As countries interested in pursuing digital radio futures attempt to sift through available technologies to make well-informed decisions, the scope of the task can appear daunting.
With several generations of various systems available for consideration, objectively comparing the performance and suitability of each system for differing allocation conditions and regulatory frameworks presents a significant challenge for policymakers.
Dr. Yong-Tae Lee and his colleagues at the Electronics and Telecommunications Research Institute in South Korea have attempted to address this by conducting an extensive two-year review of the capabilities and performance of current digital radio technologies. The results were released by Lee and members of his research team at the NAB Show in April.
In general, the tests indicate that all systems deliver similar reception performance over even terrain at a comparable power level, but that performance in hilly or high-multipath areas can vary widely depending on specific error-correction or modulation methods.
As Lee explained, the focus of the tests was to assess the performance of each digital radio technology in a quantifiable way and to determine compatibility with existing analog and new digital stations, to serve as a basis for spectrum planning and allocation policy in South Korea and elsewhere.
As a precursor to the 2010 field measurements, Lee’s ETRI team conducted lab testing on all systems in 2009. This lab work allowed the team to develop and validate comparable measurement metrics for each system, which were used to develop an “apples-to-apples” field test methodology.
The field tests were conducted in and around the eastern coastal city of Gangneung, approximately 100 miles east of Seoul. Transmission facilities for the tests were constructed on Gwaebangsan mountain, with colocated antennas for the in-band systems — analog FM, HD Radio and DRM+ at 103.5 MHz — and the out-of-band — DAB, DAB+and terrestrial digital multimedia broadcasting (T-DMB) at 195 MHz.
The digital-only systems operated with 100 W, while the analog transmissions for FM, HD Radio hybrid (at–10 and –20 dBc) and DRM+ simulcast were operated at 1 kW. In addition, the team evaluated two modulation schemes, QPSK and 16-QAM, as a part of the DRM+ trials.
The team selected mobile test routes selected to give a clear indication of each system’s performance at various distances from the transmitter and in a variety of local reception conditions. Six shorter “horizontal” routes were selected on roadways that are oriented roughly east-west at distances ranging from 1.6 to 28 miles from the transmitter site. One long “vertical” route runs roughly north-south, passing near the transmitter, and was divided for data-reporting purposes into three segments representing different terrain conditions.
To ensure comparable performance metrics throughout the tests, the mobile test platform was designed to measure the threshold of availability in a manner appropriate for each system.
For the DAB, DAB+ and T-DMB systems, coverage was determined by measuring the Character Error Rate (CER) in the receiver and validating the presence of audio by monitoring decoded audio. The results of the ETRI 2009 lab tests indicated that loss of audio will occur at a different CER for each system, with the DAB+ and T-DMB systems being generally more robust than the original DAB system due to the use of Reed-Solomon coding.
The team evaluated HD Radio system coverage by capturing the blend point for hybrid transmissions and audio signal loss for digital-only operation. Since the performance of multicast (SPS) channels will differ from the main (MPS) channel depending upon which subcarrier partitions carry the data, the team tested several channel configurations.
For tests of the extended hybrid (MP3) mode, the main channel occupied the P1 partition and an HD2 channel was assigned to the P3 partition. All-digital (MP5) operation of HD Radio also was tested, with the main program in the P1 partition and two additional (HD2 and HD3) channels in the P2 and P3 partitions, respectively.
Coverage of the DRM+ system was determined by monitoring for audio Cyclic Redundancy Check errors, which cause audio dropouts. To evaluate the quality of analog audio for the DRM+ simulcast mode, HD Radio hybrid mode and analog FM reference transmissions, the total harmonic distortion of a 1 kHz test tone was measured and compared to the threshold level of 1.8 as determined in the ETRI lab tests.
Results for the three Eureka-147 family systems were fairly consistent, with only a slight advantage in performance for the DAB+ and T-DMB systems due to their use of Reed-Solomon error correction.
Distance from the transmitter was the single largest factor in reception performance, with quality exceeding 97 percent up to about 7 miles away and better than 90 percent at 13 miles.
Signal quality steadily decreased beyond that, to about 10 percent at 28 miles from the transmitter. The mountainous terrain around the southern section of the vertical route was also challenging, with reception quality of about 50 percent to 55 percent for all three systems.
For the HD Radio system in MP3 mode, the difference between –10 and –20 dBc operating level for the digital signal was dramatic. While coverage of the lower power digital signal fell below 90 percent at 13 miles away from the transmitter and continued to drop at greater distances, the addition of 10 dB to the digital carriers delivered 100 percent signal quality out to 13 miles and better than 90 percent quality to 21 miles.
Only out at the 28-mile test route did –10 dBc coverage fall significantly, to around 37 percent. Performance along the vertical route was consistent with the horizontal route measurements, but indicated that the system had difficulty in rough terrain, performing with about 62 percent to 67 percent signal quality at –10 dBc and about 32 percent at –20 dBc.
Tests of the all-digital HD Radio (MP5) mode turned up quite a surprise.
While the overall system performance was similar to that of the hybrid mode with –10 dBc power, matching or exceeding its signal quality along most of the vertical test route, coverage in the rough terrain at the southern end of the route exceeded 98 percent for the program in the P1 partition. Since performance of the channels in the P2 and P3 channels was similar to the other systems, this dramatic improvement can be attributed to the more robust diversity coding scheme used for the P1 channel in the MP5 mode.
Finally, the DRM+ system was evaluated in simulcast mode with a 400 kHz spacing between the analog and digital signals. The QPSK modulation scheme fared significantly better than 16-QAM, delivering signal quality approaching 100 percent out to 13 miles and better than 95 percent at 16 miles. With 16-QAM, quality dropped to 94 percent at 13 miles and quickly fell off as the distances increased. QPSK also performed well in hilly terrain, delivering better than 70 percent signal quality vs. 52 percent for 16-QAM.
Rather than draw any specific conclusions about the suitability of any particular system for a particular application, Dr. Lee preferred to let the data speak for itself. With the results of these tests in hand, regulators around the world now have a useful benchmark as their countries evaluate their digital radio options.
What can we conclude?
Although the ETRI test report focused on presenting data without drawing conclusions about the suitability of a particular system, several conclusions seem evident from the results:
•Excluding differences in coding and error correction, RF coverage over even terrain was generally comparable among the most robust mode tested for all systems operating at the same power. Since all of the systems except DRM+ are using COFDM modulation, this is not surprising.
•Performance of Eureka-147 (legacy DAB) was slightly poorer than the other digital systems due to its older coding scheme, which cannot be upgraded without making existing receivers obsolete.
•–10 dBc sidebands were necessary to operate the hybrid HD Radio test station at a comparable power and performance level as the other systems.
•Error correction and modulation were two key differentiators in performance of the systems over hilly terrain, where multipath interference becomes a significant challenge.
•The all-digital (MP5) mode of HD Radio performed particularly well overall for the primary data channel due to its error-correction.
•16-QAM modulation for DRM+ delivers a significant coverage penalty relative to the QPSK modulation mode tested. The QPSK mode also performed better in hilly terrain than most other systems.
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