A study of development of transmission systems for next-generation terrestrial 4 K UHD and HD convergence broadcasting

Based on the required data rate in Section 3, optimized transmission parameters for transmitting the PLP layer data through a 6-MHz bandwidth was drawn and the receiving performances of each parameter were analyzed using computer simulations under various channel conditions. The data rate of each PLP layer depends on the transmission mode (constellation point, code rate, FFT and CP size, pilot mode, the number of sub-carriers, the number of active sub-carriers, the portion of system information, etc.) [5]. Thus, the optimized parameters for a 6-MHz bandwidth to meet the requirements in Table 2 are shown in Table 3. Note that if extended transmission mode is available by FFT size [5], extended transmission mode is applied.

Every supported constellation point in the DVB-T2 systems was considered for the 4 K UHD and HD layer. However, it was found that the required data rate (Section 2–2) could be met when 256 QAM was used for the 4 K UHD layer data and 16 QAM for the HD layer data. In addition, the 16 K and 32 K FFT sizes were excluded when drawing the optimized parameters to consider HD layer data transmission in a mobile channel environment. Due to the limit of a 6-MHz bandwidth, the minimum FFT size of the optimized parameters was restricted to 8,192 with a high code rate. In addition, the maximum guard interval size of the optimized parameters was restricted to 1/32. Thus, the degradation of receiving performances in time-varying channel environments can be estimated when employing optimized transmission parameters.

The number of total and active sub-carriers depends on the FFT and CP size, the number of OFDM symbols in a frame, and the pilot mode. Only active sub-carriers are used for real data transmission, while residual sub-carriers are used for pilot or system information transmission. Figure 9 shows the number of total and active sub-carriers when employing transmission parameter no. 1. A single transmission frame is composed of a P1 symbol, which consists of 2,048 sub-carriers, several P2 symbols, of which the size depends on the FFT size of the OFDM symbol, and data OFDM symbols. The P1 and P2 symbols are used for system synchronization and system information (FFT and CP size, the number of OFDM symbols in a frame) transmission. The number of sub-carriers for the P1 symbol, K _P1, is always 2,048, and the number of P2 symbols, N _P2, is 2 when the FFT size, K _FFT, is 8,192. In transmission parameter no. 1, the CP size was 1/128, the frame closing symbol was not used, and the number of OFDM data symbols per frame, N _data, was 200. Thus, the number of total sub-carriers per frame, K _frame, was calculated as in (1) and shown in Figure 9:

Although all 8,192 sub-carriers in an OFDM symbol can be used for pure data transmission, sub-carriers of the edge band, referred to as virtual carriers [12], are reserved to prevent adjacent channel interference. In addition, sub-carriers are used for transmitting pilots, and extended transmission mode is applied. Thus, the number of active sub-carriers per OFDM symbol, C _data, is 6,788 [5], and the number of active sub-carriers per frame, C _frame, is written as follows:

Elementary period, T, for 6 MHz (which indicates the time interval of the transmission sample) is 7/48 μs, and the period of time of a frame, T _F, can be written as follows:

As in (3), 1,357,600 active sub-carriers are transmitted for 243.5067 ms, and the number of active sub-carriers for 1 s, C _frame/sec, can be calculated as in (4):

Using (4), when the portion of the 4 K UHD layer in active sub-carriers is 60%, 256 QAM (8 bits per constellation point) is employed and the code rate is 4/5, for which the 4 K UHD layer data rate R _4KUHD is calculated as:

Moreover, when the portion of the HD layer in the active sub-carriers is 40%, 16 QAM (4 bits per constellation point) is employed and the code rate is 4/5, for which the HD layer data rate, R _HD, is calculated as:

Using the optimized transmission parameters, the receiving performances were analyzed using computer simulations under environmental conditions in Table 4. In Table 5, TU-6 channel profile which is employed for computer simulations is listed, and time-varying Rayleigh fading channel model is implemented by the method in [13]. Note that the decoding of the received system information and receiver synchronization was assumed to be ideal.

Figures 10, 11, and 12 show the bit error rate (BER) performance versus carrier-to-noise ratio (CNR) of transmission parameter nos. 1 to 3 in Table 3 for 6-MHz bandwidth in AWGN and TU-6 channels. The TOV performance of the optimized transmission parameters is presented in Table 6. The TOV data indicates both the receiving performances of less than 3 × 10^{− 6} BER [14] and the limit of reliable broadcasting performance at the transmission system level. Generally, a return path does not exist in the broadcasting environment. Thus, only when the CNR is greater than the TOV can seamless broadcasting services be provided to viewers. For the TOV of optimized transmission parameter no. 2 in Table 6, reliable HD broadcasting can be achieved when the velocity of the receiver is 70 km/h and the CNR is greater than 15.6 dB.

The TOV of a 4 K UHD layer when the velocity of the receiver was 40 km/h was in the range of 26.6 to 28.6 dB, while the range of 30.8 to 40.8 was found when the velocity of the receiver was 60 km/h. Reliable receiving of the 4 K UHD layer data in mobile channels may not be achievable because the TOV’s at every transmission parameter were above 26 dB. However, fixed receivers were assumed for the 4 K UHD layer transmission, as described in Section 3–1, and a TOV at 4 to 5 dB higher CNR than the AWGN may be required for a static channel situation of the fixed receivers. In the case of HD layer data transmission, BER performances under TU-6 are more important than for 4 K UHD layers. When the code rate of 4/5 was used in transmission parameter nos. 1 and 2 of Table 3, the TOV of 15.6 dB (70 km/h of receiver velocity) and 20.3 dB (140 km/h of receiver velocity) was observed. Thus, reliable receiving can be achieved at a receiver velocity environment of under 140 km/h. For the no. 3 transmission parameter, the guard interval size was increased but the code rate was also increased, and the receiving performance was more degraded than in the case of parameter nos. 1 and 2.

In Europe, an 8-MHz bandwidth is allocated for terrestrial channels, and the extension of the bandwidth for terrestrial channels is currently being discussed among governments, broadcasters, and mobile service providers globally. To consider these points and overcome the limitations of the 6-MHz bandwidth, the optimized transmission parameters were re-drawn for the 8-MHz bandwidth. The optimized parameters for the 8-MHz bandwidth which meet the requirements in Table 7 are listed in Table 8.

Every supported constellation point in the DVB-T2 systems was considered for the 4 K UHD and HD layer. However, similar to the case of a 6-MHz bandwidth, the required rate could be met when 256 QAM was used for the 4 K UHD layer data and 16 QAM for the HD layer data. The maximum code rate was restricted to 3/4, and the pilot modes 4, 5, 6, 7, and 8 were excluded in consideration of the HD layer under the mobile channel environment.

By extending the bandwidth from 6 to 8 MHz, the maximum FFT size of the transmission parameters in Table 8 could be limited to 4,096. While the code rates of the HD layer for a 6-MHz bandwidth were 4/5 and 5/6, which are quite high, the lowest code rates of the HD layer and 4 K UHD layer were 2/3 and 3/4, respectively. In pilot pattern, the frequency of the pilots was increased by selecting PP2 and PP4 mode. In transmission parameter nos. 2 and 3 and 4 and 5, FFT size was the same at 2,048 and 4,096, respectively, and the code rate was lowered by reducing the guard interval. Commonly, the extended transmission mode was not used, and the frame-closing symbol (last OFDM symbol of the data OFDM symbols, which carries more pilots than normal data OFDM symbols) was used.

Using optimized transmission parameters for the 8-MHz bandwidth, the receiving performances were analyzed using computer simulations. Figures 13, 14, 15, 16, and 17 show the BER performances versus CNR of the transmission parameters in Table 8 under AWGN and TU-6 channels.

Figures 13, 14, 15, 16, and 17 show the BER performances versus CNR of transmission parameter nos. 1 to 5 for an 8-MHz bandwidth, listed in Table 8, under AWGN and TU-6 channels (70, 140, and 210 km/h). The TOV performances of the optimized transmission parameters in Table 8 are summarized in Table 9. For all transmission parameters under the TU-6 channel, the TOV of the 4 K UHD layer was 24.3 to 28.1 dB, while that of the HD layer was 12.8 to 16.0 dB. From the results, reliable receiving of the 4 K UHD layer data under a static channel can be achieved, and reliable receiving of HD layer data under a very fast fading TU-6 channel at a maximum velocity of 210 km/h can be achieved at a reasonable TOV. It is significant that there was not a large performance difference between parameter no. 1 and no. 2, implying that the receiving performance does not differ between 1024 and 2048 FFT size at the 210 km/h maximum velocity of the receiver. In addition, when the code rate was decreased as a result of decreasing the guard interval size, as in parameter nos. 2 and 3 and 4 and 5, more improvement of the receiving performance was obtained under a mobile situation than in the case of the other parameters. Lastly, when the FFT size was sufficiently small, the receiving performances of the very fast fading channel (receiver velocity: 210 km/h) were better than those of the slow fast fading channel (receiver velocity: 70 km/h).