GB2371728A - Powerline modem - Google Patents

Abstract

A method and means of communicating over low voltage power supply lines uses a powerline modem. The modem may comprise a CPU 31 and DSP 18. A digital data transmission is a modulated baseband waveform which consists of a number of waveforms each mutually orthogonal over a symbol period to a carrier frequency. The nominal symbol period of the digital transmission is exactly half the cycle period of the mains supply or an integer multiple thereof. Various carrier frequencies can be chosen and frequency hopping techniques may be employed.

Description

Mains communications technique This invention relates to communications equipment which uses the Low Voltage (LV) Mains Supply as a medium for signaling and communications. Such equipment is referred to herein as a powerline modem.

The powerline modem for which the invention is provided is an item of communications equipment capable of transmitting and recieving over the LV mains supply and programmed to operate under the control of a central processing unit (CPU).

The"LV mains suppiyn is used for the distribution of electrical energy in the form of an AC supply voltage, nominally 230V in most of Europe, at with a nominal AC cycle frequency fM of 50 Hz, to premises including most homes and businesses throughout the region. Outside Europe other voltages and cycle frequencies prevail, such as 120V at 60 Hz which is used in North America. It has long been realised that the LV mains supply is a potentially widely available communications medium, and that there are applications which could exploit this capability such as remote meter reading, load control and home automation.

Within Europe there is a European Standard that applies to communications devices which use the LV mains supply, EN50065. This specifies certain aspects of the behaviour of communication devices using the LV mains supply, including the carrier frequency which is in the range 9 to 148.5 kHz.

In this description the commonly accepted terminology is used to describe the process of data transmission over a communications channel using a digital modulator, as follows. An information source, containing data which a user wishes to transmit via the communications channel, emits a continuous stream of binary information bits, the "source data" at the "source data rate"/ ? s, which is measured in bits per second. The source data is presented to the "channel coder", which transforms successive blocks of K bits in the source data into N "coded

data bits", emitting coded data bits at the coded data rate Rc = NIK bits per second. The coded data rate is typically larger than the source data rate allowing the coded data to contain some redundancy for purposes such as error correction, error detection and synchronisation : F ? e P.. The successive coded data bits are collected into groups of M before being presented to the modulator (M 1); groups of M bits are known as channel symbols. Channel symbols are presented to the modulator at a rate RB (the Ubaud rate") where RB = RclM baud. Each channel symbol is modulated for a time duration of 1/Rm (one"symbol period"), after which the modulator performs modulation for the next channel symbol. To modulate a channel symbol, the modulator generates an analogue signal of duration 11 RM which typically depends on that channel symbol, the Lp preceding channel symbols, and the Ls succeeding channel symbols where Lp. Ls O. Note that the modulator can modulate a symbol only after it had been presented Ls succeeding symbols. The nature of the analogue signal depends on the nature of the communication channel : in the specific case of the domestic mains, the analogue value represents the phase and amplitude of an electrical signal.

When using the domestic mains as a communication channel, it is desirable to ensure that the baud rate Ra is equal to 2fM i. e. twice the AC cycle rate or some integer submultiple thereof. For example, in the UK where the AC cycle rate is 50 Hz, it is desirable to use a rate of 100 baud, i. e. twice the AC cycle rate or some integer submultiple thereof. This is for a number of reasons, among which is the empirical observation that using this baud rate leads to better performance.

At the same time the desire for higher data rates means that there is a need to use a coded data rate rate Rc which is above twice the AC cycle rate.

The following method uses a the baud rate RB of twice the AC cycle rate (e. g. 100 symbols per second in most of Europe) while allowing a coded data rate Rc to exceed RB is to use multifrequency modulation using BPSK (binary phase shift keyed) waveforms with a frequency separation at multiples of fM (e. g. 50 Hz), all with the same symbol clock. Optionally the frequency separation between BPSK waveforms can be set to some integer multiple q of the AC cycle rate, ie qfM. Using this technique a coded data rate ? e of 2MfM bits per second is available from M BPSK waveforms each spaced by qfM Hz. Preferably the BPSK waveform is coherently demodulated using well known techniques described in [PROAKIS].

The transmitter transmits one channel symbol each symbol period where the channel symbol is an M bit binary value capable of taking 2M different values in the range 0 to 2-1 The hth channel symbol is transmitted during the time interval [hT, (h+l) 7], h > 0. For the hth channel symbol, the following set of M waveforms sk (f), 0 < < M- ? is defined for values of t, where h7' < t : 5 (h+l) Tand where T= 1/2fM is the symbol period (half the AC cycle period). exp () represents the exponential function, and i= -1.

Sk (t) = exp {ik- (t-r)/ r} 1) Let the coded data bits to be transmitted during the h'th symbol period be denoted by the sequence fdhk) = dhi. dh2,..- dhk... dhM. where dhk =0 or 1, 1 < k : 5 M. Then the bipolar coded symbols {bW = bm, bk2.... bkM corresponding to ldhklare derived from the sequence (dki) via the bipolar transformation bjk = 2d, -1 (this has the effect of mapping the data bit zero into the bipolar value-1, and the data bit 1 into the bipolar value 1). The complex waveform transmitted during the h'th symbol period Wk < (f) is defined as follows :

More generally the frequency separation between different members of the waveform set can set to an integer multiple q of fM Hz where in many cases-2 is preferred. Then the following

set of Mwaveforms siJf), 05 k5 M would be used : Sk (t) = exp {i! (t-hT)/T} (3) The transmitter modulates this baseband signal to a carrier frequency and generates the following function: (4) set) = re[exp(2#fct)wk(t)] where re [x] represents the real part of a quantity x. The signal s(t) is transmitted over the channel.

One example of the invention will now be described with reference to figures 1,2 and 3. Figure 3 provides an overview of the powerline modem which consists of a CPU (31) which interfaces and controls a DSP (18). The CPU controls the function of the DSP, sending data to be transmitted via the powerline and processing received data. The DSP in turn controls the function of the powerline transmit and receive path (33) which converts analogue received signals into digital baseband Q samples on receive, and on transmit converts digital IQ baseband Q samples to an analogue transmit signal. The transmit and receive path is coupled to the powerline via a line coupling network (25) which bidirectionally couples signals at the transmission frequencies while rejecting the AC line voltage in accordance with well known methods described in reference [1]. A zero crossing detector (34) constructed in accordance Mi. th (ref (l)) indicates the time at which the AC cycle crosses zero volts to the CPU (31).

The receiver path and transmitter path sections (33) is are further illustrated in figures 1 and 2 respectively. Referring to figure 1, the receive path is connected to the mains coupling network (1) from which the received signal is obtained. The received signal is passed through an attenuation network (2) and band limited to the signal frequency range by a band pass filter (3).

The signal is then amplified by a progammable gain amplifier (4) and passed through a capacitor (5) before being presented as the analogue voltage reference to multiplying digital to analogue convertors (6) and (7) whose digital input signals are the phase and quadrature signals from a sinusoidal digital local oscillator (8) generated by the DSP (18). The output of the digital to analogue convertors (6) and (7) is filtered by antialisaing filters (9) and (10), amplified by amplifying circuits (12) and (13). Any DC offsets are corrected using circuitry (14) and (15) which is controlled by the DSP (18) and converted to digital form by 8 bit analogue to digital convertors (16) and (17) the output of which is passed to the DSP (18).

Referring to figure 2, the transmit path consists of a digital representation of the transmitted signal which is generated by the DSP (18), converted to analogue form by an analogue to digital convertor (22), amplified by variable gain amplifier (23), filtered by antialiasing filter (24) and output to the mains coupling network (25).

References [1] Texas Instruments : Using the TLE2301 power operational amplifier for signal transmission on the mains network : application report, 1993

Claims (11)

CLAIMS :

1. A method and means for communicating via the low voltage mains supply using a powerline modem, the powerline modem containing either or both of a transmission and reception means, the transmission means which modulates a baseband waveform consisting of the sum of a plurality of waveforms each mutually orthogonal over a symbol period to a carrier frequency and the reception means which detects the said waveforms.

2. A method and means for transmitting the low voltage mains supply as in claim 1 where the nominal symbol period of the digital transmission method is exactly half the AC cycle period, or some integer multiple thereof.

3. A method and means of transmitting on the low voltage mains supply as in claim 1 or 2 where the baseband transmitted waveform is defined as the summation of exponential functions according to (equation 1) or (equation 3) with mutually separated by the AC cycle frequency or an integer multiple thereof.

4. A method and means of transmitting on the low voltage mains supply as in any one of claims 1 to 3 in which the signal can be transmitted on a carrier frequency selected from a plurality of possible frequencies.

5. A method and means of transmitting on the low voltage mains supply as in anyone of claims 1 to 4 which uses frequency hopping techniques between successive transmission.

6. A method and means of transmitting on the low voltage mains supply as in claim 2 in which the AC cycle period is detected using a zero crossing detector and the nominal symbol period is automatically varied according to the AC cycle period.

7. A method and means of receiving on the low voltage mains as in claim 1 where the nominal signal period is as in claim 2.

8. Apparatus for enabling communication via an AC mains supply, comprising transmission means arranged to modulate a baseband waveform comprising a plurality of mutually orthogonal waveforms over a symbol period to a carrier frequency.

9. Apparatus for enabling communication via an AC mains supply, comprising reception means arranged to receive a baseband waveform comprising a plurality of mutually orthogonal waveforms modulated over a symbol period to a carrier frequency.

10. Apparatus according to claim 8 or 9, wherein the baseband waveform is a sum of exponential functions, as set out in equation 1 or 3 above, separated by the mains AC cycle frequency or an integer multiple thereof.

11. Apparatus according to any one of claims 8 to 10, having the features set out in any one or any combination of claims 2 to 7.