How NLOS systems work

The most important technology in a point-to-multipoint (non-mesh) NLOS system is its smart base-station antenna. Instead of a single omnidirectional or sectorized antenna, these systems use an array of radiating elements. Each element is fed a version of the signal to be transmitted that differs from the others only in its amplitude and phase (time delay). The signals radiated by the array elements combine with each other in space to form one or more beams of carefully calibrated strength propagating in specific directions. The directions are so chosen that the beamsafter bouncing off assorted objects in the environment, like mountains, buildings, motor vehicles, and even aircraftall reach the location of the intended subscriber at the same time and in phase with one another [see figure, Exploiting Multipath Distortion].

Illustrations: Steve Stankiewicz

Exploiting Multipath Distortion: In just about any environment, radio communication is complicated by multipath distortion in which signals bounce off various objects and reach their intended target out of phase because they've traveled more than one path. Thanks to modern digital signal processors, a smart antenna array, like the one on the tower, turns this distortion into an asset. By analyzing the complex signal received from each subscriber during routine handshaking, the smart array decomposes it into a number of simpler signals, each characterized by its strength, direction, and time of arrival. The array then transmits signals back along the same paths, but with complementary properties (strong where they were weak and early where they were late). Thus, after negotiating the same terrain, they all come together at the subscriber's location strongly because they're in phase.

When the beams combine constructively, the result is a strong signal at the receiver, which can therefore use an indoor antenna.

Sounds good, but how do they do it? The answer is by first monitoring signals received from the subscriber unit to determine the characteristics of the environment and then by generating a complementary signal. For example, if the subscriber unit has a simple omnidirectional whip antenna, the signal it transmits will, in general, undergo multipath distortionthat is, it will take multiple paths to the base station, bouncing off various objects, being attenuated to various degrees, and undergoing various delays, depending on the different path lengths.

Say the base station receives two signals, one from the north and, 2 µs later, one from the east that is 8 dB weaker. Then the base station transmitter will format its signal into two beams, first a strong one to the east, and 2 µs later, one 8 dB weaker to the north. Of course, since the environment is constantly changing, the base station must keep monitoring subscriber transmissions, analyzing them, and updating its picture of the environment. Small wonder, then, that this sort of technology could not even be considered for commercial applications until cheap and powerful digital signal processors became available.

Taking advantage of those processors, Navini Networks uses the technology in its Ripwave product line, versions of which operate in both the licensed and unlicensed bands in the vicinity of 2.5 GHz. According to Sai Subramanian, director of marketing and product line management, the base station antenna has eight elements, but is not very large because all eight elements are within a wavelength of each other, and at 2.5 GHz, a wavelength is just 120 mm.

Although Navini Networks' subscriber premises equipment has two antennas, they don't work together as a phased array. Rather, they provide spatial diversity: it is less likely that two antennas will simultaneously find themselves in a dead spot than it is for one antenna. Navini has several U.S. trials under way.

In another approach, Iospan Wireless uses two transmit antennas at the base station and three receivers at both ends of its links. Iospan's multiple antenna technology, which was developed by the company's founder, Arogyaswami Paulraj, professor and head of the Smart Antenna Research Group at Stanford University, is known as MIMO, for multiple-input, multiple-output.

Referring to multipath distortion, Asif Naseem, vice president of business operations, marketing, and business development for Iospan, says, "Multipath is our friend. In the best conditions, we get six separate data streams out of the frequency chunk [there are six paths between three receivers and two transmitters] and realize multiples on the user data rate. In our tests from our base station on the roof to a customer over 1.5 km away, we are measuring over 13 Mb/s downstream and 6 Mb/s up. At 6 km we get over 6 Mb/s down and 4 Mb/s up. This is usable capacity." Iospan's multiple antenna enhancements of the OFDM modulation technique are being standardized in the IEEE 802.16 Working Group on Broadband Wireless Access Standards. The company is aiming the price of customer premises equipment at less than $500, and has begun trial deployments with partners in the United States and internationally. (Iospan will rely on others to manufacture, market, and install its systems; it simply provides ASICs, software, and reference designs.)