Spread spectrum communication method for power line

Electronics and Communications in Japan, Part 1, Vol. 75, No. 6, 1992 Translated from Denshi loho Tsushin Gakkai Ronbunshi, Vol. 74-B-II. No. 5 , May 1991, pp. 234-243

Spread Spectrum Communication Method for Power Line

Kaoru Endo, Member, Tadashi Nakamura, Nonmember, Soichi Tsumura and Shichiro Tsuruta, Members

NEC Home Electronics, Ltd., Kawasaki, Japan 213

SUMMARY

Spread spectrum (SS) communication is proposed as a high-speed data transmission system which uses the power line as the transmission channel. However, the power line is not originally a transmission line, the quality of transmission is not guaranteed, and the characteristics are not ideal in general. Especially, the phase characteristics may fluctuate greatly and rapidly in synchronization with the power frequency.

In the traditional SS system, the sequence inversion keying (SIK) and the synchronization control by digital lock loop (DLL) are employed. However, there is a problem in that the stable retaining tracking of the synchronization is not realized for such a channel, thereby preventing normal demodulation. Code shift keying (CSK) is proposed to replaced SIK. An effective synchronization method to replace DLL in the application to power line communication has not been proposed.

This paper proposes the synchronization control method which can establish stable synchronization at the initial stage. Also, the stable synchronization tracking in the deteriorated transmission environment is realized with a fast phase characteristics fluctuation similar to that in the power line, in addition to the demodulation method which demodulates the information stably even if the correlation function is distorted. By an experiment using

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the artificial power line and an actual home, the effectiveness is verified and the practical usefulness is demon- strated well.

Key words: Synchronization control method; code shift keying (CSK); spread spectrum (SS) communication; electromagnetic wave law; synchronization tracking; sequence inversion keying.

1. Introduction

When a power line is regarded as a channel for information transmission rather than a power supply route, it has the following features.

Most home electric and electronic equipment is provided power from a power line.

The connecting terminal is provided in each room in the standardized form of ac outlet.

As a result of the foregoing, a very useful and economical system will be constructed if the power line can be utilized as the information transmission line. A study to standardize the use of the home power line as the information transmission channel has been conducted by a survey committee for the use of the power line camer communication.

ISSN8756-6621/92/O006~50$7.50/0 @ 1992 Scripta Technica, Inc.

Table 1. A part of draft standard for power line carrier system (high-speed)

Modulation method: Transmitted power: Received voltage: Input impedance: 120 0 or above Output impedance: 10 n Frequency band: 10 - 450 kHz Data transmission rate: 9600 bitis (an example)

Spread spectrum communication system 10 dBm or less (10-0 load and measurement bandwidth 10 kHz)

-10 dBm or less (total received power for 10-0 termination)

The spread spectrum (SS) communication was adopted by the committee in 1986 [l] as the standard for high-speed systems. The system has the following features [2-51:

The communication is robust against interferences and disturbances from other systems;

the system causes little interferences and disturbances to other systems;

the security of information is maintained; and

the codedivision multiplex is possible.

Corresponding to this decision, the execution de- tails of the electromagnetic wave law was revised in 1987. Table 1 shows the outline of the high-speed standard system.

This paper proposes the SS communication system. This system realizes the high-speed and highly reliable data transmission using the power line under the condition of Table 1.

To utilize the features of SS communication, in general, not only in the power line, the spreading and despreading codes must be synchronized between the transmitter and the receiver. Consequently, it is required in the SS communication system that the synchronization between the two should be established first (initial synchronization establishment), and then the established synchronization should be maintained (synchronization tracking). In general, the sliding correlator or the correlator by SAW device is used for the forementioned synchronization control [7-151.

However, when the power line is used as the transmission channel, the relatively low frequency band must be used which makes the SAW correlator impractical. The actually reported SS communication system on the power line uses the sliding correlator which requires a long time to establish the synchronization [8-131.

Another point is that the modulation/demodulation system (sequence inversion keying, SIK) is employed which transmits information based on the polarity of the spreading code [ 171. This makes the transmission suscep- tible to the phase fluctuation in the transmission path. It is proposed that code shift keying (CSK) replaces sequence inversion keying (SIK) [ 18, 191 which can realize the information transmission robust against the fluctuation of phase characteristics.

On the other hand, the traditional DLL [20-221 as well as the extended DLL [23] proposed as a synchronization method useful in the CSK system are weak to the rapid fluctuation of the phase characteristics, as are observed often in the power line. When the channel has a phase fluctuation of large amplitude as in the case of the power line, the synchronization may escape from the lock range of the synchronization loop, thereby preventing the stable detectiodtracking of the synchronization ~ 4 1 .

This paper proposes a new synchronization method and the information decoding method which are useful in the SS power line communication system based on CSK. In the proposed system, the synchronization problem is solved by calculating the correlation function between the received signal and the code in real-time using the matched filter, and detectingltracking the maximum of the correlation function using the window. Also, by using the window, the information is decoded more securely.

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The equipment is actually constructed for evaluation and the effectiveness is verified.

This paper describes first the transmission characteristics of the power line, and points out the problems when SS communication is executed on the lower line. Then the new synchronization method and the demodulat- ing method to solve those problems are described. Final- ly, the evaluation equipment constructed to demonstrate the usefulness of the system is outlined, and the result of evaluation for the power line communication modem using the evaluation equipment is presented.

2. Transmission Characteristics of Power Line

2.1. Noise

The noise level of the power line generally decreas- es with the increase of the frequency. The noise is divided into the following three kinds according to its property.

0 Continuous noise. The continuous noise is generated from the equipment containing an oscillator, such as electromagnetic cooker, as well as the brush motor. The noise level incfeases with the decrease of the frequency.

0 Periodic noise. The periodic noise is generated from the thyristorcontrolled equipment, such as the variable incandescent lamp. The noise level is high, but the duration is short compared to the generation period.

@ One-shot noise. The one-shot noise is generated by the onloff of the thermostat or switching of M

equipment. The noise level is much higher than those of other kinds, sometimes reaching several 10 V.

2.2. Impedance

The impedance of the power line varies greatly depending on the connected equipment. Especially, the impedance may be decreased greatly when the capacitor in the equipment resonates with the inductance of the power line.

2.3. Attenuation

The power line installed in the home is either the single-phase, three-wire or the single-phase, two-wire

system. The attenuation for the single-phase, two-wire system is the same as that of the in-phase component in the single-phase, three-wire system.

Q III-phase attenuation. The attenuation is 20 to 30 dB on the average over the whole frequency band (10 to 450 kHz) [7].

@ Different phase attenuation. The attenuation between different phases in the single-phase, three-wire system sometimes exceeds 50 dB on the average. This is due to the coupling loss of the pole transformer and the transmission loss [7].

2.4. High-speed fluctuation of transmission CharaCteriStiCS

The forementioned transmission characteristics are always varying because of the following reasons:

onloff of equipment and inlout of plugs;

connection of equipment, where ac is directly rectified; and

connection of equipment with inverter.

Here, fluctuations occur suddenly. In particular, the fluctuation by the second factor occurs, i.e., it is synchronized to the source frequency (50160 Hz), and af- fects greatly the communication [24].

3. Problems in Traditional System

The transition characteristics of the power line are far from satisfactory. The characteristics cannot be anticipated and fluctuate very rapidly. Consequently, the following problems exist in the traditional system [8-131, 17-23].

3.1. Problems in synchronization capture and tracking

The synchronization is important in SS communication. There are two problem in synchronization: 1) the initial synchronization where the synchronization should be established as fast as possible at the start of communication; and 2) the synchronization tracking where the once established synchronization should be maintained in a stable way until the communication is completed.

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The sliding correlation method is the simplest for the initial synchronization. In this method, the correlation between the received signal and the despreading code is calculated. The clock frequency for generating the despreading code is shifted slightly from the clock frequency generating the spreading code in the transmitter until the correlation value exceeds a certain threshold.

Because there is a frequency difference, the relative phase between the spreading code in the transmitter and the despreading code in the receiver is varied gradually (called sliding). The correlation reaches the maximum when the two phases coincide. The synchronization detector detects that the correlation value exceeded the threshold. Then the clock for generating the despreading code in the receiver is set at the same frequency as that of the clock generating the spreading code in the transmitter. Thus, the initial synchronization is achieved. However, in principle, the frequency difference cannot be set as a large value, which limits the sliding speed. Consequently, a long time is required in establishing the initial synchronization.

When a correlator based on the SAW device is employed, it is easy to find the maximum correlation value since the whole correlation function is produced as the output by receiving a period of the spreading code. In principle, the initial synchronization is established in the time corresponding to a period of the spreading code. In the communication using the power line, however, relatively lower frequencies of 10 to 450 kHz must be used. This increases the size of the device, which is not practical.

For synchronization tracking, the delay lock loop (DLL) is employed in general [20-221. However, the range of synchronization control by DLL is within the phase difference of T = + b between the spreading/despreading codes (a = a period of spreading code/spreading code length = 1 chip length). When the power line is used as the transmission channel, the transmission characteristics change suddenly as is already described, and sometimes it is observed that the synchronization point shifts by more than k2b [24]. In such a case, the synchronization tracking cannot be executed by DLL, and the initial synchronization must be started from the beginning.

3.2. Problems in data demodulation

It is difficult in power line SS communication always to maintain the accurate synchronization. Even if

the stable and accurate synchronization is maintained, a waveform distortion is generated since the power line does not have a flat frequency characteristic. This makes the demodulation in the receiver difficult if the modulation is of a scheme where the information is carried by the transmitted signal waveform. It may seem promising to equalize the distorted waveform by an equalizer, but then a very high-speed response is required for the equalizer since a sudden change of transmission characteristics arises in the power line.

4. Proposed System

4.1. Realization of synchronization system by a high-speed correlator

4.1.1. Establishment of synchronization

The slow establishment of the synchronization in the sliding correlation method is due primarily to the fact that the correlation value is calculated only for one value of the time difference in a period of the spreading code. If all values of the correlation function between the received signal and the despreading code are calculated by receiving the signal for a period of the spreading code, the synchronization can be established with a high speed by determining the maximum of the correlation function. In the proposed system, the correlator com- posed of the digital matched filter (3, 191 is used for this purpose. The correlation function calculated by the digital matched filter is represented as in Eq. (1).

The synchronization point is determined as r,,, which is the time for the maximum of the correlation function c(t) in an observation period

where c(t) is the correlation function, PN(t) is the despreading code, k ( t ) is the received signal, bn is 6 / n , n is an arbitrary integer, b is a chip time, N is nvn, and m is the spreading code length (a period of spreading code T = rn-a).

In practice, the existencelnonexistence of the transmitted signal must be determined. Another point is that the synchronization point may migrate due to the fluctuation of the transmission characteristics. Consequently, the position of the maximum of the correlation output is observed for q periods. If p or more maxima are observed within the time interval of Ta, it is decided that

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there exists the transmitted signal. The position of the maximum satisfying the foregoing condition is decided as the synchronization point to. Similarly, once the synchronization is established, it is checked whether or not p or more maxima of the correlation are observed within f Ta/2 of the synchronization point in q periods. If this condition is not satisfied, it is decided that the transmission is over.

Thus, Ta should be set as follows. Let the shift of the synchronization point due to the fluctuation of the transmission characteristics Ta > Td. Furthermore, to prevent the inCOKect synchronization due to the disturbance such as noise, Ta-Td should be set as small as possible.

4.1.2. Synchronization tracking

Using the correlator based on the digital matched filter, the whole correlation function can be determined for each period of the received signal. Then the synchronization control can be executed in the range of a period.

The synchronization point may migrate, either by the deviation between the clocks of the transmitter and the receiver or by the change of the transmission characteristics. The former is reflected on the relatively slow shift of the synchronization point, and the synchronization tracking by DLL is sufficient to cope with the shift. However, the latter is reflected on a sudden and large shift, and a desynchronization may occuf in DLL [24].

To compensate for those shifts of the synchronization point, a method of synchronization point tracking is proposed in which a synchronization control window is provided for the time width of Ta around the synchronization point to, and the position of the window is adjusted so that the point of maximum correlation (= synchronization point) is kept in the window.

More precisely, if the maximum correlation stays in the interval to - to + Ta /2 consecutively for Cn times, to is retarded. Conversely, if it stays in the interval to - Tat2 - b consecutively for Cn times, to is advanced. By this scheme, the synchronization is controlled so that the synchronization point is always close to 4,. The reason for providing the condition that the maximum should stay in the interval consecutively for Cn times is to prevent the incorrect synchronization control by the noise.

Transm i t ter output E1-f

Fig. 1. CSK method.

Letting the shift width of the synchronization point due to the fluctuation of the transmission characteristics be Td, Ta must satisfy Ta > Td. In addition, to prevent the incorrect control due to the disturbance such as noise, Tu - Td should be as small as possible. The fluctuation width Td of the synchronization point is the same whether in the synchronization capture period or in the synchronization tracking period. Consequently, Ta for the tracking period is usually set as equal to Ta in the capture period.

4.2. Extension of demodulator in CSK system

As was discussed in section 3.2, the demodulation is difficult in the modulation method, where the information is carried by the waveform of the transmitted signal even if the synchronization is maintained accurately. From such a viewpoint, the CSK (code shift keying) method is used in this study as a modulation method to replace the forementioned method. In the method, a set of spreading codes with sufficiently low cross-correla- tions are prepared in the same number as that of the information symbols. The code to be transmitted is se- lected from those codes according to the information symbol. The receiver decides which of the spreading codes is received and restores the information.

Figure 1 shows the principle of the CSK method for the case of two information symbols. The transmitter transmits PNO or PN 1, corresponding to the information symbols "0" and " 1. " In the receiver, the received signal is given to the correlators for PNO and PN 1. The maximum correlator outputs are compared for each period, and the information is restored by deciding whether the received signal is PNO or PN 1. Even if the phase characteristic of the transmission channel obtains a fluctuation and the positionof the maximum correlation changes, the information is restored correctly as long as the maximum correlation is detected correctly. However, in the case of the power line communication, it is difficult to restore

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i T 1 NO Corr. &

FNL Corr.

Fig. 2. Correlation function.

the information in a stable way by a simple comparison of the maximum correlation outputs, since the transmission characteristics are very much deteriorated, as was discussed previously.

The following conditions should be satisfied by the set of spreading codes used in the CSK system:

0 The code length should be the same.

@ There is no unnecessary peak in the autocorrelation function.

@ The spectrum distributions should be as similar as possible.

he crosscorrelation is sufficiently low near the synchronization point.

The reason for imposing condition 0 is as follows. The demodulation in the CSK system is based on the relative comparison of the correlator outputs, and the effect of the transmission channel on the set of codes must be the same for any code. As the codes that satisfy conditions @ to @, the M-sequence code and Gold sequence code are known.

Examining the autocorrelation and the crosscorrelation of those codes, it is seen that the former is low except for the synchronization point, and the latter is relatively high except for the synchronization point, as is

Fig. 3. Demodulation method for two-symbol CSK.

shown in Fig. 2. By skillfully utilizing this property, the proposed demodulation method positively uses the auto- and crosscorrelation values at points other than the synchronization point. Then a demodulation with lower error rate is expected.

Figure 3 shows the block diagram of the proposed demodulation method for the case of two information symbols. In this system, the result of demodulation is given by

S G N ( P ~ W-SOE- POW-S~E) (2)

where

POW:

PIW:

SOE:

S1E:

SGN(x) 0 ; (I < 0) 1 : (x20)

the absolute correlation peak value of correlator 0 in W period;

the absolute correlation peak value of correlator 1 in W period;

the sum of absolute correlation of correlator 0 in E period;

the sum of absolute correlation of correlator 1 in E period.

By the property of the auto- and crosscorrelations near the synchronization point as was discussed previously, it is expected that W W > PlW and SOE < SlE hold when the information is "0." Then, (PlWeSOE) < < (POW-SlW). Using this demodulation method, the demodulation is made easier than in the simple comparison of maximum values.

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Fig. 4. SS module block diagram.

5. Evaluation Equipment

To demonstrate theoretically the usefulness of the proposed synchronization captudtracking method using the window, as well as the demodulation method by CSK employing the window, it is necessary that the transmission characteristics of the power line should be described and modeled quantitatively. However, at the present stage, the quantitative study of the transmission characteristics model of the power line is still insufficient, which makes it difficult to evaluate the proposed system theoretically. From such a viewpoint, an evaluation equipment was constructed and the communication experiment actually is executed for evaluation using the power line.

5.1. Configuration of evaluation equipment

The constructed evaluation equipment is divided largely into three functional units: the CSK modulator; the CSK demodulator; and the synchronization controller. Figure 4 shows the whole block diagram of the evaluation equipment.

5.1.1. CSK modulator

The transmitter produces the output by select- ing PNO or PNl, according to the data TXD to be

transmitted. The transmission request terminal RTS is the terminal to control onloff of the transmission. To comply with the requirement of the electromagnetic wave law, the modulation output is passed through a lowpass filter (LPF) and then sent to the power line.

5.1.2. CSK demodulator

(1) Correlator

The correlation between the despreading code and the received signal is calculated. The correlator PNO Corr calculates the correlation between the received signal and PNO, and the correlator PNl Corr calculates the correlation between the received signal and PN 1. The correlation outputs CO and C1 from those two correlators go to the absolute value circuit ABS. The absolute values of the correlation outputs I CO I and I C 1 I are used for the synchronization control and data demodulation.

(2) Demodulator

The demodulation is executed by deciding whether the received signal is PNO or PN 1. The maximum-value- hold circuit MAX Hold detects the maximum values POW and P1W in the demodulation observation period (W period in Fig. 2, i.e., the demodulation window).

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using the summing circuit X, the sums SOE and S l E outside of the demodulation observation period (E period in Fig. 2) are calculated. The multipliers calculate PIW-SOE and POW-SIE. The comparator decides which of the results are larger to execute the demodulation.

' 4 5.1.3. Synchronization controller

This unit executes the synchronization detection and tracking. For the synchronization detection and tracking, it is necessary that the maximum point Tp of the correlation waveform should be determined. The maximum point Tp is detected by the maximum point detection circuit Peak Det. Peak Det determines the maximum points TpO and Tpl as well as the maximum values VpO and Vpl of the absolute values of two correlation outputs I COI and I C11, respectively. The maximum point for the larger maximum value is produced as the output Tp.

The synchronization is detected by the synchronization detector Sync Det. When Sync Det decides that there exists a transmitted signal, it produces the carrier detection signal DSR. The synchronization tracking is executed by the synchronization tracker Sync Track.

5.1.4. Other units

The control units, i.e., Mode Control and Timing Gen, control the whole evaluation equipment, its opera- tion, generation of the clock, switching of the transmission rate, and the windows for synchronization control and demodulation.

5.2. Determination of parameters

Based on the condition in Table 1, to realize the transmission rate of 9600 bit/s in the band of 10 to 450 kHz. the constructed evaluation equipment employs the Manchester version of the M-sequence code of code length 31. Based on the result of experiment on the actual transmission channel, the Manchester version of the following codes are adopted:

PN 0: feedback tap (5, 4, 3, 1) initial value (1, 1, 1, 0, 1) = 1011111010001001010110000111001

PN 1: feedback tap (5, 2) initial value (1, 1, 1, 1, 1 ) = 11 11 1001 10100 1000010 101 1101 1000

Fig. 5. p', p" vs. q characteristics.

The result of the actual measurement of the fluctuation width of the synchronization point due to the fluctuation in the transmission channel in the power line indicates that it is approximately k4.5 ps at the maximum. Margin is added so that the window length for the synchronization is set approximately three times, i.e., as Ta = 27 ps (when the transmission rate is 9600 bitls, 6 = 3.4 ps, and it is set that Ta = 86 = 26.9 ps).

The parameters y and q for establishing the synchronization are set as follows. When the transmitted signal exists, let the minimum number that the correlation maximum is within the range of Ta in arbitrary consecutive q periods bep'. When there is no transmitted signal, let the maximum number that the correlation maximum is within the range of Ta in arbitrary constxu- tive q periods be p"; p' and p" are examined by varying q. Figure 5 shows the relations between q and pf as well as q andp".

When q is small, p' < p". which results in the incorrect synchronization. When q is increased, p' > p" is established and the correct synchronization is realized. On the other hand, the time required for the initial synchronization is increased with the increase of q. Conse- quently, q should be set as small as possible, while leaving a sufficient margin p' - p". The threshold p for the synchronization decision is set as @' + p")/2.

A situation is examined for the actual power line with various loading conditions, and it is concluded that q = 32 and p = 18 is the optimal. In other words, in establishing the initial synchronization, the maximum of the correlation output is observed for 32 periods; and if 18 or more maximum are observed within the range of Ta. it is decided that the synchronization is established. In this setting, the time for establishing the initial synchronization is 18/(transmission rate) to 64/(transmission

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Table 2. Specification of SS module

Item SS module

Modulation method code modulation (CSK)

Code length 31

Transmission rate 9600 - 4800 bit/s

Bandwidth 10 - 450 kHz

Communication mode semidoubled

Synchronization capture time 6.7 ms (9600 bitis) -

20 25m ! Om 'P !

A

B

C

D

+ ; A C Outlet Tx : Transmitter Rx : Receiver

Fig. 6. Test bed.

rate). More precisely, it is calculated as follows, for each transmission rate:

9600 bitls: 1.875 - 6.667 [ms]

6400 bit/s: 2.813 - 10.00 [ms]

4800 bit/s: 3.750 - 13.33 [ms]

In the synchronization tracking, it is Seen as sufficient that the control is exerted when the correlation maximum shifted from the anticipated synchronization point is retarded or advanced consecutively for eight times. Consequently, it is set that Cn = 8. Table 2 shows the specifications for the evaluation equipmeot.

1 ine

1 ine

1 ine

1 ine

6. Result of Evaluation

The packet error rate is measured for the artificial home power line in the laboratory as well as for the general home power line transmission channel. Then the communication performance of the evaluation equipment is examined.

6.1. Evaluation in laboratory

An artificial home power line transmission channel is constructed in the laboratory, and the evaluation

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9 : : Header Code

10:

1 1 :

1 2 :

1 3 :

14 :

Fig. 7. Packet fonnat.

experiment is conducted with home electrical equipment as the load. This has a great effect on the power line communication (equipment generating a high-level noise, equipment producing a rapid change in the transmission state, and equipment with a sharp lowpass cutoff).

Figure 6 shows the system diagram of the artificial power line used in the experiment. The evaluation equipment is placed at Tx and Rx, and loading equipment is connected to ac outlets. Two-way communication is executed for 1 OOO times between the two pieces of evaluation equipment with the packet length of 14 Bytes (according to the standard for the low-speed power line camer communication system [ 13). If the correct information is not restored in either or both pieces of equipment, it is counted as a packet error.

Figure 7 shows the packet format used in the experiment. The error rate is defined as (the number of packet errors) / lO. Table 3 shows the result of evaluation. In the table, Rx. Tx. a, c, i, j and I indicate the placement of the ac outlet shown in the system diagram of the artificial power line in Fig. 6. Equipment Nos. 1 to 7 indicate that the following electrical equipment is connected to the ac outlet.

4

r- c, c,

I

0 Equipment No. 1: TV 1.

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Table 4. Result of evaluation in general home

~ less than 1 0 )

Packet error rate

~ less than 10.'

9600 bit/s 6400 bitls 4800 bit/s

10.' or more

@Equipment No. 2: TV 2 (with transmission characteristics varying with 10-ms period).

0 Equipment No. 3: Cleaner.

@ Equipment No. 4: Electromagnetic cooker (generating a large noise).

@ Equipment No. 5: High-frequency fluorescent lamp 1.

@ Equipment No. 6: High-frequency fluorescent lamp 2.

6.3. Discussions

6.3.1. Discussion of evaluation result in laboratory

The following observations are made concerning the synchronization.

0 The synchronization establishment time can be made 6.7 ms or less (for %OO bit/s transmission).

Q The synchronization is established and tracked with a high reliability. Thus, it is seen that the following two problems in the traditional system are improved by the proposed system. One is that the time required for establishing the synchronization, which is 68 ms in the traditional system, is now reduced. The other point is the stability of the synchronization. In the traditional system, the packet error rate is 50 to 95 percent when No. 2 T V is connected.

0 Equipment No. 7: High-frequency fluorescent lamp (with low impedance at high frequency and transmission characteristics with a sharp lowpass cutoff. Fluctuation of the transmission characteristics as well as a large noise are generated).

In the observed in error rate is

table, 0 indicates that no packet error is lo00 times measurement, i.e., the packet less than lW.

6.2. Evaluation in general home

The evaluation equipment is placed in 18 actual homes, and evaluation is made at 129 placements. The evaluation procedure is basically the same as in the experiment in the laboratory, but the electrical equipment actually in use in each home is employed as the loads. Table 4 shows the result of evaluation.

Examining the packet error rate, it is seen that the packet error rate is increased to 88 to 100 percent, when equipment Nos. 5 and 6 with a sharp lowpass characteristic (high-frequency fluorescent lamps), equipment No. 4 generating a large noise (electromagnetic cooker) and equipment Nos. 1 and 2 producing a large fluctuation in the channel characteristics (TV) are connected simul- taneously.

Examining the bitwise error, it is seen that the bursty demodulation error is produced at the sudden change of the transmission characteristics, while the synchronization itself is maintained. The reason for this seems to be as follows. The level of the SS signal is

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reduced at high frequencies due to the low-pass characteristics of the channel, and a wideband and large amplitude noise is generated due to the distortion of the noise waveform at the sudden change of the transmission channel characteristics. This greatly deteriorates the SN ratio of the receiving end and results in an error. It is anticipated based on those observations that the error can be avoided by the combined use of the code which is robust against the bursty error.

6.3.2. Discussions of evaluation result in actual home

The evaluation is made for 18 actual homes. In no case is the error rate worse than 10' for the transmission rate of 4800 bit/s, which is a better result than that of the evaluation in the laboratory. The error tends to be generated less for lower transmission rate, as in the case of the laboratory. This seems to be due to the fact that the signal is attenuated more in the high-frequency band in the power line, also in the actual home. The error also is produced in a burst. Consequently, it is anticipated also from the result for the actual home that the practical and highly reliable power line communication can be realized by the combined use of the code, which is robust against the bursty error.

7. Conclusions

This paper proposed the synchronization method using the window which can establish the initial synchronization with a high speed and maintain the stable synchronization in the deteriorated environment such as the power line, as well as the demodulation method for CSK system using the window. An experimental equipment is constructed and the evaluation experiment is conducted. The following performances are verified as the result.

The synchronization is established with a high speed in 6.7 ms or less.

Even if there exist a fluctuation in the transmission characteristics, the stable synchronization detectionltracking can be realized.

As to the demodulation method, it is verified that a more secure and reliable communication can be realized if another means such as burst error-correcting code is combined.

The proposed technique will be applied to a wide range of problems, such as the highly reliable home control/security system with stable synchronization and low error rate, the small-scale LAN in office and other environments, and the automatic measuring/billing system by the connection to the public telephone line.

As future problems, it will be necessary to verify the reduction of the error rate by combining the proposed technique with the burst errorcorrecting code to model the power line transmission channel and to provide the theoretical analysis of the proposed system based on the model.

Acknowledgement. The authors are grateful for the discussions regarding this study by Mr. Hirosaki, Director, and Mr. Hasegawa, Head, NEC C&C System Res. Lab., as well as the suggestions by Mr. Morishita, President of the company's R&D Lab. They also ac- knowledge the assistance in the construction of the evaluation equipment and the evaluation experiment by Mr. Takahashi. 2nd Dev. Div. of the R&D Lab.

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(from left to right)

Kaoru Endo graduated in 1983 from the Dept. Electrical Eng., Fac. Eng., Tokyo Metrop. Univ., and affiliated with NEC Home Electronics, Ltd. He is engaged in development of home bus system, spread spectrum power line communication system, etc. Presently with 2nd Dev. Div., R&D Lab.

Tadashi Nakamura graduated in 1971 from the Dept. Electrical Eng., Miyakonojo Technical College, and affiliated with Shin Nihon Denki (presently NEC Home Electronics), Ltd. He is engaged in development of home telephones, home bus system, spread spectrum power line communication system, etc. He is presently with 2nd Dev. Div., R&D Lab. He is a member of IEEJ.

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AUTHORS (continued, from left to right)

Soichi Tsumura graduated in 1986 from the Dept. Electronic Eng., Fac. Eng., Nagaoka Tech. University, where he obtained a Master’s degree in 1988 and affiliated with NEC Home Electronics, Ltd. He is engaged in development of home bus system, spread spectrum power line communication system, etc. Presently, he is with 2nd Dev. Div., R&D Lab.

Shichiro Tsuruh obtained a Master’s degree in Electrical Eng. in 1971 from Keio University and affiliated with NEC Corp. He is engaged in development of speech recognition system at Central Res. Lab. Since 1987, he has been affiliated with NEC Home Electronics, Ud., where he is engaged in R&D of home systems. Presently, he is the Director of 2nd Dev. Div., R&D Lab. He is a member of Inf. Proc. Soc., Jap., and of Acoust. Soc., Japan.

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Spread spectrum communication method for power line