DIGITAL TELEPHONY USING CONTROL MESSAGES TRANSMITTED IN TIME SLOTS FOR RF FREQUENCY ALLOCATION

Abstract: In a system in which control messages are transmitted in predetermined time slots within fixed length time frames from a first transmitting and receiving unit to a second transmitting and receiving unit, the second unit listens on a selected RF carrier frequency for an incoming message, the selected RF carrier frequency being selected from the class of RF frequencies available and preferred for use, the RF frequencies being classified dependent on whether communications on previously selected carrier frequencies have been successful.

Description: DIGITAL TELEPHONY USING CONTROL MESSAGES TRANSMITTED IN TIME SLOTS FOR RF FREQUENCY ALLOCATION This invention r lates to digital data and control message transmission in p redetermined time slots within fixed length time frames. The present invention relates to a method of transmitting control messages in predetermined time slots within fixed length time frames from a first transmitting and receiving unit to a second transmitting and receiving unit, the control messages being for controlling the second unit, the second unit listening on a selected RF carrier frequency for an incoming message at a time waiting to receive, the selected RF carrier frequency being selected from the class of RF frequencies both available and most preferred for use, the RF frequencies being classified dependent on whether communications on previously selected carrier frequencies have been successful. Preferably the carrier radio frequencies assigned to a first unit are classified as preferred for use, less preferred for use, and least preferred for use. If an RF carrier frequency classed as preferred for use is not selectable, an RF carrier frequency less preferred is attempted to be selected. If no RF carrier frequency classed as less preferred is selectable, the second unit scans all RF carrier frequencies assigned to the network of first and second units to determine an RF carrier frequency usable for transmissions by the first unit on which to listen. The second unit is preferably a subscriber unit, and the first unit is preferably a base station. Transmission is preferably by radio. The second unit preferably listens for call set up requests or short messages. Where there is a plurality of second units for communication with the first unit, the carrier radio frequencies are classified for each second unit, for example, a particular radio frequency can be preferred for use in communication by one second unit but not another. The invention also relates to a corresponding method of receiving digital data signals, a receiver, and communication means comprising a first unit and a plurality of second units. A preferred embodiment of the invention will now be described, by way of example, and with reference to the accompanying drawings in which: Figur 1 is a schematic diagram illustrating the system including i base station (BTE -Base Terminating Equipment) and subscriber unit (NTE - Network Terminating Equipment); Figure 2 is a diagram illustrating frame structure and timing for a duplex link; Figure 3 is a diagram representing a slot-list message from a base station. The Basic Svstem As shown in Figure 1 , the preferred system is part of a telephone system in which the local wired loop from exchange to subscriber has been replaced by a full duplex radio link between a fixed base station and fixed subscriber. The preferred system includes the duplex radio link, and transmitters and receivers for implementing the necessary protocol. There are similarities between the preferred system and digital cellular mobile telephone systems such as GSM which are known in the art. This system uses a protocol based on a layered model, in particular one having the following layers: PHY (Physical), MAC (Medium Access Control), DLC (Data Link Control), NWK (Network). One difference compared with GSM is that, in the preferred system, subscriber units are at fixed locations and there is no need for hand-off arrangements or other features relating to mobility. This means, for example, in the preferred system, directional antennae and mains electricity can be used. Each base station in the preferred system provides six duplex radio links at twelve frequencies chosen from the overall frequency allocation, so as to minimise interference between base stations nearby. The frame structure and timing for the duplex link is illustrated in Figure 2. Each duplex radio link comprises an up-link from a subscriber unit to a base station and, at a fixed frequency offset, a down-link from the base station to the subscriber unit. The down-links are TDM, and the up-links are TDMA. Modulation for all links is π/4 - DQPSK, and the basic frame structure for all links is ten slots per frame of 2560 bits, i.e. 256 bits per slot. The bit rate is 512kbps. Down-links are continuously transmitted and incorporate a broadcast channel for essential system information. When there is no useful information to be transmitted, the downlink transmission continues to use the basic frame and slot structure and contain a suitable fill pattern and essential broadcast channel. For both up-link and down-link transmissions, there are two types of slot: normal slots, which are used after call set-up, and pilot slots which are used during call set-up. Each down-link normal slot comprises 24 bits of synchronisation information followed by 24 bits designated S-field which includes an 8 bit header, followed by 160 bits designated D-field. This is followed by 24 bits of Forward Error Correction, and an 8 bit tail, followed by 12 bits of broadcast channel. Each broadcast message consists of ten 12 bit segments taken from each downlink slot. When assembled into a 120 bit message, the broadcast message consists of 104 bits of data, 14 bits of Forward Error connection and 2 padding bits. In the 104 bits of data, there is a 16 bit Cyclic Redundancy Check (CRC). The broadcast channel consists of segments in each of the slots within a frame which together form the down-link common signalling channel which is transmitted by the base station, and contains control messages containing link information such as slot lists, multi- frame and super-frame information and other information, connectionless messages and information basic to the operation of the system. During call set-up, each down-link slot contains frequency correction data and or a training sequence for receiver initialisation with only a short S- field and no D- field information. Up-link slots basically contain two different types of data packet. The first type of packet, called a pilot packet, is used before a connection is set up, for example, for an ALOHA call request and to allow adaptive time alignment. The other type of data packet, called a normal packet, is used when a call has been established and is a larger data packet, due to the use of adaptive time alignment. Each up-link normal packet contains a data packet of 244 bits which is preceded and followed by a ramp of 4 bits duration. The ramps and the remaining bits left of the 256 bit slot provide a guard gap against interference from neighbouring slots due to timing errors. Each subscriber unit adjusts the timing of its slot transmissions to compensate for the time it takes signals to reach the base st?fion. Each up-link data packet comprises 24 bits of synchronisation data followed by an S-field and D-field of the same number of bits as in each down-link normal slot. Each up-link pilot slot contains a pilot data packet which is 92 bits long preceded and followed by 4 bit ramp defining an extended guard gap of 60 bits. This larger guard gap is necessary because there is no timing information available, and without it, propagation delays cause neighbouring slots to interfere. The pilot packet comprises 64 bits of sync followed by 104 bits of S-field which starts with an 8 bit header and finishes with a 16 bit Cyclic Redundancy Check 2 reserved bits, 14 forward error correction FEC bits, and 8 tail bits. There is no D-field. The S-fields in the above mentioned data packets can be used for two types of signalling. The first type is MAC signalling (MS) and is used for signalling between the MAC layer of the base station and the MAC layer of a subscriber unit in which timing is important. The second type is called associated signalling, which can be slow or fast, and is used for signalling between the base station and the subscriber units in the DLC or NWK layers. The D-field is the largest data field, and in the case of normal telephony contains digitised speech, but can also contain non-speech data samples. Call requests are made in random access Aloha slots as described in more detail below. Provision is made in the preferred system for subscriber unit authorisation using a challenge response protocol. General encryption is provided by combining the speech or data with a non-predictable sequence of cipher bits produced by a key stream generator which is synchronised to the transmitted super-frame number. In addition, the transmitted signal is scrambled to remove dc components. Contention Protocol A slotted Aloha protocol is used by subscriber units to send call set-up requests and short information messages (Datagrams) to the base station. These requests and messages are sent in a pilot packet in one of a list of slots, known as Aloha slots, that have been designated as available for this purpose in a Slot-List broadcast by the base station. A typical slot-list broadcast is illustrated in Figure 3. A slot-list is valid for a multiframe, which consists of sixteen consecutive frames numbered 0 - 15, and is broadcast in one multiframe to be applicable to the following multiframe. The number of Aloha slots in a Slot-List may vary, and thus the message containing this information is of variable length and may need to be segmented into several slot-list messages for transmission in the broadcast channel. Each slot-list message may designate up to three Aloha slots, as shown in the Figure 3. A minimum number of slot-list messages are used, and these are each assigned a sequence number and are sent in ascending order, and are re-transmitted in the same order following complete transmission within the same multiframe. Sometimes, however, a single slot-list message will be sufficient to transmit the complete Slot-List. The broadcast channel itself comprises a predetermined portion of each slot in every frame of the multiframe, and is used to broadcast various messages besides slot-list messages. Furthermore, different priorities are allocated to these different types of messages. Thus in frames 0 to 7 of a multiframe, slot-list messages have the lowest priority, and may not be transmitted because of the volume of other messages. However, slot-list messages are given a high priority in frames 8, 9, 12 and 13 so that transmission of at least a part, and most probably all, of the Slot-List is guaranteed. The information in the Slot-List is encoded for reduced bandwidth, and has to be defined by reference to a Carrier-List which includes details of bearer frequencies. This Carrier- List is also transmitted in the broadcast channel in frame 15 of the multiframe preceding that in which the Slot-List is transmitted. The base station sends information specific to each subscriber unit of which RF carrier frequencies are: (i) preferred (the so-called 'white' channels) (ii) to be used if a preferred rf frequency is not available (the so-called 'grey' channels) (iii) not to be used (the so-called 'black' channels). Each subscriber unit stores this information. Specifically, each subscriber unit maintains a Classified Carrier-List corresponding to the information received from the base station indicating radio frequencies that are preferred ("white"), frequencies that are only to be used due to the poor quality they provide if no preferred slot is available ("grey"), and frequencies which are not to be used ("black"). RF frequencies are categorised as 'black' if, for example, their use by a subscriber unit in one sector is likely to cause interference with transmissions from subscriber units in neighbouring sectors. RF frequencies are categorised as 'grey' where they provide poor quality but acceptable propagation. Updating the Classified Carrier List dependent on Call Success The base station monitors, over time, the success of calls, which can be Public Switched Telephone Network (PSTN) calls and or test calls on each RF frequency to each subscriber unit with which it communicates by radio. Where necessary an RF frequency is recategorised. For example, an RF frequency categorised as 'white' can be automatically reclassified as 'grey' should the frequency be unavailable for successful call transmission for more than a predetermined percentage of time. Conversely, 'grey' frequencies which are monitored as becoming sufficiently reliable are re-categorised as 'white'. RF frequencies which are sufficiently unreliable, or cause interference are re-categorised as 'black'. The updated Classified Carrier list for each subscriber unit is updated periodically. The Waiting to Receive Mode of a Subscriber Unit When a subscriber unit is not being used for calling, it remains in a state whereby it listens on a preferred RF carrier frequency for incoming paging messages on the broadcast channel. As mentioned above, the broadcast channel consists of segments of each of the time slots within a frame which together form a down-link (base station to subscriber unit) common signalling channel. Receiving a paging message indicates that a call should be set up. The preferred RF carrier frequency on which to "camp", ie. listen for paging messages is normally one categorised as "white", however if that RF carrier frequency fails, a search of the other RF carrier frequencies categorised as "white" is automatically undertaken to determine whether any of these other RF carrier frequencies are suitable to be "camped" on. If so, one is selected. If no RF carrier frequencies categorised as "white" are available, a search of RF carrier frequencies categorised as "grey" is undertaken. If this also draws a null result, all of the possible RF carrier frequencies available to all base stations in the network are scanned to find an RF carrier frequency usable by the particular base station for communication with the subscriber unit. Whether a particular RF carrier frequency should be camped on is determined by the subscriber unit which counts the number of cyclic redundancy check (CRC) errors in messages sent in the downlink common signalling channel (broadcast channel). If there are more than a threshold number of errors in a predetermined time then that RF carrier frequency is deemed unsatisfactory, and a different RF carrier frequency is tried. In an alternative embodiment, errors are detected in Forward Error Correction (FEC) codes in messages sent on the broadcast channel to determine which RF carrier frequency is acceptable.

Claims: CLAIMS 1. A method of transmitting control messages in predetermined time slots within fixed length time frames from a first transmitting and receiving unit to a second transmitting and receiving unit, the control messages being for controlling the second unit, the second unit listening on a selected RF carrier frequency for an incoming message at a time waiting to receive, the selected RF carrier frequency being selected from the class of RF frequencies both available and most preferred for use, the RF frequencies being classified dependent on whether communications on previously selected carrier frequencies have been successful. 2. A method of transmitting control messages according to claim 1, in which the control messages comprise data of the availability of RF carrier frequencies to the second unit for communication with the first unit, the availability data being updated dependent on whether communication on the or each selected RF carrier frequency has been successful, the availability data being retransmitted from time to time to update the second unit. 3. A method of transmitting control messages according to claim 1 or claim 2, in which the carrier radio frequencies assigned to a first unit are each classified as preferred for use, less preferred for use, or least preferred for use. 4. A method of transmitting control messages according to any of claims 1 to 3, in which if an RF carrier frequency classed as preferred for use is not selectable, an RF carrier frequency less preferred is attempted to be selected. 5. A method of transmitting control messages according to claim 4, in which if no RF carrier frequency classed as preferred or less preferred is selectable, the second unit scans all RF carrier frequencies assigned to the network of first and second units to determine an RF carrier frequency usable for transmissions by the first unit on which to listen. 6. A method of transmitting control messages according to any preceding claim in which the second unit is a subscriber unit, and the first unit is a base station. 7. A method of transmitting control messages according to any preceding claim, in which transmission is by radio. 8. A method of transmitting control messages according to any preceding claim, in which the second unit listens for a call set-up request or short message. 9. A method of transmitting control messages according to any preceding claim, in which there is a plurality of second units for communication with the first unit, the RF carrier frequencies being classified separately for each second unit. 10. A method of receiving control messages in predetermined time slots within fixed length time frames from a first transmitting and receiving unit at a second transmitting and receiving unit, the control messages being for controlling the second unit, the second unit listening on a selected RF carrier frequency for an incoming message at a time waiting to receive, the selected RF carrier frequency having been selected from the class of RF frequencies both available and most preferred for use, the RF frequencies being classified dependent on whether communications on previously selected carrier frequencies have been successful. 11. A method of receiving control messages according to claim 10, in which the control messages comprise data of the availability of RF carrier frequencies to the second unit for communication with the first unit, the availability data being updated dependent on whether communication on the or each selected RF carrier frequency has been successful, the availability data being received from time to time to update the receiver. 12. A method of receiving control messages according to claim 10 or claim 1 1, in which the carrier radio frequencies assigned to a first unit are classified as preferred for use, less preferred for use, and least preferred for use. 13. A method of receiving control messages according to any of claims 10 to 12, in which if an RF carrier frequency classed as preferred for use is not selectable, an RF carrier frequency less preferred is attempted to be selected. 14. A method of receiving control messages according to claim 13, in which if no RF carrier frequency classed as preferred or less preferred is selectable, the second unit scans all RF carrier frequencies assigned to the network of first and second units to determine an RF carrier frequency usable for transmissions by the first unit on which to listen. 15. A receiver operative to receive control messages in predetermined time slots within fixed length time frames from a transmitter, the control messages being for controlling the receiver, the receiver listening on a selected RF carrier frequency for an incoming message at a time waiting to receive, the selected RF carrier frequency being selected from the class of RF frequencies both available and most preferred for use, the RF frequencies being classified dependent on whether communications on previously selected carrier frequencies have been successful. 16. A receiver according to claim 15, in which the carrier radio frequencies are classified as preferred for use, less preferred for use, and least preferred for use. 17. A receiver according to claim 15 or claim 16, in which if an RF carrier frequency classed as preferred for use is not selectable, an RF carrier frequency less preferred is attempted to be selected. 18. A receiver according to claim 17, in which if no RF carrier frequency classed as less preferred is selectable the receiver scans all RF carrier frequencies assigned to a network including transmitters and other receivers to determine an RF carrier frequency on which to listen. 19. Communication means comprising a first transmitting and receiving unit and a plurality of second transmitting and receiving units, the first unit transmitting control messages in predetermined time slots within fixed length time frames to each second unit, the control messages being for controlling the receiving second units, each second unit listening on a selected RF carrier frequency for an incoming message at a time waiting to receive, the selected RF carrier frequency being selected from the class of RF frequencies both available and most preferred for use, the RF frequencies being classified dependent on whether communications on previously selected carrier frequencies have been successful. 20. Communication means according to c'- m 19, in which the control messages to a second unit comprise data of the availability of RF carrier frequencies to the second unit for communication with the first unit, the availability data being updated dependent on whether communication on the or each selected RF carrier frequency has been successful, the availability data being retransmitted from time to time to update the second unit. 21. Communication means according to claim 19 or claim 20, in which the carrier radio frequencies assigned to a second unit are classified as preferred for use, less preferred for use, and least preferred for use. 22. Communication means according to any of claims 19 to 21, in which if an RF carrier frequency classed as preferred for use is not selectable, an RF carrier frequency less preferred is attempted to be selected. 23. Communications means according to claim 22, in which if no RF carrier frequency classed as less preferred is selectable for transmission to one of the second units, the second unit scans all RF carrier frequencies assigned to the first and second units to determine an RF carrier frequency usable for transmissions by the first unit on which to listen. 24. Communication means according to any of claims 19 to 23, in which the RF frequencies are classified separately for each second unit.

Inventors: GOODINGS RUPERT LESLIE ALEXAND, MARTIN PAUL MAXWELL, SHORE CHRISTOPHER JOHN, GOODINGS, RUPERT, LESLIE, ALEXANDER, MARTIN, PAUL, MAXWELL, SHORE, CHRISTOPHER, JOHN
Applicants: GOODINGS RUPERT LESLIE ALEXAND, IONICA INT LTD, MARTIN PAUL MAXWELL, SHORE CHRISTOPHER JOHN, GOODINGS, RUPERT, LESLIE, ALEXANDER, IONICA INTERNATIONAL LIMITED, MARTIN, PAUL, MAXWELL, SHORE, CHRISTOPHER, JOHN
Application year: 1996
Main classification: H04B7
IPC codes: H04B7, H04J3, H04Q7