Digital Signal Processing Reference
In-Depth Information
Adaptive modulation and coding are also successfully employed for new-generation
WLAN systems. HiperLAN2 and IEEE 802.11a, both of which use OFDM technology
at the physical layer, allow four different modulation options (BPSK, QPSK, 16-QAM,
and 64-QAM) with different coding rates. The coding rates are obtained with different
puncturing patterns to a mother convolutional code, resulting in eight different modu-
lation and coding options [66]. Similar to link adaptation in EGPRS, an appropriate
modulation and coding scheme is used depending on the link quality. Therefore, a data
rate ranging from 6 to 54 Mbit/s can be obtained by using various modes. BPSK, QPSK,
and 16-QAM are used as mandatory modulation formats, whereas 64-QAM is applied
as an optional mode.
Although only a couple of cases are given above, adaptive modulation and coding
have attracted many new-generation wireless standards to consider them as options
to increase the data rates, and there has been a significant amount of research in this
area. Especially in conjunction with the advanced receiver algorithms that reduce the
required SINR to lower values, the better link quality values can be exploited to increase
the data rates further. Combining adaptive modulation with multiantenna transmitter
and MIMO schemes based on the feedback-related channel estimates, channel quality,
channel correlation, etc., is one of these interesting research areas. Based on the channel
feedback information, the modulation type on multiantenna transmitters can be varied.
Similarly, adapting the source coding with the channel coding or modulation is another
interesting area of focus for link adaptation. For example, adaptive multirate (AMR)
codec allows changing of the compression rate of speech depending on the link quality,
as in GSM AMR. For weak link conditions, where heavy FEC is required, AMR has the
ability to decrease the codec rate (more speech compression) to allocate more bits for
FEC [49].
1.4.2.3 Adaptive Cell and Frequency Assignment
As mentioned before, radio spectrum is very expensive and limited. Efficient use of
radio spectrum is very important to maximize the system capacity. The introduction of
cellular technology was a major step toward efficient usage of finite spectrum through a
concept called
frequency reuse
. The capacity of cellular systems is interference limited,
dominated by co-channel interference (CCI) and adjacent channel interference (ACI).
Early cellular systems aimed to avoid these major interference sources by designing
systems for the worst-case interference conditions along with fixed channel allocation.
This is often achieved by employing higher-frequency reuse and by allowing enough
carrier spacing between adjacent channels. Both of these reduce the spectral efficiency.
Later, more efficient spectrum usage strategies were developed that dynamically assign
frequencies relative to current interference, propagation, and traffic conditions. In tra-
ditional cellular system designs, the allocation of frequency channels to cells is fixed,
which means that each cell can use only a set of frequencies. Even if the other cells are
not fully loaded, the cell that does not have any available frequency (fully loaded cell)
cannot take advantage of it. In dynamic channel allocation, all the channels belong to a
global pool and the channels are assigned according to a cost function that considers the
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