Example Downlink Cell Loading for Voice Service

In this
example, we calculate the cell loading as a function of the number of users
assuming that all users are using standard voice service.
Assumptions:
aj  0.65 for all users
Rj  12.2 Kbps for all users
Eb/No  4 dB ( 2.512) for all users (because all users are voice-only in
this example).
a  0.4
i  0.5
Because of the fact that all users in this example have the same characteristics,
Equation 12-15 becomes
Load factor  N  (1
a  i)/[C/(a  R(Eb/No))] Equation 12-16)

Using the previous assumptions, the load factor for one user is
(1
0.4  0.5)/[3,840,000/(0.65  12,200  2.512)]  0.0057  0.57%.
Thus, for a downlink load factor of 50 percent, we can accommodate
approximately 88 simultaneous voice users. For a load factor of 60 percent,
we can accommodate approximately 105 simultaneous voice users, and so
on.
As is the case for the uplink, the downlink link budget needs to include
an interference margin equivalent to the noise rise. The required interference
margin is a function of the cell load factor, and the same figures as in
Table 12-4 apply. In other words, for a 50 percent load factor, we need a 3 dB
interference margin in the downlink.
Assume, for example, a downlink link budget as shown in Table 12-5,
where there is a base station transmitter output power of 10 W.
This link budget does not show an interference margin. Such a margin
must be included, however. The exact value of the interference will equate
to the noise rise, which increases with increasing cell load—that is,
throughput. Using the example assumptions outlined previously, Figure
12-6 shows the cell load as a function of the number of users and also
the noise rise/required interference margin as a function of the number of
users. Figure 12-7 shows the allowable downlink path loss as a function of
the number of users. If we compare Figure 12-7 with Figure 12-5, we can
determine whether the system is uplink limited or downlink limited for a
given number of voice users.
The foregoing examples show how cell loading, in terms of numbers of
voice users, can impact uplink and downlink coverage. Using voice service
is a convenient example to show how the calculations can be performed. In
reality, however, we can expect a significant mix of services—with some
subscribers using voice service and some subscribers using data services of
one kind or another. Thus, the calculations should be performed individually
for each type of service.
While, for a service like voice, the coverage is likely to be uplink limited
rather than downlink limited, the same might not apply for data service.
With UMTS, data services can be asymmetric—that is, different date rates
in the uplink compared to the downlink. Moreover, for many data services
(such as Web browsing), we will find that the downlink data rate is far
greater than the uplink data rate. Consequently, the effect of interference in
the downlink may well be greater than in the uplink, which means that the
downlink load may become the limiting factor.

If we find that we are downlink limited, then we may be able to increase
the base station output power and/or add an additional RF carriers subject
to spectrum availability. As mentioned in Chapter 6, “Universal Mobile
Telecommunications Service (UMTS),” however, the addition of a second
carrier will mean that compressed mode must be used (where the MS can
tune to other carriers for potential hard handover). Compressed mode
means an aggregate lower throughput per carrier, so that, although a second
carrier does provide significant additional capacity, it does not mean a
capacity increase of 100 percent.
Another downlink limiting factor for a single carrier base station is the
availability of downlink channelization (spreading) codes. Recall from
Chapter 6 that channelization codes are chosen from a code tree. Recall also that the use of a particular for a channelization code can pre-empt the use
of other channelization codes on the same branch of the code tree. For example,
consider the channelization code Cch,4,0. This code is simply the sequence
1,1,1,1 repeated over and over. Consider the channelization code Cch,8,0. This
is simply the sequence 1,1,1,1,1,1,1,1 repeated over and over. Clearly, if the
base station is using either of these codes in a transmission to a particular
mobile, then it cannot use the other code (or any other code that is a series
of all ones) in transmission to any other mobile. One way to overcome this
limitation, however, is for the base station to use multiple scrambling codes.
A given cell can use up to 16 downlink scrambling codes. 476