Even with the current economic climate, wireless operators continue to invest in their infrastructure. For example, AT&T recently announced a $1 billion investment. While the timing of upgrades to 4G technologies such as Long-Term Evolution (LTE) are the subject of debate, the demand for data services is driving coverage fill-in for existing 3G networks.

Anyone who has ridden on a train in Tokyo, where it is socially forbidden to talk on the phone, has witnessed the explosion of data services. Instead of using voice services, Japanese consumers furiously text, play games, and even watch TV on their mobile phones while riding on trains. This scene is repeated in many other dense urban population centers throughout Asia.

To meet these demands, Asian operators are rapidly deploying wireless repeater technologies to expand the coverage of their 3G data services. ABI Research predicts that revenues for distributed antenna systems and in-building wireless systems will exceed $15 billion by 2013.

The Infrastructure

Distributed antenna systems, in-building wireless systems, and wireless repeater systems generally share similar characteristics. Each typically connects to a wireless basestation, often a picocell, via RF. The systems distribute the RF signal to numerous antennas throughout the coverage area via fiber-optic, coax, or even CAT-5 cable. These systems can be completely analog, completely digital, or a hybrid of both.

In the U.S., in-building wireless systems predominate and are sold to the owners and operators of large venues such as airports and convention centers. These venue owners deploy distributed antenna systems throughout their venue and then charge fees to mobile operators to connect.

With mobile phones obviating the need for payphones within these venues, the venue owners view these connection fees as a way of making up for lost payphone revenues. In contrast, distributed antenna systems and wireless repeater systems are sold directly to the mobile operators, who view these systems as a means of gaining a competitive advantage in terms of coverage.

The requirements for distributed antenna systems and wireless repeaters vary depending on how they are deployed. When deployed by the venue owner, the distributed antenna system must be able to transport the entire 800/900-MHz, 1800/1900-MHz, and 2100-MHz bands, requiring bandwidths of 60 to 75 MHz per band.

Because multiple mobile wireless technologies may be employed within each band, the analog performance requirements are high, especially when GSM technology is involved, with its extreme blocker specification. For wireless repeater systems, which are typically sold to a single operator, the bandwidth requirement is limited to that operator’s licensed bandwidth, which is 20 MHz or less.

In addition, these systems typically carry a single type of wireless technology within a frequency band (e.g., four W-CDMA carriers within a 20-MHz band) with a common subscriber power control technique. Hence, the lower bandwidths and smaller variation in input signal levels reduce the analog performance requirements of the single-operator repeater system.

Achieving Performance

The digital bit rates required to transport these sampled signals drive fiber-optic transport costs. Achieving the analog performance of a distributed antenna system with 75-MHz bandwidth with a high-IF sampling radio architecture requires 14-bit analog-to-digital converters (ADCs) with sample rates over 180 MHz. The bit rate of a dual-band system requires 6.3 Gbits/s (180 Msamples/s/band • 14 bits/sample • two bands / 8B/10B). This data rate requires expensive FPGAs and small form-factor pluggable (SFP) fiber-optic transceivers.

Wireless repeater systems that serve a single operator have more economical fiber transport costs. The lower analog performance requirements and 20-MHz bandwidth can be met with 12-bit ADCs operating around 60 Msamples/s. Hence, a dual-band system can fit easily within the bandwidth of 2.5-Gbit/s optical networks (60 Msamples/s/band • 12 bits/sample • two bands / 8B/10B = 1.8 Gbits/s) and serializer-deserializer (SERDES) interfaces available on lower-cost FPGAs.

In both cases, a novel signal compression technology has the potential to reduce the cost of digital transport. For distributed antenna systems, compressing the data rate by 1.5:1 would reduce the bit rate to less than 4.25 Gbits/s, enabling the use of fiber-optic transceivers that have been commoditized by the storage-area networking (SAN) market. For wireless repeater systems, the same compression ratio of 1.5:1 would reduce the data rate requirements to 1.2 Gbits/s, enabling the use of low-cost CAT-5 twisted pair cabling, eliminating the need for fiber optics altogether for distances less than 100 m.

Signal compression technology can play a key role in reducing the cost of deploying distributed antenna systems and wireless repeater systems. The data rate requirements for each system can be reduced to lower or even eliminate the fiber-optic transport costs. By lowering the acquisition costs of distributed antenna systems and wireless repeater systems, they can deliver their promise of providing coverage fill-in for data services.