The long-neglected frequency band between 5 and 15 GHz is gaining new respect in broadband wireless applications. Developers are looking at the 5-GHz portion of the band for lower-cost implementations of 28- and 38-GHz local multipoint distribution service (LMDS) radio systems, as well as for digital wireless local-loop (WLL) and multichannel multipoint distribution service (MMDS) systems.
At the same time, players in the wireless local-area network (LAN) arena are looking above the 2.4-GHz spectrum at 5 GHz, contending that 2.4 GHz has become a junk band because of overcrowding and interference from the proliferation of devices using the Bluetooth standard, for example. Those vendors--including Radiata Communications (San Jose, Calif.), which has a contributed article in this section, believe that broadband home or enterprise LANs should shift to the 5-GHz spectrum, in particular the FCC's campus-area Unlicensed National Information Infrastructure (UNII) networks and the industrial/scientific/medical (ISM) bands.
|Troy Trenchard, of Cisco's wireless group, sees the concept of multipoint gaining strength over the past five years, allowing for shared users and shared bandwidth
Point-to-point solutions have been common in broadband fixed wireless for several years, said Troy Trenchard, director of marketing for the wireless access business unit of Cisco Systems Inc. (San Jose). But in the last five years, the concept of multipoint arose, allowing for shared users and shared bandwidth. Typically, the microwave band for broadband fixed wireless is from 2 to 6.7 GHz, and the millimeter-wave band is primarily between 24 and 40 GHz. So far, most point-to-multipoint solutions have been in evaluation or trial phases. The market is coalescing now due to a combination of several factors. On one hand, there are licensees with dollars to spend, business plans in place, and motivation, such as a license expiration date. On the other hand, vendors have begun providing some robust-appearing second-generation products.
Cisco heads up the Broadband Wireless Internet Forum (BWIF), a program of the IEEE Industry Standards and Technology Organization (IEEE-ISTO), begun late last year to promote a coding scheme that could lower costs of 2.5- and 5-GHz MMDS technology into the mass-market realm. Although microwave MMDS services were promoted in the early 1990s as a one-way broadcast method with nonline-of-sight transmission, the technology experienced only limited urban success. But the forum's vector orthogonal frequency-division multiplexing (VOFDM) coding scheme promises to reduce the costs of 30-Mbit/s wireless connections to under $50, close enough for competition with cable modems and digital subscriber lines.
BWIF members will cross-license to each other the technologies required to implement the VOFDM specifications on a worldwide, royalty-free basis. The forum's goal is to facilitate cost-effective, broadband wireless access solutions, with the performance and reliability needed for high-speed Internet access, streaming audio and video content, and voice. Cisco has been shipping point-to-point products based on VOFDM since earlier this year, said Trenchard, and the company is in trial on point-to-multipoint systems.
In point-to-multipoint broadband wireless, trade-offs must be considered at both higher and lower frequencies. Higher frequencies yield greater bandwidth, which means that statistical multiplexing can be improved and, consequently, more subscribers reached. But transmitting distances decrease as frequencies rise. In addition, at frequencies above 10 GHz, line-of-sight (LOS) is required between basestations and subscriber stations, said Michael Sanderson, director of product management for Ensemble Communications Inc. "So it will be some time before we see millimeter-wave and LOS technologies used in small-office/home-office and residential applications," he said.
Lower frequencies under 10 GHz don't require LOS but have less spectrum to offer, with channels about 6-MHz wide. Current MMDS deployments use 2.5 to 2.6 GHz in the United States and 3.5 GHz elsewhere. To achieve good statistical multiplexing, subscribers must use relatively low bandwidths.
"Up to now, the best trade-off seemed to be in the higher frequencies, above 24 GHz or so," said Sanderson. "In Europe, they've been talking about allocating the 40- to 43-GHz range in 3-GHz-wide blocks."
Network operators are choosing different technologies-whether point-to-multipoint, LOS or MMDS-depending on their business. Those in commercial applications are gravitating toward higher-frequency technologies, such as LMDS, with its higher per-subscriber bit rates and quality-of-service (QoS) guarantees. At the other end, residential applications need less bandwidth in the upstream direction and are less stringent about QoS, so MMDS is a common solution.
There hasn't been much activity in the frequencies between 10 and 15 GHz for several reasons, said Eric Wilson, vice president of systems management for Vyyo Inc. (Cupertino, Calif.). The bands between 10.7 and 24 GHz are either useless or already allocated to satellite communications (11.5 to 14 GHz), point-to-point (11 GHz and 14 to 18 GHz) or radar (18 to 24 GHz). The band between 10.2 GHz and 10.6/10.7 GHz has been a ham radio band, but some countries are reallocating it for point-to-multipoint Internet access data distribution.
Thanks to the recently formed Wireless DSL Consortium, of which Vyyo is a member, standards based on an extension of the data-over-cable service interface specification (Docsis) are becoming available for MMDS and WLL in the spectrum between 2.1 and 10 GHz, said Wilson. Although the RF side differs considerably from the Docsis hardware cable standard, "the data side of the interface and the protocol interface have adopted the Docsis standard one-for-one," Wilson said.
The consortium's six founding members are ADC, Conexant Systems, Gigabit Wireless, Intel, Nortel Networks and Vyyo. The group plans to provide a forum for developing a set of open interfaces for 3.5-GHz MMDS products, and for testing and verifying products. It will pursue a next-generation, multifaceted standard that increases system capacity and coverage.
Aside from increasing frequency, another potential solution for certain applications is unlicensed bands. Everything between 5 and 15 GHz is licensed except for the 5.3- to 5.8-GHz UNII band, said Wilson.
Unlicensed ISM bands include 2.4 and 5.7/8 GHz, said Trenchard. At 5.7, UNII bands also come into play. "The 5.7 band is not very congested right now, so there's a big opportunity for service providers who haven't licensed any spectrum," he said.
A mass market for 3.2-GHz and 5-GHz services is developing naturally, at least in outdoor applications, as the 2.4/5 band gets increasingly crowded, said Lynn Chroust, director of product marketing for Proxim Inc. Proxim, which has products in the 2.4/5-GHz band-but not the 3.2-GHz band, because the 3.2 band is not readily available worldwide-has always operated on the model of unlicensed spectra, she said.
Links are becoming crowded and less reliable in outdoor wireless applications, which are primarily confined to licensed bands, to the extent that no more outdoor building-to-building links are being put up in New York City, for example. Indoors, interference is much less of an issue, since indoor WLANs are contained in-building.
The increased interest in higher frequencies has been jump-started by the fact that wireless ISPs are trying to get high-speed Internet access in places where the wired infrastructure is either thin or missing, such as rural areas or residential areas lacking DSL and cable access.
Fixed wireless systems for the unlicensed 5-GHz band at higher, 100-Mbit/second speeds and in larger volumes just began shipping this year, said Chroust. In the 2.4-GHz band, 10-Mbit/s speeds have been reached, but going to higher speeds in this band could result in more expensive, less robust systems. "Products based on 22 Mbits/s may happen, but 2.4 GHz will probably not be stretched further than that," she said.
Streaming video is one of the main applications driving higher indoor speeds, said Chroust. Another factor is problems with 802.11b-based wireless LANs caused by Bluetooth systems.
ICs for midrange broadband wireless tend to cluster in a group at 5 GHz and a group at 10.5 GHz, with more specialized chips for satellite or point-to-point usage, said Wilson. Prices of radio ICs in the 10.5-GHz spectrum are still on a par with the higher prices of chips for 28-GHz-and-up LMDS systems, because volumes aren't there yet.
Most systems integrate the radio with the modem, so that when the radio IC changes for different frequencies, so must the wireless modem and the modem termination system. Yet it's only the radio and antenna that really must be different for different bands.
Developments in fixed wireless chips are focusing on getting modem schemes such
as QPSK and QAM or RF techniques like VOFDM algorithms out of discretes and into ICs for cost reduction, said Trenchard. "On the RF side, improvements include gallium arsenide-based MMICs microwave monolithic ICs starting to drive some of the economics," he said.
Infineon Technologies Inc. has developed a MMIC chip set for 3.5-GHz wireless access, using GaAs upconverter and downconverter chips, along with a separate power amplifier. Paul Thesing, group marketing manager for Infineon's wireless-products business unit, said that although the narrowband upconverter and downconverter chips fit these new markets in the 3.4- to 3.8-GHz range, the power amplifier used in narrowband applications doesn't.
For 802.11 2.4-GHz WLAN radio ICs, ParkerVision Inc. uses a direct-conversion method to eliminate the IF stage, cutting the bill-of-materials cost by 30 percent to 50 percent, said Al Petrick, vice president of strategic alliances. In 5-GHz WLANs, several vendors are developing single-chip RF ICs or RF modules, including Atheros Communications Inc. and Intarsia Corp.
Contributions to this week's special section from Atheros, Intarsia and others take the pulse of design activity in midrange-frequency components. Additional articles in the section round out the topic by exploring telecommunications infrastructure topics such as Compact PCI, Internet Protocol gateways and DSP for wireless designs.