Digital Signal Processing Reference
In-Depth Information
cannot be established. In order to establish a reliable link while minimizing interfer-
ence to other users, the transmitter should continuously control the transmitted power
level. Power control is a simple form of adaptation that compensates for the variation of
the received signal level due to path loss, shadowing, and sometimes fading. Numerous
studies on power control schemes have been performed for various radio communica-
tions systems (see [
1
]
and the
references
listed therein). In code division multiple-access
(CDMA) systems, signals having widely different power levels at the receiver cause
strong signals to swamp out weaker ones in a phenomenon known as the near-far effect.
Power control mitigates the near-far problem by controlling the transmitted power.
It is possible to trade off power for bandwidth efficiency; i.e., a desired BER (or FER) can
be achieved by increasing the power level or by reducing the bandwidth efficiency. One
way of establishing a reliable link is to add redundancy to the information bits through
FEC techniques. With no other changes, this would normally reduce the information
rate (or bandwidth efficiency) of the communication. In the same way, high-quality
links can be obtained by transmitting the signals with spectrally less efficient modula-
tion schemes, like binary phase shift keying (BPSK) and quaternary PSK (QPSK). On
the other hand, new-generation wireless systems aim for higher data rates made possible
through spectrally efficient higher-order modulations. Therefore, a reliable link with
higher information rates can be accomplished by continuously controlling the coding
and modulation levels. Higher modulation orders with less powerful coding rates are
assigned to users that experience good link qualities, so that the excess signal quality
can be used to obtain higher data rates. Recent designs have exploited this with adaptive
modulation techniques that change the order of the modulation [1, 2], as well as with
adaptive coding schemes that change the coding rate [3, 4]. For example, the Enhanced
General Packet Radio Service (EGPRS) standard introduces both Gaussian minimum
shift keying (GMSK) and 8-PSK modulations with different coding rates through link
adaptation and hybrid automatic repeat request (ARQ) [5]. The channel quality is esti-
mated at the receiver, and the information is passed to the transmitter through appro-
priately defined messages. The transmitter adapts the coding and modulation based on
this channel quality feedback. Similarly, variable spreading and coding techniques are
present in third-generation CDMA-based systems [3], cdma2000 and wideband CDMA
(WCDMA, or Universal Mobile Telecommunications System [UMTS]). Higher data
rates can be achieved by changing the spreading factor and coding rate, depending on
the perceived communication link qualities.
Adaptive antennas and adaptive beam-forming techniques have also been studied
extensively to increase the capacity and to improve the performance of wireless com-
munications systems [6]. The adaptive antenna systems shape the radiation pattern in
such a way that the information is transmitted (for example, from a base station) directly
to the mobile user in narrow beams. This reduces the probability of another user expe-
riencing interference in the network, resulting in improved link quality, which can also
be translated into increased network capacity. Although adaptive beam forming is an
excellent way to utilize multiple-antenna systems to enhance the link quality, recently
different flavors of the usage of multiantenna systems have gained significant interest.
Space-time processing and multiple-input multiple-output (MIMO) antenna systems
are some new developments that will allow further usage of multiple-antenna systems in
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