A field test for tracking DVB‐T signals was run in Marseille, France [54]. The spectrum of an ideal DVB‐T signal in the 8K mode is compared to that of sampled signals as shown in the top and bottom plots of Figure 40.12(a), respectively. Within the effective bandwidth of 8 MHz, the null margins used to avoid out‐of‐band emissions and pilot subcarriers with boosted power are clearly visible. The cross‐correlation of the cyclic prefix in the guard interval with that at the end of the symbol useful part, averaged over four OFDM symbols, is shown in Figure 40.12(b), where the correlation peak is located at the 1564th sample (the top plot), and the factional CFO is estimated to be 0.00012 rads/s from the corresponding differential phase (the bottom plot).
After cyclic prefix removal, the FFT is applied to the samples in the useful part. The continual pilot pattern is used to estimate the integer CFO over two consecutive OFDM symbols, while the scattered pilot pattern for each OFDM symbol is detected after CFO correction. Figure 40.12(c) shows the CIR (the blue curve) estimated from an OFDM symbol as a snapshot of multipath acquisition. The threshold (the black dash line) is set as 80% of the total power within the acquisition region to detect possible paths (the red circled line). The first path is declared among all acquired paths according to their rate of occurrence. In this particular case, the paths arriving at 1564.5, 1565.5, and 1566.5 in samples are the three most frequently detected ones with their occurrence probability equal to 1, and the earliest arrival is at the 1564.5th sample. This path is then used to initiate the DLL tracking with the 20 s tracking results shown in Figure 40.12(d). As shown, the 95% accuracy is within 0.95 m with an estimated C/N0 of 57.97 dB‐Hz.
In general, the carrier phase of OFDM signals is not tracked for at least two reasons. First, the dc component of most baseband OFDM symbols is a null subcarrier to avoid the effect of dc bias at reception. Second, generation and transmission of OFDM symbols are independent from one symbol to the next. As a result, no phase continuity is required to be maintained at any subcarriers. As analyzed earlier, for communications, demodulation of OFDM symbols with cyclic prefix is tolerant to small timing errors and depends on the relative phase at data subcarriers, which can be easily calibrated with the help of pilot subcarriers. However, the OFDM signaling adopted by DVB‐T retains the dc component. Besides, the cyclic prefix duration is specified in such a way that a whole number of cycles is ensured for the middle carrier [44]. It happens in DVB‐T that the middle carrier is assigned as a continual pilot subcarrier, which has a constant value across OFDM symbols. As a result, the baseband center frequency (dc component) has no phase discontinuity, which gives rise to the opportunity for carrier phase tracking. Carrier phase tracking has the potential to provide more accurate timing for ranging and ultimately for positioning than cross‐correlation of cyclic prefix and pilot subcarriers currently used for coarse and fine TOA estimation, respectively. The possibility of carrier phase tracking for DVB‐T signals was recently shown in [62] with in‐the‐air DVB‐T signals collected in experimental tests.
40.2.3 ISDB‐T Signals for Timing and Ranging
The Terrestrial Integrated Service Digital Broadcasting (ISDB‐T) is one of the earliest standards for digital TV, digital audio, and data, developed by Japan’s Association of Radio Industries and Business (ARIB) [63]. Also adopting OFDM, ISDB‐T groups its subcarriers within a transmission channel into 13 segments, which explains the name: band segmented transmission (BST‐OFDM). Thus, ISDB‐T supports hierarchical transmission using hierarchical layers where each layer has one or more segments with their own transmission parameters (such as different inner coding rate, modulation scheme, and time interleaving length). In this way, different services such as high definition television (HDTV), multi‐channel simple definition television (SDTV), and data can be transmitted in one frequency channel. For example, an ISDB‐T implementation has 13 segments over a channel bandwidth of 6, 7, or 8 MHz. For audio and data program transmissions, ISDB‐TSB (SB stands for sound broadcasting) uses only one or three segments in the channel while ISDB‐Tmm (Terrestrial Mobile Multimedia) can use up to 33 segments by concatenating blocks of the 13‐segment (Type A) and the 1‐segment (Type B) over a maximum band of 14.5 MHz.
Figure 40.12 Test results of pilot‐carriers‐based delay tracking for refined TOA estimation [54].
Source: Reproduced with permission of IEEE.
As shown in Figure 40.13(a), each channel of 6 MHz has 13 segments with each segment occupying a bandwidth of 6 MHz/14 = 428.6 kHz. The 6 MHz channel allows for three operating modes, which differ in the number of carriers and carrier spacing Δf as well as the effective bandwidth. Also shown in the figure is an example allocation of segments into Layer A with 1 segment for partial reception at headheld receivers, Layer B with 7 segments for mobile reception of SDTV, and Layer C with 5 segments for fixed reception of anbotehr SDTV. The 13 segments in the channel can also be allocated into Layer A with 1 segment for partial reception at headheld receivers and Layer B with 12 segments for mobile and fixed reception of HDTV.
As shown in Figure 40.13(b), each ISDB‐T frame has 204 OFDM symbols. Each symbol has an effective symbol part with duration Tsym = 252 μs, 504 μs, and 1008 μs for modes 1, 2, and 3, respectively, and a guard interval with duration TGI = 1/4, 1/8, 1/16, or 1/32Tsym. As a result, the duration of a symbol ranges from the shortest 53.0145 ms (mode 1 with 1/32 guard interval (GI)) to the longest 257.04 ms (mode 3 with ¼ GI). At the sampling rate of fs = 512/63 MHz, the size of FFT/IFFT is 2048 (2K), 4096 (4K), and 8192 (8K) for modes 1, 2, and 3, respectively.
Figure 40.13 Layered segments of ISDB‐T channel and OFDM symbols in a segment configuration.
Figure 40.13(c) shows the OFDM segment configuration in mode 1 with 108 carriers for differential modulation (left) and synchronous modulation (right), respectively. In differential modulation, a continual pilot (CP) occupies the carrier 0. In addition, there are continuous carriers dedicated to transmission and multiplexing configuration control (TMCC) and auxiliary channel (AC) to convey control information. According to [63], there are 1 CP, 2 AC1 and 4 AC2, and 5 TMCC in mode 1; 1 CP, 4 AC1 and 9 AC2, and 10 TMCC in mode 2; and 1 CP, 8 AC1 and 19 AC2, and 20 TMCC in mode 3. Similarly, in synchronous modulation, a scattered pilot (SP) is inserted once every 12 carriers in the frequency direction and once every 4 symbols in the time direction. In addition, there are 2 AC1 and 1 TMCC in mode 1, 4 AC1 and 2 TMCC in mode 2, and 8 AC1 and 4 TMCC in mode 3, respectively, which appear in every symbol but are arranged pseudorandomly in the frequency direction.
As in DVB‐T, both CPs and SPs are produced by PRBS generators with a unique initial condition for each segment [63]. A detailed comparison of ISDB‐T with ATSC‐8VSB and DVB‐T can be found in [35]. From the viewpoint of timing and ranging, the methods