Direct Communication between devices in case of disasters

Yesterday, a discussion started after I read this article on RCR Wireless News:

As in every major disaster, communications networks quickly showed their inherent weakness in times of greatest need. Japan's NTT Communications reported outages affecting Internet voice data that relies on IP-VPN technology.

In a brief statement, the operator apologized for the "trouble and inconvenience," following the string of earthquakes and significant aftershocks that rattled nerves and buildings throughout much of Japan. Some communication services are no longer available, NTT said, and telephone service, particularly long-distance service, is showing strain as well.

Service disruptions have been reported by all three of the major mobile operators in Japan, according to BusinessWeek.

This prompted me to ask on Twitter about which technologies are available that can help the mobile network cope with these problem.

Here are few approaches:

I blogged earlier about Multihop Cellular Networks (MCN) and ODMA. These technologies have their own limitations and problems and I have not heard of anything more about them being standardised or adopted.

Another post was on Ad-Hoc Networks that can be formed in case of failures resulting in Mobile devices being able to communicate directly without the need for network or base stations. The slight problem is that this approach replies on WiFi being available which may not always be the case.

A colleague suggested that in Tetra, Direct Mode of operation is available that is intended for situations like these. A presentation is embedded below:

Steven Crowley on twitter suggested that 802.16m has already started working in this direction. I got a related presentation on that which is embedded below:

Finally, Kit Kilgour mentioned about DSAC (Domain Specific Access Control) whose intention is to discontinue the voice service in emergency (to avoid congestion) but continue the packet domain normally. I have not looked at DSAC on this blog but in LTE instead Service Specific Access Control (SSAC) is used since LTE is PS only. See the blog entry here.

Please feel free to add any more information on this topic in the comments.

Multihop Cellular Networks and ODMA

While going through September issue of IEEE Communications magazine, I came across Multihop Cellular Networks (MCNs). The concept sounded familiar and another article confirmed my suspicion. MCNs is similar to ODMA for those who remember the early 3GPP specs. ODMA or Opportunity Driven Multiple Access was revolutionary concept but it was too advanced for that time and the chipset (and battery) technology was not that advanced to have it implemented successfully.

The best place to understand ODMA is in 3GPP TR 25.924. Also see this.

So what exactly is multihop cellular network (MCN)?

To quote from IEEE Communications Magazine (Sep 07):

MCN incorporates the flexibility of ad hoc networking, while preserving the benefits of using an infrastructure.

The salient feature of MCN is that communications are not restricted to single hop; multihop transmissions are allowed.

The advantages of using MCN include capacity enhancement, coverage extension, network scalability, and power reduction. However, there are still a number of open research issues that need to be solved in order to provide efficient and effective multihop transmissions in cellular networks in the future.

From another article in the same issue:

Existing architectures and protocols proposed for MCNs are very diverse and different in several aspects. Relay Stations (RSs) can be preinstalled by network operators or simply be other idle MHs who are not transmitting their own data. Also, depending on how radio resources are allocated for routing paths of active connections, different protocols at the medium access control and routing layers can be designed. Radio resources forMobile Hosts (MHs) at different hops may be allocated in timedivision duplex (TDD) or frequency-divisionduplex (FDD) mode. Frequency bands other than the cellular frequency band may be used for relaying. Finally, advanced techniques using cooperative diversity can be employed to enhance network performance compared to simple relaying schemes.