GB2285724A

GB2285724A - Selecting a channel in a TDMA system according to a variable threshold value

Info

Publication number: GB2285724A
Application number: GB9400880A
Authority: GB
Inventors: Nicholas William Whinnett; Bertrand Adrien Cormier
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1994-01-18
Filing date: 1994-01-18
Publication date: 1995-07-19
Anticipated expiration: 2014-01-18
Also published as: GB2285724B; GB9400880D0

Abstract

In a TDMA communications system where mobile stations communicate directly with each other without the help of a base station or system controller a mobile periodically samples the signal level 56 on each available channel. The samples are used to plot a cumulative probability distribution of carrier - Interference ratio 58 and to select a threshold value 59. To set-up a call a channel is selected and compared with the threshold value. If no channels offer a C/I greater than the threshold then the channel which meets the criterion for the longest time is selected 72. Alternatively, BER may be measured. <IMAGE>

Description

COMMUNICATIONS SYSTEM Field of the Invention This invention relates in general to a method for communicating in a TDMA communications system where a plurality of mobile stations communicate directly to one another over a communications resource.

Backerround to the Invention Trunked radio systems are well known and are generally referred to as communication systems that have a plurality of mobile stations that communicate via infrastructure. The infrastructure includes a base station and a system controller. The system controller may be located at the base station.

Radio communications systems make use of radio frequency communications channels for communication between the mobile stations and the base stations. Radio communications systems that make use of time division multiple access (TDMA) as an access mode are well known.

For the centrally controlled TDMA system, the mobile stations in the system will have access to a number of radio frequency carriers or carrier frequency pairs for communication, one or more of which may be available at each base site of the system. The carriers support control and traffic channels.

In TDMA communications systems a communications frequency or frequencies are divided into time frames and time slots. Mobile stations are assigned communications channels to support their communication. A communications channel is a frequency and time slot. The mobile stations communicate on their assigned communications channel through the system controller of the radio communications system.

The system controller effectively minimises co-channel and adjacent channel interference by centrally providing synchronisation information, allocating communications resources and controlling transmitting power.

Mobile stations may communicate directly to one another without the benefit of infrastructure which is referred to as direct mode operation. In direct mode operation there is no system controller. Thus, there is no centralised timing synchronisation or power control to help minimise interference.

In direct mode mobile stations communicate over dedicated frequencies. A direct mode mobile station may manually select a dedicated frequency or the direct mode mobile station may scan the available dedicated frequencies to find an available frequency based on signal strength measurements and a fixed threshold. Both methods of selecting and scanning are inefficient when there are a large numbers of users in the system. There is a high level of interference in such direct mode communications.

In some direct mode environments there may be a pool of communications channels available and the direct mode mobile station may scan the available communications channels and monitor the received signal levels of the channels and choose a communication channel based on the received signal level. In a TDMA system, scanning and decision making based on the received signal level and a fixed threshold is inefficient because the received signal level varies sporadically when monitoring frequencies and time slots over any period of time. In addition, interference is dependent upon the number of users and the number of users varies from time to time on a system.

Thus, it is desired to have mobile stations that normally access TDMA systems be capable of communicating in a direct mode operation without having to add components or circuitry to the mobile station. It is also desirable to have mobile stations operating in direct mode use the same coding scheme, modulation scheme, and channel definitions as a TDMA communications system and use a method for selecting a communication resource based upon a variable interference threshold value in order to minimise interference and maximise the communications range for a given density of transmissions.

Summarv of the Invention According to the present invention, there is provided a method for selecting an available communications resource for communication between direct mode mobile stations including periodically monitoring at least one communications channel available in the TDMA system to find a variable threshold value and selecting a communications resource for communication based on the variable threshold value dependent upon the level of interference present in the channel.

Brief Description of the Drawing FIG. 1 shows a communication system in different operating modes.

FIG. 2 shows one example of a direct mode environment.

FIG. 3 shows cumulative probability distributions of interference measurements for various transmission densities.

FIG. 4 is a flow chart for a preferred embodiment of a method in accordance with the present invention.

Detailed Description of the Preferred Embodiment Referring to FIG. 1, there is shown three operating modes of mobile stations in a communication system. FIG. la shows typical trunked mode communication between two mobile stations 2, 4 over trunked communication channels 11, 13. The mobile stations 2, 4 communicate via a system controller 10 which provides synchronisation and allocates the communications resources in a main TDMA communication system.

FIG. 1b shows a direct mode communication between mobile stations 2, 4 whereby the mobile station 2, 4 communicate directly to one another without the benefit of the system controller 10. FIG. ic shows another direct mode communication where one mobile stations 2 communicates directly to a number of mobile stations 4, 6, 8. Mobile stations that are able to operate in both direct mode and trunked mode are dual mode mobile stations.

The mobile stations and the base station communicate using a TDMA access mode. The direct mode mobiles communicate using the same frequencies, channel definitions, and modulation as in the centrally controlled TDMA system. Thus, a direct mode mobile may also employ the same time slot structure and coding schemes as in the centrally controlled TDMA system. Synchronisation may be provided by one of the mobile stations. For example, the mobile station that is initiating a communication may simply pass synchronisation information to receiving mobile stations before communication is actually set-up. Thus, the same mobile equipment may be used in both direct mode and system controlled mode without needing to add circuitry or components.

FIG. 2 shows one example of a direct mode system environment including a plurality of direct mode mobile stations. A receiving mobile station 21 is placed in the centre of a number of other mobile stations 22, 23.

A transmitting mobile station 23, is located some distance (transmitting distance) from the receiving mobile station 21. The transmitting mobile station 23 transmits a carrier signal 26 to the receiving mobile station 21.

All of the other mobile stations 22 are transmitting signals 25 that could interfere with the carrier signal 26. Thus, a resultant signal is received at the receiving mobile station 21 from the transmitting mobile station 23 and is called an interfering signal 27.

A C/I (carrier signal/interfering signal) ratio value may be determined for a particular time slot and frequency by measuring a interference signal (I) level received at the mobile station 21 on the particular time slot and frequency. The C value is constant for a particular transmitting distance.

Alternatively, a noise level may be used as a parameter instead of C/I.

FIG. 3 shows cumulative probability distributions of C/I for 1 kilometre transmitting distance and various transmission densities. The C value used for the calculations of the cumulative probability distributions of FIG. 3 is 10-13 W. The receiving mobile station 21 measures interference signal levels on a number of different frequencies and time slots (communications channels) for a period of time. A C/I value is calculated for each measurement and a cumulative probability distribution is plotted as in FIG. 3. In this example, the resulting cumulative probability distribution is a Gaussian distribution. The cumulative probability distribution varies dependent upon the density of transmissions in the system at the time when the interference measurements are taken. A threshold value may be selected dependent upon the resulting cumulative probability distribution.

For example, if curve 32 is plotted and the desired probability is 50% then the threshold value selected should be 20 dB.

The distribution curves shift to the left on the plot of FIG. 3 as the density of transmissions increases. The curve to the far left is based on a density of transmissions (d) value equal to 7 with a transmitting value equal to lkm. The curve to the far right is based on a density of transmissions value equal to .5 with a transmitting distance equal to lkm. Thus, as the density of transmissions increases the C/I values decrease therefore selected threshold values must decrease to be optimum.

FIG. 4 shows a method of selecting a communications channel according to the present invention. The receiving mobile station 21 (hereinafter mobile station) 21 must be in direct mode as in step 50. The mobile station 21 may select direct mode manually such as by a conventional switch mechanism on his mobile station or the mobile station 21 may automatically switch into direct mode when it unable to communicate to a valid base station. If the mobile station user needs to make a call immediately as in step 52 then the mobile station 21 communicates on any available communications channel as in step 74, otherwise the mobile station 21 operates in idle mode, step 54.

The mobile station 21 samples signal levels on various communications channels to determine interference on a number of different frequencies and time slots for a period of time as in step 56. The mobile station 21 plots a cumulative probability distribution of the sampling as in step 58. The mobile station selects a C/I threshold level based on the cumulative probability distribution as in step 59. The mobile station does not have to actually plot a cumulative probability distribution but may simply chose a maximum threshold value from the measurements.

If the mobile station wants to set up a call as decided in step 66, the mobile station scans a communications channel for a period of time and measures interference signal levels as in step 60. If the interference level is below the threshold value all of the time as decided in step 62, the communications channel is selected for communication as in step 64.

If the interference level is not below the threshold value all of the time as decided in step 62, and if the mobile station has not scanned a predetermined maximum number of channels as decided in step 63, the mobile station scans another communications channel as in step 60.

If the mobile station has scanned a predetermined number of communications channels, step 63, and has not found an available communications channel where the interference level is below the threshold all of the time the mobile station communicates on a communications channel that has an interference level below the threshold the highest proportion of time as in step 72.

If the mobile station does not want to initiate communication as decided in step 66 the mobile station waits in idle mode a period of time as in step 68. After a maximum period of waiting time, step 70, the mobile station repeats steps 56, 58 and 59 to determine a new maximum threshold value.

Since the threshold value is dependent upon the density of transmissions and the density of transmissions does not remain constant it is a good idea to change the threshold value often. Thus an adjustable threshold value determines the best communications channel available to communicate during a particular time.

In an alternative embodiment of the present invention the C/I parameter may be converted to a bit error rate (BER) value. Since the present invention includes mobile stations that do not have the benefit of a system clock and thus the C/I varies sporadically, the (BER) would be an appropriate measure of selecting a communications channel. Similarly, the mobile station measures an interference signal level for a number of different frequencies and time slots for a period of time. If the C value is known then a BER value may be calculated for each interference signal level measured. Depending on the modulation method being employed there is a respective f(C/I) that calculates the BER. A communications channel that has an average BER over a period of time that is less than a predetermined BER threshold may be selected or the communications channel with the lowest BER may be selected. Similarly, the BER will increase as the density of transmissions increases, thus, the threshold value could be increased.

The C value could be measured on a signalling channel prior to selecting the traffic channel.

Thus, there is provided a method for selecting an available communications resource based on the variable threshold value for a direct mode mobile station that normally communicates in a system controlled TDMA communications system.

Claims

1. A method for selecting an available communications resource for communication in a TDMA communications system including a plurality of mobile stations that communicate directly to one another without the benefit of infrastructure using the same coding scheme, modulation scheme, and channel definitions as a main TDMA communications system involving mobile to mobile communication via radio infrastructure, the method for communicating comprising the steps of: periodically monitoring at least one communication channel available in the TDMA system to find an adjustable threshold value dependent upon the level of interference present in the channel; and selecting a communications resource for communication based on the adjustable threshold value.

2. The method of claim 1 wherein the step of monitoring is performed when the mobile station is in an idle mode of operation.

3. The method of claims 1 or 2 wherein the step of monitoring at least one communication channel further comprises the step of plotting a probability distribution level of interference.

4. The method of claims 1 or 2 wherein the step of monitoring at least one communication channel further comprises the step of calculating a Bit Error Rate estimate.

5. The method of any of the preceding claims further comprising the step of monitoring signals from a system controller of the TDMA communication system by the mobile station periodically monitoring the at least one communication channel.

6. A method for selecting an available communications resource for communication substantially as herein described with reference to FIG. 4 of the drawing.