Position, Navigation, and Timing Technologies in the 21st Century. Группа авторов

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 of cross‐correlation of cyclic prefix and pilot‐based correlation as well as the CIR estimated from pilots described for DVB‐T in Section 40.2.2 are applicable to ISDB‐T. A system for the use of ISDB‐T signals for position location is disclosed in [64].

      40.2.4 DTMB Signals for Timing and Ranging

      Similar to the European DVB‐T described in Section 40.2.2, the Chinese DTMB in its multi‐carrier modulation mode also adopts OFDM for robustness against frequency‐selective fading [65, 66]. Unlike DVB‐T, however, DTMB uses known PN sequences in the GI between two consecutive OFDM symbols instead of a cyclic prefix. In addition to serving as GI, the PN sequence is also used for channel estimation and synchronization in the time domain. It is therefore called time‐domain synchronous OFDM (TDS‐OFDM). As such, TDS‐OFDM can provide fast acquisition and may not need to insert scattered and continual pilots to OFDM symbols, thus increasing the spectrum efficiency by 10–15% as compared to DVB‐T. However, without cyclic prefix in GI, the circular shift property of an OFDM symbol is lost, and special processing is required at the receiver to reconstruct the cyclic property of the signal and to ensure perfect removal of the PN sequence before demodulation [67].

From the viewpoint of ranging, a definite advantage of DTMB is its framing structure, which is aligned exactly with the absolute time (Beijing time) such that a receiver can determine the time of transmission once the current frame number is known. As shown in Figure 40.14(a), the DTMB frame has four layers: the calendar day frame, which is reset at 00:00:00 hours (Beijing time) every day to start a new frame; a minute frame, which consists of 480 super frames; a super frame, which is 125 ms long, and eight super frames make up 1 s; and finally, a signal frame, which is the basic unit of the DTMB frame structure.

      Each signal frame has a frame header and a frame body, both at the same symbol rate of 7.56 million‐symbols per second (Msps). Note that the term symbol used here by DTMB is similar to that used by ATSC‐8VSB, whose duration is equivalent to the elementary period (a sample) of DVB‐T. As shown in Figure 40.14(b), the frame body is fixed with 3780 symbols over 500 μs, while the frame header has three modes to support services under different conditions. In mode 1, a frame header contains 420 symbols over 55.56 μs (FH420), which is made of an 82‐symbol front synchronization sequence (preamble), a 255‐symbol PN sequence (denoted by PN255), and an 83‐symbol rear sync sequence (post‐amble). Both the front and rear sync sequences are cyclic extensions of PN255, known as the cyclic prefix and the cyclic suffix, respectively. As a whole, a signal frame in mode 1 has 4200 symbols over 555.56 μs at a rate of 225 signal frames per super frame. The generator polynomial for PN255 is G₂₅₅ = 1 + x + x⁵ + x⁸ with a different initial condition (phase) for each signal frame to serve as an index for the signal frame [65]. When the signal frame is not indexed, the initial condition (index 0) is D₁…D₈ = 00001101. The average transmission power of the frame header in mode 1 is doubled as compared to the frame body.

      In mode 2, the frame header contains a single 595‐symbol PN sequence (PN595) over 78.70 μs (FH595), resulting in a signal frame of 4375 symbols over 578.70 μs at the rate of 216 signal frames per super frame. PN595 takes the first 595 symbols of a 1023‐symbol m‐sequence specified by the generator polynomial G₁₀₂₃ = 1 + x³ + x¹⁰ with the initial condition being D₁…D₁₀ = 0000000001, which is reset for each signal frame [65]. In other words, the PN595 sequence has the same phase for all signal frames. In mode 2, the frame header and frame body are transmitted using the same average power.

Finally, in mode 3, a frame header has 945 symbols over 125 μs (FH945), which is made of a 217‐symbol front synchronization sequence (cyclic prefix), a 511‐symbol PN sequence (PN511), and a 217‐symbol rear sync sequence (cyclic suffix). Again, the phase of the PN511 sequence in each signal frame differs, serving as an index for the signal frame. As a whole, a signal frame in mode 3 has 4725 symbols over 625 μs at a rate of 200 signal frames per super frame. The generator polynomial for PN511 is G₅₁₁ = 1 + x² + x⁷ + x⁸ + x⁹, again with a different initial condition for each signal frame as its index [65]. When the signal frame is not indexed, the initial condition (index 0) is D₁…D₉ = 111011111. The average transmission power of the frame header in mode 3 is again doubled as compared to the frame body.