【 01 INTRODUTIO 】
By the Growing number of smart phone users, the demand for year-on-year increases in voice and data capacity in cellular systems is a problem faced by every operator worldwide and it will not disappear with the advent of new technologies and spectrum. The extraordi-nary growth in wireless data traffic such as video conferencing, media streaming and mobile-TV continually increase the demand for capacity in the cellular.
Especially for the stadium, square and other places which are crowded with smart phone users, the demands for capacity in voice and data are extraordinary. To request more capacity, the service providers are exploring several ways to expand the network capacity.
1） Adding Cell Sites is an effective but expensive approach to add capacity. In general adding new real estate is time consuming and increasingly prohibitive. With median inter-site distances dropping from 5km to 2km and recently to less than 200m in dense urban areas, the operator has less choice in selecting affordable property. Doubling the number of cell sites approximately doubles the network capacity and the throughput per user (assuming the user density stays constant), and greatly improves the peak user and the aggregate throughput per km2.
2）Adding Carriers (or more accurately, bandwidth) directly adds to capacity. The LTE standard is particularly adept at utilizing increased bandwidth. In addition, in the USA, the FCC permits increasing radiated power with the bandwidth in the PCS, AWS, and lower 700 MHz bands providing improved penetration and coverage. Doubling bandwidth at least doubles throughput.
3）Reducing noise. In 3G and 4G LTE networks, noise containment in the RF path is critical. External noise from a variety of sources — including multi-path reflection, environmental noise and interference from adjacent or nearby cells — can significantly decrease receiver sensitivity at the base station. As noise within the sector increases, mobiles increase their signal power levels, creating more uplink interference. Noise within the RF path is also problematic, with thermal noise and passive inter-modulation (PIM) being the major culprits.
4）Increasing frequency reuse. Another way to increase the capacity is to create more opportunities for frequency reuse through higher order sectorization.
5）Among a number of strategies shown above, the traditional way of adding cells and purchasing of additional spectrum both presents significant cost and time issues. Normally, site acquisition and site construction can be taken up to 2-3 years. And the total cost is more than 0.2 million USD due to the acquisition, construction and commission. And for adding more spectrums, if it is available, can be easily cost billions of dollars.Small cell deployment is also being touted as an excellent way to add network capacity. However, it could not satisfy service providers’ immediate need for more capacity.
【 02 TRADITIONAL SECTORIZATION 】
In order to avoid interference in channelized systems, frequency channels need to be separated geographically. With sectorization the number of sectors (cells) subject to interfer-ence is reduced as the power of the sector focused forward.
In the last 50 years, wireless capacity has increased by a factor of about 1,000,0006. This growth has come from better spectral efficiency, more spectrum and more cells/sectors. Since the 1990s, one of the most popular and effective strategies for increasing site and network capacity has been sectorization.
The first sectorized systems replaced standard 360-degree omni-directional antennas with three separate directional antennas. The most commonly deployed configuration uses three antennas, each with a nominal azimuth beamwidth of 65-degrees. While the antennas within a sectorized cell share a common base transceiver station (BTS), each is managed and operated independently with its own power level, frequencies and channels.
The use of three directional sector antennas versus one omni-directional antenna substantially reduces co-channel cell interference and triples the opportunity for frequency reuse. As a result, the service providers realize significant gains in capacity.
【 03 HIGH ORDER SECTORIZATION 】
More than ten years ago, the service providers began to explore the capacity potential with higher order sectorization, which is splitting the conventional three-sector system into six-sector system. A six-sector deployment is using two 33° narrow beams to replace one 65° beam sector antenna. Due to the narrow azimuth beamwidth, higher order sectorization reduces not only the overlap interference, but also the soft hand-off area to improve the frequency reuse efficiency.
Comparing the 33° narrow azimuth beam antenna with normal 65° sector antenna, the narrow azimuth beam antenna provides rapid roll-off pattern, better sidelobe and backlobe suppression. Moreover, when a 120 ° sector is split into two small sectors equally by replac-ing one normal sector antenna with two narrow beam antennas, two split sectors can be con-trolled independently to optimize the network and customize the footprint of the cell site.
Figure 1 illustrates the significant reduction of inter-sector overlap in switching from a 65-degree to a 33-degree antenna. Reducing the overlap decreases the soft handoff area and provides additional capacity gains.
For the six-sector deployment, two crossover level points between split sectors can be optimized to -6 to -10 dB, which is good crossover level to handover between sectors for 2G, 3G and LTE systems.
【 04 THE DRAWBACK OF HIGHER ORDER SECTORIZATION BY 】
The traditional way of deploying six-sector site is to replace three 65° sector antennas with six narrow beam panel antennas, which means double the number of antennas when using higher order sectorization. The increasing number of antennas means double the costs including purchasing, packaging, transportation and deployment cost.
Normally, a 33° narrow beam antenna is much wider in size than a 65° panel antenna due to its physical requirement for an additional antenna array to provide a narrower azimuth beam width. So the wider in size creates a lot of visual impact for the site. Moreover, the large surface of panel antenna creates significantly more wind loading comparing with the 65° panel antenna. Besides that, the larger narrow beam antenna also adds more weight to the tower than the 65° panel antenna.
Another issue by using two narrow beam antennas to replace the 65° panel antenna is alignment errors during deployment. Two narrow beam antennas need to be aligned precisely to replace a 120° sector for optimizing overlapped area, intra-sector interference, and crossover level points.
For the reason mentioned above, the six-sector site by using narrow beam antennas is not gained much attention in telecom market.
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