Wireless Network Design: Optimization Models and Solution ...

114 Eli Olinick
5.3.5 Additional Design Considerations
The models presented thus far consider the key aspects of 3G network design. In
this section we briefly describe some of the other decisions that network designers
face and how the they may be incorporated within the framework developed in
this chapter. The ACMIP model and its extensions assume symmetric requirements
for uplink (mobile-to-tower) and downlink (tower-to-mobile) communication. However,
future cellular systems are expected to have larger data rate requirements on
the downlink than on the uplink. The common belief is that asymmetric applications
such as web browsing will dominate traffic leading to larger data throughputs
on the downlink. The models described above are readily modified to achieve desired
downlink performance as described below, and in Amaldi et al. [2, 6, 8] and
Rajan et al. [51].
Let SIR ↓
min be the minimum acceptable signal-to-interference ratio for the downlink.
For simplicity, we assume a single path channel and perfect orthogonality of
the spreading codes assigned to users connected to the same base station. Thus, at
each mobile, the interference is only from base station signals that do not service that
particular mobile. Moreover, we assume that the base station transmits at constant
power for each mobile that it serves. The downlink from tower ℓ to subscribers at
test point m is subject to ambient noise at location m, Nm. Thus, the SIR requirement
for the downlink from tower ℓ to subscribers at test point m is
P target
∑i∈M\{m} ∑ j∈Li\{ℓ} P target
gm j
gi j xi
≥ SIR
j + Nm
↓
min . (5.41)
To add this to the models described above, let vmℓ be a binary variable such that
vmℓ is one if and only there is at least one subscriber at a test point m who is served
by tower ℓ and add the following downlink SIR constraints to the model:
gm j
∑ ∑ xi j ≤
gi i∈M\{m} j∈Li\{ℓ} j
1
SIR ↓
−
min
Nm
+ (1 − vmℓ)Γmℓ ∀m ∈ M,ℓ ∈ Lm,
Ptarget (5.42)
where Γmℓ is a suitably large constant such that (5.42) is non-binding (i.e., automatically
satisfied) when vmℓ is zero. Thus, the Γ ’s play the same role in the downlink
SIR constraints as the β’s play in the uplink QoS constraints. The following constraints
must also be added to ensure a logical connection between the x’s and v’s,
and to define the v’s as binary:
xmℓ ≤ dmvmℓ ∀m ∈ M,ℓ ∈ Lm, (5.43)
vmℓ ∈ {0,1} ∀m ∈ M,ℓ ∈ Lm. (5.44)
Interference between users assigned to the same tower in a CDMA system can
be reduced by a technique known as sectorization. Sectorization uses directional