency comprises a physical channel supporting a plurality of logical channels (i.e., time slots) therein. The availability of multiple frequencies per cell, with multiple logical channels per frequency, permits base stations to simultaneously handle many telephone conversations with many mobile stations. The frequencies allocated to a cell are preferably spaced apart across the frequency spectrum of the cellular band. This serves to minimize the instances of adjacent channel interference.
Because only a limited number of frequencies are available in the cellular band, an allocation of the same frequencies is repeated (i.e., reused) in other cells in a distant part of large service areas with many cells. No adjacent cells, however, are assigned the same frequency. Furthermore, the power levels of the signal transmissions on any given frequency are limited in strength. The foregoing precautions serve to minimize the likelihood of co-channel interference caused by reuse of that same frequency in a distant cell.
The mobile switching center further functions to dynamically assign the traffic channels available in any one cell among the plurality of mobile stations located within the cell area that desire communications. In this regard, the traffic channels comprise time slots in digital time division multiple access (TDMA) formatted frequencies which are allocated to the cell. Commands that assign a logical channel for a mobile station use a certain digital traffic channel assigned to a given cell and are transmitted from the mobile switching center to the base station for that cell. The commands are then relayed by the base station to the certain mobile station over one of the channels to direct mobile station selection of the assigned traffic channel for handling the call.
Assignment by the mobile switching center of a particular digital traffic channel to a particular mobile station for carrying a cellular communication (i.e., a call) primarily occurs in three instances. The first instance is at call set-up when the subscriber activates the mobile station to initiate a call and the system selects for assignment the traffic channel to carry that new call. The second instance is at call hand-off when the subscriber, while engaged in a call, moves from one cell in the service area to another cell, and the system selects for assignment the traffic channel in the new cell that will continue the handling of the on-going call. The third instance is at an intra-cell hand-off, which occurs when signal strength is good, but quality is bad. In all three cases, it is important that the traffic channel selected for assignment at call set-up or hand-off be of the highest quality possible.
Conventionally, the selection and assignment by the system of a traffic channel to carry a call has been performed by randomly choosing a logical channel from the available (idle) digital traffic channels allocated to the cell currently serving the mobile station. Alternatively, the system maintained a record of historic traffic channel use for each cell (normally in the form of a FIFO queue), and the traffic channel selected to carry the call was the channel allocated to that cell that had not been assigned for mobile station calling use in the longest period of time. While each of these selection methods was successful in assigning a traffic channel to a mobile station for carrying the call, the selection failed to take into consideration whether the traffic channel is the best quality channel available to carry the call.
An improved prior art method for assigning traffic channels to carry a call in response to a call set-up, or hand-off gave primary consideration to whether the traffic channel was the best quality channel, based upon a set of metrics, available to carry the call. Examples of such metrics used to evaluate channel quality include: interference level on the uplink or downlink, power levels used by the mobile and base stations, and efficient logical channel usage (i.e., frequency packing). One known method for metric-based quality channel selection comprises interference driven channel selection (IDCS). Signal strength measurements are made by a signal strength measurement device in each cell base station on those frequencies allocated to that cell which have idle traffic channels. From these measurements, the injected uplink interference (comprising either adjacent channel, co-channel, or other type of interference) on the frequencies for each of the idle traffic channels is determined and reported to the mobile switching center. The reported interference measurements are then sorted in relative order of idle traffic channels having the best measured interference to the worst measured interference. The sorted list is then used by the system in selecting, for a base station, a traffic channel from the frequency with the best interference for assignment at call set-up or call hand-off.
A voice communication normally occupies only a single logical channel. This is referred to as a full rate (FR) channel call. A single full rate channel may not, however, have sufficient bandwidth to efficiently support some kinds of data communications. These communications may be better (and perhaps only) handled by simultaneously using plural logical channels. This is referred to as a multi-full rate channel call. It is recognized that different cellular systems utilize a different number of time slots per full rate channel, and further support a different maximum number of full rate channels (nFR) within a frequency channel.
There is a need for a system to intelligently assign multi-full rate calls, up to nFR, to available traffic channels. Preferably, such a system would take advantage of metric-based quality channel selection in making these call assignments.
SUMMARY OF THE INVENTION
Calls handled by a cellular phone system specify whether they request multiple full rate service. These calls are assigned to traffic channels by a process that first sorts the available and acceptable traffic channels for a cell. Second, sorts the first sort identified full rate channels into multi-full rate channel combinations meeting the requested service rate. Third, assigns the call in that cell to one of the second sort identified multi-full rate channel combinations.
More specifically, the multi-full rate channel assignment process of the present invention responds to a received call request by performing a full rate channel sort to identify available full rate channels that meet a requested service type (comprising frequency band, modulation parameters, and potentially any other radio link parameters) below an operator allowed cost threshold related to measured radio condition metrics. A multi-full rate sort is then performed on the id
Desgagne Michel
Lachance Matthieu
Cumming William
Jenkens & Gilchrist
Telefonaktiebolaget LM Ericsson (publ)
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