Ask the Expert

You can think of WiMAX either as Wi-Fi on-the-move, or a cellular system which delivers DSL / office Internet speeds. Although Wi-Fi (also known as 802.11) works well in offices, cafes, and outdoor “hotspots,” if you move across town you’ll quickly lose coverage. WiMAX, however, is a wireless data network that can cover an entire city, just like cell phone networks do today. Even better, WiMAX (or 802.16) provides speeds comparable to Wi-Fi - between 1 and 10 million bits per second (Mbps) for individual users. So with WiMAX, you have the best of both worlds—mobility and speed.

WIMAX is gaining significant momentum worldwide. Many WiMAX networks are already deployed. South Korea already has a small mobile WIMAX network in Seoul. India has fixed WIMAX networks in Bangalore and Mumbai. Many other network launches are happening. In the U.S., Sprint and Clearwire are planning to launch nationwide networks later this year. Like cell phone networks, WiMAX networks will involve expensive “infrastructure”— locating base stations radios on towers and building backend core networks.

Users can buy a variety of WIMAX terminals: coffee cup-sized  terminals for fixed applications, USB dongles or built-in WIMAX for laptops and sub-notebooks, or standard smart phones like Blackberrys. Soon WIMAX will be embedded into many consumer devices from cameras to cars. Once you have a WIMAX terminal, you will have to sign up with a carrier like Sprint for ubiquitous and fast conenctivity. WiMAX transmits data packets using the standard Internet Protocol and also supports voice services using the voice over packet (VOIP) protocol.

There are several technologies in WiMAX that make it capable of handling high speed transmission for mobile users. I’ve worked on some of these both in my research at Stanford and in the two companies I’ve founded, Iospan (now part of Intel) and Beceem.

The first is called Multiple Input Multiple Output (MIMO). This technology, which I and my group developed in the early 1990s, uses multiple antennas in the receiver and the transmitter to enable both dramatically increased speeds and to make them more reliable.  Wi-Fi systems have also begun to adopt MIMO technology. MIMO can increase throughput from 100% to 300%.

Another technology in WIMAX is called Orthogonal Frequency Division Multiple Access (OFDMA). In OFDMA we use an OFDM modulation technique that is friendly for broadband applications and also allows the terminals to use the bandwidth flexibly. OFDMA improves network coverage since terminals can choose reduced bandwidth transmission to increase their range and reach a distant base station. OFDMA is not available in WI-Fi.

A third element in WiMAX is called Opportunistic Scheduling. Typically in a wireless system, the signal strength bobs up and down (known as fading), and in WiMAX, we monitor these swings and schedule transmission when the signal is strong. This can improve throughput by about 50%.

And finally, since WiMAX is a mobile network, it needs the capability of executing “handoffs” between base stations. As you drive along, the network will keep track of which base stations can provide the strongest signal. When appropriate, the network will switch your connection from a weaker station to a stronger station without interrupting service.

Although WiMAX deployments are just beginning, researchers in academia and industry are already thinking about the next step—a “fourth generation” or “4G” network. The current WiMAX standard (802.16e) allows for transmission speeds of 100 Mbps, but the next standard (802.16m) will allow for 1,000 Mbps. Getting to this point will require further research in managing interference and in other technologies. The era of fast, ubiquitous and mobile wireless access is almost here and it will profoundly change the way we communicate.