Mobile ad hoc networks (MANETs) have become the backbone of tactical military communication because of their exceptional flexibility, security, and reliability. While MANETs have many unique capabilities, however, they have not yet realized their full potential. A new technology, self-interference cancellation (SIC), could greatly enhance both the performance and functionality of MANETs.

One of the essential features of a MANET is that it has no fixed infrastructure and no single point of failure. A MANET can be deployed anywhere, at any time, and are self-configuring and self-healing. These capabilities make MANETs particularly attractive for tactical military communication.

The primary capability that enables mobile ad hoc networking is that every mobile device is dual-purpose: It is both an access device for the mobile user and a mesh node for relaying traffic. Ideally, the mobile user should be able to communicate even as that user’s device is forwarding traffic to other nodes. In practice, however, it’s difficult for the mobile device to receive other channels while it is transmitting because the roar of its transmitter desensitizes its receiver, severely limiting the receiver’s ability to hear signals from remote nodes.

The essentials of MANETs

In fixed deployments, local access and network backhaul channels can be assigned different frequencies in different parts of the same band, and filters can be employed to provide isolation between the access and backhaul radios. However, MANETS are ad hoc networks and, consequently, frequencies can’t be planned. Channels are assigned as needed based on current network topology and frequency availability. Every radio must be able to communicate on any channel in the MANET’s operating band (typically in the UHF spectrum). Conventional filters are of little use, because conventional filters isolate specific frequencies.

The need for frequency agility applies to all mobile ad-hoc networks. Military MANETs, however, involve added complexity that makes segregating the band using traditional filters virtually impossible: They employ frequency-hopping technology to thwart jamming, mitigate multipath fading, and prevent eavesdropping. Military MANETS create “channels” by assigning specific (pseudorandom) hopping sequences to users. These channels can’t be isolated using filters because the transmit and receive frequencies are constantly changing. Nor can the hopping sequences be in any way coordinated to minimize interference; frequency-hopping is jam-proof precisely because the hopping sequences appear random to outside observers.

Inevitably, trade-offs are made so that each mobile unit can act both as an access device and mesh relay node. These concessions may negatively impact response times and throughput. That is a major problem as today’s military MANETs are expected to handle a mix of voice, data, and video.

Empowering MANET devices with SIC

Self-interference cancellation technology was originally developed to enable what researchers call “in-band full-duplex” operation. SIC permits a two-way radio to transmit and receive on the exact same channel at the exact same time, doubling spectral efficiency. This feat is accomplished by canceling the noise that the transmitter in a two-way radio produces in its receiver. It works somewhat like a noise-canceling headset, except that SIC can handle much louder noises and is much more accurate. SIC observes and models the noise and uses the information gathered to produce an inverted copy of the noise. The noise is canceled out by combining it with a continually updated, inverted copy at the input of the receiver. (Figure 1.)

Figure 1: SIC chip adds cancellation signal to the input of the receiver.

As useful as in-band full-duplex operation is, there are more immediately compelling applications, such as canceling the noise that a local transmitter produces on other channels in the same frequency band. (Figure 2.) This enables a device containing two or more radios to listen on other channels while one of the radios is transmitting. The ability to “listen while talking” can benefit not only military MANETs but also consumer products such as whole-home Wi-Fi networks.

Figure 2: SIC technology can eliminate the interference a transmitter creates on other channels in the same frequency band.

All wireless mesh networks face the same basic challenge. How can unit “B” relay data from unit “A” to unit “C”? Unit “B” can’t transmit to unit “C” while it is receiving from unit “A,” because when unit “B” turns on its transmitter its receiver can no longer hear unit “A.” Because of this limitation, large-scale wireless mesh networks have not been successful in commercial markets. There are small-scale wireless mesh networks on the market – such as those used to create whole-home Wi-Fi networks – that solve the problem using one of the following techniques:

A single radio switches rapidly between receiving from unit “A” and transmitting to unit “C”: This store-and-forward approach avoids interference between the transmitter and receiver, but throughput is reduced 50 percent with each hop. Dual-band Wi-Fi mesh products typically use this approach. However, if there are multiple hops to the destination node, then end-to-end throughput falls off a cliff.
Two radios operate on different frequency bands: A mesh network can be configured so that neighboring nodes transmit and receive on different bands. For instance, unit “B” could listen on 2.4 GHz and transmit on 5 GHz. This approach was tried by a few wireless mesh vendors but never caught on. It is also a type of frequency planning and can’t be employed by mobile ad hoc networks for the reasons mentioned earlier.
Two radios operate in different parts of the same band: So-called “tri-band” Wi-Fi routers typically use the UNII-1 and UNII-3 sub-bands in the 5 GHz band. The high-end Netgear Orbi home Wi-Fi system is an example of this approach. Again, this is a form of frequency planning, so it can’t be used with MANETs.

None of the above approaches is suitable for large-scale ad hoc networks carrying time-sensitive voice, video, and data in military environments. However, self-interference cancellation solves the problem by providing a more flexible way to isolate transmit and receive channels. While conventional filters offer only fixed responses – high-pass, low-pass, or band-pass for specific frequencies – SIC offers a dynamic response under software control. Rather than blocking only predefined frequencies, it cancels the noise produced by a local transmitter on any channel, on command. Moreover, while conventional filters require a transition band – they can’t suddenly go from blocking to passing signals as the frequency changes – SIC can produce a very sharp response, enabling the transmitter and receiver to operate even when they are on adjacent channels. (Figure 3.)

Figure 3: SIC’s sharp response enables use of the immediately adjacent channel, as shown by this spectrum analyzer screen shot.

Employing self-interference cancellation offers three major advantages for military MANETs. First, by allowing a second radio to receive while the mesh radio is transmitting (“listen while talking”), SIC enables each node to act as both an access device for the mobile user and a mesh node for relaying traffic. Second, it minimizes latency and maximizes throughput by enabling every mesh node to transmit and receive at the exact same time. Third, SIC is the only approach that has the agility and speed to isolate frequency-hopping channels – an essential requirement for today’s military communication.

SIC for on the move

Military MANETs promise exactly what military users need: a robust communication solution that can deploy anywhere at any time, and that is self-configuring and self-healing. However, interference between the two (or more) radios contained in each node limits their performance. While conventional filters can reduce interference in other environments, they are inappropriate for mobile ad hoc networks. Only self-interference cancellation technology can eliminate the interference with high precision and allow MANETs to finally achieve their full potential for military users on the move.

Joel Brand is the VP, product management, for Kumu Networks. For more than 25 years Joel has actively participated in defining and building infrastructure solutions for the telecom and wireless industries. He previously held executive positions at ConteXtream (acquired by HP), Bytemobile (acquired by Citrix) and Ruckus Wireless (Now part of Arris). Joel holds a B.Sc. and M.Sc. in Engineering from the Israeli Institute of Technology (the Technion) and an MBA from National University in San Diego.

Kumu Networks www.kumunetworks.com