METHOD OF RESOLVING COLLISIONS IN DIRECT MODE OPERATION

Abstract: A method of resolving collisions between transmissions in a system in which users use transceivers such as half-duplex RF transceivers to talk to each other on a common channel. During transmission, each transceiver detects pauses in its user's speech, for example using a Voice Activity Detection algorithm. During a pause, the transceiver monitors the common channel for transmissions of other transceivers. If the transceiver detects such transmissions, it disables itself temporarily from transmitting and generates a busy signal to the user.

Description: METHOD OF RESOLVING COLLISIONS IN DIRECT MODE OPERATION FIELD OF THE INVENTION The present invention relates to radio communications and, more particularly, to a method for resolving collisions between several two-way radio units trying to transmit on the same channel. BACKGROUND OF THE INVENTION Systems of two-way radio units may use one of two different modes of operation for managing communication among the units. In trunked mode operation (TMO), the communication is managed by one or more base stations. The units communicate with each other only via the base stations. In direct mode operation (DMO), the units communicate directly with each other on a common channel. For clear communication in DMO, only one unit may transmit on the common channel at any one time. If two or more units try to transmit simultaneously, the transmissions interfere with each other, a situation that is called a "collision". Therefore, each unit monitors the channel before transmitting, and transmits only if the channel is clear, i.e., if no other unit is transmitting. This monitoring usually is effected by means of periodic received signal strength indication (RSSI) measurements or by similar signal quality estimator measurements. Two units, having monitored the common channel and having determined that the common channel is clear, may nevertheless start transmitting simultaneously, thereby producing a collision. For units that are capable of transmitting and receiving simultaneously, standards such as the Aloha standard and the Slotted Aloha standard provide protocols for resolving such collisions. Basically, if a unit receives any signal that is different from the signal that that unit is transmitting, that unit stops transmitting. These protocols are not usable by systems of low-cost two-way radio units that are not capable of simultaneous transmission and reception. Furthermore, two mobile units may determine the common channel to be clear, and then start transmitting, because the two units are out of range of each other. If the two units subsequently move within range of each other, a collision arises without being recognized as such. There is thus a widely recognized need for, and it would be highly advantageous to have, a method of resolving collisions in a communication system based on two-way radio units that do not transmit and receive simultaneously. SUMMARY OF THE INVENTION According to the present invention there is provided a method for communication among a plurality of users, including the steps of: (a) providing each user with a transceiver, all the transceivers operative to transmit and receive on a common channel; and (b) for each of at least one transceiver, the user whereof wishes to transmit speech: (i) detecting a pause in the user's speech; (ii) during the pause, monitoring the common channel for a transmission by any other transceiver; and (iii) if the transmission by the any other transceiver is detected, reversibly disabling the transceiver. Communications systems that are based on half-duplex two-way radio units, that do not transmit and receive simultaneously, typically alternate between time slots used collectively for transmission and time slots used collectively for reception. The duration of these time slots typically is from several hundreds of microseconds to several tens of milliseconds. For example, under the TETRA standard, the slot duration is 14.1 milliseconds. The present invention is directed at collision resolution in systems for speech communication, and is based on the fact that ordinary speech includes many pauses on the time scale of a transmission/reception time slot. During a pause, a unit that is attempting to transmit monitors the common channel. If that unit detects a transmission by another unit during the pause, then the pausing unit stops transmitting. Although the present invention is directed primarily at wireless cornmunications such as two-way radio, the present invention is applicable to any communications system in which multiple users converse with each other via transceivers on a common channel. For example, the transceivers could be telephone sets and the common channel could be a common copper wire line or fiber optic line shared by all the telephone sets. BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1 is an illustration of how the present invention resolves a collision of two transceiver units that start transmitting simultaneously; FIG. 2 is an illustration of how the present invention resolves a collision of two transceiver units that start transmitting sequentially. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is of a method of resolving collisions in a communication system in which several transceivers share a common communication channel. Specifically, the present invention can be used to resolve collisions in a system of two-way radios that do not transmit and receive simultaneously. The principles and operation of collision resolution according to the present invention may be better understood with reference to the drawings and the accompanying description. Referring now to the drawings, Figure 1 shows the activities of three transceiver units, for example two-way radio units, as functions of time, illustrating several features of the operation of the present invention. At time To, unit 1, having monitored (RX) the common channel and found the common channel to be clear, attempts to transmit (TX) to unit 3. At the same time, unit 2, having monitored (RX) the common channel and found it to be clear, attempts to transmit (TX) to a fourth unit (not shown). Thus, starting at time To, there is a collision between the transmissions of unit 1 and the transmissions of unit 2. The transmissions are in the form of digital packets that include digitized compressed representations of the speech of the users of units 1 and 2, and also address information identifying the intended recipients of the packets. For example, the packets transmitted by unit 1 identify unit 3 as the intended recipient. It is assumed for illustrational purposes in Figure 1 that the signal received by unit 3 from unit 1 is sufficiently stronger than the signal received by unit 3 from unit 2, for example because unit 3 is significantly closer to unit 1 than to unit 2, that unit 3 can recognize the packets transmitted by unit 1 despite the interference from unit 2, although in general this is not the case. The speaker of unit 3 initially is OFF, but upon receiving and recognizing packets transmitted by unit 1, unit 3 turns its speaker ON and translates the packets into audible speech that is rendered to the user of unit 3 on the speaker of unit 3. At time Ti, the users of both units 1 and 2 pause in their speech. Both units 1 and 2 detect these pauses, using a standard technique such as Voice Activity Detection (VAD). During the pauses, from time Ti to time T 2 , both units 1 and 2 stop transmitting and instead monitor the common channel. Because neither unit 1 nor unit 2 transmits from Ti until the time T 2 at which the users of units 1 and 2 both resume speaking, both unit 1 and unit 2 determine the common channel to be. clear, and both unit 1 and unit 2 resume transmitting at time T 2 . Typically the duration of the time interval (T 2 -Tι) during which a transmitting unit pauses to monitor the common channel is between 2 reception slots' duration and several seconds. Thus, under the TETRA standard, in which the alternating transmission and reception slots have durations of 14.5 milliseconds, the duration of the pause is between 110 milliseconds and 4 seconds. Conventionally, a transceiver unit, that stops receiving transmissions from another transceiver unit, waits a predetermined time interval ("inactivity time") for resumption of transmissions from the other unit. If the transmissions do not resume during this time interval, the receiving transceiver unit assumes that the transmissions are over, and acts accordingly. For example, the receiving transceiver unit turns off its speaker, and the receiving transceiver unit may resume monitoring the common channel for the purpose of initiating its own transmission. To indicate to unit 3 that the pause at time Ti is not a true cessation of transmission, at the start of the pause, unit 1 transmits to unit 3 a pause message to that effect. Unit 3 then waits a longer time than the standard inactivity time before concluding that the transmissions from unit 1 are over. A typical value of the inactivity time is 0.5 seconds. An illustrative value of the time that unit 3 waits for resumed transmission from unit 1 after receiving a pause message is 2.0 seconds. Preferably, during the pause from time Ti to time T , the speaker of unit 3 renders "comfort noise", represented in Figure 1 by cross-hatching, to reassure the user of unit 3 that unit 3 is still operating, and the pause is merely a temporary cessation of transmission from unit 1. Preferably, the pause message includes values of parameters that describe the background noise level at unit 1, so that unit 3 can generate comfort noise that corresponds to that background noise level. As noted above, at time T 2 , both units 1 and 2 resume transmission. Unit 3, upon detecting the resumption of transmission from unit 1, resumes translating the packets transmitted by unit 1 into audible speech and rendering this speech to the user of unit 3 on the speaker of unit 3. The speech of the user of unit 1 pauses at time T 3 , but the speech of the user of unit 2 does not pause until a later time T 4 . At time T 3 , unit 1 stops transmitting and starts monitoring the common channel. Because unit 2 continues to transmit, unit 1 detects transmission on the common channel, unit 1 now disables itself temporarily from transmitting, and renders a busy signal on its speaker to indicate to the user of unit 1 that the common channel is in use by another transceiver unit. Unit 3, upon detecting the cessation of transmission from unit 1 at time T 3 , again renders comfort noise on its speaker until the maximum pause time is reached at time T 4 . Unit 3 then turns its speaker OFF. Figure 1 illustrates the stochastic nature of the present invention. The method of the present invention recognizes a collision only when one of the competing transceiver units pauses. So, in the example illustrated in Figure 1, the collision between units 1 and 2 is not recognized until time T 3 , because units 1 and 2 pause simultaneously at time Tj and resume transmitting simultaneously at time T 2 . Once a collision starts, whichever transceiver unit pauses first is denied use of the common channel, and the transceiver unit that pauses last obtains exclusive use of the common channel. Figure 2 illustrates the ability of the present invention to resolve collisions that arise despite the monitoring of the common channel by the transceiver units. At time To, unit 1, having monitored the common channel and having determined that the common channel is clear, starts transmitting. Meanwhile, unit 2 also is monitoring the common channel. Unit 2 fails to detect the transmissions of unit 1. This can occur, for example, if unit 2 is shielded from unit 1, or if unit 2 is too far from unit 1 to detect the transmissions of unit 1. Consequently, unit 2 starts transmitting at time Ti, initiating a collision, at least with respect to other transceiver units that are capable of receiving transmissions from both unit 1 and unit 2. At time T 2 , unit 2 moves to a position where units 1 and 2 are capable of receiving each other's transmissions. The speech of the user of unit 1 pauses at time T 3 , and the speech of the user of unit 2 pauses at a later time T 4 . Having paused at time T 3 , unit 1 detects the transmissions of unit 2 and disables itself temporarily. Unit 2, having paused at later time T 2 , detects a clear common channel and resumes fransn_dtting at time T 5 . While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims: WHAT IS CLAIMED IS: 1. A method for communication among a plurality of users, comprising the steps of: (a) providing each user with a transceiver, all said transceivers operative to transmit and receive on a common channel; and (b) for each of at least one transceiver, the user whereof wishes to transmit speech: (i) monitoring said common channel for a transmission by any other transceiver, and (ii) if said transmission by said any other transmitter is detected, reversibly disabling said each transceiver; the method characterized by the step of: (c) for said each transceiver, the user whereof wishes to transmit speech: detecting a pause in said speech, said monitoring being effected during said pause. 2. The method of claim 1, wherein said reversible disabling of said each transceiver includes generating a busy signal. 3. The method of claim 1, further comprising the step of: (c) for each of said at least one transceiver: during said pause, transmitting a pause message prior to said monitoring. 4. The method of claim 3, wherein said pause message includes an indication of a noise level at said each transceiver. 5. The method of claim 1, wherein said detecting of said pause is effected using Voice Activity Detection.

Inventors: CARMON RAFAEL, CARMON, RAFAEL
Applicants: CARMON RAFAEL, MOTOROLA ISRAEL LTD, CARMON, RAFAEL, MOTOROLA ISRAEL LIMITED
Application year: 2000
Main classification: H04Q
IPC codes: H04L12, H04W24, H04W74