Almost exactly a year ago I first reported on WiMAX Mobile Multihop Relay (MMR). At the time, I didn't have a lot of details of how it was supposed to work. In the meantime, however, the IEEE has published an advanced draft of the spec that answer a lot of questions. Also very helpful to get an idea of MMR is this slideset. So, here's an overview of the functionality:
Especially in rural areas, there are often little or no possibilities to backhaul high bandwidths connections via a fixed line copper or fiber links. The two possibilities are then either dedicated high bandwidth microwave connections per base station or a concept in which the base stations themselves form a mesh like network to forward traffic between base stations with no dedicated backhaul connection.
In addition to rural backhauling, forwarding traffic between wireless network nodes is also an interesting method to fill coverage holes and to improve in building coverage. At first, it might seem illogical that sending a data packet over the air interface more than once actually increases the data rate. In practice, however, transmitting the packet over two or more links with a high signal to noise ratio is better than only transmitting it once but very slowly because the signal quality is low.
The 802.16j amendment to the standard covers the following points to achieve these goals without increasing the number of base stations with expensive backhaul links:
Backwards Compatibility
MMR has been specified in a way that does not require mobile devices to be aware of relay nodes. This is important as introducing relaying would otherwise not be possible in already deployed networks.
Multi Hop Capability
The standard is designed in a way that allows a packet to traverse several hops until it reaches a base station that has a backhaul connection.
Relay Station Implementation Options
From the point of view of mobile stations, relays without a dedicated backhaul connection look like a standard base station and have their own base station ID. The specification allows two kinds of relay stations (RS). A simple RS relays everything up to a real base station, including even simple messages such as ranging requests and leaves the processing of all messages to the base station. Such simple relays are also referred to as transparent relays as all links to mobile devices via relay stations are controlled by a base station. More complex relays, referred to as non transparent relays, are able to locally manage the link to the subscriber and only forward user data packets to a base station and higher layer signaling information.
Summary
An ambitious spec! Let's see if and when we see this in practice