The Second Generation (2G) wireless systems focused their effort on providing
mobile voice applications to the end-user with an acceptable quality. The evolution of
the end-users' need towards multimedia applications has pushed the wireless
community to conceive the so-called 3G systems. Wideband Code Division Multiple
Access (WCDMA) has been recommended as a strong candidate for 3G systems (Holma
and Toskala, 2000). The success in the deployment of such networks will critically
depend on how efficiently the wireless networks can support traffic flows with Quality
of Service (QoS) guarantees. Provisioning of a wide range of QoS requirements
and effectively handling them together is definitely a challenging issue.
Due to simple frequency assignment plan, the interfering mobiles in a
CDMA system reside in the same cell, and therefore, the system is
interference-limited. Hence, interference is arguably the biggest limiting factor to system capacity
(Swartz et al., 1999). In addition to this, shadowing and multipath fading
affect signal propagation largely (Chan, 1994). Evidently, link
adaptation is the only alternative in delivering the required QoS, specifically under poor radio
conditions. The basic idea of a link adaptation scheme is to operate a link as efficiently
as possible in the prevailing channel conditions by adapting the
transmission parameters (transmission power, source/channel codes) to meet the desired
QoS guarantees. In a CDMA environment, capacity and other QoS measures are
described in terms of Signal-to-Interference Ratio (SIR), which is determined by the
transmission powers of the co-users and the gains of the radio channels between the
mobile users and base stations. The SIR, being a measure of link quality, needs to
be optimized, so that the target Bit Error Rate (BER) and throughput are met
without any spectral inefficiency. To achieve satisfactory quality on a wireless link, at
least the threshold SIR must be met. An increase in the transmission power or
channel gain of a user not only increases the received power, but also increases
the interference to other users, causing a reduction in their received SIR. On the
other hand, an increase in the Spreading Gain (SG) of a user also increases its
received power, but this decreases the transmission rate of the user and prolongs the
time duration over which the user creates interference to others. Controlling
the transmission powers and SGs of the users based on the link condition will
amount to directly controlling the QoS measures and channel efficiency. Hence, the
obvious aim of the work is to maximize system performance without any loss in
spectral efficiency (i.e., at a minimum SIR level), so that the cell capacity improves. This
clearly demands a Power Control (PC) policy, which is link-adaptive in nature. |
