Non-line-of-sight issues plague broadband wireless designers

Wireless data connections are increasingly being used for building-to-building links between local-area networks and for carrier services such as subscriber connections (wireless local-loop) and back-haul services. Many applications have traditionally been "best efforts" relying on higher-level network protocols to manage the end-to-end link. However, modern data networks are carrying an increased quantity of delay-sensitive traffic such as synchronization data for secure applications like virtual private networks and thin clients, as well as real-time data for applications such as voice-over-Internet Protocol and videoconferencing. This leads to a requirement for high throughput and high availability in all parts of the network.

Achieving high availability (99.99 percent or better) with wireless has traditionally involved establishing a line of sight between the communicating stations, either directly or via an intermediate relay station. Where this is not feasible due to the cost of relay sites or the sheer number of obstacles, it is necessary to deploy non-line-of-sight equipment. In a non-line-of-sight environment, maintaining high availability at the same time as maximizing channel throughput presents significant challenges.

A non-line-of-sight link encounters obstacles such as buildings, trees and hills between the transmitting station and the receiving station. In such environments, the basic laws of physics cause a transmitted signal to take a multitude of paths via reflections, refraction and diffraction toward a receiving station. Some of these "multipaths" will be received with sufficient signal strength to be detectable and demodulated into a meaningful data stream.

However, the paths have different characteristics and the transmitted signal is received as multiple individual signals with varying amplitude, delay and signal strength. The quality of the received signal varies depending on the exact location of the receiver. If there is motion in the environment (although neither the transmitter nor the receiver may be moving), each path changes dynamically with time.

In reality, none of the current remedies provides a satisfactory solution allowing for high availability, high throughput and reasonable range. They each offer trade-offs — for instance, sacrificing range for better availability. The real challenge is to overcome the obstacles of non-line-of-sight environments at the same time as minimizing loss of availability, throughput or range. This can be achieved using technologies such as space-time coding, orthogonal frequency-division multiplexing and adaptive modulation

Space-time coding is a method of transmitting multiple data beams on multiple transmitters to multiple receivers. Space-time-coding massively increases the odds of receiving the data. If any one path is faded, there is a high probability that other paths are not. A simple analogy is that with the toss of a single coin, there is a 50 percent chance of a head. If four coins are tossed, there is a 15/16 chance of getting at least one head.

For space-time coding to be effective, the paths need to be decorrelated — that is, they need to behave differently to each other. This can be achieved using techniques such as spatial separation of the antennas or separation of the transmitted waveforms via time, polarization, frequency or modulation.

The benefit of space-time coding is the ability to recover a signal from multiple faded signals. However, the downside is in the processing overhead required, which increases the cost of the solution.

Orthogonal frequency-division multiplexing (OFDM) involves the transmission of data on multiple carriers, meaning communication is maintained should one or more carriers be affected by either narrowband or multipath interference. A key aspect of OFDM is that the individual carriers overlap to improve spectral efficiency. Normally, overlapping signals would interfere with each other. However, through special signal processing, the carriers in an OFDM waveform are spaced in such a manner that they effectively do not see each other: They are orthogonal to each other, so there is no cross-interference and hence no system loss.

OFDM flavors

Many vendors have developed variations of OFDM by changing the number of carriers and the number of pilot tones. These pilot tones provide advanced channel-equalization feedback to allow instant recovery from even the deepest of fading situations. This means OFDM systems are more spectrally efficient and can combat and recover from fading better than single-carrier systems. Again, the downside is in the complex DSPs required and their associated cost.

With adaptive modulation, the signal modulation — or the phase-shift keying and quadrature amplitude modulation — is dynamically modified according to the signal level currently being received. Since the channel may vary in intensity in less than a second, adaptive modulation allows the system to transmit the maximum amount of data possible by rapidly optimizing itself to the channel conditions. The effect is to overcome some of the effects of fading while maximizing the data throughput of the radio channel without impacting availability of the system.

The downside of adaptive modulation is that data throughput is variable depending on the modulation technique at any point in time. Where guaranteed bandwidth is required, only the lowest common denominator should be considered, leading to potentially inefficient use of the radio channel.

Without line-of-sight, traditional point-to-point wireless solutions are rendered useless. To overcome the problems of excess path loss, unpredictable fading and variable delay, a blend of advanced technologies is required. With the right technologies, it is possible to achieve link availability of 99.99 percent in more cases than ever before. But there are still challenges to be overcome.

Research into non-line-of-sight systems is ongoing. Further advances in digital signal processing will reduce the cost of space-time-coding systems, while improvements in antenna technology will increasingly allow for decorrelated transmission of multiple signals. As the technology improves, the probability of establishing high-availability, high-throughput connections over longer ranges will increase.