What are your choices? Comparing broadband wireless options

Over the last several years, considerable work has been done toward creating wireless standards for delivering broadband wireless access. This article will look at different standards and discuss how they can be used in those networks.

The IEEE 802.11 standard was designed primarily for indoor wireless LANs operating in the 2.4-GHz ISM band. It was initially released with three physical layer techniques: direct sequence, frequency hopping and infrared. One extension to the standard, 802.11b, supports data rates of 5.5 and 11 Mbits/second using complementary code keying (CCK) modulation while maintaining backward compatibility with the 1- and 2-Mbit/s rates of 802.11. Another extension to the original 802.11 standard, 802.11a, uses an entirely different physical layer using OFDM. The 802.11a standard supports data rates from 6 to 54 Mbits/s and operates in the 5-GHz band.

Let's focus on 802.11b — since the 2.4-GHz band offers more favorable propagation characteristics for wide area deployments — and examine how 802.11b equipment can be modified to meet the needs of outdoor applications.

Recently, 802.11b has gained popularity providing high-speed Internet service in hotspots. Each hotspot is connected to the Internet via a backhaul network. Users wanting to access the Internet can plug low-power PCMCIA cards into their laptops. The cards can communicate with 802.11b access points. Access points can serve cell sizes up to 300 meters under optimal conditions, although typical indoor hotspots are usually much smaller.

To extend the cell sizes to address wide-area broadband wireless access networks, a number of vendors have packaged standard 802.11b products for outdoor use together with high-gain directional antennas at the customer-premises equipment (CPE). High-gain outdoor antennas provide additional system gain and also eliminate the signal attenuation that results from in-building penetration. The resulting link budget improvements allow the range of the access point to be extended to several kilometers. But changes are usually required to the standard 802.11 MAC layer, which was not intended to deal with these longer distances. The bit rate decreases with distance from 11 Mbits/s close to the access point to 1 Mbit/s at the edge of a cell. The published data rates for 802.11b are the aggregate radio bit rates that include all of the traffic, management and overhead in both directions on the radio link. The throughput is typically 60 to 75 percent of the total bit rate, which is then shared between the uplink and downlink data traffic.

The biggest drawback to wide-area 802.11b-based networks is the cost of truck rolls and more-complex CPE installations resulting from the use of outdoor antennas. The high degree of interference present in unlicensed spectrum in many areas is also an issue.

To make wide-area residential broadband wireless cost-effective for operators, broadband wireless access products need to be standards-based to enable small, low-cost CPE terminals to be produced by multiple vendors. The air interface technology behind the standard also needs to be able to deliver broadband data rates over longer distances (several kilometers or more) to the same self-installable terminals. This will eliminate the need for truck rolls and offer the lowest overall network costs for both equipment and deployment. The 1xEV-DO technology, part of CDMA2000's evolution path and standardized by 3GPP2 and by TIA/EIA in the United States as IS-856, offers all of these capabilities and is available today. Self-installable 1xEV-DO terminals are already available from several vendors. These terminals can be purchased by end-users at the local electronics store, self-installed and activated over the air.

Dual benefits

Basically, the 1xEV-DO is a spectrally efficient air interface optimized for the transmission of high-speed packet data. Combining the benefits of both time-division multiplexing and code-division multiplexing, the air link uses adaptive modulation and coding to maximize the instantaneous transfer rate to all users. QPSK, 8-PSK and 16-QAM modulations are used on the forward link while BPSK is used on the reverse link. Turbo coding combined with hybrid ARQ is also used to optimize the system throughput. The power of CDMA's mobile standards have also been inherited by 1xEV-DO and can be deployed with a frequency-reuse factor of 1, making it easy to scale network capacity with limited available spectrum.

The high-performance air link delivers peak forward and reverse data rates of 2.45 Mbits/s and 153 kbits/s — sufficient for most residential users and business professionals. Using a single 1.25-MHz frequency pair, a cell site can provide 7.3 Mbits/s (3 sectors x 2.45 Mbits/s per sector) aggregate forward peak throughput. It is important to note that the 7.3-Mbit/s figure, in contrast to the 802.11 numbers given earlier, refers to the actual user traffic. Additional 1.25-MHz carriers can be added to the same access point to further increase capacity.

The 1xEV-DO air interface was designed from the beginning for wide-area outdoor coverage. Although developed primarily for the 3G mobile market, the technology is well-suited for fixed broadband wireless-access deployments in areas that are difficult or too expensive to serve with DSL or cable modem technologies. Also, 1xEV-DO systems are deployed in the 800-MHz and 1.9-GHz licensed bands, which have good propagation characteristics that favor larger cell sizes — especially when faced with the signal attenuation due to in-building penetration. These bands permit higher output power and antenna gains than what is normally permitted in unlicensed spectrum, and also offer a more controlled interference environment. The asymmetry of the air link is also critical to enabling large cells.

High-power amplifiers are used only at the access points to provide enough forward link system gain for large cell sizes while the structure of the reverse link allows for the use of small low-power user terminals.

Another standard worth noting is 802.16a, which was just published by IEEE last month. The 802.16a standard specifically targets wide-area broadband wireless access from 2 to 11 GHz. The standard offers great flexibility to system vendors, supporting three physical layers (single carrier, OFDM and OFDMA), both TDD and FDD duplexing and RF channels from 1.5 to 20 MHz. This flexibility actually allows the development of networks with widely varying characteristics, making it more difficult now to compare the performance and capacity of a generic 802.16a product with the others mentioned in the article.

The challenge of defining specific access profiles for 802.16a interoperability is being addressed by the WiMax forum, an organization made up of supporters of the 802.16 standards. Once the profiles are defined, WiMax will also be certifying equipment for interoperability, starting towards the end of 2004.

Each of the standards discussed offers unique strengths that make it particularly well-suited for specific types of deployments. The 802.11b standard has had tremendous success recently for both home and hotspot networks and continues to gather momentum for short-range, high-capacity networks. A more recent standard, 1xEV-DO can cover longer distances while maintaining interoperability with standard low-cost CPE terminals. Its capacity and coverage, combined with the portability inherent in the technology make it an attractive alternative to DSL for many applications. The newest of these standards, 802.16a, offers great promise for the future of wide-area broadband wireless access.