Integration key to 802.11n

Technical challenges resulting from the latest wireless LAN (WLAN) specification, 802.11n, are significant. Serving multiple frequency bandwidths (2.4 GHz and/or 5 GHz) with multiple antennas, the devices used in 802.11n WLAN must be optimized for output power as well as size.

For the designers of the circuits that make up the 802.11n radio, the things to think about include integration, power, performance, filtering, isolation and throughput. This is by no means a short list, but the market promise of 802.11n WLAN is high, and designers and manufacturers have been quick to act on the draft 802.11n specification. The 802.11n standard recently moved to draft 3.0, and the IEEE expects publication of the final standard in October 2008. Manufacturers are taking different approaches to the challenge, but all seem to agree that more integration will be the secret to success in 802.11n.

Chip status

Several vendors have already released product for 802.11n. SiGe Semiconductor Inc. (Ottawa, Canada) recently announced its latest 802.11n module, the SE2593A. This device joins a family of 802.11n modules.

Ralink Technology Corp. (Cupertino, Calif.) is currently shipping its 802.11n chip sets, and expects to begin sampling single-chip solutions at the end of this year. Broadcom Corp. (Irvine, Calif.) is in production for its 802.11n radios, and Atheros Communications (Santa Clara, Calif.) is on its third generation of 802.11n devices.

Integration

With the complexity of multiple-in, multiple out (MIMO) technology, 802.11n devices face real challenges in meeting OEMs form factor and performance expectations. The obvious way to address the form factor issue is through integration. So, how much integration is being done? Manufacturers tend to be focused on either the power amplifier (PA) or radio/baseband functionality. In terms of PAs, they are being integrated into front-end modules (FEMs), which typically include amplifiers, switches and passives. These can be used in conjunction with integrated MAC/baseband/RF chip sets or single-chip radios. If the PA is integrated with the MAC/baseband/RF functionality, there is a risk of trading off footprint for range.

SiGe Semiconductor is working on FEMs for 802.11n, one of which integrates 2.4-GHz and 5-GHz PAs and low-noise amplifiers (LNAs), power detector, transmit and receive switch, diplexers and associated matching in a package measuring 5 mm x 6 mm x 1 mm.

Designs using external PAs or FEMs are particularly well suited for applications that need to support high quality-of-service over long transmission ranges. "Routers or computers typically use an external PA or FEM to provide the added power required for reliable link performance given a greater distance," said Darcy Poulin, senior systems engineer at SiGe Semiconductor. "This is beneficial to ensure coverage throughout a home installation, for example."

Products that offer integrated MAC/baseband/RF functionality are ideal for applications where space is the top criterion. Ralink has taken the approach of integrating the MAC and baseband in one chip and the RF in a second chip. In Ralink's RT2850 transceiver chip for 2.4 and 5GHz, for instance, the company has integrated an LNA, down-conversion mixer, up-conversion mixer, predriver, voltage controlled oscillator (VCO), variable gain amplifier (VGA) , automatic level control (ALC), synthesizer, in-phase/quadrature (I/Q) modulator, I/Q demodulator, low-drop out (LDO) and three-wire serial bus in 2.4 and 5.0 GHz in a 9 x 9-mm QFN-76 package. Ralink's chips currently work with an off-chip PA, but the company reports that an integrated PA is in the works.

Until recently, Broadcom also only offered a two-chip set, but now it has integrated further with the BCM4322 single-chip 802.11n device. Kevin Mukai, senior product line marketing manager at Broadcom, reports that this new generation of product "integrates many of the external components in our current generation designs, which results in a BOM savings of 40 percent." The BCM4322 integrates two receivers, two transmitters and a CPU. It is designed to work with an external PA or front end.

Atheros is also offering a CMOS single-chip 802.11n product. The recently introduced AR9280, for instance, integrates the MAC, baseband and dual 2.4-and 5-GHz radios in a 10 x 10-mm QFN package.

Key specs

When selecting the RF front end for an 802.11n design, the key selection criteria not surprisingly boil down to size, performance and cost. Size and cost are easy to see and compare, but what are the key performance specs that can make or break the end product?

"Delivering required power while meeting error vector magnitude (EVM)--especially dynamic EVM in burst applications--and exceeding the EVM specification at low power levels," reports SiGe's Poulin. Other important considerations, he said, are delivering to spectral mask requirements and ease of system integration.

Lillian Chiu, marketing manager for Ralink Technology Corp., thinks it is most important to consider noise figure, which impacts long-range performance; output power at 1-dB compression, which affects short-range performance; and I/Q match, which affects peak throughput.

"Output power, linearity and current consumption are some of the key specifications for 802.11n front ends. Output power is a key factor in determining absolute range, and linearity determines performance in terms of throughput vs. range," said Broadcom's Mukai.

Design challenges

Certainly some impressive integration is taking place in advance of the release of the 802.11n specification. How have vendors overcome the design challenges?

"The biggest challenge has been achieving the level of integration required to meet multichannel, dual-band and stringent filtering requirements in small form factors," said SiGe's Poulin. "Other significant challenges include meeting stringent channel-to-channel isolation in MIMO systems (solved with careful RF design) and meeting more stringent out-of-band emission requirements when transmitting over multiple channels in a MIMO environment (achieved with more linear power amplifiers)."

Ralink's Chiu agreed that MIMO technology brought new challenges. She reported that the greatest challenge was "to reach peak throughput available from MIMO OFDM under adverse operating conditions."

Todd Antes, vice president of marketing at Atheros Communications, noted, "The greatest challenge has been to address and exceed key customer requirements to drive the adoption of 802.11n broadly into the mainstream market by bringing solution costs to generally accessible end pricing."

It is clear that some vendors already have a surprising depth of expertise in this new standard. As designers choose the necessary devices to develop their 802.11n designs, this expertise could prove to be very helpful.

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