the peak.
Figure 40.11 Ideal correlation functions for various components of an OFDM symbol.
When the correlation functions of scattered pilots in four consecutive symbols are coherently summed, the resulting function has a periodicity of NFFT/3 as shown in Figure 40.11(c), which is because the pattern of aggregated scattered pilots has a spacing of three carriers. The correlation peak maintains the same shape, but the unambiguous interval increases by a factor of 4.
The difference between continual pilot carrier indexes is shown in Figure 40.11(d), which exhibits repetition in frequency. The correlation function of continual pilots of an OFDM symbol is shown in Figure 40.11(e), which has the same periodicity as in Figure 40.11(c) but with a raised level of cross‐correlation due to spectral leakage of its irregular subcarrier placement. It is one reason why only scattered pilots are used in correlation tracking for refined TOA estimation.
The correlation function of a full OFDM symbol with all subcarriers is shown in Figure 40.11(f), where the continual and scattered pilots have an amplitude factor of 4/3 while data subcarriers of unity amplitude are drawn randomly from a QPSK constellation of z = (1/
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
