In Japan, the Association of Radio Industries and Businesses (ARIB) standard- ized terrestrial integrated services digital broadcasting (ISDB-T) in June 2000. Table 8.3 shows the three modes defined in the ISDB-T for television and Table 8.4 for audio [3]. The commercial service-in is scheduled in 2003.
It is very interesting to compare Table 8.2 and Table 8.3. The number of subcarriers in the DVB-T is a bit more than that in the ISDB-T with wider occupied bandwidth, and the DVB-T uses only coherent demodulation schemes, whereas the ISDB-T uses not only coherent but also differential demodulation schemes. The narrower occupied bandwidth and the use of differential detection in the ISDB-T are both designed for mobile reception, because they can give it robustness against frequency selective fading even with low SNR.
In addition to these, the ISDB-T uses an interesting pilot symbol insertion method to support mobile reception. Figure 8.1 shows the time- frequency structure of an ISDB-T-based OFDM signal. In the pilot symbol layout, there are two kinds of pilot symbol patterns, such as the continuous pilot pattern on the subcarrier with the highest frequency and the scattered pilot pattern where pilot symbols are inserted at certain subcarriers and certain time samples. The channel transfer function essential for coherent demodulation is estimated from interpolation of the scattered pilot symbols along the frequency axis, and the scattered pilot symbols are transmitted at
162 Multicarrier Techniques for 4G Mobile Communications
Table 8.3
ISDB-T Parameters (Television)
Parameter Mode Mode 1 Mode 2 Mode 3
Bandwidth 5.575 MHz 5.573 MHz 5.572 MHz
Number of subcarriers 1,405 2,809 5,617
Modulation QPSK 16 QAM 64 QAM DQPSK
Useful symbol length (ts) 252s 504s 1,008s Subcarrier separation (⌬f) 3.968 kHz 1.984 kHz 0.992 kHz Guard interval length (⌬G) ts/4 (63s) ts/4 (126s) ts/4 (252s)
ts/8 (31.5s) ts/8 (63s) ts/8 (126s) ts/16 (15.75s) ts/16 (31.5s) ts/16 (63s) ts/32 (7.875s) ts/32 (15.75s) ts/32 (31.5s) FEC (inner code) Convolutional code (R=1/2, 2/3, 3/4, 5/6, 7/8) FEC (outer code) Reed-Solomon Code (204, 188)
Interleaving Time-frequency domain bit interleaving Information transmission rate 3.65–23.2 Mbps
Required C/N 3.1 dB–20.1 dB
every time sample, so they can easily track the time variation of the channel transfer function.
8.3 5 GHz-Band Wireless LANs
In 1998, the IEEE 802.11 standardization group decided to select OFDM as a basis for its new 5-GHz wireless LAN standard, which supports data transmission rates from 6 to 56 Mbps. In the DVB-T and ISDB-T, which are mentioned in Section 8.2, OFDM is used in continuous transmission mode for the purpose of broadcasting. This new standard, called ‘‘IEEE 802.11a,’’ is the first to use OFDM in packet transmission mode [4].
Following the IEEE 802.11 decision, ETSI adopted OFDM in the standard of HIPERLAN/2 [5], as well as ARIB in the standard of MMAC [6, 7]. Since then, the three bodies have worked in close cooperation to ensure that differences between the three standards are kept to a minimum, enabling the manufacturing of equipment that can be used worldwide.
The main difference between IEEE 802.11a and HIPERLAN type 2 is in the medium access control (MAC). The IEEE 802.11a uses a distributed MAC based on carrier sense multiple access with collision avoidance (CSMA/
CA), whereas the HIPERLAN type 2 uses a centralized and scheduled MAC based on time division multiple access with dynamic slot assignment
TE AM FL Y
Team-Fly®
Figure 8.1 Time-frequency structure of an ISDB-T-based OFDM signal.
(TDMA/DSA). The MMAC supports both of these MACs. In terms of the physical layer (PHY), there are only a few minor differences among the three standards. Table 8.5 shows the system parameters for the IEEE 802.11a [4]
and the MMAC [6] and Table 8.6 for the HIPERLAN type 2 [5] and the MMAC [7].
Figure 8.2 shows the time-frequency structure of an IEEE 802.11a- based OFDM packet [8, 9]. To achieve packet mode transmission, a preamble is attached at the head of the payload. The preamble is composed of patterns A, B, and C, which are all known training signals. Usually, pattern A (80 sample-long) is used for automatic gain control, pattern B (80 sample-long) for FFT timing synchronization and coarse frequency offset compensation, and pattern C (160 sample-long) for subcarrier recovery to carry out coherent demodulation. In patterns A and B, there are pilot symbols inserted at almost every four subcarriers, which is an extension of Schmidl’s method (see Section 5.2).
Table 8.4 ISDB-T Parameters (Audio)
Bandwidth 429 kHz*
1.27 MHz**
Number of subcarriers 109* 217* 433*
325** 649** 1,297**
Modulation QPSK 16 QAM 64 QAM DQPSK
Useful symbol length (ts) 252s 504s 1,008s Subcarrier separation (⌬f) 3.968 kHz 1.984 kHz 0.992 kHz Guard interval length (⌬G) ts/4 (63s) ts/4 (126s) ts/4 (252s)
ts/8 (31.5s) ts/8 (63s) ts/8 (126s) ts/16 (15.75s) ts/16 (31.5s) ts/16 (63s) ts/32 (7.875s) ts/32 (15.75s) ts/32 (31.5s) FEC (inner code) Convolutional code (R=1/2, 2/3, 3/4, 5/6, 7/8) FEC (outer code) Reed-Solomon Code (204, 188)
Interleaving Time-frequency domain bit interleaving Information transmission rate 280.8–840 Kbps
(* 1-segment transmission, ** 3-segment transmission)
Table 8.5
IEEE 802.11a and MMAC Parameters
Channel spacing 20 MHz
Bandwidth 16.56 MHz (−3 dB)
Number of subcarriers 52
Number of pilot subcarriers 4 Useful symbol length (ts) 3.2s Subcarrier separation (⌬f) 312.5 kHz Guard interval length (⌬G) 800 ns
FEC Convolutional code
Interleaving Frequency domain bit interleaving (within one OFDM symbol)
Information transmission rate/ 6 Mbps (BSPK,Rc=1/2) Modulation/coding rate 9 Mbps (BSPK,Rc=3/4) 12 Mbps (QSPK,Rc=1/2) 18 Mbps (QSPK,Rc=3/4) 24 Mbps (16 QAM,Rc=1/2) 36 Mbps (16 QAM,Rc=3/4) 48 Mbps (64 QAM,Rc=2/3) 54 Mbps (64 QAM,Rc=3/4) Multiple access method CSMA/CA
Table 8.6
HIPERLAN/2 and MMAC Parameters
Channel spacing 20 MHz
Bandwidth 16.56 MHz (−3 dB)
Number of subcarriers 52
Number of pilot subcarriers 4 Useful symbol length (ts) 3.2s Subcarrier separation (⌬f) 312.5 kHz Guard interval length (⌬G) 800 ns
FEC Convolutional code
Interleaving Frequency domain bit interleaving (within one OFDM symbol)
Information transmission rate/ 6 Mbps (BSPK,Rc=1/2) Modulation/coding rate 9 Mbps (BSPK,Rc=3/4) 12 Mbps (QSPK,Rc=1/2) 18 Mbps (QSPK,Rc=3/4) 27 Mbps (16 QAM,Rc=9/16) 36 Mbps (16 QAM,Rc=3/4) 54 Mbps (64 QAM,Rc=3/4) Multiple access method TDMA/DSA