can operate in all three modes by constructing a local replica encompassing the three PN codes [71], via partial accumulation of delay‐multiply of samples in pre‐ and post‐ambles [74], and through detection of the frame header mode and symbol [75]. There are also methods that utilize up to three consecutive frames to acquire the PN code and estimate the frame number in a way that is insensitive to the CFO and Doppler as well as the sampling frequency offset [76].
Pseudoranges between a DTV receiver and DTV transmitters can be calculated from the decoded transmission time of DTMB frame headers and their arrival time at the receiver, measured by correlating the sampled signals with a local PN sequence [77]. Although it does not contain pilot subcarriers, the TDS‐OFDM employed by DTMB uses TPS with 36 subcarriers as shown in Figure 40.14(c) to convey system information such as the constellation mapping scheme, coding rate, and interleaving mode. The TPS is BPSK‐modulated with a much lower demodulation threshold than other higher‐order modulation schemes. Unlike SPs, the TPS of TDS‐OFDM occupies the same frequency‐domain location, unchanged from frame to frame. More importantly, the TPS does not change frequently, remaining the same for several hours and even days. Once correctly detected from previous signal frames, it can be regarded as known symbols in the subsequent symbols without the need for repeated detection, thus serving as continual pilots. Indeed, the combination of time‐domain processing of the PN sequence and the frequency‐domain processing of TPS (as pilots) provides more accurate TOA estimation for time‐frequency joint positioning [78, 79].
40.2.5 Next‐Gen ATSC 3.0 Signals for Timing and Ranging
The final digital TV technology described in this chapter is the upcoming next‐generation DTV technology called ATSC 3.0, which has great potential as a signal of opportunity for timing, ranging, and positioning applications. The standard candidates [80, 81] were announced in 2015, and the latest version was released in early 2020. Without the restriction of backward compatibility, ATSC 3.0 is purportedly “built to last” in the sense that it will avoid any future disruptive technology by allowing its layers/components to evolve gracefully and is intended to serve as a reference for future DTV worldwide!
For the end users, ATSC 3.0 will provide higher audio and video quality for both fixed and mobile reception with interactivity and personality. DTV will be made part of the Internet in three transmission modes, namely, broadcast, broadband, and push in advance. In a sense, continuous video/audio streams are broken down into data file segments accompanied by a playlist, thus allowing for watermarking‐based content recognition and easy personal advertising insertion. ATSC 3.0 will employ the state‐of‐the‐art pragmatic approach of bit‐interleaved coding and modulation (BICM), achieving the best performance as compared to A/53 [37], A/153 [41], and DVB‐T [44] in terms of the link efficiency (capacity in bits/s/Hz) as a function of signal‐to‐noise ratio (SNR) (dB) for a 6 MHz channel.
In ATSC 3.0 signal transmission, the video and audio as well as other data from content providers and studios arrive as IP packets, which are then processed in three steps. In the first step, feedforward error correction is applied, followed by bit interleaving and mapping into a modulation constellation. In the second step, symbol interleaving in time and frequency is applied prior to framing. Finally, in the third step, pilot subcarriers are added, followed by IFFT to convert it from the frequency domain to the time domain, where time guards (cyclic prefix and suffix) are added to form OFDM symbols. Preamble symbols and bootstrap symbols are finally added ahead of the payload symbols to form an ATSC frame. After DAC, the waveform undergoes power amplification and then transmission.
Figure 40.15(a) shows the time‐frequency structure of an ATSC 3.0 frame, which lasts 50 ms (min) to 5 s (max) and occupies a bandwidth of 4.5 MHz to 6–8 MHz. It consists of three components: (i) a bootstrap located at the beginning of each frame as specified in [80], (ii) a preamble immediately following the bootstrap, containing L1 (the lowest layer of the ISO 7 layer model) control signaling applicable to the remainder of the frame, and (iii) one or more subframes as specified in [81]. A subframe contains an integer number of OFDM symbols (FFT size and GI duration) in the time dimension and spans the full range of configured carriers (scattered and/or continual pilot patterns and useful carriers) in the frequency dimension. Other attributes defining a subframe type include whether or not frequency interleaving is enabled and whether a subframe is single‐input single‐output (SISO) or MIMO. A frame may contain multiple subframes of different subframe types.
Figure 40.15 Bootstrap signaling within an ATSC 3.0 frame.
ATSC 3.0 introduces a new, unique feature called bootstrap. Providing a universal entry point into a digital transmission, the bootstrap is fixed in configuration (e.g. sampling rate, signal bandwidth, subcarrier spacing, and time‐domain structure) and is known to all receiving devices. The bootstrap signal has a fixed bandwidth of 4.5 MHz as compared to 6–8 MHz for the preamble and payload. As shown in Figure 40.15(b), the bootstrap consists of a number of symbols, beginning with a synchronization symbol positioned at the start of each frame period to enable signal discovery, coarse synchronization, frequency offset estimation, and initial channel estimation. The remainder of the bootstrap contains sufficient control signaling (information about the preamble, system bandwidth, sampling rate, and time to the next similar frame as well as services such as emergency alert) for reception and decoding of the rest of the frame. Each bootstrap symbol has three parts: A, B, and C, comprising complex‐valued time‐domain samples. Part A with NA = NFFT = 2048 is derived from the IFFT of the frequency‐domain structure with an appropriate cyclic shift. Part B with NB = 504 and Part C with NC = 520 are samples taken from Part A with a frequency shift ±fΔ and a possible phase shift e−jπ. As shown in Figure 40.15(b), the initial symbol (bootstrap symbol 0) has the time‐domain structure CAB provided for synchronization, while the remaining bootstrap symbols use BCA up to and including the final one (bootstrap symbol Ns), which indicates field termination with a phase inversion of subcarrier values (180°).
The values used for Part A of a bootstrap symbol originate from its frequency‐domain specification as a Zadoff–Chu (ZC) sequence modulated by a PN sequence, with the frequency‐domain structure shown in Figure 40.15(c). The ZC root and PN seed are used to signal the major and minor versions of the bootstrap, respectively. The length of the ZC sequence is NZC = 1499, which is the largest prime that results in a channel bandwidth no greater than 4.5 MHz with a subcarrier spacing of Δf = 3 kHz. The ZC sequence has a natural reflective symmetry about the DC subcarrier at which the mapped ZC sequence value is set to zero (the DC subcarrier is null). The ZC sequence remains the same for every symbol.
A length‐16 linear feedback shift register (LFSR) is used to generate the PN sequence of length 65,535. The sequence is initialized with the seed, representing the minor version, continues from one symbol to the next within a bootstrap (749 used per symbol), and is only re‐initialized with the seed for a new bootstrap. As shown in Figure 40.15(c), the PN sequence values assigned to the subcarriers below the DC subcarrier are mirror‐reflecting of those above the DC subcarrier to ensure a reflective symmetry such that the desirable constant amplitude zero autocorrelation (CAZAC) properties are retained for the product sequence. With bootstrap symbols, information is signaled through the use of cyclic shift
