Digital Signal Processing Reference
In-Depth Information
won't skyrocket as it does in a single-carrier system. Remark that the idea of
using multiple parallel subcarriers has only recently found its way to a practi-
cal implementation. Thanks to the availability of abundant processing power,
a discrete-time mathematical representation is used to build up the multicarrier
baseband signal before it is converted to the analog domain.
Imagine how difficult it would be to generate and modulate 52 carriers in the
analog domain, thereby controlling their frequency in such a way that these
carriers are orthogonal to each other! After conversion to the analog domain,
the only task that remains for the analog front-end is to upconvert the base-
band signal to the appropriate frequency band. It seems that wireless commu-
nications have merely become a matter of digital processing, while the role of
analog circuitry is ever more reduced.
However, in a world ruled by digital virtuality, it should not be forgotten that
sooner or later, the digital processor has to interface with the analog world.
Ironically, a nice example of the imminent danger of forgetting about the ana-
log aspects is provided by the 802
.
11a/g wireless lan standard itself. The
combination of all the 52 qam-modulated carriers will result in anything but a
constant-envelope signal in the time domain. The sum of the amplitudes of all
subcarriers results in a signal with a large peak-to-average power ratio (papr)
[Han03]. An am-modulated signal with a considerable modulation depth is
thus applied to the analog front-end of the transceiver. To process such a sig-
nal, a very linear transmission chain with a considerable dynamic range is re-
quired. The dynamic range of the signal affects the loading factor of the dac
of the transmitter (adc in the receiver). Since the average magnitude of the
ofdm modulated baseband signal is less than its occasional peak value, the
full quantization range of the converter is not used very efficiently. A higher
number of bits is thus required to represent an ofdm signal with the same level
of accuracy as a signal that uses the full-scale range of the converter.
The same situation is encountered in the rf power amplifier. The amplifier
must be able to cope with the peak demands of the ofdm system, but most of
the time it is driven at an average level which is considerably lower. For exam-
ple, the papr of the 52-subcarrier 802
.
11a system requires that the amplifier is
driven with a power back-off of at least 8 dB from the 1 dB compression point
(P
1dBc
[Hei73]). In other words, the rf power stage is being biased for oper-
ation under high-strain conditions, but handles much lower output levels for
most of the time (see Figure 1.6). The reduction in the power-added efficiency
(pae) of the amplifier under such operating conditions has a direct impact
on the power consumption, heat production and battery lifetime of portable
devices.
