OFDM: Choice of the key parameters
Contributed by Dusan Matic
Useful symbol duration
The useful symbol duration T affects the carrier spacing and coding
latency. To maintain the data throughput, a longer useful symbol duration
results in increase of the number of carriers and the size of FFT (assuming
the constellation is fixed). In practice, carrier offset and phase stability
may affect how close two carriers can be placed. If the application is
for the mobile reception, the carrier spacing must be large enough to make
the Doppler shift negligible. Generally, the useful symbol duration should
be chosen so that the channel is stable for the duration of a symbol.
Number of carriers
The number of subcarriers can be determined based on the channel bandwidth,
data throughput and useful symbol duration.
The carriers are spaced by the reciprocal of the useful symbol duration.
The number of carriers corresponds to the number of complex points being
processed in FFT. For HDTV applications, the number of subcarriers are
in the range of several thousands, so as to accommodate the data rate and
guard interval requirement.
Modulation scheme
The modulation scheme in an OFDM system can be selected based on the requirement
of power or spectrum efficiency. The type of modulation can be specified
by the complex number
dn=an+jbn
,
defined in The use of FFT in OFDM. The symbols an
and bn can be selected to (± 1, ±
3) for 16QAM and ± 1 for QPSK. In general, the selection of the
modulation scheme applying to each subchannel depends solely on the compromise
between the data rate requirement and transmission robustness. Another
advantage of OFDM is that different modulation schemes can be used on different
subchannels for layered services.
Coded OFDM
By using frequency and time diversity OFDM provides a means to transmit
data in a frequency selective channel. However, it does not suppress fading
itself. Depending on their position in the frequency domain, individual
subchannels could be affected by fading. This requires the use of channel
coding to further protect transmitted data. Among those channel techniques,
trellis coded modulation (TCM), combined with frequency and time interleaving
is considered the most effective means for a selective fading channel.
TCM combines coding and modulation to achieve a high coding gain without
affecting the bandwidth of the signal. In a TCM encoder, each symbol of
n bits is mapped into constellation of n+1 bits, using a
set-partitioning rule. This process increases the constellation size and
effectively adds additional redundancy to the signal. A TCM code can be
decoded with a soft decision Viterbi decoding algorithm, which exploits
the soft decision nature of the received signal. The coding gain for a
two-dimensional TCM code over a Gaussian channel is about 3 dB for a bit
error rate (BER) of 10-5.
It should be mentioned that one of the advantages of OFDM is that it
can convert a wideband frequency selective fading channel into a series
of narrowband and frequency non-selective fading subchannels by using parallel
and multicarrier transmission. Coding OFDM subcarriers sequentially by
using specially designed TCM codes for frequency non-selective fading channel
is the major reason for using the COFDM for terrestrial broadcasting. However,
the search of the best TCM code is still ongoing.
Although trellis codes produce improvements in the signal-to-noise ratio
(S/N), they do not perform well with impulsive or burst noise. In general,
transmission errors have a strong time/frequency correlation. Interleaving
plays an essential role in channel coding by providing diversity in the
time domain. Interleaving breaks the correlation and enables the decoder
to eliminate or reduce local fading throughout the band and over the whole
depth of the time interleaving. Interleaving depth should be enough to
break long straight errors.
Flexibility and scalability
Based on the information theory, the channel capacity is a function of
the signal-to-noise ratio and channel bandwidth. The concept of graceful
degradation has been implemented in the analog TV systems. It is believed
that the joint source/channel coding is the best way to achieve flexibility
and scalability. COFDM has been considered very flexible for the layered
and scaleable transmission. Different groups of COFDM subchannels can be
assigned to different orders of modulation, power levels, and channel coding
schemes.
