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WiMAX advantages bring about new challenges

Posted: 01 Dec 2005 ?? ?Print Version ?Bookmark and Share

Keywords:wireless? wimax? broadband? telecommunications? wireline?

WiMAX is a form of broadband wireless access based on the IEEE 802.16 standard for wireless metropolitan-area networks. Widespread deployment of the technology is expected over the next three to five years, driven by WiMAX's ability to deliver affordable last-mile broadband Internet services. Many of the companies entering the WiMAX market include those that have dominated the WLAN arena. As engineers schooled in WLAN tackle this emerging standard, they face many different system considerations, especially in terms of RF requirements and architectures.

How does the 802.16 WiMAX standard compare to the 802.11 WLAN standard? Both are based on OFDM, use multiple pilot tones and support modulations ranging from BPSK to 64QAM.

But there are some major differences as well. For instance, rather than a fixed 20MHz bandwidth with 52 subcarriers as in 802.11, WiMAX systems can use variable bandwidths from 1MHz to 28MHz with 256 subcarriers (192 data subcarriers) in either licensed or unlicensed spectrum. The first WiMAX rollouts are expected to use 3.5MHz and 7MHz channel bandwidths.

WiMAX supports subchannelization, meaning that instead of transmitting on all 192 data subcarriers, you can transmit on just one subset. In this scenario, by using the same amount of power over fewer carriers, the system achieves greater range. As WiMAX CPE evolves into in-building devices, it'll be necessary to make up for the power loss incurred when transmitting the signal outside the building. Since CPE is typically limited in power, concentrating the power over fewer subcarriers in the uplink can balance the power in the uplink and downlink and enable greater range.

While the larger number of subcarriers gives WiMAX an advantage over 802.11, the resulting challenge to the system design is that the subcarriers are spaced more closely together, so there are tighter requirements for phase noise and timing jitter. This translates to a need for higher-performance synthesizers.

WiMAX also uses a variable-length guard interval to improve performance in multipath environments. The guard interval is a time delay at the beginning of the packet to compensate for multipath interference. With a clear channel, the guard interval can be shortened, increasing the throughput. With more subcarriers and a variable-length guard interval, a WiMAX system's overall spectral efficiency will be 15-40 percent higher than a WLAN system. For instance, WiMAX achieves a spectral efficiency ranging from 3.1-3.8Mbps/MHz, compared to only 2.7Mbps/MHz for 802.11a/b/g.

Error-vector magnitude (EVM) requirements for 802.11 are specified at -25dB, which is required to achieve a 10 percent packet error rate. For 802.16, EVM is held to -31dB, which is based on a 1 percent packet error rate. This lower error rate helps contribute to WiMAX's longer range, along with the receiver noise figure, which is more stringent for 802.16. Specifically, 802.11's maximum noise figure is 10dB, while 802.16 operates at 7dB.

Overall challenges
When designing a new WiMAX system, the first question is whether the system will be TDD, FDD or H-FDD. Countries such as Canada and much of Europe are generally adopting an FDD structure. In the United States, if the system is used in licensed spectrum, then the duplexing will already be specified. If the system is FDD, two complete radios (including synthesizers) operating simultaneously on different frequencies will be required.

This type of system will need extensive external filtering to prevent the transmit power from leaking into and interfering with the receiver. In addition to cost, the dual radios and required filtering become a concern in terms of board space. Many industry leaders expect that base stations will use full FDD mode due to its higher throughput, while subscriber stations will use lower-cost H-FDD or TDD.

Given a choice, H-FDD can be an attractive alternative because it has a single radio (and single synthesizer) and similar costs to TDD. The key concern with H-FDD is that the synthesizer must be able to switch between the transmitter and receiver within 100ms. Since the system isn't simultaneously transmitting and receiving, filtering requirements are significantly relaxed compared to FDD.

Perhaps the 802.16 specification that has the greatest impact on system design is EVM, because the EVM must be 6dB higher for 802.16 than for 802.11. This has a number of implications. First, all the system blocks must be more linear. Second, phase noise must be considerably better than in an 802.11 design. Tighter phase-noise requirements have implications for the synthesizer, which result in a longer settling time. Third, if an I/Q interface is chosen, then I/Q balance must also be tighter and will likely require I/Q calibration.

RF architectures
When selecting an RF architecture for a WiMAX design, the basic choice is between a superheterodyne or direct-conversion architecture. In terms of satisfying the stricter transmitter regulatory requirements, a superheterodyne architecture is advantageous because of off-chip filtering of unwanted emissions.

There are two different kinds of superheterodyne baseband interfaces: IF and I/Q. With an IF interface, the signal at the baseband processor is at a low (but not zero) frequency. Typical IF frequencies range from 10-50MHz. With an I/Q interface, the signal at the baseband processor extends to DC. In this case, any I/Q imbalance will result in images that fall directly on top of the desired signal and appear as noise. Thus, I/Q balance is critical for an I/Q interface and it's likely that external I/Q calibration will be required. Thus, the IF interface is preferable because it doesn't require any external calibration.

A direct-conversion transmitter architecture, on the other hand, takes the two I/Q inputs at baseband and directly modulates them up to the RF. This architecture is attractive because it leads to a smaller and less expensive radio design. It removes the need for an IF local oscillator and eliminates the SAW filter. The challenge with this approach is that performance is harder to maintain. For instance, any small DC offsets that occur will degrade system performance. I/Q balance is also critical. Therefore, DC and I/Q calibration will be required. Also, without SAW filtering, spurious transmissions may result in spectral mask emission failures.

- Darcy Poulin
SiGe Semiconductor Inc.

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