Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B14/00—Transmission systems not characterised by the medium used for transmission
  • H04B14/02—Transmission systems not characterised by the medium used for transmission characterised by the use of pulse modulation
  • H04B14/06—Transmission systems not characterised by the medium used for transmission characterised by the use of pulse modulation using differential modulation, e.g. delta modulation

Definitions

• the invention relates to digital data transmission in time slots within a fixed length time
• time division multiplexing/time division multiple access TDM/TDMA
• a known TDM/TDMA communications system initially establishes all call connections using a single time slot per frame (a single "bearer") at 32 kilobits per second (kbps) encoded according to an Adaptive Differential Pulse Code Modulation (ADPCM) modulation scheme.
• ADPCM Adaptive Differential Pulse Code Modulation
• This provides good quality transmission for speech data, but the rate of information flow is insufficient for voice-band modems.
• the communications system adds a
• bearers together provide a 64 kbps rate of data transmission using Pulse Code Modulation
• PCM Physical Coding Coding
• a service change is any change whereby the transcoding of input signal data into digital data for transmission is changed.
• An other example of a service change is conversion from ADPCM to PCM, or vice versa. This might be done to provide a better quality of transmission.
• the known communications system chooses to assign multiple independent slots to the call connection and inter leaves the digital data between the slots. For example, when two slots per frame are used, with a 64 kbps data rate, one of the slots transmits all of the even samples and the other transmits all of the odd samples. This assignment of samples to slots is undertaken assuming minimum delay.
• TDM/TDMA communications systems transmit data in bursts at a rate higher than the data
• Minimum delay is when the data samples
• Figure 1 The assignment of data samples to time slots in the known system described in WO96/08934 is shown in Figure 1. This shows a call connection using two slots per frame, in which even samples, denoted sample 16 to sample 54 are transmitted in slot 3 of a ten slot frame. The odd samples, denoted 29 to 67 are correspondingly transmitted in slot 6 of the same frame. Later odd and even samples are similarly transmitted in slot
• the slots are transmitted at different predetermined times, and the time relationship of the samples to the slots is known; for
• slot 6 transmits samples which are 13 samples later in time than slot 3. This is because slot 6 is transmitted 12 samples later in time, and transmits the odd samples.
• the data can be decompressed and reassembled at the receiver.
• the digital data for transmission is created using a transcoding function, which takes audio data and converts it into a digital data stream. It is this digital data stream which is compressed into one or more predetermined slots for transmission. This process occurs both in the uplink direction, ie. communications from a subscriber unit to a base station,
• Figure 2 shows an audio input signal switchably connected via a first transcoding function
• FI FI or a second transcoding function F2 to a TDM transmitter.
• the signal is received by a TDMA receiver to determine the received digital data and this is switchably reverse- transcoded via reverse transcoding function Fl' or reverse transcoding function F2' to provide an audio output signal.
• the digital data is transmitted across a radio link, and there are delays both due to the TDM TDMA protocol itself and delays in transcoding. As a result, the switching between transcoding functions must be timed accurately so that the data passes through the correct
• the known TDM/TDMA communications system as described in WO96/08934 defines a period in time when the data changes and executes a handshake between the transmitter and receiver.
• the sequence of signals involved in the handshake is shown in Figure 3.
• the base station which initiates the service change mutes its output of digital data received from a subscriber unit corresponding to information such as audio signals, and transmits a control signal to the subscriber unit requesting a change in transcoding.
• the subscriber unit (NTE) mutes its output and changes its transcoding, and transmits an
• the base station then sends a message indicating completion to the subscriber
• the present invention relates to a method of transmitting a digital data message in time
• the indication of the frame at which transcoding change will occur is sent more than one time frame in advance. This has the advantage that should the indication not be successfully transmitted, re-transmission in a subsequent frame can still be made, and in
• the indication can be sent up to X time frames in advance, where X can be, for example, between 10 and 20 inclusive, for example 15.
• Agreeing the time frame at which to change transcoding has the advantage that a mute period is no longer required, in particular when switching from a call connection involving a single slot per frame to a call connection involving two more slots per frame.
• the slot or slots in the time frame at which the transcoding function changes and used for transmission according to the old transcoding function, or the new transcoding function contain data such that a first part of a first slot will contain data encoded according to the old transcoding function if the first slot is used for transmitting data
• first part occupies the first (N-S) x B/N bits of the slot, where S is the slot number, B is the number of bits transmitted in the slot, and N is the number of slots in the time frame,
• the receiving unit can deduce which samples result from the old transcoding function and which result from the new transcoding function.
• the change in transcoding function occurs earlier at the transmitting unit than at the receiving unit to account for processing and transmission delays.
• the change in transcoding function occurs at a different time in the uplink direction than in the
• change in transcoding function preferably occurs within 10 to 20 time frames of being indicated. This is to ensure that, where a modem is connected to a receiving unit, the change is complete before the end of an answertone transmission from the modem, but allows sufficient time for the receiver to understand the frame at which the transcoding change will occur and to react in time.
• the transcoding function is changed one frame earlier for data transmitted in the uplink direction, that is, from a subscriber unit to a base station, than in the downlink direction, that is, from the base station to the subscriber unit.
• the present invention also relates to a corresponding method of reception.
• the present invention also relates to a corresponding transmitter.
• the present invention also relates to
• the present invention also relates to a corresponding communications means including a sending unit and a receiving unit.
• the present invention also relates to a corresponding communications means including a sending unit and a receiving unit.
• invention also relates to a corresponding base station.
• Figure 1 is a diagram illustrating an example of how samples are assigned to time slots in a frame (prior art)
• Figure 2 is a simplified functional diagram indicating switching between transcoding functions (prior art)
• FIG 3 is a diagram illustrating communications between a base station (BTE) and a subscriber unit (NTE) for changing transcoding function (prior art),
• FIG 4 is a schematic diagram illustrating the system including a base station (BTE - Base Terminating Equipment) and subscriber unit (NTE - Network Terminating Equipment),
• Figure 5 is a diagram illustrating frame structure and timing for a duplex link
• Figure 6 is a schematic diagram illustrating an example of transcoding function change by adding a second slot per frame to a call connection
• FIG. 7 is a diagram illustrating communications between a base station (BTE) and a
• NTE subscriber unit
• 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 unit.
• the preferred system includes the duplex radio link, and transmitters and receivers for implementing the necessary protocol. More specifically, there is a base station BTE connected via a access network an exchange
• TE such as a telephone handset, answering machine, facsimile machine or
• a layered model in particular the following layers; PHY (Physical), MAC (Medium Access Control), DLC (Data Link Control), NWK (Network).
• PHY Physical
• MAC Medium Access Control
• DLC Data Link Control
• NWK Network
• GSM Global System for Mobile communications
• 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
• 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 ⁇ /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
• the down-link transmissions continue to use the basic frame and slot structure and contain a suitable fill pattern.
• normal slots which are used after call set-up
• pilot slots 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 filler, followed by 12 bits of the broadcast channel.
• the broadcast channel consists of segments in each of the slots of 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, connectionless messages and other information basic to the operation of the system.
• each down-link pilot slot contains frequency correction data and 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
• 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
• Each up-link normal 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 192 bits long preceded and followed by 4 bits ramps defining an extended guard gap of 60 bits. This larger guard gap
• the pilot packet comprises 64 bits of
• the S-field 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
• the second type is called associated signalling, which can be slow or fast and is used for signalling between the base station and 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 a non-speech data samples.
• 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.
• the transmitted signal is scrambled to remove dc components.
• the transcoding function in a way that avoids the need for a mute period, the transcoding function must be changed:
• the slot at which the transcoding function change occurs includes data transcoded according to the old function in its first part and data transcoded according to the new function in its second part.
• the number of binary bits in the first part is chosen to be a multiple of the sample size resulting from the old
• the number of binary bits in the second part is chosen to be a multiple of the sample size resulting from the new transcoding function. For example, a 32 kbps ADPCM transcoding produces 4 bit samples whereas a PCM transcoding produces 8 bit samples. Accordingly, the transcoding change in the slot at which ADPCM
• transcoding is changed to (or from) PCM transcoding occurs after a multiple of 8 bits in
• the transmitting unit and receiving unit are respectively one of a base station and a subscriber unit.
• the base station sends a message to the subscriber unit indicating a future
• indicated frame is designated as a time reference for when the new transcoding function
• the second part contains data encoded according to the new service if the slot is in use in that service.
• the first part occupies the first (N - S)*B/N bits of the slot, where S is the slot number, B is the
• rule (3) above ensures that the boundary between the old and new service is positioned such that all data is transmitted. This is because where there are 160 bits per slot and 10 slots per frame, rule (3) above indicates that the first part must consist of 160 - 16S bits where S is the slot number (an integer between 0 and 9 inclusive).
• transcoding change must occur with some multiple of 16 bits in the first part to satisfy rule
• FIG. 6 An example is shown in Figure 6 where a digital data stream (a) is compressed into selected time slots (b) for transmission and them decompressed upon reception to reconstitute the digital data stream B.
• slot 3 Before the transcoding function change, slot 3 carries samples numbered 16 to 55 encoded using ADPCM. Thus slot 3 carries 20 bytes of digital data.
• function change slot 6 does not carry data from this digital data message. This occurs in
• slot 6 constitutes 6 bytes of data.
• slot 6 carries nothing in its first part (8 bytes) and odd PCM samples numbered 85 to 107 in its second part, which is equivalent to 12
• slot 3 carries the even PCM samples numbered 96 to 134 and slot 6 carries the odd PCM samples numbered 109 to 147.
• the sending unit has to change its transcoding function approximately one frame earlier than the receiving unit.
• the first step is that the base station selects a frame at which the transcoding function will change. This frame is denoted F.
• the base station sends a control signal namely an enable service control signal which includes an indication of the frame F rather than sending a complete identifier it is sufficient to send F modulo 2 K where K is chosen so as to trade-off the size of the signal transmitted against
• F modulo 2 K is the remainder when F is divided by 2 K , and is the equivalent to the last K binary bits of F, which is more
• the subscriber unit On receipt of the enable service request, the subscriber unit prepares to effect the
• transcoding change at frame F which is calculated as the next frame number at which the
• the base station receives the enable service acknowledgement and sends a further control signal to indicate receipt of the acknowledgement, at which point the signalling exchange is complete.
• the signalling exchange is not completed until the transcoding function change has been completed. More specifically, in this alternative embodiment the base station does not send the final control signal until it has both received the enable service acknowledgement and has actually effected the transcoding change.
• phase shift will be a phase shift in the signal. If the phase shift is significant, it must be compensated
• the subscriber unit may need to delay transmission uplink to the base station using PCM coding by one sample so as to compensate for the ADPCM transcoding delay. This is because PCM transcoding delay is less than ADPCM coding delay. So as to ensure that the delays with old and new
• transcoding are matched, the delay of the transcoding function which is greater is applied as the delay to both. It is possible to decide for particular transcoding functions whether
• the encoded data is dependent on the current input signal and previous signals also.
• Parameters which represent longer term variations (such as general amplitude) in the signal are independently derived at the transmitting unit and receiving unit from the previous signals.
• the encoded data contains information about the short term variations only.
• the received encoded data is combined with the derived parameters to reconstitute the signal.
• both transmitting and receiving unit derive the same values for each parameter, and ensues that these values be optimal.
• PCM is not a transcoding function which takes time to converge. This means for an ADPCM to PCM transcoding change the time required to converge is not a cause of significant distortion. For a PCM to ADPCM change, there is about a lmS distortion of the signal. This distortion is handled by arranging for both the transmitting unit and receiving unit to run their transcoders for the new transcoding function for an agreed number of samples before the transcoding change, so that they are already convergent. This requires both transmitter and receiver to have the capacity to encode or decode using both transcoding functions at the same time.
• the subscriber unit cannot independently control switching of transcoding functions used for uplink and downlink but must change both at the same time. Also, it is not possible to precisely assign within a slot data encoded according to more than one transcoding function, in particular as described by rule (3) above. However, causing the transcoding function to change one frame later on the uplink compared to the downlink is useful in minimising the mute period and hence minimising the risk of a modem attached to a subscriber unit not establishing a call correctly. In consequence, the period for which the subscriber unit mutes output to the subscriber terminal equipment
• the transcoding happens as soon
• the transcoding must be changed at the transmitting unit A around the start of slot 1 of frame F - 1, and the inverse transcoding changed at the
• the transcoding at the uplink encoder of the subscriber unit occurs roughly 9 slots earlier than at the downlink decoder of the subscriber unit.
• the main other factor is that the uplink is transmitted one whole slot in advance, see Figure 5, so this makes the difference close to 1 frame.
• Another factor is the adaptive timing advance (the amount by which the subscriber unit advances its transmit timing to compensate for propagation delay) which can take a value between 0 bits and 104 bits.
• a third factor is that the transmitted data occupies only 160 of the 256 bits in the slot; this
• a subscriber unit according to the second embodiment can only change its transcoding both uplink and downlink at the same time, by offsetting the uplink switch by 1 frame, the period of corruption of data received on the downlink and uplink is reduced
• the and subscriber unit both mute their transmission of digital data corresponding to information, such as audio signals, for a period of 1 frame so as to avoid corrupted data being sent.
• the mute period is sufficiently short that

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Mobile Radio Communication Systems (AREA)
• Time-Division Multiplex Systems (AREA)
• Communication Control (AREA)

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• 1997
  • 1997-04-16 GB GBGB9707726.7A patent/GB9707726D0/en active Pending
• 1998
  • 1998-04-15 WO PCT/GB1998/001104 patent/WO1998047297A2/en not_active Application Discontinuation
  • 1998-04-15 EP EP98917376A patent/EP0983695A2/en not_active Withdrawn
  • 1998-04-15 BR BR9809102-6A patent/BR9809102A/pt not_active IP Right Cessation
  • 1998-04-15 JP JP54363798A patent/JP2001521693A/ja not_active Withdrawn
  • 1998-04-15 AU AU70616/98A patent/AU7061698A/en not_active Abandoned
  • 1998-04-15 RU RU99123710/09A patent/RU99123710A/ru not_active Application Discontinuation

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