Orthogonal Frequency-Division Multiplexing and Multiple Access
4.3 Coded Orthogonal Frequency-Division Multiplexing: COFDM
4.3.3 Some Classical Applications of COFDM
Early applications of MCM and OFDM techniques have been in HF military communications systems [17]. The COFDM idea was conceived in the 1950s, however, widespread applications emerged in the 1980s–
2010s. For OFDM/COFDM to come to technological prominence, a period of 30 years was needed in order to overcome the practical difficulties that existed in their hardware implementation. The ability to define the COFDM signal in the Fourier domain, the advances made in modulation, channel coding, equalization techniques, and digital signal processing in the 1980s, and the ease with which software implementation is carried out in DSP processors, or dedicated VLSI circuits, has made OFDM an attractive choice for many applications, particularly in wireless telecommunication systems. Early on in the 1980s, OFDM Technology gave birth to a new generation of telephone modems of data rates above 14 kbps which were then considered as“ultrahigh-speed voiceband data modems” [18]. Trellis coded modulation and OFDM were among tech-niques that made these developments possible [19]. We now address few forerunner applications of COFDM in wireless networks whose evolved versions have become standard signaling for WMAN (wireless metro-politan area network), such as WiMAX and AeroMACS, and WWAN (wireless wideband area networks) of the twenty-first century.
4.3.3.1 COFDM Applied in Digital Audio Broadcasting (DAB)
One of the early applications of COFDM in radio broadcasting is in DAB for mobile users within the framework of EURIKA 147 Project in Europe [14]. The DAB standard was initiated as a European research project in the 1980s and it was launched for the first time by the Norwegian Broadcasting corporation in June 1995. BBC (British Broad-casting Company) launched its first DAB digital radio broadcasts in September 1995. BBC had provided DAB coverage for over 60% of the UK population as of 1998. By mid-1998 more than 150 million people around the world were within the coverage of a DAB transmitter [20]. An upgraded version of DAB was released in February 2007, which is designated as DAB+.
OFDM and rate-compatible punctured convolutional coding (RCPC) form the core of COFDM signaling applied in DAB and DAB+. A major reason for the selection of OFDM modulation is the possibility to use single-frequency network. In a single-frequency network, the user receives several copies of the same signal from a number of transmitters with different delays corresponding to the distance between the receiver and the transmitters. As long as the time difference between the arrivals of two signals is less than OFDM guard interval, no ISI or ICI will occur. The
presence of two or more time-shifted copies of the signal provides a diversity advantage in the network, in the sense that the probability that the sum of the signals has an unacceptably low power because of propagation, shadowing, andflat fading losses is much lower than that of an individual signal [4].
The DAB signal occupies a bandwidth of 1.536 MHz with a gross transmission rate of 2.3 Mbit/s. The net data rate, however, vary between 0.6 and 1.8 Mbit/s depending on the convolutional code rate, which is application dependent. The individual carriers are modulated using differential QPSK (DQPSK). To protect subcarriers fromflat fading effect, convolutional code of constraint length 7 is applied. To allow for both equal and unequal error protection, the code rate varies from 1=3 to 3=4, which gives rise to an ensemble code rate average of 1=2. DAB operates on four different transmission modes, using four different sets of OFDM parameters, as shown in Table 4.1. Modes I through III are optimized for performance in different operating frequency ranges, while mode IV is designed to provide larger area coverage at the expense of more sensitivity to Doppler shift.
The DAB waveform consists of a number of audio signals, sampled at 48 kHz with an input resolution of up to 22 bits that can provide CD quality audio output [20].
4.3.3.2 COFDM Applied in Wireless LAN (Wi-Fi): The IEEE 802.11 Standard Since early 1990s, wireless local area networks (WLAN) designed and developed over so-called ISM bands (industrial, scientific, medical band over 0.9, 2.4, and 5 GHz) have been around. In June 1997, the IEEE approved an international interoperability standard: the IEEE 802.11 Standards. In July 1998, the IEEE 802.11 working group adopted OFDM signaling for their new 5 GHz standard that supports data rates 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. To accommodate various supported
Table 4.1 COFDM parameters for DAB operating modes.
Parameter Mode I Mode II Mode III Mode IV
Nominal frequency <375 MHz <1.5 GHz <3 GHz <1.5 GHz
No. of subcarriers 1536 386 192 768
Useful symbol duration 1 ms 250μs 125μs 500μs
Guard interval 246μs 62μs 31μs 123μs
Subcarrier modulation DQPSK DQPSK DQPSK DQPSK
Channel coding RCPC RCPC RCPC RCPC
data rates, the standard allows for the use of four different modulation schemes, namely, BPSK, QPSK, 16-QAM, and 64-QAM for subcarriers.
The selected channel coding scheme is convolutional coding with variable rate that is dependent on the data rate. Thus, the choice of modulation parameters and channel coding rate depend on the selected data rate and is set according to Table 4.2 [21].
Table 4.3 lists other important timing parameters of COFDM signaling used in the IEEE 802.11 standard. Chapter 5 reveals that the same COFDM ideas that are applied in IEEE 802.11 standards are carried over to IEEE 802.16 standards and WiMAX networks.
A key parameter that predominantly affects the selection of other OFDM parameters is the guard interval of 800 ns. Clearly, this guard interval creates robustness against delay spread up to several hundred nanoseconds, depending on the coding rate and the applied modulation scheme. Practically speaking, this implies that the signaling is robust enough for the system to be used in any indoor environment, including large factory buildings. It can be also used in outdoor environments, although directional antennas may be needed to reduce the delay spread.
To limit the SNR effect of the guard interval to 1 dB, the symbol duration is selected to be 4μs. The useful symbol duration, therefore, as shown in Table 4.3, is 3.2μs, which is equal to the inverse of subcarriers frequency spacing, 0.3125 MHz. By using 48 subcarriers and applying various modulation scheme provided in the Standard, uncoded data rates of 12–72 Mbps are achievable. In addition to 48 data subcarriers, each OFDM symbol contains 4 pilot subcarriers that is used to track the residual carrier frequency offset remaining after the initial carrier fre-quency correction. To protect the signal against deep fade, a ½ rate, Table 4.2 Data rate-dependent parameters of COFDM in IEEE 802.11 Standard.
Data rate M bits=s
Modulation format
Convolutional code rate
Coded bits per OFDM symbol
Data bits per OFDM symbol
6 BPSK 1=2 48 24
9 BPSK 3=4 48 36
12 QPSK 1=2 96 48
18 QPSK 3=4 96 72
24 16-QAM 1=2 192 96
36 16-QAM 3=4 192 144
48 64-QAM 2=3 288 192
54 64-QAM 3=4 288 216
constraint 7, convolutional code is used. The code rate can be adjusted to 2/3 and¾ through the process of puncturing. This provides coded data rates in the range of 6–54 Mbps [5].