Digital mobile station using presettable timeslot counter for compensating for propagation delay time

Abstract: In a TDMA cellular mobile communication system, a mobile station receives a TDM signal from a cell site and detects a signal indicating the propagation delay time of signals between the cell site and the mobile station. From an assigned timeslot, a time interval from a detected frame sync to the time of transmission of a burst signal from the mobile station is determined. The propagation delay time indicated by the detected signal is subtracted from the determined time interval. When a frame sync is detected from the received TDM signal, clock pulses are counted until it corresponds to the subtracted time interval, whereupon a timing pulse is generated to cause a burst signal to be transmitted from the mobile station to the cell site.

Description: The present invention relates generally to mobile communications systems, and more specifically to a mobile station which transmits burst signals at such times that the round-trip propagation delay time of signals between each cell site and the mobile station is compensated. In time division multiple access digital communications systems, the propagation delay time of a signal sent from a cell site and the same amount of delay time involved with a signal sent in response to it from a mobile station are the factor that must be taken into account to allow the cell site to precisely detect signals from preassigned timeslots. Conventional mobile stations are provided with a plurality of delay elements which are controlled by a detected propagation delay time in such a manner that the round-trip propagation delay time is compensated. However, the operating performance of such delay elements are sensitive to ambient temperatures and variabilities exist among the delay times of different mobile stations. Difficulty thus arises to precisely control and maintain the propagation delay time factor for each mobile station. In the specification of European patent application number 87103186.0, which was published under number 236968 on September 16 1987, there was proposed a method of transmitting digital signals in a TDMA transmission between a fixed station and a mobile station using a time-slot pattern in which information was transmitted in bursts. Because of transit time differences caused by reflections, an instruction, containing, in encoded form, the necessary phase position information in relation to the transmitted frame, was sent from the fixed station to each mobile station. A feature of a mobile station to be described below, as an example, is that it has propagation delay time compensation circuitry that minimises the effects of ambient temperature and variability problems. In a digital mobile station which receives a time-division multiplex (TDM) signal from a cell site of a cellular mobile communications system, the TDM signal contains a sequence of frames each having a frame sync signal, and a signal indicating a propagation delay time from the cell site to the mobile station. In an arrangement to be described below, by way of example, a timeslot assignment signal indicative of the position of a timeslot assigned in a frame, the delay time indicating signal and the timeslot assignment signal are detected by the mobile station from the received TDM signal. The detected propagation delay time signal is subtracted from the timeslot assignment signal to produce a difference time signal. When the frame sync is detected from the received TDM signal, clock pulses are counted to produce a clock count value. A timing pulse is generated when the clock count corresponds to the subtracted time interval. A burst signal is transmitted from the mobile station to the cell site in response to the timing pulse. The following description and drawings disclose, by means of an example, the invention which is characterised in the appended claims, whose terms determine the extent of the protection conferred hereby. In the drawings:- Fig. 1 is a block schematic circuit diagram of a mobile telephone, and Figs. 2 and 3 are timing diagrams useful for describing the operation of the arrangement shown in Fig. 1. Referring now to Fig. 1, there is shown a digital mobile station for use in a cellular mobile communications system. The system includes a plurality of base stations located at cell sites. Each base station is capable of calculating the amount of propagation delay time between the cell site and each mobile station using signals received from mobile stations in a known manner. A signal indicating the propagation delay time is transmitted together with a destination identifier, a time slot assignment signal indicative of the position of an assigned timeslot, and an information-bearing signal in a TDM (time-division multiplex) frame format. The mobile station shown in Fig. 1 receives such a TDM signal from a nearby cell site through diplexer 10 and receiver 11, in which it is converted to an intermediate frequency signal and applied to a timing recovery circuit 12. Following carrier and clock recovery procedures performed in a manner well known in the art, timing recovery circuit 12 supplies its output to a data recovery circuit, not shown, and to a delay time signal detector 13 for detecting the delay time indicating signal contained in a timeslot destined to the mobile station. The output of detector 13, which is indicative of the propagation delay time from the cell site to the mobile station is fed into a subtractor 16 in which it is subtracted from a timeslot count value representing the amount of time from the reception of a frame sync to the time of transmission of a burst signal on an assigned timeslot in which the mobile station is allowed to transmit. This timeslot count value is derived from a known timeslot assignment circuit 17 by decoding a timeslot assignment signal contained in the transmitted signal. The subtracted timeslot count is of a propagation delay-time compensated value and is used to preset the full count value of a presettable timeslot counter 19. A frame sync detector 18 is connected to the output of timing recovery circuit 12 for detecting a frame sync code from the beginning of each frame and enabling the counter 19 to start counting clock pulses from a clock source 20. When the clock count reaches the preset value, timeslot counter 19 supplies a timing signal to a burst generator 21 to allow it to generate a burst signal containing transmit information-bearing and feeds it to a transmitter 22 in which the burst signal is converted in frequency and amplified for transmission through diplexer 10 to the cell site. In operation, assume that a propagation delay time T exists between a cell site and the mobile station, and that the latter is assigned a k-th timeslot by the timeslot assignment circuit 17. A propagation delay time signal is extracted by the detector 13 to detect the delay time T, which is subtracted by subtractor 16 from the count value given by assignment circuit 17 representing the interval between a detected frame sync pulse and the start timing of the assigned k-th timeslot. Timeslot counter 19 is preset by the output of subtractor 16. Frame sync detector 18 extracts a frame sync pulse from the received TDM signal at time t 0 (see Fig. 2) and, in response, timeslot counter 19 is enabled to start counting clock pulses from clock source 20. Timeslot counter 19 thus produces a timing pulse at time t 1 when the count value preset by the subtractor 16 is reached. It is seen in Fig. 3 that transmit information-bearing signal or data 30 from the cell site is received as shown at 31 after the elapse of delay time T and, at the end of k-timeslot interval minus 2T from the end of receive data 31, transmit data 32 is sent from the mobile station and received by the cell site as shown at 33 at time which is k timeslots spaced from the transmit data 30. In this way, the round-trip propagation delay time 2T is automatically compensated for. It will be understood that, although a particular arrangement has been described, by way of example, in illustration of the present invention, variations and modifications thereof, as well as other embodiments may be made within the scope of the appended claim 1.

Claims: A digital cellular mobile communication system in which a cell-site station transmits in a time-division multiplex frame format a timing instruction to a mobile station, characterised in that the cell-site station transmits, as the timing instruction, a frame synchronization signal indicating reference timing, a timeslot assignment signal indicative of an assigned timeslot, and a delay time signal indicating a propagation delay time between the cell-site station and the mobile station, in that the mobile station includes frame synchronization signal detector means (18) for detecting the frame synchronization signal, delay time signal detector means (13) for detecting the delay time signal, timeslot assignment signal detector means (17) for detecting the timeslot assignment signal and producing a timeslot count value representing the amount of time from the time of detection of the frame synchronization signal by the frame synchronization signal detector means (18) to the time of transmission of a burst signal, subtractor means (16) for subtracting the delay time signal detected by the delay time signal detector means (13) from the timeslot count value detected by the timeslot assignment means (17) and producing a subtracted timeslot count value, a clock pulse source (20) for generating clock pulses, and a presettable timeslot counter (19) arranged to be set by the subtracted timeslot count value supplied from the subtractor means (16) for counting the clock pulses in response to the frame synchronization signal and producing a burst transmission timing signal when the count of the clock pulses is equal to the set count value, and in that the mobile station transmits the burst signal in response to the burst transmission timing signal.

Inventors: NAKAHARA KENJI, TSUNODA JIRO, NAKAHARA, KENJI,, TSUNODA, JIRO,, Nakahara, Kenji, c/o NEC Corporation, Tsunoda, Jiro, c/o NEC Engineering, Ltd.
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H04B7
IPC codes: H04B7, H04L7