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Spectrum Management

Cognitive radio technologies offer new methods for spectrum management beyond simple collision avoidance.

Bruce Fette, Chief Scientist, Communication Networks Division, General Dynamics C4 Systems

The immediate interest to regulators in fielding cognitive radios is to provide new capabilities that support new methods and mechanisms for spectrum access and utilization now under consideration by international spectrum regulatory bodies. These new methodologies recognize that fixed assignment of a frequency to one purpose across huge geographic regions (often across entire countries) is quite inefficient.

Today, this type of frequency assignment results in severe underutilization of the precious and bounded spectrum resource. The Federal Communications Commission (FCC; for commercial applications) and the National Telecommuni­cations and Information Administration (NTIA; for federal applications) in the United States, as well as corresponding regulatory bodies of many other countries, are exploring the question of whether better spectrum utilization could be achieved given some intelligence in the radio and in the network infrastructure.

This interest also has led to developing new methods to manage spectrum access in which the regulator is not required to micromanage every application, every power level, antenna height, and waveform design. Indeed, the goal of mini­mizing interference with other systems with other purposes may be reasonably automated by the cognitive radio. With a cognitive radio, the regulator could define policies at a higher level, and expect the equipment and the infrastructure to resolve the details within well-defined practical boundary conditions such as available frequency, power, waveform, geography, and equipment capabilities. In addition, the radio is expected to utilize whatever etiquette or protocol defines cooperative performance for network membership.

In the United States, which has several broad classes of service, the FCC has held meetings with license holders, who have various objectives. There are license holders who retain their specific spectrum for public safety and for other such public purposes such as broadcast of AM, FM, and TV. There are license holders who purchased spectrum specifically for commercial telecommunications pur­poses. There are license holders for industrial applications, as well as those for special interests.

Many frequencies are allocated to more than one purpose. An example of this is a frequency allocated for remote control purposes—many garage door opener companies and automobile door lock companies have developed and deployed large quantities of products using these remote control frequencies. In addition, there are broad chunks of spectrum for which NTIA has defined frequency and waveform usage, and how the defense community will use spectrum in a process similar to that used by the FCC for commercial purposes.

Finally, there are spectrum commons and unlicensed blocks. In these frequen­cies, there is overlapping purpose among multiple users, waveforms, and geogra­phy. An example of spectrum commons is the 2.4 GHz band. The following sections touch on new methods for spectrum management, and how they lead to spectrum efficiency.

Managing Unlicensed Spectrum

The 2.4 and 5 GHz band are popularly used for wireless computer networking. These bands, and others, are known as the industrial, scientific, medical (ISM) bands. Energy from microwave ovens falls in the 2.4 GHz band. Consequently, it is impractical to license that band for a particular purpose. However, WiFi (802.11) and Bluetooth applications are specifically designed to coexist with a variety of interference waveforms commonly found in this band as well as with each other. Various types of equipment utilize a protocol to determine which fre­quencies or time slots to use and keep trying until they find a usable channel. They also acknowledge correct receipt of transmissions, retransmitting data pack­ets when collisions cause uncorrectable bit errors.

Although radio communication equipment and applications defined in these bands may be unlicensed, they are restricted to specific guidelines about what fre­quencies are used and what effective isotropic radiated power (EIRP) is allowed. Furthermore, they must accept any existing interference (such as that from microwave ovens and diathermy machines), and they must not interfere with any applications outside this band.

Bluetooth and 802.11 both use waveforms and carrier frequencies that keep their emissions inside the 2.4 GHz band. Both use methods of hopping to frequen­cies that successfully communicate and to error correct bits or packets that are corrupted by interference. Details of Bluetooth and 802.11 waveform properties are shown in Table 1.

Table 1: Properties of 802.11, Bluetooth, ZigBee, and WiMAX waveforms

Standard

Name/description

Carrier frequency

Modulation

Data rate

Tx Pwr; EIRP

802.11a (802.11g = both 802.11a and 802.11b)

WiFi; WLAN

5 GHz, 12 channels (8 indoor, 4 point to point)

52 carriers of OFDM, 48 data, 4 pilot, BPSK, QPSK, 16 QAM, or 64 QAM, carrier separation = 0.3125 MHz, symbol duration = 4 μs, with cyclic prefix = 0.8 μs, Vitterbi R = 1/2, 2/3, 3/4

54, 48, 36, 27, 24,1,8,12, 9, and 6 Mbps

12-30 dBm

802. 11b

WiFi; WLAN

2.4 GHz, 3 channels

CCK, DBPSK, DQPSK with DSSS

11, 5.5, and 1.0 Mbps

12-30 dBm

802.15.1

Bluetooth; WPAN

2.4-2.4835 GHz, 79 channels each 1 MHz wide, adaptive frequency hopping at 1600 Hops/s

GFSK (deviation = 140-175 KHz)

57.6, 432.6, and 721 Kbps, 2.1 Mbps

0-20 dBm

802.15.4

ZigBee; WPAN

868.3MHz, 1 channel; 902-928 MHz, 10 channels with 2 MHz spacing; 2405-2483. 5 MHz, 16 channels with 5 MHz spacing

32 chip symbols for 16 ary orthogonal modulation with OQPSK spreading at 2.0 Mcps (2.4 GHz); DBPSK with BPSK spreading at 300 Kcps (868MHz) or 600 Kcps (9 15 MHz)

250 Kbps at 2.4 GHz; 40 and 20 Kbps at 868 and 915MHz

-3-10 dBm

802.16

WiMax, Wibro;
WMAN

2-11 GHz (802. 16a), 10-66 GHz (802.16); BW= 1.25,5,10, and 20 MHz

OFDM, SOFDM: 2048, 1024, 512, 256, and 128 FFT; carriers, each QPSK, 16 QAM, or 64 QAM; symbol rate = 102.9 μs

70 Mbps

40 dBm; EIRP = 57. 3 dBm

The 802.11 waveform can successfully avoid interference from microwave ovens because each packet is of sufficiently short duration that a packet can be delivered at a frequency or during a time period while the interference is minimal. Bluetooth waveforms are designed to hop to many different frequencies very rap­idly, and consequently the probability of collision with a strong 802.11 or micro­wave is relatively small and correctable with error correcting codes.

The regulation of the 2.4 and 5 GHz bands consists of setting the spectrum boundaries, defining specific carrier frequencies that all equipment is to use, and limiting the EIRP. As shown in Table 1, the maximum EIRP is 1W or less for most of the wireless network products, except for the metropolitan WiMAX serv­ice, and the FCC-type acceptance is based on the manufacturer demonstrating EIRP and frequency compliance.

It is of particular interest to note that each country sets its own spectral and EIRP rules with regard to these bands. Japan and Europe each have regulatory rules for these bands that are different from those of the United States. Consequently, manufacturers may either (a) make three models, (b) make one model with a switch to select to which country the product will be sold, (c) make a model that is commonly compliant to all regional requirements, or (d) make a model that is capable of determining its current location and implement the local applicable rules. Method (d) is an early application of cognitive techniques.

Noise Aggregation

Communication planners worry that the combined noise from many transmitters may add together and thereby increase the noise floor at the receiver of an impor­tant message, perhaps an emergency message. It is well understood that noise power sums together at a receiver. If a receiver antenna is able to see the emis­sions of many transmitters on the same desired frequency and time slot, increasing the noise floor will reduce the quality of the signal at the demodulator, in turn increasing the bit error rate, and possibly rendering the signal useless. If the inter­fering transmitters are all located on the ground in an urban area, the interference power from these transmitters decays approximately as the reciprocal of r3.8. The total noise received is the sum of the powers of all such interfer­ing transmitters. Even transmitters whose received power level is below the noise floor also add to the noise floor. However, signals whose power level is extremely small compared to the noise floor have little impact on the noise floor. If there are 100 signals each 20 dB below the noise, then that noise power will sum equal to the noise, and raise the total noise floor by 3 dB. Similarly, if there are 1000 trans­mitters, each 30 dB below the noise floor, they can raise the noise floor by 30 dB. However, the additional noise is usually dominated by the one or two interfering transmitters that are closest to the receiver.

In addition, we must consider the significant effect of personal communication devices, which are becoming ubiquitous. In fact, one person may have several devices all at close range to each other. Cognitive radios will be the solution to this spectral noise and spectral crowding, and will evolve to the point of deployed science just in time to help with the aggregated noise problems of many personal devices all attempting to communicate in proximity to each other.

Aggregating Spectrum Demand and Use of Subleasing Methods

Many applications for wireless service operate with their own individual licensed spectra. It is rare that each service is fully consuming its available spectrum. Studies show that spectrum occupancy seems to peak at about 14 percent, except under emergency conditions, where occupancy can reach 100 percent for brief periods of time. Each of these services does not wish to separately invest in their own unique infrastructure. Consequently, it is very practical to aggregate these spectral assignments to serve a user community with a combined system. The industry refers to a collection of services of this type as a trunked radio. Trunked radio base stations have the ability to listen to many input frequencies. When a user begins to transmit, the base station assigns an input and an output frequency for the message and notifies all members of the community to listen on the repeater downlink frequency for the message. Trunking aggregates the available spectrum of multiple users and is therefore able to deliver a higher quality of service while reducing infrastructure costs to each set of users and reducing the total amount of spectrum required to serve the community.

Both public safety and public telephony services benefit from aggregating spectrum and experience fluctuating demands, so each could benefit from the abil­ity to borrow spectrum from the other. This is a much more complex situation, however. Public safety system operators must be absolutely certain that they can get all the spectrum capacity they need if an emergency arises. Similarly, they might be able to appreciate the revenue stream from selling access to their spec­trum to commercial users who have need of access during times when no emer­gency conditions exist.

Priority Access [1]

If agreements can be negotiated between spectrum license holders and spectrum users who have occasional peak capacity needs, it is possible to define protocols to request access, grant access, and withdraw access. Thus, an emergency public service can temporarily grant access to its spectrum in exchange for monetary compensation. Should an emergency arise, the emergency public service can with­draw its grant to access, thereby taking over priority service.

In a similar fashion, various classes of users can each contend for spectrum access, with higher-priority users being granted access before other users. This might be relevant, for example, if police, fire, or military users need to use the cel­lular infrastructure during an emergency. Their communications equipment can indicate their priority to the communications infrastructure, which may in turn grant access for these highest-priority users first.[2]

By extension, a wide variety of grades of service for commercial users may also prioritize sharing of commercially licensed spectrum. Users who are willing to pay the most may get high priority for higher data rates for their data packets. The users who pay least would get service only when no other grades of service are consuming the available bandwidth.

General Dynamics C4 Systems
Scottsdale, AZ
(480) 441-3033
www.gdc4s.com

Footnotes
1Cellular systems already support priority access; however, there is reported to be little control over the allocation of priority or the enforcement process.
2This technique is implemented in code division multiple access (CDMA) cellular communications.

This article was first published in the March/April issue of Portable Design. Reprinted by permission.

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