sequence of symbols. Details about relative and absolute cyclic shifts using Gray code mapping as well as other information about preamble and subframes can be found in [80, 81].
Similar to other OFDM signals, pilot signals, whose location and amplitude are derived from a reference PN sequence, one for each transmitted carrier on any given symbol, can be used for synchronization, channel estimation, transmission mode identification, and phase noise estimation, among other uses. Pilots used in ATSC 3.0 include scattered, continual, edge, preamble, and subframe boundary pilots, which may be transmitted at a boosted power level and whose transmitted value is known to the receiver. ATSC 3.0 also specifies an optional technology of transmitter identification (TxID) that uniquely identifies each individual transmitter via an RF watermark, which enables system monitoring and measurements, interference source determination, geolocation, and other applications. For example, the TxID signal can be used to measure the CIR of each transmitter independently to support in‐service system adjustments, including the power levels and delay offsets of individual transmitters [80, 81]. Methods for timing and ranging using PN codes, cyclic prefix/suffix, pilot subcarriers, and watermarking signals described in previous sections for ATSC‐8VSB, DVB‐T, ISDB‐T, and DTMB signals are herein all applicable.
40.3 Pseudorange Measurements from Broadcasting Signals
As described in Section 40.2, TOA measurements from DTV signals (e.g. ATSC 8VSB, DVB‐T, and DTMB) are made relative to the receiver’s local timeline, which may differ from that of a transmitter by clock errors such as bias and drift. The TOA measurements can be taken at a fixed rate (periodic) or at a variable rate whenever a particular event such as the start of a field sync or an OFDM symbol occurs (aperiodic to the receiver). Besides TOA, TOT measurements are required to form pseudoranges from TOA measurements. Note that DTV transmitters can be synchronous as in a SFN or asynchronous, when each transmitter maintains its own clock loosely coupled to a common timeline like UTC.
To derive the pseudorange equations, we first establish the relationship between the timelines at the transmitter (labeled as TX time) and receiver (labeled as RX time) as shown in Figure 40.16. The event of interest for our ranging purpose is the leading edge of the ATSC 8VSB’s field sync segments (or of the useful part of OFDM symbols) represented by up‐pointing arrows in Figure 40.16.
The time at which this leading edge leaves the transmitter antenna is the TOT. The successive times of transmit are related by
where n = 0, 1, 2, … is the number of fields, and Tfield is the nominal period of a field, which is about 24.2 ms (at a field rate of 41.32 Hz) for ATSC‐8VSB signals.
Assume that the receiver’s time ticks at a sampling rate of, say, 10 MHz. The TOA of the leading edge of the field sync segments is estimated by determining the location of the correlation peaks as detailed in Section 40.2.1. Referring to the RX time, we estimate the TOA by counting the samples between successive correlation peaks (denoted by Pn) and the first peak relative to the first sample (denoted by P0).
The first sample is set to be zero for the receiver clock, which differs from the transmitter time by an offset, denoted by t0. As a result, the TOA can be expressed in terms of the correlation peak locations as
(40.2)
where t0 is different for each transmitter using an independent clock.
If we calculate the pseudorange for and at each time of arrival TOAn, the measurements will not be on a uniform scale due to the random nature of the TOA caused by relative movement and noise. Hence, they are called aperiodic pseudoranges, denoted by APRn, and given by
The time of measurement for the aperiodic pseudoranges is the same as the TOA. But aperiodic pseudoranges are not available regularly on a uniform time scale. In order to integrate these pseudoranges with other sensor measurements, interpolation may be required. Alternatively, we can form periodic pseudoranges [23].
In addition to the initial clock offset t0, the clocks may drift in frequency, leading to Tfield and Ns (the number of samples per field) off their nominal values. For the stationary transmitter and receiver, there is no Doppler frequency shift. The changes in symbol rate and sampling rate are due to the clock frequency instability, and the combined effect is observed at the receiver.
Figure 40.16 Relationship of timelines at transmitter and receiver and aperiodic pseudoranges.
For asynchronous transmitters, each pseudorange equation contains at least an unknown of its own related to the transmitter (i.e. the initial clock offset t0). No instantaneous position fixing is possible with such pseudorange measurements for a stand‐alone solution unless additional information such as TOT and LOT is encoded on broadcasting signals (add‐on services). Nevertheless, there are different positioning mechanisms that can be employed to deal with the unknowns in pseudoranges, including differential ranging, relative ranging, and self‐calibration, among others.
Differential ranging involves a reference receiver at a known location that provides an estimate of the TOT or TOA of the same event via a data link to a user in order to cancel out the common TOT at the user receiver, leading to spatial difference of pseudoranges [7, 19, 20]. Relative ranging accumulates changes in range to a transmitter from a starting location [25]. As long as the signal tracking is maintained, the displacement from the starting point can be estimated from the temporal differences of pseudoranges to several transmitters in a process known as radio dead reckoning [23, 24, 82]. If the transmitter locations are known and the receiver starts from a known initial location, the method of self‐calibration can be used to estimate the unknown TOT [17].
As an example, consider the case of self‐calibration with an aperiodic pseudorange (Eq. 40.3). We first form the range between a known transmitter and our receiver at the initial known location as APRn, and count the samples between successive correlation peaks Pn. Since we do not know t0 and Tfield (except its nominal value), we can reformulate Eq. 40.3 as
(40.4)
Assume that the receiver is stationary (or its location known if it is moving). We collect N+1 measurements of APRn = APR and Pn and obtain the following
