RF technology was well represented at this year's 56th annual IEEE Broadcast Symposium. This month, I'll look at two of the many topics discussed at the symposium-the new DTV standard in China, and digital on-channel repeater technology.
More than 10 years ago, the FCC adopted the ATSC standard for terrestrial digital TV. The DVB-T standard was adopted in 1997. If we had the opportunity to create a new terrestrial DTV standard, what would it look like?
At the IEEE 2006, a paper by Jian Song and other engineers working on the project in the Electronic Engineering Department at Tsinghua University, described the Chinese CDMB-T system. It is quite different than either ATSC, or COFDM-based standards such as DVB-T.
This new DTV standard uses advanced forward error correction techniques, can use either multiple or single carrier transmission modes, has a robust frame header with a known PN (pseudorandom noise) sequence and, for multiple carrier implementations, uses TDS-OFDM (time-domain synchronous orthogonal frequency division multiplexing). Frame timing is based on calendar days and minutes.
Although CDMB-T supports modulation constellations ranging from 4QAM through 64QAM, the most robust mode-4QAM-is always used for transmitting the frame header. Performance of the standard is excellent. In addition to the different modulation methods, it allows choosing different code rates and data rates depending on the payload data rate.
Laboratory testing at a code rate of 0.4 with a frame header PN sequence length of 420 symbols, using multiple (3,780) carriers showed a receiving sensitivity of -97 dBm with a carrier-to-noise ratio of only 1.9 dB!
At this coding rate, system payload data rate was approximately 5 Mbps. At the other end of the scale, again using multiple carriers, but with a code rate of 0.8 and a frame header PN sequence length of 945 symbols, the maximum data rate is over 28 Mbps. Receive sensitivity drops to -79 dBm with a carrier-to-noise ratio of 19.8 dB.
These measurements are for an 8 MHz terrestrial channel. Although the paper does not provide any information on performance in a 6 MHz channel, it is reasonable to assume that payload data rate would be less than 75 percent of the 8 MHz data rate, provided the frame header size wasn't changed.
Use of outer BCH (Bose, Ray-Chaudhuri, Hocquenghem) coding and inner low-density parity check, or LDPC coding is a major contributor to the performance of CDMB-T.
CDMB-T includes another mode, 4QAM-NR, which was not discussed in detail in the paper, using 4QAM modulation and NR (Nordstrom-Robinson) coding.
China has filed for patents on the system and many have been granted. Three inventions are fundamental to the CDMB-T system: TDS-OFDM technology; the protection methods for the frame header and body; and a frame structure synchronous to real time, to facilitate automatic wake-up and power saving.
Although European Union countries have settled on DVB-T and the United States on ATSC, it will be interesting to see how many countries outside China adopt this standard.
Given the performance and flexibility of the standard, I expect to see at least some countries consider it, especially if the patent licensing fees are competitive. For more information on the coding methods used in CDMB-T, do a Google search on "LDPC coding," "BCH coding" and "Nordstrom-Robinson coding."
DIGITAL ON-CHANNEL REPEATERS
While a single-channel distributed transmission system offers the most flexibility and the best performance, it requires distributing programming to each site through a microwave or fiber connection. Digital on-channel repeaters, or DOCRs, appear to be much simpler, since they receive the primary station off-air and retransmit it on the same channel, eliminating the need for the microwave or fiber connections.
However, two factors add complexity. Since timing can't be changed, delay through the DOCR has to be kept to a minimum. Even then, some older ATSC receivers are likely to have problems if the main transmitter creates a "pre-echo" to the DOCR output that's outside the range of the adaptive equalizer.
The second complication is that the output of the DOCR tends to interfere with reception since it's on the same channel as the received signal. Papers at this year's symposium offered solutions to these problems.
Kok-Keong Loo presented a paper he wrote with Karim Nasr and others at Brunel University's School of Engineering and Design describing the performance of an echo cancellation system for on-channel repeaters. The echo canceller inserts a training sequence in the received DTV signal. This known training sequence is used by a channel estimator connected to the output of the receiver's low-noise amplifier to generate taps for an FIR filter, which then removes the loopback signal generated by the DOCR from the primary station's signal that's amplified and provided to the antenna.
While the system was designed and tested for DVB-T and DVB-H networks, it should be able to be adapted for ATSC. In a DVB-T system, the guard interval duration sets the maximum processing time and therefore the number of taps possible on the FIR filter. This may not be as much of an issue for ATSC implementations, assuming the use of fifth-generation receiver chipsets.
DELAY SPREAD
The paper has a table showing the delay spread in microseconds for different areas. They range from 0.110 µs for rural areas, to 0.932 µs in typical urban, 2.47 µs in "bad urban" areas and finally, the worst case, hilly terrain with a 6.22 µs delay spread.
As delay spread increased, more taps are needed to handle the echoes. For an 8,000-carrier COFDM system, the maximum processing time can be as much as 8 µs, allowing use of 80 taps at a 10 MHz sampling rate for cancellation. For COFDM systems using 2,000 carriers, the maximum allowable delay is around 2 µs. Even with this constraint, the technique can achieve echo cancellation above 20 dB for all but the bad urban and hilly environments.
According to the paper, this system has been simulated but not yet implemented in hardware.
One technology that has been implemented in hardware is the equalized DOCR developed by the Korean Broadcasting System and ETRI. This DOCR includes digital signal processing technology that removes RF loopback signal. It also corrects linear and nonlinear distortions, improving the output SNR (signal-to-noise) ratio.
Young-Woo Suh presented a paper he wrote with other engineers from KBS and ETRI describing "Field Test Results of Digital On-Channel Repeaters in the DTV Transmission Network in Korea." Suh explained that repeaters are essential for the implementation of DTV in Korea, as the country currently has 20 transmitter stations and more than 300 analog TV repeaters.
The DOCRs designed by KBS and ETRI now allow two modes. Mode 1 includes the circuitry needed to cancel RF-coupled signals and correct received channel impairments as well as distortions (linear and nonlinear) in the DOCR high-power amplifier. This mode is suitable for high-power systems and those with transmit/receive isolation below 70 dB. It is capable of transmitting signals with signal-to-noise ratios greater than 30 dB, but delay can be as much as 5.7 microseconds. It allows higher power and wider coverage, but requires repeaters that are more complex and costly than the mode 2 DOCR.
The mode 2 DOCR uses a low-pass filter to reduce adjacent channel interference and a linearity corrector to remove amplifier distortions, but includes no equalization circuitry to cancel the RF loop-back signal. SNR is typically less than 20 dB, but delay is under 3.9 µs using a low-pass filter with 256 taps. This mode is suitable for low power, small coverage areas where low delay is needed. It also is less complex and, as a result, less expensive.
How well do the Korean DOCRs perform? As expected, in most of the tests, the ability of the receiver to handle pre-echoes (the echo arrives before the main signal) was critical. This meant fifth-generation receivers, which handle pre-echoes over the same range as post-echoes, were required to take full advantage of the DOCR.
In the second phase of testing at Suwon, South Korea in 2005, a transmitter output power of 20 watts was used, while there was minor improvement when second generation (1999) DTV receivers were used.
Adding the DOCR provided only minor improvement with the second generation (1999) DTV receiver-out of 24 sites, eight had coverage with the DOCR on, one more than with the DOCR off. The fifth generation (2004) receiver provided better reception with or without the DOCR. With the DOCR off, it worked at all but four of the sites. With the DOCR turned on, all sites had reception.
For both receivers, ease of reception as measured by antenna pointing angles improved with the addition of the DOCR. Similar improvements were noticed during testing at the KwangMyung site.
This test compared performance of third- and fifth-generation ATSC tuners with the DOCR on and off. The improvement in this test was greater with the third-generation receiver, which had successful reception at 19 of the 26 sites with the DOCR off, and 24 of the 26 with the DOCR on, than with the fifth-generation receiver, which had reception at all but two of the sites even with the DOCR off, and worked at all 26 sites with the DOCR on.
The study concluded a DOCR system was effective in improving coverage, not only by increasing the number of sites with successful reception, but by significantly improving the ease of reception as measured by an increase in reception angle from tens of degrees to almost 360 degrees when the DOCR was switched on.
If you agree with me that providing good reception with indoor antennas and portable devices, especially in urban areas, will be essential for the success of over-the-air HDTV, in many markets DOCRs or distributed transmission systems will be needed to fill in the shadows. Engineering these systems to reduce interference to older DTV receivers, however, will not be easy.
As always, comments and questions are welcome.
