By Ramzi Bargouthi
The coming of IS-95 networks promised much in terms of new capabilities,
but with a new technology came new approaches and new tasks
for cellular engineers to learn. What are the major challenges
of cdmaOne network planning and optimization? Ramzi Bargouthi,
CDMA technical director, MSI
Code Division Multiple Access (CDMA) technology has come
a long way since the late 1980s when it was first proposed by Qualcomm
Inc. as an alternative to TDMA and FDMA techniques to enhance the capacity
of cellular networks. Initially, CDMA was regarded with skepticism, but
it was a skepticism mingled with excitement-an excitement that derived
from two main factors.
First there was the promise of high capacity, and hence
a reduction in the number of base stations, compared to the existing analog
or proposed TDMA systems. The second factor was the inherent CDMA capabilities
that on one hand make the design and optimization of the network seem
less involved than that of other systems, and on the other make the real-world
operation of the network less complicated. These capabilities are:
o frequency reuse: all users on a CDMA carrier transmit
over the same RF channel, eliminating the challenging tasks associated
with frequency planning;
o cell breathing: as the traffic loading in a CDMA cell increases, the
cell will automatically shrink in size while surrounding cells expand
to compensate for the resulting drop in the coverage area;
o soft capacity limit: as a cell shrinks to its minimum size allowed by
the infrastructure, it can still accommodate more users with a graceful
degradation in call quality; and
o soft handoff: a user may enter handoff with another cell and continue
to be connected to the original cell for some time before dropping it.
This reduces dropped calls due to handoff failures, eliminates 'ping-pong'
effects and also improves system capacity and call quality while in soft
handoff when the links act as a form of diversity.
Today numerous IS-95 based CDMA networks, now known as cdmaOne
systems, are operational worldwide, and most of these expectations
have proved true. However, the new systems brought with them
a whole new set of challenges for cellular engineers involved
in designing, and optimizing the performance of, cdmaOne networks.
The major tasks are:
o maximizing system capacity;
o maximizing system performance as it applies to improved call quality,
sufficient cell coverage and minimum dropped calls; and
o minimizing cost (both deployment and operational).
Below, we discuss some of the principles and challenges associated
with the design and optimization of cdmaOne cellular networks.
It is assumed that the reader is familiar with the basic principles
of these systems.
Frequency reuse efficiency and cdmaOne capacity
cdmaOne systems employ universal frequency reuse where all
users on a cdmaOne carrier transmit over the same RF channel.
Along with this simplicity of frequency planning come the
challenges of designing the network and setting the system
parameters to optimize the frequency reuse efficiency. Simply,
the capacity of a cdmaOne cell drops as adjacent cdmaOne cells
are deployed. The reason for this is that the users of these
cells are sharing the same RF channel and thus increasing
the noise floor seen at the receiver. In the absence of signals
from other cells, the noise floor would only result from users
within the cell; in this case, the cell capacity is at a maximum.
This represents a cell with a frequency reuse efficiency of
100 per cent. As signals from other cells share the RF channel,
the frequency reuse efficiency drops in accordance with the
amount of interference they contribute as compared to the
interference from within the cell. This is described in more
For a single, isolated cdmaOne cell, the noise floor seen
at the receiver is the summation of the background noise,
control channel power and traffic channel power of users within
the cell. The cell capacity in this case is limited (soft
limit) by the noise floor and can be determined by solving
for N users in the equation below. This equation applies to
the cdmaOne reverse link with power control. A similar equation
can be derived for the forward link:
Desired SNR = S / [(N-1) v S + No] = S / [Isc + No]
Isc = Interference power from users within the cell
S = Received signal power, assumed equal for all users due to power control
v = Voice activity factor
No = Background noise power
Now, for a multi-cell network, the noise floor is increased
by the interference received from users in other cells, Ioc, and the capacity
Desired SNR = S / (Isc + Ioc + No) and the frequency
reuse efficiency is, by definition:
f = Isc / (Isc + Ioc)
Notice here that the SNR, and cdmaOne system capacity, drop
as a function of Ioc; the challenge is thus to design cell
site locations, antenna orientations and system parameters
to minimize Ioc. This does not mean, however, that Ioc can
be minimized to zero by isolating the cdmaOne cell. The reason
for this is that in cdmaOne some overlap between cells is
needed to allow soft handoff and hence a minimum value of
Ioc would always be present at the receiver. Therefore a compromise
between cell coverage and soft handoff parameter settings
needs to be achieved to minimize Ioc.
It is also noteworthy, looking at the second equation above,
that the desired SNR and system capacity improve with a drop
in the total interference seen at the receiver. That brings
us to one of the major inherent capabilities of cdmaOne: a
technique that results in a reduction in system interference
translates directly into an improvement in system performance
and capacity. This is the principal behind the improvements
in cdmaOne systems performance when deploying interference
cancellation techniques, antenna sectorization, smart antennas,
power control and more.
Soft handoff and handoff parameters optimization
In a cdmaOne network, a mobile station (MS) travels between
the coverage areas of multiple cells and constantly monitors
the strengths of the pilot channels. Following IS-95 protocols
the MS is expected to enter handoff when it detects a pilot
Ec/Io that exceeds T_add and similarly drops a handoff link
if the pilot Ec/Io drops below T_drop ( these being the threshold
level settings for adding or dropping a soft handoff). The
process of adding or dropping a handoff link does not take
place instantaneously, but rather following exchange of some
messaging with the base station and after waiting for a pre-defined
time threshold to verify the validity of the decision.
The set of thresholds associated with the handoff process represents
some of the most challenging parameters to be set for a cdmaOne
network. For instance:
o T_add must be set low enough (~ -16 dB) to ensure that handoff is initiated
and completed before the signal strength of the original link drops below
accept able signal quality. On the other hand, T_add must be set large
enough (~ -12 dB) to ensure that handoff does not take place too early,
resulting in inefficient utilization of channel elements at the cell site.
Therefore, a compromise between the percentage of MSs in handoff and the
utilization of channel elements needs to be achieved. It is proposed that
on average about 30 percent of the MSs be in soft handoff utilizing two
channel elements at different cell sites.
o T_add and T_drop must be set such that an MS does not enter in and out
of handoff very frequently. This results in excessive messaging and data
processing at both the mobile and base station.
o T_Tdrop must be set to avoid unnecessary dropping out of handoff. This
may occur in cases when the MS travels through fast variations in signal
strength due to shadowing.
o link balancing: as the cell loading varies, the reverse link will vary
in size accordingly. It is required that the
forward pilot link follows to maintain link balance. This can be achieved
by varying the pilot power or varying the handoff thresholds so that the
handoff window will move accordingly. This also works to avoid creation
of what is known as an 'orphan'-a cell with no soft handover into adjacent
In order to reduce unnecessary handoffs, IS-95B introduces
a new dynamic threshold on top of T_add and T_drop; it is referred to
as T_Dyn-add and T_Dyn-drop and varies based on the current Active Set.
Here an MS would wait for the dynamic threshold to be satisfied before
sending the PSSM message.
Frequency planning and hard handoff
In a cdmaOne network, the total allocated spectrum is divided
into multi-cdmaOne carriers (1.25 MHz for IS-95 systems).
Initially, the system is deployed with one carrier and users
are assigned to that carrier. As the networks mature and the
demand for capacity grows, operators are faced with deploying
a second, a third, or even more carriers. It does not make
economical sense to deploy multi-carriers through out the
whole network; instead, additional carriers can be deployed
in areas where the capacity exceeds that capable of being
supported on the existing carriers deployed at an acceptable
level of quality.
The challenges associated with deployment of multi-carriers
are related to the handoff of MSs between carriers as they
cross from a multi-carrier region to another or vice versa.
In cdmaOne systems, this is referred to as hard handoff between
cdmaOne carriers. It is required that an MS entering/exiting
a multi-carrier region determines the available carriers,
initiates and completes the handoff process in time in order
not to drop the call. The system data messaging and processing
associated with this process can be extensive.
Many techniques are employed by different infrastructure
manufacturers to accomplish smooth hard handoff. One technique employs
what is referred to as beacons. These are sectors at the boundary region
transmitting only the pilot signal at the other unavailable carrier frequencies.
The MS, with the help of the infrastructure, would then switch carriers
Other techniques propose deploying sectors pointing in
opposite directions at the boundaries of the region. By smart-guessing
the direction in which the MS is traveling, the infrastructure decides
whether or not to instruct the MS to switch carriers. For example, if
the MS is determined to be traveling outside of a multi carrier region,
it is instructed to switch carriers.
In cdmaOne systems, every sector is constantly transmitting
its pilot signal as a reference for its MSs to maintain synchronization
with the sector. In addition, MSs measure and compare pilot
signal strengths to determine potential handoff candidates.
A problem arises when an MS is in an area where it can see
many strong pilots. Currently, cdmaOne MSs can serve up to
three pilots; this causes the extra pilots to act as strong
interference or pilot polluters which can lead to reduced
system performance in the form of dropped calls and inferior
call quality in addition to reduced system capacity.
Some of the techniques one may employ to reduce the effects
of pilot pollution are:
o maintain link balancing by reducing pilot power as the reverse link
shrinks due to an increase in loading;
o antenna downtilting and rotation; and
o lowering base station heights.
PN offset planning
In cdmaOne systems, all sectors employ the same PN sequence
on the forward link-each, however, with a different time shift
from the other. This time shift is referred to as the PN offset
associated with that sector. All cdmaOne transmissions start
at the same instant of time referenced to GPS time. Signals
from the different sectors are received at an MS out of sync
and the MS needs to only synchronize to its intended sector's
PN offset to demodulate its intended signal.
However, due to multipath propagation, variations in
propagation delays and propagation loss, three potential problems need
to be taken into account in network planning and performance optimization:
o adjacent offset interference: this occurs when two pilots with adjacent
PN offsets, due to propagation delay difference, arrive at the MS within
a short time shift from one another (within the MS search window). This
causes the MS to think they both are a result of multi-path of the same
signal and hence combines the two signals leading to a corrupted signal;
o co-offset interference: this occurs when two pilots that are assigned
the same PN offset but located at a large geographical distance from one
another are received at the MS within the search window. This occurs if
the undesired signal is strong enough such as having line of site to the
MS. Similar to adjacent offset interference, the receiver would attempt
to combine the two signals leading to a corrupted signal; and
o neighbor ambiguity: This occurs when an MS tries to measure the pilot
strength of a sector in the neighbor list at a time that another pilot
assigned the same PN offset is received strongly at the MS. The MS would
then report the pilot strength of the wrong sector and then be instructed
to enter handoff with it while it is not the intended sector.
Therefore, in developing a PN offset plan, the designer
needs to factor in the effects of propagation delays and signal propagation
loss to minimize the potential problems described above.
These, then, are some of the network planning challenges
offered by cdmaOne networks today. But the standard is still
developing. Currently, an IS-95-based standard is being proposed
to IMT-2000 for third generation personal communications systems.
3G systems will support many advanced applications and services
o low-speed voice to high-speed data services ranging from
8kbit/s, 144 kbit/s, 384kbit/s, up to 2mbit/s ;
o packet-switched and circuit-switched services;
o short message services, emails, internet applications, and
With these applications come new challenges to CDMA-based
o faster and more accurate power control algorithms on the
reverse link and the forward link;
o improved mobile station receivers combining more resolvable
o coherent reverse link demodulation techniques;
o multi-carrier planning of wideband width 3G systems overlaid
on 2G systems; and
o different design requirements for voice and multi-rate data
Clearly, as we move from primarily voice-based services to
the true multimedia cdmaOne environment, the challenges of
effective planning and optimization will be increased. Products
such as MSI's Planet enable operators to model and understand
the complex trade-offs necessary to achieve such optimized
performance, and further enhancement of MSI products will
help to ensure that the needs of the CDMA technology networks
of the future will also be fully addressed.