Capacity Smarts

Many cellular networks already are exceeding capacity limits in today's predominately voice environment. The added pressure of additional voice traffic and new data services only will exacerbate the difficulty. Migrating to 3G technology will help, as 3G provides improvements in capacity. However, 3G technologies also enable increased user data rates, thus allowing a single user to extract a larger fraction of available network resources. Hence, the high-rate data services enabled by 3G may actually contribute to capacity problems.

In today's CDMA networks, adding network capacity traditionally is done by adding additional carrier frequencies or increasing the cell-site density in the network. Adding carrier frequencies has associated costs and deployment difficulties. Issues with the reliability of carrier-to-carrier hard handoffs force operators to deploy additional carrier frequencies in clusters or, in some cases, network-wide. Such a deployment is both costly and time consuming, especially considering that the network-capacity issues typically are in strategic hot spots of the network. Further, the addition of carriers simply is not possible when available spectrum is exhausted.

More cells can be added to networks to offload traffic. However, the logistics and additional maintenance costs of new sites often make this solution unattractive, if not impossible. Further, in dense urban areas cell radii already may be so small that cell splitting often is not an alternative.

Smart Antennas in CDMA

Smart antennas are another technique for increasing capacity in CDMA networks. Smart antennas have been in existence for considerable time in military applications, but only in recent years have they gained acceptance in the commercial wireless industry. Current CDMA smart antennas use sector synthesis (or equivalently, sector beamforming) to balance traffic across sectors, fine-tune coverage areas and reduce handoff overhead leading to an increase in the sectorization efficiency. This approach of synthesizing sectors using an antenna array can be characterized as beamforming across an aggregate of users as opposed to beamforming on a per-user basis. The advantage of such an approach is that the beamforming is done at the RF level and, therefore, the architecture can be designed as a non-invasive add-on to an existing base station. This type of smart antenna then can support the existing base of deployed cell sites without significant modifications to the base-station hardware.

Figure 1 on page 32 shows the cell-efficiency improvement offered for a specific network-deployment example using the sector-synthesis smart antenna. This figure plots the cell-site traffic loading, as measured in primary code usage in Erlangs, against the overload blocking duration in seconds (out of a 1-hour = 3,600-second period). Primary code usage implies that only a single Walsh code is counted for each call despite the fact that multiple codes might be used due to soft handoff. Overload blocking duration is the duration that the forward-link power exceeds a blocking threshold and is considered a grade-of-service (GOS) measure. The scatter plot in the figure shows with the smart antenna, the cell site exhibits lower levels of overload blocking for a given amount of carried traffic compared to the case without the smart antenna. Or conversely, for a given level of overload blocking (i.e. GOS), the smart-antenna-equipped cell site carries more traffic. The smart antenna increases the capacity by carrying more traffic at a given GOS, or by improving the GOS at a given traffic load.

Six-Sector Applications

Further, the flexibility of the smart-antenna platform permits migration from 3- to 6-sector applications. Smart antennas use antenna arrays, which can form narrow sectors providing excellent sector-to-sector isolation. The antenna array provides extra degrees of freedom over a traditional antenna that yields increased optimization flexibility. This helps mitigate interference problems that have limited 6-sector applications in the past.

Figure 2 displays the performance of a cell under 3-, 4-, 5-, and 6-sector configurations with the smart antenna. The cell under study is configured so that all calls are carried on the first CDMA carrier frequency. When this carrier is full, new calls overflow to the second CDMA carrier. Thus, all calls that originate on the second carrier would have been blocked if only a single carrier were present.

A blocking rate is defined as the percentage of calls originating on the second carrier over the total number of calls. The baseline case represents performance with a traditional 3-sector configuration. As the plot indicates, increasing sectorization with the smart antenna increases capacity by improving the GOS at a given level of offered traffic.

To increase capacity further, the smart-antenna processing can be moved from the RF domain down to the baseband digital domain. This allows beamforming to be done on an individual traffic channel vs. the sector synthesis described previously.

However, on 2G CDMA systems, the handset requires that the pilot channel be used as a coherent source for demodulation. This presents a challenge as, ideally, the pilot channel would be broadcast using an antenna-array response producing a wide beamwidth (nominally 120 degrees for a 3-sector cell) while the traffic channel would use an antenna-array response producing a narrow beam-width. This yields a phase mismatch, which, if severe enough, will erode any gains associated with using a narrow beam. However, through accurate angle-of-arrival information and careful narrow-beam and wide-beam design, the coherence between the traffic and pilot channels is maintained. Field-trials for this approach demonstrate a 6.4dB reverse link gain and a 6.7dB forward link gain using a 16-element circular array.

3G systems attempt to solve this by providing auxiliary pilots, which are used in addition to the common pilot to provide a phase reference. They may be used in either a dedicated fashion or to form spot or fixed beams within a sector. This allows a traffic channel to use a narrow beam with an auxiliary pilot as the phase reference. However, the cost of the auxiliary pilot is additional power and network complexity. The same approach applied in 2G networks can be applied to 3G networks, eliminating the need for auxiliary pilots and maximizing the performance provided by a smart antenna.

By providing smart-antenna solutions for both 2G and 3G CDMA air interfaces, you can add capacity in all configurations. This is important because the existing base of cdmaOne handsets likely will be in use for many years and will aid the migration process from 2G to 3G networks.

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Feuerstein (martyf@metawave.com) is Metawave senior vice president & general manager, embedded data products.