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wired packet networks. To make things worse, routers
have no information about the current number of users
or cell loads exactly. The L4 router could make it pos-
sible, but it is not widely deployed yet. The IP QoS is
the other approach to measure the service quality. It
is classified into integrated and differentiated services
(Y. Tang, 2001). However, the IP QoS cannot be eas-
ily guaranteed in wireless systems because wireless
resources are much lesser than wired ones (S. Sen,
1999; M. Ricardo, 2002). The IP QoS cannot main-
tain service qualities of all calls. This scheme sim-
ply rejects calls to obtain the QoS of other calls and
it cannot resolve traffic congestion problems. More-
over, QoS is not yet widely available even on the fixed
networks.
The use of AMC is one of the state-of-the-art tech-
niques in the standards for third and fourth gener-
ation wireless systems that have been developed to
achieve high spectral efficiency on fading channels
(A.J. Goldsmith, 1998). The principle of AMC is to
change the modulation and coding format adaptively
depending on instantaneous variations of the channel
conditions. These conditions are subject to the sys-
tem restrictions. The AMC extends the system capa-
bility to adapt to good channel conditions. Channel
conditions should be estimated based on the feedback
from the receiver. For the AMC-OFDMA systems,
higher order modulation with higher code rates (i.e.,
16 QAM with R=5/6 Turbo Codes) are typically as-
signed to users near the cell site. On the other hand,
lower order modulation with lower code rates (i.e.,
QPSK with R=1/3 Turbo Codes) are assigned to users
at the cell boundary. The AMC allows various data
rates to be assigned to different users depending on
their channel conditions. Since the channel conditions
change over time, receivers collect a set of channel
statistics which are used by both the senders and the
receiver to optimize system parameters such as mod-
ulation and coding, signal bandwidth, signal power,
training period, channel estimation filters, automatic
gain control, etc (Lu, 2004).
Most of the handoff algorithms are designed to per-
form handoff based on the link gain or the received
signal strength (RSS). These algorithms can find tar-
get base stations in terms of the link gain or the signal
strength before the current signal power is too weak to
sustain communications (M. McGuire, 1997). Thus,
the conventional handoff algorithms can help to get
services from a base station which has the better sig-
nal strength, but they cannot optimize the service
quality in the packet networks. In the circuit switch-
ing networks, the received signal strength can be a
metric to measure the service quality for the allocated
resources, because resources are dedicated. However,
new factors besides the link gain or the received sig-
nal strength have to be considered in case of packet
networks (N.D. Tripathi, 1999; Cao, 2003). There-
fore, we need new handoff methodology in terms of
the service quality each terminal gets instead of the
legacy handoff algorithms which only considers the
link gain or the received signal strength.
3 HANDOFF PROCEDURE
In this chapter, we propose new handoff algorithm
based on the service quality which is related with two
components; the service rate and the data rate.
3.1 System Environments
Our handoff algorithm operates on the shared channel
with a lot of users in the packet-based networks. How
many users exist in the same cell determines how of-
ten each user can be served because the channel is al-
located by the segment, the fundamental transfer unit
scheduled by there exist data to send. If the number of
users is small, then users can be served frequently. On
the other hand, intervals each user is served become
longer as the number of users increases.
7
6
5
4
3
2
1
Figure 1: Example of the cell layout with AMC
We consider the cell layout with the adaptive mod-
ulation and coding to enlarge the system capacity as
Figure 1 and Table 1.
Table 1: Example of adaptive modulation and coding
Fraction Code Rate Modulation
1 1/3 QPSK
2 1/2 QPSK
3 2/3 QPSK
4 5/12 16QAM
5 1/2 16QAM
6 2/3 16QAM
7 5/6 16QAM
The AMC helps more users can be served with the
higher data rate in a permissible range of the bit error
rate (BER) by applying the appropriate modulation
and coding method depending on the signal to inter-
ference plus noise ratio (SINR). Hence the actual data
rate each terminal gets differs with the received signal
strength and the amount of noise which varies with
the traffic condition. Generally, mobile terminals near
A SMART HANDOFF PROCEDURE IN 4G NETWORKS
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