DIGITAL DATA COMMUNICATION SYSTEM

Abstract: A digital data communication system comprises a plurality of transmitting stations (TS1...TSN) and at least one receiving station (RS) connected by a multi-access non-destructive channel. In the figure the channels are provided by frequency division multiplexing over optical fibres which are coupled by a star coupler (SC) although other methods of channel separation may be applied. The transmitting stations each comprise a data source (DS1) which provides data to a coder (C1) and the coded signals provided by the coder are transmitted by a transmitter (Tx1). The coder is arranged to provide sufficient redundancy to allow the receiving station to receive substantially all of the data signals from all of the transmitting stations to be received regardless of their respective times of transmission. In this way a "send it and forget it" communication system may be provided which does not require channel allocation, system-level acknowledgement or expensive receiving stations. Transmission efficiency is maximised for a given throughput and reliability if the coders are arranged to provide Maximum Distance Separable Coding (MDS).

Description: Digital data communication system The present invention relates to a digital data communication system comprising a plurality of transmitting stations and at least one receiving station, each transmitting station comprising a source of a digital data signal and means for transmitting the digital data signal to the at least one receiving station on a distinct channel and the at least one receiving station comprising a number of receivers. Such a system is disclosed in United Kingdom Patent Application GB 2 241 847 A which relates to a passive optical network for communicating on a number of different optical frequencies. In this prior art arrangement each node of the system comprises a receiver for every transmitting station which in most cases would be prohibitively expensive. Alternatively a communication system could be constructed in which specific channels are assigned prior to data communication but this results in a control overhead, a need to assign channels and re-attempt failed connections which also adds to the complexity of the system. It is an object of the present invention to ameliorate these disadvantages. According to the present invention a system as defined in the first paragraph is characterised in that the number of transmitting stations is larger than the number of receivers at each receiving station, in that each receiver comprises means for receiving a digital data signal from any single transmitting station, and in that each transmit¬ ting station further comprises means for applying a sufficient quantity of redundancy to the digital data signal before transmission to permit the receiving station to receive substantially all of the digital data signals from all of the transmitting stations irrespective of their relative times of transmission. By transmitting each data signal with a significant amount of redundancy (typically more than 5 times) a 'send it and forget it' communication system may be provided in which the transmitting stations transmit their data and its associated redundancy only once. They then do not have to request a channel, wait for a channel to be free or retransmit unsucessful channel requests. Furthermore, the receiving stations will not require the large number of receivers provided in GB 2 241 847 A. Typically only one receiver will be provided at each receiving station although in certain circumstances a second receiver may be provided in order to enhance performance. The present invention is particulary applicable to a passive optical network (PON) communication system which has wide bandwidth so that a large number of channels may be provided by wavelength division multiplexing (WDM) or coherent multi-channel (CMC) arrangements. Alternatively sub-carrier multiplexing, where each transmitting station uses the same optical frequency but a unique electrical frequency, may be used. The invention is not limited to optical systems, however, and may also be applied to wired systems (using space or frequency division multiplexing) and to radio systems. A system in accordance with the present invention may be further characterised by no inherent acknowledgement of received signals. The distinction to no inherent acknowledgement rather than to no acknowledgement at all is made because in normal system use there would probably be user level acknowledgement in the form of relevant responses to previous messages. A communication system in accordance with the present invention may advantageously be provided as an asynchronous system to provide still further simplification, albeit at some cost to signal throughput. The large redundancy required by the present invention is admittedly a drawback, particularly if any of the transmitters are to be operated from a battery source but this drawback may be reduced to a minimum (for optical, wired and radio systems alike) by use of Maximum Distance Separable (MDS) Codes to provide the redundancy. Such codes provide the best performance for a given quantity of redundancy. The invention also relates to a transmitting station for use with a data communication system in accordance with the invention and to a method of communicating digital data. The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a block schematic diagram of an optical embodiment of the present invention, Figure 2 shows a diagram for assisting in the description of the present invention, Figure 3 shows a graph of cell loss probability against cell division factor for the present invention, Figure 4 shows a graph of the cell division factor against system throughput, Figure 5 shows a graph of the cell division factor against coding rate for various values of system throughput, Figure 6 shows a block schematic diagram of a switched embodiment of the present invention, and Figure 7 shows a graph of the cell division factor against coding rate for a system having two receivers per receiving station. In the drawings, like features have been identified using the same reference signs. Figure 1 shows a multi-access network of a type to which the present invention is applicable. An optical receiving station RS is coupled via an optical fibre to a passive star coupler SC which is also connected via optical fibres to a number of transmitting stations TS1, TS2,..TSN (three stations only are shown for clarity). Each transmitting station TS, for example TS1, comprises a data source DS1, a coder Cl and a transmitter Txl connected in cascade. Each transmitter is arranged to transmit on a particular optical frequency fl, f2,..fN which is different from that of any other transmitter. The receiving station RS comprises a receiver Rx which can tune to any one of the frequencies of the transmitters and thus receive any signal being transmitted by that transmitter. The receiver Rx may be of the heterodyne variety in which case the optical frequencies of the transmitters may be closely spaced, such a frequency division system usually being referred to as coherent multi-channel (CMC). Alternatively the frequencies may be more widely spaced which would permit the use of optical filtering to separate a channel which can then be directly detected. This latter frequency division technique is usually referred to as wavelength division multiplexing (WDM). The hardware details for implementing either of these optical communication techniques are well known to those skilled in the art. The operation of the coders Cl, C2...CN is discussed below. A slotted case is considered as an example in which all of the coded data signals from the transmitting stations are synchronised at the coupler SC and hence at the receiving station RS. One technique for achieving this synchronisation is to measure the round trip delay from every transmitting station to the receiving station and assign trans¬ mission output timing for each transmitter. This is in effect a time delay which is equal to half of the difference between round trip delay for a particular transmitting station and round trip delay for the transmitting station most distant from the receiving station. No additional delay is then applied to this most distant transmitting station. Another technique assigns the time delays relative to a notional transmitting station at the largest distance possible but this will result in unneccesarily long delays in most systems. In this straightforward example, each coder C is arranged to simply repeat the digital data signal from the respective data source DS five times so that the signal trans¬ mitted by the transmitter Tx is simply six consecutive versions of the same signal. The receiver at the receiving station simply has to tune to the respective frequencies of the trans- mining stations in any arbitrary order to receive all of the data. Figure 2 is a diagram for explaining the operation of the system in which the vertical axis represents transmission activity from the plurality of discrete transmitters as well as receiver activity and the horizontal axis represents time. A worst case slotted arrangement is considered in which six transmitting stations TS1 - TS6 transmit their data signals six times in succession simulta- neously on six different frequencies. This repetition code is the simplest type of redundancy that may be used in accordance with the invention. The receiver may thus receive the complete data signal from each transmitting station by tuning to the frequency of all of the transmitting stations in any order and receiving their transmitted signals. In the example shown the receiver tunes to the signals from the six transmitters in the order: Tx3, Txl, Tx4, Tx6, Tx5, Tx2. Because the data is transmitted on different channels, contemperaneous signals are not damaging to one another giving what is termed a non-destructive collision. By adding the large redundancy to each of the signals from the data sources, all of the data signals can be communicated between the transmitting stations and the receiving station without any signalling overhead whatsoever. Clearly the time taken for the receiver to tune to each new frequency must be taken into account and for this reason a fast-tuning receiver is required. In addition or as an alternative to such a receiver, the transmitters may be arranged to provide a small amount of time between successive transmissions of their data signal, in other words a guard slot, to ensure that all of the data signals are received in their entirety. Communication systems in accordance with the invention will not normally have the luxury of adding sufficient redundancy to each digital data signal that the quantity of signal plus redundancy is equivalent to the number of transmitting stations in the system. If the digital data to be communicated is considered to occur in cells, as the size of the system increases, the random characteristics of a large population may be exploited to provide an acceptably low cell loss probability by statistical multiplexing. Clearly if a large number of channels should happen to be active simultaneously then a cell from one or more transmitting stations will not be received. What probability of cell loss will be acceptable is determined by the application so that while the following discussion takes a cell loss probability of 10 "9 as an acceptable level, this figure may be increased or decreased according to the application. Somewhat more efficient use of the available transmission time in the communication system will result from the use of more sophisticated redundancy coding, in particular Maximum Distance Separable (MDS) coding which is described in detail in Chapter 11 of The Theory of Error Correcting Codes by F.J. MacWilliams and NJ.A. Sloane and published by the North Holland Publishing Company, Amsterdam. These codes possess the maximum possible distance between codewords and any k symbols can be taken as message symbols. Thus, to apply these codes in the present communication system, a data cell is divided into a plurality k of micro-cells and then coded using a MDS code to provide n coded micro-cells. Any k of the n coded micro-cells may be used to reconstruct the original k micro-cells and hence the original data signal. In the present description k is referred to as the cell division factor. The benefits of such codes in the present communica¬ tion system will be readily apparent from the following discussion. A synchronous system is considered for simplicity of analysis in which the receiver is assumed to be able the receive a micro-cell in every time slot from any channel (in other words a fast receiver or some guard time in the micro-cells). Micro-cells are considered to be received at random from those channels which contain a signal. As is usual in analysis of communication systems, the ultimate aim is to maximise throughput. Consider that each one of N transmitting stations transmits data cells at random times with a rate a cells per second and the time required to transmit the cell at a rate of B bits per second is β seconds. The probability of receiving an uncoded, undivided cell is: P = (number of cells received in unit time)/(number of cells transmitted in unit time) P = (1 . e - Nαβ )/(Nα3) Redundancy is now added using a Maximum Distance Separable code. The cell is divided into k pieces called micro-cells, which are encoded into n micro-cells such that any k of the n micro-cells are sufficient to reconstruct the original cell. The probability of receiving a micro cell is analogous to P above and is given by: p = (1 - e - Nα J n/ >)/(NαjS(n/k)) The cell can be succesfiilly reconstructed if any k or more micro-cells are received, therefore the probability of receiving a cell is now: -1 p = x ■ Σ (? ) p ! d -p ) " " - 1 j-0 3 In practice it is reasonable to assume that the number n of coded micro- cells is large and that the required cell loss probability is small. Consequendy the product of n and p (the probability of receiving a micro-cell) is very much greater than one and np(l-p) is greater than 1. In such circumstances the binomial distribution can be approximated by a Gaussian distribution function G to give: P - l -G ( ( - l -np) / np ( l -ρ) ) ) = l- _i = f x e ^dy in which x = (k - 1 - npW ψO - P))- In the limit as n tends to infinity and provided that k/n, the coding rate, is less than the probability p of receiving a micro-cell, x tends to minus infinity and the integral term in the equation for P tends to zero. Thus the probability P of receiving a cell thus tends towards 1 provided that: which simplifies to NαjS < (1 - e - e flW) Thus for small coding rates (meaning that n/k is large) the throughput N β approaches 1. This result accords with intuition because if each transmitting station transmits one coded cell at a very low coding rate, the receiver will be able to receive data virtually constantly. It has been assumed that the receiver is aware of the channels containing data to be received. This could be acheived, for example by sensing the energy in all of the channels continously or by including a very fast scanning receiver which, in a frequency division multiplex system, would derive a frequency spectrum with a number of 'blips' indicating those channels containing a signal. Suitable channel sense arrangements will be familiar to those skilled in the art. When the invention is applied to an asynchronous communication system the calculated throughput will deteriorate slightly because further micro-cells will be lost due to the unslotted transmission of data. The deterioration is, of course, compensated by the lack of global timing requirements in an asynchronous system. It will be appreciated that in a practical system that n cannot be infinite and consequently the probability P of receiving a cell will not be exactly equal to 1. Thus it is necessary to determine what practical values of n can be applied to communication systems with acceptable cell loss probability and throughput. Figure 3 shows a graph of cell loss probability on the vertical (logarithmic) axis against k, the cell division factor, on the horizontal axis for a coding rate of 1/8 and a throughput NαjS of 0.5. It will be seen that large values of k are required for a reasonable value of cell loss probability. What is an acceptable cell loss probability will vary widely depending upon the application and is taken here to be 10 *9 . It is desired to minimise the value of k and to maximise the coding rate and the throughput to provide a practical system. These requirements conflict with each other and have to be balanced for any particular application. The following graphs will assist in acheiving this balance. Figure 4 shows k, the cell division factor on the vertical axis against throughput Nα3 on the horizontal axis for several different values of coding rate. The highest (dashed) curve corresponds to a coding rate of 1/3, the next curve (dot-dashed) corresponds to a coding rate of 1/100 and the thickened line is actually three curves corresponding to coding rates of 1/5, 1/7 and 1/9 in descending order. As can be seen, for a given throughput, as the coding rate is decreased, the value of k first decreases and then increases. This implies that the value of k can be minimised with respect to the coding rate for a given value of throughput. Figure 5 shows k, the cell division factor on the vertical axis against the inverse of the coding rate on the horizontal axis for various values of throughput. The highest curve corresponds to a throughput of 0.6 and the remaining curves to throughputs of 0.5, 0.4 and 0.3 respectively. One of the interesting areas on the graph relates to a through¬ put of 0.5 with a coding rate of around 1/8 and a cell division factor of approximately 60. By using these graphs the parameters of a communication system in accordance with the present invention may be defined to suit user requirements. Although completely random access has been described, the receiver behaviour can be altered within the scope of the present invention, for example to take account of widely differing traffic densities from the plurality of transmitting stations. If a transmitting station TSl has an average traffic density which is greater than that of transmit¬ ting station TS2 by a factor of five, the coding rates of the coders Cl, C2 and the behaviour of the receiver may be arranged as follows. A rate 1/2 code is applied to the data traffic from TSl, in other words n is only twice k. A rate 1/10 code is applied to d e data from TS2, in odier words n is ten times k. For a given amount of data, TS2 will transmit five times as many micro-cells as TSl but since their respective traffic densities differ, they will be transmitting for an equal proportion of the time. The receiving station is provided with intelligence to bias its receiver in favour of receiving from TSl. The proportion of time it will spend receiving from TSl will be five times greater than it will spend receiving from TS2. Note that the receiver behaviour is still random, however. The different coding rates will be compensated by the receiver behaviour to provide equal cell-loss probabilities for cells from both TSl and TS2. However the throughput from TSl to the receiving station will be five times that from TS2. Alternatively, the receiver may be arranged to operate in a purely random manner and the different coding rates will then provide different error probabilities rather than different throughputs. This principle can be extended further, with or without alteriung the behaviour of the receiver, to cope with a large number of different traffic rates and could even be arranged to be adaptive under the control of network management. The communication system may be arranged with enhanced performance at little extra cost by adding a second receiver at the receiving station. Figure 6 shows a switched embodiment of the invention in which a number of transmitting stations TSl, TS2, TS3....TSN are each coupled to a respective transmission line LI, L2, L3...LN. The transmission lines from the transmitting stations are all connected to a receiving station RSI. Each transmitting station, as in the previous example, comprises a data source DS which provides digital data to a coder C which in turn provides coded data to a transmitter Tx. The transmitter Txl transmits the data and redundancy along the transmission line LI. The remaining transmitting stations TS2...TSN are constructed in the same manner and transmit data and redundancy along respective transmission lines L2...LN. Receiving station RSI comprises two receivers R l and Rx2 which each may be connected to any one of the N transmssion lines L via the switches SW1 and SW2. The manner in which the data and redundancy are transmitted in this embodiment may take any suitable form. The receivers Rxl and Rx2 are best arranged to receive from mutually different channels in order to avoid waste of the receiver resource as described above. Alternatively each receiver may be arranged to receive signals from a subset of the transmitting stations but this in effect provides two separate communication systems which will not benefit from statistical multiplexing as much as a single system would. In operation the hard wired embodiment of the present invention shown in Figure 6 behaves in a broadly similar manner to the optical embodiment but one in which the channel separation is provided physically rather than in the frequency domain. The diffi- culties encountered in the optical domain with regard to receiver channel switching times are somewhat less problemmatic in a wired embodiment since fast solid state switches are readily available and channel energy sense is easy to provide. In addition, d e hard wired embodi¬ ment (provided that the two receivers are arranged to receive from mutually different channels) will not suffer from the bit rate degradation due to receiver loading that the optical embodiment of Figure 1 will. Figure 7 shows a graph of cell division factor against coding rate for the two receivers per receiving station system of Figure 6. Values of coding rate as low as 2 or 3 can be seen to provide good results. The uppermost curve corresponds to a throughput of 0.95 and the remaining curves relate, in descending order, to throughput values of 0.9, 0.8 and 0.6 respectively. These throughput values are based on the assumption that the receiving stations always receive from different transmitting stations. In order to apply this graph to the optical case the throughput values for this graph have to be halved because the trans¬ mitted signal power has to be divided between the receivers. By comparison with the graph of Figure 5 it can be seen that the use of a second receiver allows a reduction in the cell division factor k of approximately 25 % for the same system throughput. Further receivers may be added but the balance of throughput, reliability and cost will mean that a small number is optimal. Two receivers are particularly attractive because a law of diminishing returns applies and the first extra receiver provides the greatest performance gain for a given additional cost. The present invention will also find application in the radio field, particularly where the transmitting stations are powered from a mains or a vehicle voltage source. The channel separation will most likely be by way of frequency division multiplex but applications using, for example, code division multiplex (CDM) to provide the distinct transmission channels can be envisaged. In such a case eash transmitting station TS has its own unique spreading code or key which provide a signal orthogonal to all of the other transmittined signal thus giving a non-destructive multi-access channel.

Claims: CLAIMS 1. A digital data communication system comprising a plurality of transmit¬ ting stations and at least one receiving station, each transmitting station comprising a source of a digital data signal and means for transmitting the digital data signal to the at least one receiving station on a distinct channel and the at least one receiving station comprising a number of receivers, characterised in that the number of transmitting stations is larger than the number of receivers at each receiving station, in that each receiver comprises means for receiving a digital data signal from any single transmitting station, and in that each transmit¬ ting station further comprises means for applying a sufficient quantity of redundancy to d e digital data signal before transmission to permit the receiving station to receive substantially all of the digital data signals from all of the transmitting stations irrespective of their relative times of transmission. 2. A communication system as claimed in Claim 1, further characterised in that each receiving station comprises a maximum of two receivers. 3. A communication system as claimed in Claim 1 or Claim 2, further characterised in that the system has no means for assigning communication channels in operation. 4. A communication system as claimed in Claim 1, Claim 2 or Claim 3, further characterised in that each receiving station has no means for providing any inherent ackowledgement of received digital data signals. 5. A communication system as claimed in any one of the preceeding Claims, further characterised in that the plurality of transmitting stations are arranged to transmit each digital data signal and d e associated redundancy only once. 6. A communication system as claimed in any one of the preceeding Claims, further characterised in that the distinct transmission channels are provided by means of frequency division multiplexing. 7. A communication system as claimed in any one of me Claims 1 to 5, further characterised in that the distinct transmission channels are provided by means of space division multiplexing. 8. A communication system as claimed in any one of the preceeding Claims, further characterised in diat the transmitting stations have no relative timing relationship. 9. A communication system as claimed in any one of die preceeding Claims, further characterised in that the means for applying redundancy to d e digital data signals are arranged to apply Maximum Distance Separable coding to die signals. 10. A communication system as claimed in any one of the preceeding Claims, further characterised in that at least two of the plurality of transmitting stations are arranged to apply different quantities of redundancy to their respective digital data signals. 11. A communication system as claimed in Claim 10, further characterised in that the at least one receiving station is arranged to respond to the different levels of redundancy by biassing its receiver in favour of reception from transmitting stations with lower levels of redundancy. 12. A transmission station for use with a communication system as claimed in Claim 1, the transmission station comprising a source of a digital data signal, means for adding redundancy to d e digital data signal to provide a channel signal and means for transmitting the channel signal. 13. A receiving station for use with a communication system as claimed in Claim 1. 14. A method of communicating digital data signals, comprising adding a sufficiently large quantity of redundancy to die digital data signals from a plurality of data- senders on distinct channels to permit a single receiver to recover substantially all of the digital data signals from each of the plurality of data-senders irrespective of the time relation of the digital data signals.

Inventors: KHOE GIOK DJAN, SINHA ATUL NARENDRANATH, VAN DRIEL CAREL JAN LEENDERT, KHOE, GIOK, DJAN, SINHA, ATUL, NARENDRANATH, VAN DRIEL, CAREL, JAN, LEENDERT
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1995
Main classification: H04J14
IPC codes: H04J14