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WiMedia UWB and WiMax Coexistence

Tue, 06/19/2007 - 10:22am
By Jim Lansford, Alereon, Inc.
The ongoing debate is not over whether or not UWB can cause interference with other incumbent signals in the same band, but the effectiveness of techniques used to protect the victim service.
The recent history of Ultra wideband (UWB) communications technology has seen great debate over whether UWB causes unacceptable interference to existing users of the same and nearby bands. This paper begins with the current state of regulations, followed by details of methods for coexistence of WiMedia UWB with existing systems in the 3-5GHz band, and limitations on the ability to detect these incumbents.

Since UWB covers such a large bandwidth, coexistence with incumbent users, both WiMax and others, is important, and may in fact be required for regulatory

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Figure 1. Distance at which UWB is at the noise floor of an incumbent receiver (assuming 6 dB noise figure in receiver).
approval in most countries outside the United States. The spectrum of a WiMedia UWB signal can be "sculpted" based on a priori decisions about restricting emissions in certain bands such as the radio astronomy bands in Japan. The multiband OFDM (Orthogonal Frequency-Division Multiplexing) signal permits flexibility in this "spectral sculpting" that is unique to MB-OFDM UWB (Ultra-Wideband)signals.
Regulatory Concerns
The original concept of UWB is that of a spectral "underlay" in other words, the UWB signal would have such a low power spectral density that its interference potential would be negligible. The relevant FCC rules, under which UWB was authorized in a Report and Order dated February 14, 2002, state that Part 15 devices such as UWB shall not cause "harmful interference." Unfortunately, this term is never defined. In an effort to protect incumbent services in the UWB bands while still enabling the personal area network (<10 meter) usage model, the FCC chose to allow UWB to radiate at a power spectral density of -41.3 dBm/MHz, which is the same level allowed for Class B unintentional radiation — the same power level that is considered "background noise" for consumer electronics devices.

As the UWB industry has undertaken the task of working with the regulatory bodies around the world to get approval outside the US, the debate over what constitutes harmful interference has been heated, especially in Europe. The central issue is not whether UWB can cause interference; it is possible to construct usage models of UWB that do indeed interfere with other systems. The debate is over whether these usage models are relevant and whether the interference is serious enough to warrant protection of the victim service. In this article, we consider the case where the incumbent signal is in the same band as the UWB signal is considered, and examine mitigation strategies for minimizing the interference potential of UWB.
Interference — When Can it Occur?
Before analyzing this case in detail, it is useful to analyze the amount of power that can be coupled into another radio’s receiver from a UWB transmitter. Since the FCC rules specify the UWB power as a spectral density, we merely have to examine the effective bandwidth of the incumbent receiver to determine the total power coupled into the receive chain, as long as the UWB transmitter has a constant power spectral density over that incumbent receiver’s bandwidth. This can be written as an equation as follows:

Equation 1. Peff = -41.3+10Log10(Beff)

Where:
Peff = the effective power available before any path loss or other losses Beff = the effective bandwidth of the incumbent receiver

The power available to be coupled into an incumbent’s receiver is more complicated than this, however. As the Peff increases, so does the noise floor; the noise power that the incumbent receiver will observe in its bandwidth is, at a minimum, the thermal noise due to electron movement, which is given by:

Equation 2. N0 = kTBeff

Where:
N0 = the noise power at the receiver’s antenna
k = Boltzmann’s Constant (8.617 339 × 10-5 electron-Volts/°K)
T = temperature in degrees Kelvin, typically taken as 300°K for room temperature And Beff is the same as above.

There are convenient typical values of N0 that are commonly used; in a 1 Hz bandwidth at room temperature (27°C or 300°K), N0= -174 dBm, so in a 1 MHz bandwidth over which UWB power spectral density is defined, N0 = -114 dBm.

In addition, we need to estimate the path loss between a UWB transmitter and a potential receiver. Based on studies done in IEEE 802.15.3a, we can estimate the path loss in a UWB link assuming free space path loss (FSPL) and far field propagation. The path loss is given by:

Equation 3. PL = 45+20Log10(d) Where: PL is the path loss in dB d = distance in meters

These factors taken together can be used to estimate the potential for interference to an incumbent receiver due to a UWB transmission. If we assume that the incumbent has a 6 dB noise figure1and is within line of sight of the UWB transmitter such that the path loss equation given by Equation 3 applies, then we can calculate the distance at which the received power will equal the noise floor of the incumbent receiver, for a given transmitter power in the UWB system. For example, if the UWB system is transmitting at a constant power of -10 dBm over a 528 MHz bandwidth, and the incumbent system has a bandwidth of 27 MHz, then an effective power level of -27 dBm (calculated using Equation 1) would be available. The 27 MHz bandwidth receiver would have an N0 of -100 dBm, and using Equation 3, we can calculate the distance at which the UWB signal will be 6 dB (the assumed noise figure of the receiver) above N0 as being 12.6 meters. Thus, an incumbent receiver will see the UWB receiver at or below its noise floor at any distance beyond 12.6 meters, even assuming free space propagation. If we inspect Equations 1, 2, and 3, we can see that this distance is only related to power level and is independent of bandwidth, as long as the UWB system has a flat power spectral density across the incumbent’s bandwidth. Figure 1 shows this noise floor distance as a function of the power spectral density of UWB. As this clearly shows, the distance at which UWB signals can interact with an incumbent receiver falls off dramatically as the UWB signal power drops.

With this as a starting point, we can look at a number of existing services and determine the distance at which the UWB system will begin to interact with incumbent services.
Coexistence with Systems in the 3 to 5 GHz Band
Downlink (receive) only systems such as C-band satellite and radio astronomy pose unique problems for coexistence because there is no practical way to detect the presence of a receiver. Fortunately, the power levels of UWB are very low by definition, so the potential for interference is small; more importantly, both of these systems require highly directional antennas, which must be pointed into space, and the antennas must be located outdoors. For these reasons, UWB is not likely to cause significant interference to these systems.

Transceiver systems such as WiMax in the 3 to 5 GHz band are an interesting case to analyze for interference; WiMax systems (also called Broadband Fixed Wireless Access) are similar to

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Figure 2. Collaborative coexistence for traffic arbitration of collocated systems.
cellular systems in that there is a hub mounted on a tower that communicates with subscriber terminals scattered around a cell. Thus, there is a downlink signal that is relatively weak by the time it reaches a subscriber terminal, and an uplink signal that is fairly strong in the local environment. The uplink-downlink signals can either be on separate frequencies (called frequency division duplex – FDD) or can be on the same frequency, but separated in time (called time division duplex – TDD). The interference issue is one of proximity; at the distances that UWB and WiMax could interact, there are at least two solutions that could be employed: Detect and Avoid (DAA), and collaborative coexistence.

DAA can be used at distances of a few meters of separation between the WiMax-enabled device and the UWB-enabled device; it is based on the idea that the WiMedia UWB PHY can be used to perform spectral analysis of the local environment. There have been a number of papers published on this topic2 which show that WiMax signals can be detected at reasonable power levels (above -70 dBm) and the WiMedia PHY can either drop a band or create a "notch" within a band whose depth is approximately 15 to 20 dB, although greater notch depths are under study.

The WiMax detection problem is significant and will not be addressed in this paper. Unfortunately, much of the discussion in regulatory meetings has focused on detection levels of -75 dBm/MHz and below, a level at which the interference around a personal computer will cause significant numbers of false alarms because of the inability of the UWB receiver to distinguish between WiMax downlink signals and spurs or other noise signals in the local PC environment.

For WiMax and UWB devices that are collocated, it is more effective to manage interference by controlling access to the wireless medium in the time domain; in other words, arbitration between the WiMax and UWB systems can prevent collisions from happening at all. These techniques have been studied extensively for coexistence between Wi-Fi and Bluetooth in IEEE 802.15.2 and IEEE 802.19; the same techniques could be employed directly for WiMedia UWB and WiMax coexistence. In general, these time arbitration techniques, such as Packet Traffic Arbitration (PTA) in 802.15.2, analyze traffic in both systems and decide which should be allowed on the air. A way to visualize this conceptually is as a traffic model, as shown in Figure 2. As in any traffic arbitration system, there has to be a rule to decide which packets are allowed on the medium; for a simple traffic light, the two systems take turns allowing traffic to flow, which would correspond to alternating between UWB and WiMax, where one system is completely shut off while the other is operating, and this alternation happens at periodic intervals. This clearly works well, but limits the throughput in either system. There are more sophisticated techniques such as PTA3 that examine each packet and decide which should be allowed on the air based on the packet type (beacon, QoS, best effort), and can be more highly optimized. Since these techniques are designed for collocated systems, it is up to the developer to decide the level of performance required based on assumed usage models.
Summary and Future Directions
This article has shown that there is minimal impact on incumbents in most usage models; for systems such as WiMax that must operate in close proximity to the UWB system. There are a variety of techniques, including MAC layer arbitration, that allow performance to be optimized to very high levels of coexistence. About the Author Jim Lansford, Ph.D. is chief technical officer of Alereon, Inc., 7600 North Capital of Texas Hwy., Austin, TX; (512) 345-4200; www.alereon.com

References: 1 The WiMax specification calls for a maximum of 7dB noise figure. 2 See "Multiband-OFDM and Cognitive Radio for WiMax Coexistence," Lansford, Jim, ICUWB-2005, Yokosuka, Japan, December, 2005. 3 The 802.15.2 Recommended Practice can be downloaded from IEEE at standards.ieee.org/getieee802/download/802.15.2-2003.pdf

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