Phased-Array Smart Antennas Increase Capacity in CDMA Networks
Multibeam antennas offer service providers the ability to create completely different sector mappings for CDMA and AMPS service while using the same antenna array.
By: Martin Feuerstein, VP of Product Management and Advanced Technology, Metawave Communications
Contents
What Is A Smart Antenna?
Sector Mapping
Capacity Gains
Reduced Handoff Overhead
Urban Settings
Future Advances
Multibeam smart antenna systems, long known to provide interference reduction benefits in analog networks, are now being used to increase capacity in CDMA cellular networks. For CDMA networks, non-invasive smart antenna add-ons, or appliqués, can play valuable, and in many ways non-traditional, roles in improving network performance and capacity. These non-traditional applications of phased-array antenna deployments involve network-wide interference control, traffic load balancing, handoff management, and optimum resource allocation.
The design of multibeam smart antennas for CDMA applications is non-traditional in that it does not attempt to create an optimum antenna pattern for each traffic channel. Rather, the system uses beam-forming algorithms to synthesize optimum antenna patterns on a per-sector basis; each CDMA sector is assigned radiation patterns exhibiting different beamwidths, azimuth pointing angles, and customized sculpting characteristics. This approach integrates easily with existing smart antenna architectures for analog FM frequency division multiple access (FDMA) services, such as AMPS, enabling the development of dual-mode CDMA/AMPS smart antennas that complement the hybrid digital/analog networks prevalent today.
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What Is A Smart Antenna?
A multibeam smart antenna is a highly configurable antenna system that senses the radio environment, including traffic and interference levels, and then alters its operation to optimize performance for specific local conditions.
The fundamental element of a multibeam smart antenna system is a phased-array antenna that creates 12 narrow beams (although other options are possible). These narrow beams, either individually or in combination, are intelligently exploited by the system to reduce interference in both analog and digital networks.
The smart antenna system connects to CDMA and AMPS/narrowband AMPS (N-AMPS) base stations as a non-invasive overlay. From an operational perspective, the functions of the smart antenna are transparent to the cell site radios and to the rest of the network, including the switch.
In the CDMA operating mode, a multibeam smart antenna makes use of the phased array to create custom sector antenna patterns through a process known as sector synthesis. Under software management, sector synthesis allows independent control of sector azimuth pointing angles, sector beamwidths, and per-beam gains, providing operators with unprecedented flexibility for fine-tuning a cell site's sector configuration and footprint
The software-defined smart antenna allows engineers to tailor sector radiation patterns to a cell's traffic patterns, topology, and RF environment. For example, Figure 1 shows a three-sector cell with a 60 deg. beamwidth on one sector, 90 deg. beamwidth on another, and a custom pattern with a 4 dB per-beam notch on the third sector. To adjust this pattern, the RF engineer simply downloads a new configuration file, updating the system either at the cell site or from a remote location such as the switch or network operations center.
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Sector Mapping
Smart antennas offer service providers the ability to create completely different sector mappings for CDMA and AMPS service while using the same antenna array. Operators can select different sector beamwidths and azimuth pointing angles for digital and analog signals, thereby improving the capacity and performance of their CDMA systems while reducing antenna clutter and loading on the tower.
Figure 2 shows a block diagram of a CDMA phased-array smart antenna system. The antenna system's "smarts" reside in the spectrum management units (SMUs), which contain the control algorithms and phased-array interface functions and perform the core processing in the system.
Each SMU is an intelligent unit designed to make real-time decisions about optimum mappings between the phased-array antenna and the cell site. Each routing decision is implemented within a multi-dimensional RF switch and vector control matrix. The system also includes a pooled matrix of multicarrier linear power amplifiers (LPAs), low-noise amplifiers (LNAs), filters, and a transmit combiner/driver assembly. The entire smart antenna system is administered and monitored through a computer interface that allows static or dynamic parameter control, as well as monitoring of real-time system performance.
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Capacity Gains
The chief advantage of smart antennas is their ability to increase the capacity of individual CDMA cell sites by up to 40% per CDMA carrier frequency. Smart antennas achieve these capacity gains through two primary benefits: balanced traffic loading and reduced handoff overhead.
Balanced Traffic Loading
Statistics derived from commercial cellular networks indicate that traffic loads are unevenly distributed across cells and sectors (Figure 3). After analyzing traffic data from a number of cellular and PCS markets, we find that on average the most heavily loaded sector has roughly 140% of the traffic it would carry if all sectors were evenly loaded or "balanced." By contrast, the two more lightly loaded sectors have 98% and 65% of the traffic relative to a uniformly loaded case.
Due to these traffic load imbalances, significant under-utilized capacity exists in some sectors even while other sectors are blocking. The objective of traffic load balancing is to tap into this extra capacity by shifting excessive traffic load from heavily loaded sectors to under-utilized sectors.
Smart antennas support this objective by allowing operators to configure sector patterns that result in relatively equal sector loading levels. For example, by adjusting sector point angles, an engineer can split a hot spot between two or more sectors, making more effective use of a site's available capacity than if the hot spot were served by a single, capacity-strained sector (Figure 4). Load leveling with smart antennas reduces the blocking rate at a given level of carried traffic, which translates into increased cell site capacity.
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Reduced Handoff Overhead
In CDMA networks, some level of handoff is desirable, but too much handoff can extract a significant performance penalty from the network. Excessive handoff results in higher transmit power requirements, more interference, increased likelihood of dropped calls, and most importantly, degradation of forward link capacity.
For optimal forward link capacity, CDMA network operators strive to tightly manage the amount of handoff activity. Typical networks run at handoff overhead levels between 85% and 100% (i.e., 1.85 to 2 average handoff links per subscriber). One key to controlling handoff overhead is the sector antenna pattern itself. The beamwidth and rolloff characteristics of the radiation pattern play a critical role in determining the amount of soft/softer handoff in the network because they affect the size of intra- and inter-cell handoff regions.
With phased-array smart antennas, it is possible to synthesize sector radiation patterns that have a sharper rolloff characteristic than conventional off-the-shelf sector antennas (Figure 5). The sharper rolloff provided by smart antennas allows operators to maintain optimal coverage while reducing handoff overhead. In addition, operators can use the system's per-beam gain feature to fine-tune sector coverage in 30-deg. increments.
When using phased-array smart antenna approaches, transmit power can be turned up in specific directions to enhance coverage in traffic hot spots and inside building, or to create dominant servers in multiple pilot regions. In other directions, transmit power can be reduced to minimize interference, control handoff activity, or tame severe cases of coverage overshoot.
Using synthesized phased-array patterns to control sector footprints is significantly more flexible than the alternatives of employing antenna downtilts or adjusting sector transmit powers—adjustments that impact the entire coverage area of the sector rather than confining changes to the specific problem area.
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Urban Settings
The benefits of traffic load balancing and reduced handoff overhead were demonstrated in the following smart antenna deployment at a cell site in a major metropolitan network.
The cell's coverage area included several major freeways and commuter routes. Before installation of the smart antenna system, it was determined that the cell's three sectors—alpha, beta, and gamma—differed significantly in traffic loading. The beta sector, which provided service for two major highways, carried a much higher load than the alpha and gamma sectors. Its average load was calculated to be 140% of ideal, where 100% loading on all sectors is ideal (Figure 6).
Following deployment of the smart antenna, the azimuth of each sector was rotated 60° clockwise. This adjustment in cell sectorization split the high usage area between two sectors and brought all three sectors within 14% of ideal loading (Figure 7).
A comparison of switch statistics collected before and after load balancing was made. Despite carrying a slightly higher level of total cell site traffic, the smart antenna-equipped site experienced a 60% reduction in forward power overload control (a measure of blocking duration). In addition, peaking loading in the beta sector was reduced by approximately 20%, while handoff overhead for the cell site was reduced by over 7%. Based on these findings, it was calculated that the phased-array smart antenna system increased the site's CDMA capacity nearly 30%.
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Future Advances
How much intelligence or "smarts" can smart antennas provide? In the very near future, smart antennas will support dynamic traffic load balancing, automatically altering a site's sectorization in response to fluctuations in call volumes. As traffic levels change over time, smart antennas will resculpt and reconfigure the cell based on the RF performance and interference data logged by a bank of high-speed, frequency-agile scanning receivers.
Variations in the geographic distribution of traffic due to such factors as driving conditions, commuting patterns, or changes in usage patterns will trigger changes in a cell site's configuration so that the site's traffic-carrying capabilities are optimized at all times. In effect, dynamic load leveling will deliver capacity on demand, directing cell site capacity where it's needed, when it's needed.
For even greater capacity gains, you can expect to see smart antenna systems networked together to enable inter-cell coordination. Smart antenna-equipped cell sites will exchange information on interference levels, traffic loading, and other performance parameters, using an operator's T1 backhaul network for data transmissions. Imagine the ability for one smart antenna system to request that an antenna system on a neighboring cell contract coverage on one or more of its beams because the offending site is causing excessive interference. Inter-networking smart antenna-equipped sites will usher in a new era of intelligence in the wireless network.
Finally, smart antennas are emerging as a significant component of new wider bandwidth CDMA (W-CDMA) standards for third-generation (3G) mobile systems across North America, Europe, and Asia. The proposed 3G W-CDMA common air interface standards include built-in support for smart antenna systems.
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About the author: Marty Feuerstein is the VP of Product Management and Advanced Technology at Metawave Communications. He can be reached at 10735 Willows Rd. NE, P.O. Box 97069, Redmond WA 98073-9769 Tel: 425-702-5600, Fax: 425-702-5970.