RTC Magazine

While many technologies currently available for fixed broadband wireless can only provide line-of-sight (LOS) coverage, the technology behind WiMax has been optimized to provide excellent non-line-of-sight (NLOS) coverage. WiMax’s advanced technology provides the best of both worlds—large coverage distances of up to 50 kilometers under LOS conditions and typical cell radii of up to 5 miles/8 km under NLOS conditions.

The radio channel of a wireless communication system is often described as being either LOS or NLOS. In an LOS link, a signal travels over a direct and unobstructed path from the transmitter to the receiver. A LOS link requires that most of the first Fresnel zone is free of any obstruction (Figure 1). If this criteria is not met, then there is a significant reduction in signal strength. The Fresnel clearance required depends on the operating frequency and the distance between the transmitter and receiver locations.

In an NLOS link, a signal reaches the receiver through reflections, scattering and diffractions (Figure 2). The signals arriving at the receiver consist of components from the direct path, multiple reflected paths, scattered energy and diffracted propagation paths. These signals have different delay spreads, attenuation, polarizations and stability relative to the direct path. The multi-path phenomena can also cause the polarization of the signal to be changed. Thus using polarization as a means of frequency re-use, as is normally done in LOS deployments, can be problematic in NLOS applications.

Leveraging Multi-Path Signals

How a radio system uses these multi-path signals to an advantage is the key to providing service in NLOS conditions. A product that merely increases power to penetrate obstructions (sometimes called “near-line-of-sight”) is not NLOS technology because this approach still relies on a strong direct path without using energy present in the indirect signals. Both LOS and NLOS coverage conditions are governed by the propagation characteristics of their environment, path loss and radio link budget.

There are several advantages that make NLOS deployments desirable. For instance, strict planning requirements and antenna height restrictions often do not allow the antenna to be positioned for LOS. For large-scale contiguous cellular deployments, where frequency re-use is critical, lowering the antenna is advantageous to reduce the co-channel interference between adjacent cell sites. This often forces the base stations to operate in NLOS conditions. LOS systems cannot reduce antenna heights because doing so would impact the required direct view path from the CPE to the Base Station.

NLOS technology also reduces installation expenses by making under-the-eaves CPE installation a reality and easing the difficulty of locating adequate CPE mounting locations. The technology also reduces the need for pre-installation site surveys and improves the accuracy of NLOS planning tools.

The NLOS technology and the enhanced features in WiMax make it possible to use indoor customer premise equipment (CPE). This has two main challenges; firstly, overcoming the building penetration losses and secondly, covering reasonable distances with the lower transmit powers and antenna gains that are usually associated with indoor CPEs. WiMax makes this possible, and the NLOS coverage can be further improved by leveraging some of WiMax’s optional capabilities.

NLOS Technology Solutions

There are seven aspects of WiMax technology that together solve or mitigate the problems resulting from NLOS conditions. These include orthogonal frequency division multiplexing (OFDM) technology, sub-channelization, directional antennas, transmit and receive diversity, adaptive modulation, error correction techniques and power control.

OFDM technology: OFDM technology provides operators with an efficient means to overcome the challenges of NLOS propagation. The WiMax OFDM waveform offers the advantage of being able to operate with the larger delay spread of the NLOS environment. By virtue of the OFDM symbol time and use of a cyclic prefix, the OFDM waveform eliminates the inter-symbol interference (ISI) problems and the complexities of adaptive equalization. Because the OFDM waveform is composed of multiple narrowband orthogonal carriers, selective fading is localized to a subset of carriers that are relatively easy to equalize. Figure 3a shows a comparison between an OFDM signal and a single carrier signal, with the information being sent in parallel for OFDM and in series for single carrier.

The ability to overcome delay spread, multi-path and ISI in an efficient manner allows for higher data rate throughput. As an example (Figure 3b), it is easier to equalize the individual OFDM carriers than it is to equalize the broader single carrier signal. For all of these reasons recent international standards such as those set by IEEE 802.16, ETSI BRAN and ETRI, have established OFDM as the preferred technology of choice.

Sub-channelization: Sub-channelization in the uplink is an option within WiMax. Without sub-channelization, regulatory restrictions and the need for cost-effective CPEs typically cause the link budget to be asymmetrical, which causes the system range to be up-link limited. Sub-channeling enables the link budget to be balanced such that the system gains are similar for both the up and down links. Sub-channeling concentrates the transmit power into fewer OFDM carriers; this is what increases the system gain that can either be used to extend the reach of the system, overcome the building penetration losses or reduce the power consumption of the CPE. The use of sub-channeling is further expanded in orthogonal frequency division multiple access (OFDMA) to enable a more flexible use of resources that can support nomadic or mobile operation.

Antennas for fixed-wireless apps: Directional antennas increase the fade margin by adding more gain. This increases the link availability as shown by K-factor comparisons between directional and omni-directional antennas. Delay spread is further reduced by directional antennas at both the Base Station and CPE. The antenna pattern suppresses any multi-path signals that arrive in the sidelobes and backlobes. The effectiveness of these methods has been proven and demonstrated in successful deployments, in which the service operates under significant NLOS fading.

Adaptive antenna systems (AAS) are an optional part of the 802.16 standard. These have beam-forming properties that can steer their focus to a particular direction or directions. This means that while transmitting, the signal can be limited to the required direction of the receiver, like a spotlight. Conversely when receiving, the AAS can be made to focus only in the direction from where the desired signal is coming from. They also have the property of suppressing co-channel interference from other locations. AASs are considered to be future developments that could eventually improve the spectrum re-use and capacity of a WiMax network.

Transmit and receive diversity: Diversity schemes are used to take advantage of multi-path and reflections signals that occur in NLOS conditions. Diversity is an optional feature in WiMax. The diversity algorithms offered by WiMax in both the transmitter and receiver increase the system availability. The WiMax transmit diversity option uses space time coding to provide transmit source independence; this reduces the fade margin requirement and combats interference. For receive diversity, various combining techniques exist to improve the availability of the system. For instance, maximum ratio combining (MRC) takes advantage of two separate receive chains to help overcome fading and reduce path loss. Diversity has proven to be an effective tool for coping with the challenges of NLOS propagation.

Adaptive modulation: Adaptive modulation allows the WiMax system to adjust the signal modulation scheme depending on the signal to noise ratio (SNR) condition of the radio link (Figure 4). When the radio link is high in quality, the highest modulation scheme is used, giving the system more capacity. During a signal fade, the WiMax system can shift to a lower modulation scheme to maintain the connection quality and link stability. This feature allows the system to overcome time-selective fading. The key feature of adaptive modulation is that it increases the range that a higher modulation scheme can be used over, since the system can flex to the actual fading conditions, as opposed to having a fixed scheme that is budgeted for the worst-case conditions.

Error correction techniques: Error correction techniques have been incorporated into WiMax to reduce the system signal to noise ratio requirements. Strong Reed Solomon FEC, convolutional encoding and interleaving algorithms are used to detect and correct errors to improve throughput. These robust error correction techniques help to recover errored frames that may have been lost due to frequency selective fading or burst errors. Automatic repeat request (ARQ) is used to correct errors that cannot be corrected by the FEC, by having the errored information resent. This significantly improves the bit error rate (BER) performance for a similar threshold level.

Power control: Power control algorithms are used to improve the overall performance of the system. It is implemented by the base station sending power control information to each of the CPEs to regulate the transmit power level so that the level received at the base station is at a predetermined level. In a dynamical changing fading environment, this pre-determined performance level means that the CPE only transmits enough power to meet this requirement. The converse would be that the CPE transmit level is based on worst-case conditions. The power control reduces the overall power consumption of the CPE and the potential interference with other co-located base stations. For LOS the transmit power of the CPE is approximately proportional to its distance from the base station; for NLOS it is also heavily dependant on the clearance and obstructions.

Suited for NLOS Duty

WiMax technology can provide coverage in both LOS and NLOS conditions. NLOS has many implementation advantages that enable operators to deliver broadband data to a wide range of customers. WiMax technology has many advantages that allow it to provide NLOS solutions, with essential features such as OFDM technology, adaptive modulation and error correction. And WiMax has many optional features, such as ARQ, sub-channeling, diversity and space-time coding that will prove invaluable to operators wishing to provide quality and performance that rivals wireline technology. For the first time, broadband wireless operators will be able to deploy standardized equipment with the right balance of cost and performance, choosing the appropriate set of features for their particular business model. Eugene Crozier is a system architect at SR Telecom, and Allan Klein is v.p. of systems and technology at SR Telecom.

SR Telecom Inc.
Montreal, QC, Canada.
(514) 335-1210.
[www.srtelecom.com].

WiMax Forum
Vista, CA.
(503) 712-2206.
[www.wimaxforum.org].