GB2439126A

GB2439126A - Non-destructive bit-wise arbitration over noisy communication channels

Info

Publication number: GB2439126A
Application number: GB0611683A
Authority: GB
Inventors: Peter Strong
Original assignee: Siconnect Ltd
Current assignee: Siconnect Ltd
Priority date: 2006-06-13
Filing date: 2006-06-13
Publication date: 2007-12-19
Also published as: WO2007144605A1; GB0611683D0

Abstract

Many protocols for distributed arbitration of medium access share the feature that a dominant bit (a burst of carrier within a bit timeslot) will win a contention battle against a recessive bit (an absence of carrier within a bit timeslot). It is therefore important to be able to distinguish the existence on a communication channel of a dominant bit as opposed to channel noise. To assist in the detection of a dominant bit, the transmitter generates a signal for transmission by combining the dominant bit (30, fig 4) with a correlation pattern (32). At a receiver, the signal is demodulated 40 into a candidate bit and a candidate correlation pattern, and the bit is signalled as a dominant bit 44 if correlation 42 of the candidate pattern indicates a correlation pattern. The correlation may involve correlating the candidate pattern with a predetermined pattern or correlating a first part of the candidate pattern with a second part (auto-correlation). The invention has particular application to power-line and wireless communications.

Description

DATA TRANSMISSION METHOD AND APPARATUS

The present application relates to a method and apparatus for transmitting data in a data network. It is particularly, but not exclusively, concerned with data communication in a data network comprising a plurality of stations interconnected by bus lines. The invention has particular application wherein the bus lines are implemented by means of electrical power lines and coaxial cables.

An overriding aim with any communications system is to provide robust communication which is capable of overcoming external interference, which could otherwise give rise to false data readings at a receiver of transmitted data.

Particularly in power line communications, noise and interference can be a significant problem. This is because, despite the fact that power lines represent a useful physical medium for implementation of communication networks, they are by their nature compromised by the fact that they may have been installed without regard to communications requirements. They may be unshielded, and may also be placed physically adj acent other electrical equipment.

As a result, power lines can be at risk of significant interference from unintentional generation of electromagnetic energy by devices adjacent to or connected to the power line network concerned. Although, recently, electromagnetic compatibility (EMC) regulations have been imposed in many countries, problems with interference may remain. For example, electromotive equipment powered by a power line intended for use for communications purposes will impose substantial unwanted components onto the power line, such as during running of a motor, or due to switching of electrical components. As a practical example, a domestic laundry machine will inevitably impose substantial spikes on a power line when the motor of the machine is switched on or off These spikes are substantially more difficult for a communications device to overcome than the effects of a running motor, as the former will be rather more transient than the latter.

The difficulty imparted by a transient effect is that a receiver may confuse this with a pulse which the receiver is expecting. This problem is particularly difficult to overcome in the preamble to a data message, as payload data is normally accompanied by a CRC check which inevitably identifies a significant false detection. In the preamble, however, false detections are less easy to identify as the information to be sent is often unprotected by error check codes.

The most significant source of confusion exists where nodes in a network are competing for access to the digital carrier. Various carrier sense multiple access (CSMA) protocols have been established which attempt to resolve the competition between nodes, with different degrees of success. Each of these to some extent relies upon received signal strength indication (RSSI). RSSI comprises a measurement of the received signal strength on the carrier, in particular within a frequency band defined for the communications protocol concerned. A convenient way of eliminating out-of-band noise is by means of a filter. The RSSI measure takes an energy integral within the frequency band of interest. However, RSSI is incapable of differentiating between the in-band noise and a start of frame signal from a node intending to transmit.

For example, an electric motor could output significant incoherent noise which might include the band within which the RSSI measure is to be taken. This noise may be sufficient to exceed an energy level threshold by which the receiving station taking the RSSI measure will consider the received signal to be a valid start of frame. This will give rise to a false detection if the in-band noise exceeds this threshold.

One example of an attempt to overcome the above problems is presented in existing PLC technology. In this technology, a communications frame commences with a header containing a sequence of bits at a first transmission rate (F 1), these bits forming a service priority field that enable stations to arbitrate for the medium in such a way that ensures that the station with the highest service priority message wins control of the medium. This tecimique is described as "non-destructive bitwise" arbitration and adopted in the CAN and CeBus communications standards.

In existing PLC technology, an Fl bit is constructed from four different carrier frequencies summed together and transmitted in a short burst. A pseudorandom chip pattern is modulated onto the carriers to spread the power of the carriers. An Fl bit represents either a logical one or a logical zero. A logical zero is represented by the existence of a short burst of modulated carrier and is known as a dominant Fl bit; conversely, a logical one is represented by the absence of this short burst of modulated carrier and is known as a recessive Fl bit.

All stations forming nodes in a network, with messages queued for transmission, will initially map the traffic class based priority accorded to the payload of the message concerned onto the plurality of service priority F 1 bits. In the next Fl bit time slot, each station with a message queued to send will begin to transmit its own service priority sequence of Fl bits onto the medium. It will be understood that stations will be transmitting simultaneously during this F 1 arbitration phase in order to perform the process of arbitration for the medium.

The process of determining which station achieves control of the medium is thereby entirely distributed. This is because a station transmitting a short burst of Fl (a dominant bit) in the current Fl bit slot will continue to the next Fl bit slot regardless of the behaviour of other stations. On the other hand, a station not transmitting an Fl burst (i.e. transmitting a recessive bit) will instead listen to the medium for the duration of that Fl bit slot and will withdraw from arbitration if another station transmits a dominant bit. That withdrawing station will not participate any further in the arbitration process and will back off and wait for the next arbitration opportunity.

At the end of the service priority arbitration sequence, one or more stations with the highest service priorities will remain in contention for the medium.

Further arbitration may be necessary in order to overcome the possibility that more than one station may have progressed through the service priority arbitration sequence and still be contending for the medium. This could, for example be resolved on a random basis or by any other means.

The technique of non-destructive bitwise arbitration has proved to be reliable on fixed wire transmission mediums such as twisted pair and coaxial cables, where the background noise levels are low and the resultant signal to noise ratio is high.

However, on media such as wireless and power lines, where the background noise level is very high, and attenuation caused by fading can result in a very low signal to noise ratio at a receiving station, the technique of arbitration using a plurality of carriers (as in the above described example) is prone to false detection. False detection occurs when in-band energy caused by noise is above the decision threshold.

As noted in relation to the RSSI detector described above, a well known technique to overcome this false detection is to employ band pass filters around the desired in-band signal, to reduce the amount of out of band noise relative to the signal seen by a detector. However, such filters can only reduce out of band noise, and caimot resolve problems associated with in-band noise.

Another well known technique is to employ probability density function algorithms to combat the problem of false detection by varying temporarily the decision threshold relative to the background noise. Although a number of different algorithms have been developed that provide improved avoidance of false detection, such algorithms require a relatively slow response with respect to the variable background noise, in order to provide a stable, rather than a rapidly varying, threshold. Clearly, there is advantage in varying the threshold over the relative long term if physical conditions (such as ambient noise levels) have altered, but it would be inherently disadvantageous to vary the threshold rapidly as this would inevitably increase the detection error rate.

Such techniques do provide material benefits in reducing the probability of false detection, but cannot eradicate it satisfactorily in time varying noisy channels, in particular in channels such as power line where high amplitude, wide band noise bursts may occur frequently. Even a combination of filters and probability algorithms can contribute little to the reduction of errors created by noise burst with frequency components that are in the signal band of interest and of sufficient amplitude to trigger a threshold level and so produce a false detection.

The invention aims to provide an improved approach to the reduction of errors brought about by false detection in a non-destructive bitwise arbitration process over a noisy channel, such as in wireless or power line communications.

A first aspect of the invention provides powerline communications apparatus comprising means for transmitting a signal in frame form, the frame containing a header defining bits for use in non-destructive bit wise arbitration, said bits being dominant or recessive bits, the apparatus further comprising means for combining a dominant bit for transmission with a correlation pattern for detection at a receiver of said dominant bit.

A second aspect of the invention provides powerline communications apparatus operable to receive a signal in frame form, the frame comprising a header containing at least one arbitration signalling bit slot for use in non destructive bitwise arbitration, the apparatus comprising dominant bit detection means operable to detect a dominant bit sent in said at least one bit slot, said dominant bit detection means comprising demodulation means for demodulating a candidate bit from a candidate correlation pattern and correlating means for correlating said candidate correlation pattern to determine if said candidate correlation pattern is a correlation pattern, said dominant bit detection means being operable to signal detection of a dominant bit on determination that a corresponding candidate correlation pattern is a correlation pattern.

A third aspect of the invention provides a method of transmitting a signal in a powerline communications network comprising transmitting a signal in frame form, the frame containing a header defining bits for use in non-destructive bit wise arbitration, said bits being dominant or recessive bits, the method further comprising combining a dominant bit for transmission with a correlation pattern for detection at a receiver of said dominant bit.

A fourth aspect of the invention provides a method of receiving a signal in a powerline communications network, the network being configured to convey signals in frame form, the frame comprising a header containing at least one arbitration signalling bit slot for use in non destructive bitwise arbitration, the method comprising detecting a dominant bit sent in said at least one bit slot, said dominant bit detection step comprising demodulating said signal into a candidate bit and a candidate correlation pattern and correlating said candidate correlation pattern to determine if said candidate correlation pattern is a correlation pattern, then signalling detection of a dominant bit on determination that a corresponding candidate correlation pattern is a correlation pattern.

It will be appreciated that the invention can also be implemented by means of computer executable instructions -such as can be delivered on a computer readable storage medium (e.g. flash memory or optical or magnetic data storage means) or by way of a computer receivable signal. The invention could also be implemented by using application specific hardware apparatus, such as a DSP, an FPGA or an ASIC.

A specific embodiment of the invention will now be described by way of example only, and with reference to the accompanying drawings, in which: Figure 1 illustrates an example communications system on which a specific embodiment of the invention resides; Figure 2 illustrates a communications frame for use in the communications system illustrated in figure 1; Figure 3 illustrates a communications device operating as a station at a node of the communications system illustrated in figure 1; Figure 4 illustrates a signal processing element within a transmit side of the communications device illustrated in figure 3; and Figure 5 illustrates a signal processing element within a receive side of the communications device illustrated in figure 3.

As shown in figure 1, a communications network 10 is illustrated with four interconnected nodes 12. The nodes are connected by means of power lines 14, such as a ring main in a domestic installation. It will be appreciated that the illustrated interconnections are symbolic and are not intended to represent the physical arrangement of conductors between the nodes -such conductors will in many cases comprise a ring to which the electronic equipment implementing the nodes are each connected.

Each node 12 comprises a communications station capable of transmitting and receiving information in accordance with a frame based communications protocol. Figure 2 illustrates a typical example of a frame structure, for use in conjunction with the specific embodiment. This packet structure is substantially consistent with existing packet structures such as the 1-lomePlug technology.

The frame consists of four fields, named Fl, F2, F3 and F4. Fl and F2 are collectively the start of the frame, used for establishing control of the physical medium (the interconnections) and signalling the nature of information to follow. F3 is provided to contain the payload to be sent in the frame, and F4 is an end of frame, containing portions for signalling acknowledgements or transmission failures.

In more detail, Fl is a header in a fixed time slot with a sequence of bits used in the arbitration phase of medium access control. Any suitable protocol for distributed arbitration of medium control can be contemplated, and the specific embodiment is not limited to any particular arrangement. However, many of these protocols share the feature that a dominant bit (a burst of carrier within a bit timeslot) will win' a contention battle against a recessive bit (an absence of carrier within a bit timeslot). It is thus important to be able to distinguish the existence on the channel of a dominant bit, as opposed to channel noise.

Figure 3 illustrates schematically a station 20 provided at each node. It will be appreciated that each element of the station is described by way of example only, and any arrangement to establish communication will be suitable. Further, the implementation of the station 20 may be by way of application specific hardware, or general purpose communications equipment (such as a computer with a suitably configured modem) under the control of software.

The station 20 comprises a data source 22 and a data sink 24. The nature of the data source and the data sink is not relevant to the present description, except to say that the data source is configured to request transmission of information to another device, and the data sink to receive information from another device. One or other of the data source and data sink could be omitted in certain stations in a network -some stations may be pure transmitters of information and not have any recourse to receive information, and vice versa. A transmitter/receiver unit 26 provides means to enable establishment of network connection with other stations residing at other nodes of the network 10.

Figure 4 illustrates the function of the transmitter/receiver unit 26 in terms of its ability to send Fl bits to contend for the medium, in order to send later portions of the frame.

The arrangement illustrated in figure 4 will be understood to be highly schematic, and may be implemented in software in a practical application.

The arrangement illustrated in figure 4 is used when the data source 22 of the station 20 generates a request to send information to another device. The transmitter/receiver unit 26 generates F 1 data corresponding to this request, containing, in particular, priority information in accordance with an arbitration policy for the network concerned. This priority information will comprises a string of Fl bits representing the relative priority of the message to be sent. This priority information is input to an Fl bit generator 30 which generates a physical string of dominant and recessive bits in accordance with the Traffic Class of the data to be transmitted in F3. The Fl generator is operable to output bursts of carrier frequency corresponding to dominant bits -in this specific embodiment, for robustness, a dominant bit comprises simultaneous bursts at four distinct carrier frequencies. Meanwhile, when the Fl bit generator generates a dominant bit, a correlation sequence generator 32 is triggered to generate a correlation sequence, which is then modulated onto dominant bits in the sequence ofFi bits.

Two possible embodiments of this correlation sequence will now be described. In a first embodiment, the correlation sequence is a sequence which will be known to all stations in the network. In this case, the correlation sequence can be used singly in a single Fl bit, in the knowledge that a receiver will recognise the correlation sequence for what it is, by cross correlation.

In a second embodiment, the correlation sequence is not necessarily known to other stations, and can indeed be randomly or pseudo-randomly generated. In this embodiment, the correlation sequence is repeated within a single F 1 bit, such that a receiver can identify the existence of the sequence on an Fl bit by means of autocorrelation.

Figure 5 illustrates a corresponding arrangement residing in the transmitter/receiver unit 26 for detection of valid Fl dominant bits. The arrangement comprises a detector and demodulator 40 which receives the raw channel signal and detects the presence of possible Fl bits in the correct carrier frequencies in the channel. Corresponding to the Fl bit generator described above, the detector/demodulator 40 detects the existence of a candidate F 1 dominant bit by the presence of bursts of carrier at the four carrier frequencies identified above.

It may be that one of these carrier frequencies becomes particularly susceptible, in a particular arrangement, to losses -depending on the robustness requirements for the system, this can be interpreted as non-detection of the bit at the receive side.

Alternatively, the system could be configured to accept a dominant bit despite less than all carrier frequencies being detected - the remaining correlation pattern detection will enhance the robustness of the system in compensation for any false detections made possible by this approach.

Alongside this, the detector/demodulator 40 demodulates the band of frequencies in which correlation patterns can be expected. The candidate correlation pattern for a detected candidate Fl bit is then forwarded to a correlator 42.

Corresponding to the two possibilities for placement of correlation patterns on the signal, as set out above, there are two examples for the correlator. In accordance with the first arrangement, whereby the correlation pattern is one which is known to all stations on the network, and could indeed be a standard pattern for all devices according to the protocol, the correlator 42 is specifically configured to correlate the pre-agreed pattern. On detection of this pattern in the Fl bit slot being tested, the correlator outputs a verification signal to a verifier unit 44.

Alternatively, the correlator could be configured to auto-correlate a first half of the detected candidate pattern against a second half. The correlator 42 in this configuration samples the first half of the pattern and attempts to match it against the second half.

It will be appreciated that in this configuration, it could be possible to arrange for something other than simple concatenation of the correlation pattern, such as transmitting the inverse thereof as the second half.

Again, after confirming the existence of a valid correlation pattern, the correlator outputs a verification signal to the verifier 44.

The verifier 44 verifies that the candidate Fl dominant bits extracted by the detector/demodulator unit 40 are in fact real dominant bits, triggered by verification signals sent by the correlator 42. The verified Fl dominant bits are then output by the verifier, for further processing.

While the foregoing provides description of the invention in terms of apparatus and processes for establishing communication, the skilled reader will understand that other aspects of the invention also exist. For instance, the invention could be implemented by means of computer executable instructions, operable to configure a suitable computer to carry out steps in accordance with any aspect of the invention. The computer executable instructions may be provided by means of a computer program product, such as embodied on a computer readable storage medium, or as embodied on a signal for receipt by a computer. The signal may be initiated by a download instruction generated by a browser on the computer, or by means of an automatic update facility at the computer concerned.

It will be appreciated that the above represents only one of many alternative configurations of the invention, and should not be treated as limiting the scope of protection sought for the present invention. The scope of protection should be read as being defined by the claims appended hereto, with reference to the foregoing

description and the accompanying drawings.

Claims

CLAIMS: 1. Powerline communications apparatus comprising means for

transmitting a signal in frame form, the frame containing a header defining bits for use in non-destructive bit wise arbitration, said bits being dominant or recessive bits, the apparatus further comprising means for combining a dominant bit for transmission with a correlation pattern for detection at a receiver of said dominant bit.

2. Apparatus in accordance with claim 1, comprising correlation pattern generation means for generating a correlation pattern for combination with a dominant bit.

3. Apparatus in accordance with claim 1 or claim 2 wherein the correlation pattern is in a frequency band substantially higher than the bit frequency of the header.

4. Apparatus in accordance with any preceding claim wherein the correlation pattern is predetermined.

5. Apparatus in accordance with any of claims 1 to 3 wherein the correlation pattern comprises a first part and a second part, the second part being dependent on the first part.

6. Apparatus in accordance with claim 5 wherein the second part is the same as the first part.

7. Apparatus in accordance with claim 5 wherein the second part is a function of the first part.

8. Powerline communications apparatus operable to receive a signal in frame form, the frame comprising a header containing at least one arbitration signalling bit slot for use in non destructive bitwise arbitration, the apparatus comprising dominant bit detection means operable to detect a dominant bit sent in said at least one bit slot, said dominant bit detection means comprising demodulation means for demodulating a candidate bit from a candidate correlation pattern and correlating means for correlating said candidate correlation pattern to determine if said candidate correlation pattern is a correlation pattern, said dominant bit detection means being operable to signal detection of a dominant bit on determination that a corresponding candidate correlation pattern is a correlation pattern.

9. Apparatus in accordance with claim 8 wherein the correlating means is operable to correlate said candidate correlation pattern with a predetermined correlation pattern.

10. Apparatus in accordance with claim 8 wherein the correlating means is operable to separate said candidate correlation pattern into two portions, and to correlate the first portion against the second portion.

11. Apparatus in accordance with claim 10 and comprising signal processing means for processing said second part before correlation.

12. A method of transmitting a signal in a powerline communications network comprising transmitting a signal in frame form, the frame containing a header defining bits for use in non-destructive bit wise arbitration, said bits being dominant or recessive bits, the method further comprising combining a dominant bit for transmission with a correlation pattern for detection at a receiver of said dominant bit.

13. A method in accordance with claim 12 and comprising generating a correlation pattern for combination with a dominant bit.

14. A method in accordance with claim 12 or claim 13 wherein the correlation pattern is in a frequency band substantially higher than the bit frequency of the header.

15. A method in accordance with any of claims 12 to 14 wherein the correlation pattern is predetermined.

16. A method in accordance with any of claims 12 to 14 wherein the correlation pattern comprises a first part and a second part, the second part being dependent on the first part.

17. A method in accordance with claim 16 wherein the second part is the same as the first part.

18. A method in accordance with claim 16 wherein the second part is a function of the first part.

19. A method of receiving a signal in a powerline communications network, the network being configured to convey signals in frame form, the frame comprising a header containing at least one arbitration signalling bit slot for use in non destructive bitwise arbitration, the method comprising detecting a dominant bit sent in said at least one bit slot, said dominant bit detection step comprising demodulating said signal into a candidate bit and a candidate correlation pattern and correlating said candidate correlation pattern to determine if said candidate correlation pattern is a correlation pattern, then signalling detection of a dominant bit on determination that a corresponding candidate correlation pattern is a correlation pattern.

20. Method in accordance with claim 19 wherein the correlating step includes correlating said candidate correlation pattern with a predetermined correlation pattern.

21. Method in accordance with claim 19 wherein the correlating step includes separating said candidate correlation pattern into two portions, and correlating the first portion against the second portion.

22. Method in accordance with claim 19 and comprising processing said second part before correlation.

23. A computer program product comprising computer executable instructions operable to configure general purpose computer controlled communications apparatus to perform a method in accordance with any one of claims 12 to 22.