TEAM FLY WIRELESS NETWORK DEPLOYMENTS phần 3 ppsx

42 Chapter 3

interference, creating dominant servers, managing handoff activity, and

handling non-uniform and time-varying traffic distributions. It also provides

the ability to decouple the analog and digital sector configurations.

With smart antennas, a single physical array antenna can be used to

synthesize completely different sector configurations for the digital and

analog services. As the following sections will illustrate, there are strong

theoretical and practical reasons that optimum CDMA sector settings are

much different from optimum analog configurations; for example it may be

desirable to implement CDMA as a 6-sector configuration while maintaining

an underlying analog 3-sector network. Smart antennas enable such

flexibility in deployment and optimization, while sharing a common antenna

array for both analog and digital services, or among multiple digital services

(e.g. vehicular voice, high rate data service, wireless local loop and private

networks).

In cellular systems where antennas are shared between analog and

CDMA, service providers are forced into fixed grid patterns due to the

underlying frequency reuse assignments of the analog network. Without a

smart antenna system, azimuth pointing angles of the sectors are locked into

a rigid hexagonal grid pattern which forces all alpha, beta and gamma

sectors—both analog and CDMA—to be aligned across the network.

However, since CDMA is based on unity frequency reuse, there is no need to

maintain a rigid grid pointing pattern across the entire CDMA network.

2.2 Traffic Load Balancing

Statistics derived from commercial cellular and PCS networks

consistently indicate that traffic loads are unevenly distributed across cells

and sectors. In other words, it’s quite common for a cell to have a single

sector near the blocking point, while the cell’s other two sectors are lightly

loaded. Traffic data from a number of cellular and PCS markets show that on

average the highest loaded sector has roughly 140% of the traffic it would

carry if all sectors were evenly loaded. By contrast, the middle and lowest

loaded sectors have 98% and 65% of the traffic relative to a uniformly

loaded case. Even though some sectors in a network may be blocking,

significant under-utilized capacity exists in other sectors. The objective of

traffic load balancing is to shift excessive traffic load from heavily loaded

sectors to under-utilized sectors. The result is a significant reduction in peak

loading levels and, hence, an increase in carried traffic or network capacity.

At a coarse level, static sectorization parameters can be adjusted for load

balancing based on average busy hour traffic distributions. For optimum

control of peak loading levels in time-varying traffic conditions, dynamic

adjustment of sector parameters can be used employed on real-time

Smart Antennas 43

measurements of traffic and interference. Under dynamic control, network

parameters (neighbor lists, search windows, etc.) must be adjusted to support

the range of dynamic sectorization control.

Both network simulation and experimental field results confirm that

traffic load balancing can reduce peak loading levels, and thus minimize air

interface overload blocking. An example presented in [5] describes a traffic

hotspot scenario in which 54% of subscribers achieved acceptable service

prior to smart antennas, while 92% of the subscribers obtained good service

with smart antennas because of the ability to change sector azimuth pointing

angles and beamwidths. Extensive commercial deployment results show an

average 35% reduction in sector peak loading with sector beam forming,

combined with additional benefits from handoff management and

interference control.

2.3 Handoff Management

Cellular service providers often have an extremely difficult time

controlling handoff activity. In CDMA networks, some level of handoff is

desirable due to gains associated with the soft handoff feature (soft handoff

allows the subscriber units to be simultaneously connected to multiple

sectors). However, too much handoff can extract a significant performance

penalty from the network. The penalty includes an increase in the total

average transmit power per subscriber, which wastes valuable linear power

amplifier (LPA) resources at the cell site, increases forward link interference

levels and decreases forward link capacity accordingly. Excessive handoff

activity can also result in dropped calls due to handoff failures.

For optimum forward link capacity, CDMA network operators strive to

tightly manage the amount of handoff activity. Typical networks may run at

handoff overhead levels between 65% and 100% (i.e., 1.65 to 2.0 average

handoff links per subscriber. Smart antennas can be used to manage handoff

activity by controlling the RF coverage footprint of the cell site to tailor

handoff boundaries between sectors and cells, and reducing rolloff of sector

antenna pattern. Figure 1 (left) illustrates the radiation pattern from a smart

antenna versus an off-the-shelf commercial sector antenna. With smart

antenna arrays, it is possible to synthesize radiation patterns with sharp

rolloff (i.e. steep transition out of the antenna’s main lobe) in order to reduce

handoff overhead, while still maintaining coverage. Sector patterns

synthesized from the phased array antenna can be much closer to an ideal

sector pie-slice or conical pattern. Commercial deployment results

demonstrate that smart antennas can reduce handoff overhead by 5 to 15%,

thus increasing forward link capacity by an equivalent amount.