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A large commercial market has emerged that offers various capabilities to satisfy the short-range communication requirements of the U.S. Navy. Some vendors are using the industrial, scientific, and medical (ISM) frequency bands to allow unlicensed use and are building their systems to comply with IEEE Standard 802.11. These systems provide capacities from 1 to 50+ megabits per second (Mbps) and operate over distances ranging from the same room to several kilometers. ISM systems provide a cost-effective way to evaluate and test wireless technology. An iterative build–use–refine model has proven to be an effective way to converge on what is needed and what is affordable. Commercial wireless systems might provide that magical 80 percent solution with a 20 percent price tag.

Navy Applications and Wireless Systems
The key to selecting the right wireless system is identifying the range of environments within which a wireless local area network/metropolitan area network (LAN/MAN) would operate. The Navy needs a reliable, easy-to-use, high-speed capability to distribute information within an amphibious force that includes units at sea and ashore. Typical operations that need more cost-effective communications capabilities include deployed battle groups in an underway formation, intraship LAN operations, and operations in the littoral.

Deployed configurations might have to operate on mobile platforms—both shipboard and vehicular—and contend with weather, normal movements of a ship, and rapid ship and vehicular maneuvers. This means that the wireless system must perform in conditions that are difficult for radio systems, such as a congested radio frequency (RF) environment. Moreover, the RF system must function within the rooms in a ship—or what is essentially an array of large metal boxes—which cause self-interference and limit communications range. Because of these structures, operation of intraship wireless LANs is likely to require a hybrid approach combining the wireless systems with “cabling” (both traditional and lossy”). Wireless systems provide flexibility for network connectivity in unanticipated locations, independent of existing LAN drops.” They also provide flexibility to introduce new shipboard capabilities without having to wait several years for a scheduled dry-dock retrofit.

Which systems best match the deployed users’ needs? To answer this question, MITRE has developed a list of evaluation criteria and examined potential issues to define the scope of a testbed activity.

Evaluation Criteria
Effective Range: A wireless network can substitute for a wired LAN, allowing quick setup and flexible operation for deployed users. It can also support LAN-to-LAN connectivity in a MAN-like application. There are several communication ranges of interest. One is between terminals within a LAN and is typically within 100 meters; another is between LAN clusters ashore, a distance usually within 10 kilometers. The range to a node afloat may be up to 20 kilometers, and there may be over 30 kilometers between nodes afloat. The antenna requirements to support these distances will differ depending on relative antenna heights (about 15 feet high for a line-of-sight distance of five miles), nearby terrain (flat or mountainous), and what the user is able to carry. Large masts and towers are not envisioned for the general wireless LAN/MAN application.

RF Spectrum: Industry currently offers two major 802.11 implementations (see sidebar, Making Wireless Local Area Networks Secure). Because of their modulation techniques and carrier frequencies, these implementations offer somewhat different capabilities. In general, direct sequence spread spectrum (DSSS) systems have greater inherent interference protection and provide greater multipath mitigation, facilitating mobile operation as well as operation in “cluttered” environments. Note, however, that interference protection and multipath mitigation decrease as data rates increase. Orthogonal frequency division multiplexing systems typically provide higher data rates: up to 55 Mbps currently and up to 100 Mbps in the future, compared with a rate of up to 11 Mbps for DSSS. The RF propagation effects on these systems operate similarly within line-of-sight distances; their range differences result largely from the transmitter and antenna constraints imposed by frequency management regulations.

Security: Most military applications require a capability to handle classified information and must be Type I encrypted. Efforts are under way by several vendors to provide an embedded Type I encryptor that is suitable for a wireless classified LAN. Systems without embedded encryption can be used with a separate Type I encryptor to interconnect wired LANs. Because the security aspects of wireless LANs have been extensively investigated, we focused on other aspects of the overall system.

Cost: The market offers many alternatives that provide significant capabilities at a relatively low cost—a fraction of the cost of many military radio systems. With embedded encryption, commercial radios suitable for short-range LAN applications cost around $25,000 per unit. This does not include the added cost for stabilization of the antennas. Using advanced antenna technologies, such as flat panel, phased array, or low probability of intercept (e.g., beam forming), could facilitate antenna stabilization. Incorporating these waveforms and capabilities into planned systems, such as the Joint Tactical Radio System or the Navy’s Digital Modular Radio, may be the best option for the future. In the meantime, the Navy could use the commercial-off-the-shelf versions for operations in which bandwidth poses a challenge.

Size/Weight: The acceptability of any system heavily depends on meeting the constraints of physical space. The equipment suite should fit into a container <1 meter³ when in a stored configuration, and its weight should not exceed 25 kilograms. In a deployed configuration, the antenna should be no larger than 1 meter, with a transceiver no larger than 0.1 cubic meters (1 cubic foot).

Two people should be able to deploy and establish a link within 30 minutes and make the system operational within an hour. Shipboard and vehicular installations are likely to be pre-installed and allow rapid deployment and use.

Data Rate: The wireless LAN should provide throughput equal to or better than that of a 10-Mbps Ethernet LAN. That performance will vary with the information system application, bit error rate, network configuration, and access technique. We plan to conduct significant modeling, simulation, and testing to determine reliable estimates.

Potential Issues
Interference: Operating frequencies should be compatible with the deployed RF environment. The most interference will probably occur in the shipboard environment; the ground environment should be relatively benign by comparison. Analysis and testing based on the class of ship are necessary to site a terminal that will reach deployed forces ashore. The wireless system must be compatible with the standard communication frequency plans for the amphibious support group. Some commercial systems offer only fixed frequencies and bandwidth, and the feasibility of employing them may significantly depend on their ability to reject interfering signals. By operating in frequency bands not directly used on the ship, we hope to reduce the risk to a level acceptable for initial evaluation. The actual impact will require further testing

Operation on the Move: Some deployment applications must function on the move, which, at a minimum, requires stabilization of the antenna system. We are examining whether this can be done at an affordable cost and whether shipboard or vehicular installation is practical. Specific configurations should be capable of supporting ship-to-shore and ship-to-ship, as well as vehicle-to-vehicle and vehicle-to-ship communication. Ship-to-ship operation, while under way, offers additional challenges beyond antenna stabilization. Since the ships are moving relative to each other, diversity techniques with multiple antennas may be needed to mitigate antenna blockage. Networking aspects, perhaps the use of ad hoc networking techniques, will need to be considered for multiple ship applications. Multipath usually occurs during mobile operation, so the capability to mitigate those effects is also important. When ships are beyond line of sight, an airborne relay capability may eventually become part of a networked configuration.

Currently this lies outside the scope of our investigations. The issue is: Can wireless systems be tailored to deal with these aspects and still be cost-effective?

Emission Control: At certain times, shipboard configurations must operate under emission control, a self-imposed condition implemented at the on-scene commander’s discretion. During this condition, a wireless LAN must continue to operate and support the LAN components, but individual segments may not be able to connect with one another; for example, they cannot connect to a deployed (shore) extension. Once emission control is terminated, the LAN segments must be able to reestablish contact with one another and resume operations quickly.

MITRE is beginning to test the use of commercial wireless LAN/MAN systems to satisfy typical requirements of deployed Navy users. We hope to establish how well commercial systems match these requirements and provide a near-term solution, determine the performance of different systems as a function of specific deployment scenarios, and quantify such aspects of performance as throughput. Using an iterative test–use–refine model, our research should help the Navy refine its needs and begin to employ the technology both onboard ship and ashore.

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For more information, please contact Alan Moyer using the employee directory.

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