Field Trial of Digital Video Transmission over Medium-Voltage Powerline with Time-Domain
Synchronous Orthogonal Frequency Division Multiplexing Technology
Jian Song, Changyong Pan, Qing Wu, and Zhixing Yang
Dept. of Electronic Engineering, Tsinghua National Lab of Information Science and Technolgy
Tsinghua University, Beijing, China, 100084
Haitao Liu, Bingzhen Zhao, and Xiao Li
Beijing ECOM Communications Technology Co., Ltd.,
Fu Wai Avenue, Xi Cheng District, Beijing, China, 100037
Abstract: Using Digital Multimedia Broadcast-Terrestrial
(DMB-T)
system
from
Tsinghua
University,
PLC system can generally be divided into two categories:
Narrow band with working frequency below 150KHz and
the physical layer (PL) transmission date rate of less than
1Mb/s; Broadband with working frequency of 2 ~30MHz
(it can be even higher than 40MHz for some systems
depending on regulation) and the PL transmission data rate
is beyond 200Mb/s. Spread Spectrum and orthogonal
frequency division multiplexing (OFDM) are two
modulation techniques used in PLC systems depend on
different applications, and recently announced broadband
PLC standards such as HomePlug AV and Opera, all adopt
OFDM technology [5-6]. In the communication area, PLC
systems is one of the last mile solutions providing
communication service over medium-voltage (MV, in China,
~10KV) cable; can provide high speed broadband Internet
access supplementary to DSL, WiFi and cable systems; can
offer home networking capability for multimedia services,
as well as can be a viable solution to the smart home such
as the automatic control of the appliances and the video
surveillance system. In the electrical industry, PLC offers
important value-added services such as automatic meter
reading (AMR), remote connection and/or disconnection,
energy/load management, equipment/device monitoring,
and power outage notification for utility companies to
enhance power grid security and improve service quality as
well as the business efficiency.
However, several hurdles, both non-technical and technical,
are needed to be overcome to assure the large-scale
deployments of PLC systems. From non-technical point of
view, policy and standardization are two major issues. As
PLC system will inevitably introduce interference to the
existing services operated in the same band, more favorable
regulations is definitely needed to lift those stringent
requirements on PLC systems. Existing industrial PLC
standards use same modulation technology of OFDM yet
two-way
(frequency division duplex) digital video transmission over
medium-voltage power cables of up to 700m-long was
successfully demonstrated. The preliminary experiments
validate the feasibility of using Time-Domain Synchronous
Orthogonal Frequency Division Multiplexing (TDS-OFDM)
technology, a core technology in DMB-T and being adopted in
Chinese Digital Terrestrial Television Broadcasting Standard,
for powerline communication. TDS-OFDM offers higher
spectrum efficiency as known pseudo random sequence is used
as the guard interval (or frame header) and also serves the
purpose of synchronization and channel estimation avoiding
the overhead to put either training symbols or pilots in OFDM
symbol.
Key words: Powerline Communication (PLC), Time-Domain
Synchronous Orthogonal Frequency Division Multiplexing
(TDS-OFDM), Digital Multimedia Broadcasting-Terrestrial
(DMB-T)
I. Introduction
Powerline communication (PLC) uses the existing electrical
power grid to provide various services to costumers. As the
communication signal (with working frequency much
higher than 50/60 Hz) can simply be carried by the power
cable, there is no need to build up new networks and hence,
saves not only investment but also time to economically
provide various services to customers who receive service
from utility company yet not covered by communication
networks. Another advantage is the overall coverage, there
is no existing network can link more customers in quite
different areas than that by the electrical power networks.
Therefore, it has drawn extensive research interests,
especially recently thanks to the latest developments in the
information technology area [1-4].
1-4244-1090-8/07/$25.00 ©2007 IEEE.
559
The FEC output is converted to nQAM (Quadrature
Amplitude Modulation with n of the constellation points)
DMB-T supports the three constellations of 64QAM,
16QAM, and 4QAM with power normalization.
D. Time interleaving
Convolutional interleaver is used across OFDM Signal
Frames (to be defined later) with number of interleaving
branches of B=52 and interleaving depth of M=0, 48, 240,
and 720, respectively (corresponding to four modes from 0
to 3). The maximal overall delay of time
interleaving/de-interleaving is ~0.3s.
E. Frequency interleaving within 3780 symbols of each
OFDM frame body
This is done in each OFDM frame body (FB) which
consists of 36 information symbols and 3744 data symbols.
F. Hierarchical Frame structure
Table I Payload in Mbps under different working modes by
DMB-T
the rest functional blocks are quite different. The lack of
universal standard makes customers hesitate to consider
PLC product and buy different products of no
interoperability issue instead, and makes manufacturers
cautious to enter into PLC industry wondering which
standard(s) to support and PLC products tend to be more
expensive. Technical issues are also quite challenging. In
PL, as power cable was originally design for power delivery,
high frequency communication signal always incurs both
large attenuation increasing with the increase of frequency,
putting strong limitation on transmission length and high
intensity noises decrease with the increase of frequency.
Also, unlike the communication cable, quite stable and
hardly interfered, PLC system is quite vulnerable to the
environmental interference and different noise, hence,
electromagnetic compatibility (EMC) must be addressed. In
this sense, channel modeling is critical and lots of papers
with very good results and insights have been published
[7-8]. For higher layers effective and flexible protocols
handling different services simultaneously are very
important to the successful implementation of the PLC
technology as the data throughput for these systems can’t be
very high given the limited bandwidth.
Recently, we have performed two-way digital video
transmission over the MV power cable using DMB-T
system by Tsinghua University. In the following, we first
highlight the DMB-T system focusing on TDS-OFDM,
coding/modulation schemes and frame structure in Section
II. We then present the fried trail results of two-way digital
video transmission over the MV power cable in Section III,
and conclude our paper in Section IV.
Payload (Mbps)
Guard
Interval
125 μs
(1/4)
55.6 μs
(1/9)
Constellation Scheme
Inner
QPSK
Code Rate
16QAM
64QAM
4/9
4.813
9.626
14.438
2/3
7.219
14.438
21.658
8/9
9.626
21.658
28.877
4/9
5.414
10.829
16.243
2/3
8.122
16.243
21.658
8/9
10.829
21.658
32.486
DMB-T system takes a hierarchical frame structure given in
Figure 3. From the top to the bottom, they are Calendar Day
Frame (starting from 00:00:00am each day, for example,
Beijing Time), Minute Frame lasting exactly one minute,
Super Frame with fixed duration of 125ms, and Signal
Frame. As the fundamental block of this frame structure is
Signal Frame, we will then focus on the Signal Frame
construction in the following.
Each Signal Frame consists of Frame Header (FH) and FB
with same symbol rate of 7.56 Msps. Each FB of the Signal
Frame lasts exactly 500μs (3780×1/7.56μs). FH uses PN
sequence and that is why this unique baseline technology is
called TDS-OFDM. FH length can be 420 and 945 symbols
(the relative guard interval length of 1/9 and 1/4,
respectively) to support different applications. FH is BPSK
modulated with average power twice as much as that of FB.
Using FH of 420 as an example, FH consists of three parts,
II. Highlight of DMB-T System
Figure 2 is the schematic diagram of the DMB-T transmitter
with major functional blocks described as follows.
A. Scrambler
Binary PN sequence of 215-1 bit long is used to randomize
the input data to facilitate the system synchronization.
B. Forward error correction (FEC) code
It is concatenation of outer Reed-Solomon RS (208, 188)
and inner recursive systematic convolutional codes of rate
4/9, 2/3, and 8/9 (all come from puncturing the same
mother code with code rate 1/2).
C. Signal constellation and mapping
560
pre-amble of 82, PN of 255 (m-sequence), and post-amble
of 83. Both pre- and post-ambles are certain cyclic
extension of PN of 255. The FH can be fixed or rotated
(making each Signal Frame within same Super Frame
identified exclusively). Those rotated PNs are carefully
picked so that the cross-correlation between two adjacent
PN sequences is minimal.
The data throughput offered by DMB-T system within
8-MHz bandwidth is summarized in Table I.
There is 6-bit physical layer system information (Spectrum
spread with BPSK mapping) in FB to inform receiver the
constellation mapping, code rate, interleaving mode, carrier
mode information, and FH information.
G. Inverse Discrete Fourier Transform (IDFT) of 3780
points
It is conducted for the FB with sub-carrier spacing of 2 KHz
and that explains why the FB duration is 500μs.
H. Post-baseband processing
Squared root raised cosine filter with roll-off factor 0.05 is
adopted to shape the baseband signal.
attenuation is 30 dB higher, significantly limits the
transmission length. For the testing purpose, other than
those two cables given in [9], we also chose MV power
cable of 700m with channel attenuation and noise behavior
in Figure 4 for the two-way digital video transmission. It
shows that the transmission attenuation is quite symmetric
while the noise characteristic at each end is quite different.
The experimental setup is given in Figure 5, one transceiver
(TX1) uses a DVD player as video source and sends signal
at the central frequency of 7.56MHz to RX2 while TR2
takes the output from a digital video and sends signal at the
central frequency of 23.56MHz to RX1, respectively. For
illustration purpose, we only use monitors instead of
MPEG-2 bit-error-rate tester to check the transmission
performance.
For all three MV cables with length of 400m, 700m and,
1Km, respectively, we first turned on two transceivers
simultaneously to see if two-way digital video can be
achieved. The two-way video transmission of 1Km failed as
noise on one side is two high (please refer to [9]) while the
other two succeeded at 1/9 guard interval, 4/9 code rate,
48-symbol interleaving mode, and QAM modulation. For
MV cables of 400 and 700m, we then changed modulation
mapping from QAM to 64 QAM and code rate from 4/9 to
2/3 with payload data rate beyond 21.6 Mbps, the
transmission is still absolutely reliable. After verifying
constellation mapping could be 64QAM (the highest
possible for DMB-T system), we conducted the test to see
the impact of the interleaver modes. In our test, we have
tried different interleaving modes and found no significant
difference, i.e., long-time interleaving important for digital
video broadcasting may be omitted to reduce both the
memory required and the processing delay. During the
entire 2-hour measurement, the system works well and no
stall picture has been found.
This field trial just demonstrated the feasibility of using
TDS-OFDM technology for PLC systems. As all the system
parameters used in the experiment are solely optimized for
the broadcasting application with channel bandwidth of
8MHz, lots of works in physical and MAC layers need to be
done to realize the communication applications. As
technologies are pretty mature, the overall modification
efforts are definitely manageable.
III. Two-way Digital Video Transmission
Based on DMB-T systems described above, we have built
two transceivers with the transmitter central frequency of
7.56MHz and 23.56MHz, respectively and each works at 8
MHz analog bandwidth. Each transceiver has two power
output levels: +13 dBmW with power-amplifier (PA) and
-10dBmW without PA. Power intensity of transmitter with
PA is -55dBm/Hz, well below -50 dBmW/Hz specification
of HomePlug 1.0 standard, and this system won’t cause
noticeable interference to existing systems. This also means
that as long as linearity of PA is not an issue, the transmit
power can be even higher. The lab test results under
AWGN channel of both carrier to noise ration (C/N) and
the receiving sensitivity under the QPSK rate 4/9 mode is
given in Table II.
Table II. The measurement of C/N and receiving sensitivity
of the transceivers under AWGN
Transceiver 1
Transceiver 2
C/N
2.0 dB
2.1 dB
Receiving Sensitivity
96 dBm
-96.3 dBm
In [9], we have reported the measurement results on MV
power cable and results show that when the power cable
length is less than tripled (from 400m to 1Km), the channel
IV. Conclusion
561
Using DMB-T system adopting TDS-OFDM modulation
technology designed for the broadcasting purpose, we have
successfully performed two-way digital video transmissions
over the MV network with the power cables of up to 700m
long. Results show that more than 21.6Mbps payload
throughput can be delivered at certain power margin and
implementing higher constellation mapping seems
necessary to carry more information bit by each subcarrier
provided the channel condition is favorable. Time
interleaving is not giving significant performance
improvement from this test. With same guard interval, FEC
coding rate, and constellation mapping, TDS-OFDM offers
higher spectrum efficiency as no pilots or training symbols
needed in OFDM symbol for synchronization and channel
estimation. Therefore, it is an attractive modulation scheme
for PLC systems of limited bandwidth. Moreover, with PN
used in each OFDM symbol, the system can perform fast
channel acquisition to track any changes within a short
period. The current work is just concept-proof and more
work in physical and MAC layers is needed to actually
implement this technology into the PLC systems with
satisfactory performance.
References
[1] S, Galli, et. al., “Broadband is Power: Internet Access through the
Power Line Network”, IEEE Comm. Mag., 2003, 41(5):82-83.
[2] N. Pavlidou, et. al., “State of the Art and Future Trends”, ibid, 2003,
41(5): 34-40.
[3] H. Hrasnica, et. al., “Broadband Poweline Communications-Network
Design”, John Weily & Sons Ltd. 2004.
[4] Y. H. Ma, et. al., “Performance Analysis of OFDM Systems for
Broadband Power Line Communications under Impulsive Noise and
Multipath Effects”, IEEE Tran. On Power Delivery, 2005, 20(2): 674-682.
[5] “HomePlug AV Specification”, Version 1.0.00, Dec. 16, 2005.
[6] “OPERA SPECIFICATION - Part 1: TECHNOLOGY”, Version 1.0,
Jan. 31, 2006.
[7] S. Barmada, et. al., “Innovative Model for Time-Varying Power Line
Communication Channel Response Evaluation”, IEEE J. SAC, 2006, 24(7):
1317-1326.
[8] T. Sartenaer, et. al., “Deterministic Modeling of the (Shielded)
Outdoor Power Line Channel Based on the Multiconductor Transmission
Line Equations”, ibid, 2006, 24(7): 1277-1291.
[9] H. Liu, et. al., “Channel Study for Medium-voltage Power Network”,
2006 IEEE International Symposium on PowerLine Communications and
its Applications, 2006, P.245-250, Florida, USA.
Copy
Copy
CP
Data+Pilot
Data+Pilot
CP
•••CP-OFDM
OFDM Symbol
PN
Data
PN
Data
••• TDS-OFDM
OFDM Frame
Figure 1. Difference between CP-OFDM and TDS-OFDM symbol in time domain.
562
Figure 2. The transmission system diagram for DMB-T.
Calendar Day Frame
䯴 24Hour䯵
0
…………
00:00:00 am
1439
…………
24:00:00 am
Minute Frame
䯴 1Minute䯵
0
…………
…………
Super Frame
䯴 125ms䯵
…………
First
Frame
…………
Signal Frame
䯴 555.6us/625us䯵
Frame
Header
PN Sequence
Frame Body
Data Block
Figure 3. The frame structure of DMB-T system.
563
479
-10
-80
5th-7th District
7th-5th District
Noise of 5th District
Noise of 7th District
-20
-90
-30
Noise level:dBm/Hz
Attenuation:dB
-100
-40
-50
-110
-120
-60
-130
-70
-80
0
5
10
15
20
Frequency:MHz
25
30
35
-140
40
0
5
10
15
20
Frequency:MHz
25
30
35
40
(a)
(b)
Figure 4 The characteristics of attenuation of two transmission directions (a) and noise at each end of the transmission (b) for
the 700m-long MV power cable.
Figure 5 The experimental setup to two-way digital video transmission over MV powerline.
564