Topic 10: License-Exempt Wireless Applications for Public Safety

Tech Topic #9 closed with a discussion of some of the issues that would accompany the application of cognitive radios in a public safety scenario. These issues illustrate the vast potential for cognitive radio to serve the public safety community while at the same time suggesting that many problems remain to be solved before the "open" use of public safety spectrum could take place. An alternative approach that is already being used by commercial and private users as well as the public safety community is to take advantage of the "common access" and minimal costs available in the license-exempt spectrum bands. This topic addresses some of the basic concepts for use of the license-exempt spectrum in addition to considering some of the issues presented by this approach to satisfying the needs of the public safety community.

In the event of localized incidents or even massive disasters, reliable, immediate and efficient public safety communications between agencies and first responders is crucial. The rapid deployment of seamless voice communications is mission critical for first responders who may also need layouts or blueprints of buildings, the transfer of real-time video at the scene, or geographical and other broadband data to carry out missions effectively. However, ubiquitous coverage of anywhere and anytime public safety communications has always been desired but more difficult in reality to accomplish. Unpredictable incidents that occur when public safety communications infrastructure may not be available challenge the network designer who must find a way (Remember; just make it happen?!).

Further, such critical communications must go beyond just voice-oriented communications and now tend towards multimedia broadband applications that require higher bandwidths during peak operations in real-time. Thus, the response may require an alternate method of broadband communications in a situation where an existing fixed public safety land mobile radio network infrastructure may not be able to support the requirements. The Commission has been very proactive in allocating addition spectrum for licensed public safety broadband use - such as the public safety allocations in the 4.9 GHz band and the 700 MHz band. However, a number of public safety agencies are turning to license-exempt spectrum as an alternative or complementary means of providing broadband capabilities.

A wireless communications technology that has gained considerable application in municipalities across the country, called wireless mesh networks (WMNs), may be able to bring an alternative rapid terrestrial connectivity, interoperability, and reliability to emergency workers and public safety officers in the field. WMNs may be used to establish a wireless communications network to support a municipal public safety agency relatively easily and cost effectively and several municipalities across the country have chosen this method to provide for communications for their first responder community. Also, WMNs may be used to rapidly extend or expand existing networks when circumstances require additional connectivity for responding fire, police, or emergency medical personnel.

A wireless mesh network is a communications network composed of radio access nodes organized in an overlapping mesh topology where each node relays or routes information from other nearby nodes.1 Because of its nature as an infrastructure-less distributed communications architecture for relaying traffic, the size and the range of a network can be dynamically elastic but it simultaneously requires small transmit power levels to restrain potential interference. In general, most wireless mesh networks utilize the technology of license-exempt and inexpensive wireless local area networks (WLANs).

Although a mesh technology itself has no restrictions that require the use of any particular spectrum band, WLAN unlicensed spectrum bands are most commonly used due to the minimal cost associated with "free" license-exempt spectrum. The typical WLAN communications range is limited by the low transmit powers to relatively short distances, approximately 100 meters, hence, mesh networks can be a complementary architecture to extend the range of a WLAN, where the mesh end-node can flexibly join and leave the network. Such dynamic network architectures combined with the possibility for multiple paths for routing of traffic between end-user destinations provide for a scalable network for on-demand and reliable communications. Therefore, WLAN radio access coupled with mesh networking may be a viable alternative for public safety communications in some situations. In addition, this architecture can provide an inexpensive method of routine remote surveillance and monitoring tasks for public safety.

WLAN was originally designed and developed as a home networking technology for nomadic users to wirelessly extend an Ethernet equivalent local area network (LAN) using shared communications media among a group of users through a wireless connection that operates at relatively short distances. WLAN uses license-exempt spectrum bands regulated by FCC rules, 47 C.F.R. Part 15.2 The FCC originally conceived the license-exempt bands to provide a no-cost slice of public access spectrum with only two provisions. First, the transmitter could cause no harmful interference to any nearby licensed services, and secondly, any receiver in this band must be able to accept any interference that may be present. Subsequently, the first wireless LAN was developed by the IEEE 802.11 standards committee (widely known as Wireless Fidelity or 'Wi-Fi' and 'Radio LAN') in 1997. Interestingly, the Wi-Fi standards were a response on the part of industry to the relatively restriction free use of the license-exempt spectrum allocation and rules. Three standards were ultimately established:3

  • IEEE 802.11b (11 Mbps in the 2.4 GHz band)
  • IEEE 802.11a (54 Mbps in the 5.2 GHz band)
  • IEEE 802.11g (54 Mbps in the 2.4 GHz band)

The standards encompass 83.5 MHz of unlicensed spectrum bandwidth in the Industrial Scientific and Medical band (2.4 GHz) and 300 MHz bandwidth in Unlicensed National Information Infrastructure (U-NII) band (5 GHz). Hence,

  • 2.4 GHz - 2.4835 GHz in ISM
  • 5.150 - 5.250 (indoor), 5.250- 5.350 (in/outdoor), and 5.725 - 5.825 GHz (outdoor point-to-point only) in U-NII
  • 5.470 - 5.725 GHz

Because no specific rules were provided for the unlicensed bands, the IEEE established the standards and in fact provided a channelization plan for the band via the standard. The channelization plan for IEEE 802.11b is shown below in Figure 1. It suggests that although the band can be used "without restriction," in fact there is a de facto standard that is used for equipment design, compliance, and certification and therefore there is a specific plan for spectrum utilization. It should be noted that each channel is 20 MHz wide and as a result, there are only 3 non-overlapping (read non-interfering) channels in the 2.4 GHz band.

Figure 1: Wi-Fi Channelization

Similarly, in the 5 GHz band, the Wi-Fi channelization for IEEE 802.11a is show in Figure 2. Again, the channels are 20 MHz wide, but in the 802.11a standard the modulation technique is Orthogonal Frequency Division Multiplexing (OFDM) instead of the spread spectrum techniques of IEEE 802.11b. The 20 MHz channel bandwidth for Wi-Fi is significantly larger than the corresponding 1.25 MHz or 5 MHz bandwidths for commercial mobile cellular systems, but the data throughput differential is significantly larger as a result.

Figure 2: IEEE 802.11a Channelization

In the case of 802.11a and 802.11g, there are five non-overlapping channels to select from. In this situation though, the modulation scheme is much more flexible and tolerant in terms of performance in a noisy or interference plagued environment due to the multiplicity of OFDM carriers. Hence, there is a bit more "forgiveness" in the selection of an operating channel. In any case, and as is true with 802.11b as well, interference issues must be resolved locally; usually done simply by changing to another channel on a local basis. (Remember that "accept any interference provision"? Well, that means the FCC won't help resolve any interference issues either!) It should be noted that any of the channels may be selected for use, but in a limited geographical location, selection of one of these five channels would minimize the potential for interference from other Wi-Fi access points in that limited area.

WLAN systems have become increasingly popular and widely deployed for a variety of unlicensed wireless broadband applications worldwide including enterprise solutions as well as extending the range of in-home networks. The growth of "hotspots," or Wi-Fi access points, to provide access to the Internet has been phenomenal. Many advanced wireless technologies such as SDR/CR (Software Defined Radio / Cognitive Radio), MIMO (Multiple-In - Multiple-Out) antennas, and more sophisticated modulation and access technologies are being adopted to support the WLAN domain. Furthermore, WLAN applications are being extended beyond just nomadic to include mobile services as well through technologies such as the site-to-site radio handoff for mobile terminals. Typical examples of this use are the Wi-Fi access provided on some railroad trains and the access provided for mobile car-mounted terminals in police vehicles. These applications bring attention to the use of WLANs as an alternative method of public safety communications either from an infrastructure perspective or as an emergency ad hoc method to gain communications at the scene of an incident.

The technical characteristics of Wi-Fi are summarized in the table below.

Summary Table of IEEE 802.11 (Wi-Fi) Family
Family 802.11 802.11b 802.11a 802.11g 802.11n
Frequency 2400-2483.5 MHz 2400-2483.5Hz 5150-5250 MHz
5250-5350 MHz
5725-5825 MHz
2400-2483.5 MHz 2.4GHz and
5 GHz
83.5 MHz 83.5 MHz 300 MHz 83.5 MHz Same
Number of Channels FHSS: 79 ch
DSSS: 3 or 6
3 12 3 Same as
Channel Width 20 MHz 20 MHz 20 MHz 20 MHz 20 MHz or
Standard year July 1997 Sep. 1999 Sep. 1999 March 2002 started Expected in October 2008
Spatial Streams 1 1 1 1 1,2, 3, 4
Max PHY rate 2 Mbps 11 Mbps 54 Mbps 54 Mbps 144 Mbps
Up to 600 Mbps
Data Throughput <1.2 Mbps < 5 Mbps < 32 Mbps < 32 Mbps <80Mbps, 11g
<160 Mbps, 11a
Data Rate
1, 2 1, 2, 5.5, 11 6, 9, 12, 18,
24, 36, 48, 54
1, 2, 5.5, 11 NG WiFi
Max. Power
50, 250, 1000mw 1000 mw Same
16 - 64 QAM
Way of Use Indoor/
Indoor, Outdoor Indoor, Outdoor Indoor, Outdoor

There are a several technical features of WLANs that should be spotlighted due to their relation to previous Tech Topics. The first is the flexible data rate that can step up/down depending on the available signal strength. Closely related to the data rate is the flexible modulation scheme. By sensing received signal strength, the Wi-Fi linked terminals negotiate the appropriate modulation scheme that in turn determines the available data throughput on the channel. This is an interesting application of cognitive radio techniques discussed previously in Tech Topics #8 and #9.

Even though the WLAN operational range is relatively short due to the 1 Watt transmit power limitation and the interference restrictions, the capacity for high data rates can be useful and has the additional advantage of lower cost. The wireless mesh networking technology extends WLAN's ability to span wider end-to-end range. Mesh networking not only extends range but also improves overall network capacity when more nodes are added. For a mesh network architecture, two schemes are well developed: Infrastructure-mesh and Client-mesh. These two types of mesh networks can be combined to form a hybrid network. In fact, Client-mesh is also known as "Ad-hoc" where each node relays other traffic to an access node (usually one of Infrastructure-mesh nodes) that routes traffic outside the Ad-hoc network, most likely IP-based Internet traffic.

Recently, public safety agencies across the country have been able to leverage the widely successful private sector wireless technology as a complementary solution for disaster and municipal public safety communications.4 Examples include Oklahoma City, Oklahoma; Silicon Valley, CA; Minneapolis, MN; and many others.5

Although Wi-Fi and Mesh Networks are a readily available solution for public safety communications requirements, there are several shortfalls to using this method for PS communications. First and foremost is the factor of network access and control. If the network is built by a municipality and the network will provide commercial access in addition to use by PS entities, then the requirement for "assured access" by PS may be questionable. The network will be available to all who wish to use it and in some circumstances access may not be available for PS. This would be especially true in a crisis situation on a local level. In addition, management of the network is critically important and PS must have the ability to control authorized and verified access.

Next, operations and maintenance of the network are key issues. If the municipality that is supplying the network is also maintaining it, then there may be some confidence that issues can be resolved expeditiously. Otherwise, portions (or all) of the network could become unserviceable to the PS community - an eventuality that would be unacceptable under any circumstances.

Finally, in depth consideration must be given to complete and thorough engineering of the network to ensure that complete radio coverage of the operational area is maintained. Because meshed Wi-Fi networks rely on the interconnection of short range Wi-Fi radio nodes, thorough engineering analysis must be accomplished to insure that complete geographical radio coverage is maintained. This is especially critical in areas where only one radio access point is available.

This Topic has extended our discussion of reconfigurable radio systems that support public safety to include the dynamically configurable Wi-Fi based Mesh Networks. In the next topic, the evolution of Wi-Fi to WiMax will be considered.

1 For a more in-depth discussion of mesh networking, see the Tropos® Networks, Inc. presentation at or the Nortel Networks, Inc. presentation at

2 See for the Part 15 rules. Note that the letter versions of the standards are not chronologically consistent since version (b) actually came before (a)!

3 See for a copy of the individual IEEE wireless standards.

4 For a concise review of public safety mesh networks, see

5 For example, see, or