B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

12.9 Real-Life Applications of OFDM and DMT 707
12.9 REAL-LIFE APPLICATIONS OF OFDM AND DMT
OFDM is arguably one of the most successful signaling techniques for digital communications.
Combined with transmitter power loading and bit loading, the benefits of OFDM include high
spectral efficiency and resiliency against RF interferences and multipath distortion. As a result
of the many advantages, there are a number of practical OFDM/DMT communication systems
ranging from the wire-line digital subscriber line (DSL) system to the wireless ultrawideband
(UWB) radio as well as satellite broadcasting.
Asymmetric Digital Subscriber Line (ADSL)
In the past few years, ADSL has replaced a vast majority of voice modems to become the
dominant technology providing internet service to millions of homes. Conventional voice
band modems use up to 3.4 kHz of analog bandwidth sampled at 8 kHz by the public switched
telephone network (PSTN). These dial-up modems convert bits into waveforms that must
fit into this tiny voice band. Because of the very small bandwidth, voice band modems are
forced to apply very large QAM constellation (e.g., 960-QAM in V.34 for 28.8kbit/s). Large
QAM constellation require very high transmission power and high complexity equalization.
For these reasons, voice band modems quickly hit a rate plateau at 56kbit/s in the ITU-T V.90
recommendation. 13
ADSL, on the other hand, is not limited by the telephone voice band. In fact, ADSL completely
bypasses the voice telephone systems by specializing in data service. It relies on the
traditional twisted pair of copper phone lines to provide the last-mile connection to individual
homes. The main idea is that the copper wire channels in fact have bandwidth much larger
than the 4 kHz voice band. However, as distance increases, the copper wire channel degrades
rapidly at higher frequency. Hence, DSL can exploit the large telephone wire bandwidth
(up to 1 MHz) only when the connection distance is short (1-5 km). 14
The voice band is sometimes known as the plain-old-telephone-service (POTS) band.
POTS and DSL data service are separated in frequency. The voice traffic continues to use the
voice band below 3.4 kHz. DSL data uses the frequency band above the voice band. As shown
in Fig. 12.16, the separation of the two signals is achieved by a simple (in-line) low-pass filter
inserted between the phone outlet and each telephone unit when DSL service is available.
Figure 12.17 illustrates the bandwidth and subcarrier allocation of the ADSL system. From
the top of the POTS band to the nominal ADSL upper limit of 1104 kHz, we have 255 equally
spaced subchannels (subcarriers) of bandwidth 4.3175 kHz. These subcarriers are labeled 1 to
255. The lower number subcarriers, between 4.3175 and 25.875 kHz, may also be optionally
used by some service providers. In typical cases, however, ADSL service providers utilize the
Figure 12. 16
Data and voice
share the same
telephone line
via frequency
division. The
data service is
provided by the
DSL central
office, situated
near the DSL
modems.
DSL
modem
Low-pass
liter