Like LTE, WiMAX is also competing for a place in IMT-Advanced 4G and shares the same fate as the current LTE standard: It is too slow. As a result, the 802.16m working group has been tasked by the IEEE to enhance the system. While only few details were available so far, the working group has published a very early draft version of the 802.16m System Description Document (SSD). Thanks to Robert Syputa of WiMAX Pro for the tip.
While there are still many gaps in the document, the main features are already described. Here's a short overview with some further background information:
Use of Several Carriers
Like other standards bodies, the IEEE has recognized that increasing the bandwidth used for data transmission is one of the best ways to increase overall data transfer rates. A multi-carrier approach, in which two or even more carriers, which are not necessarily in adjacent bands, are used for transferring data, will be used by the future WiMAX air interface. The approach used by WiMAX is backwards compatible, i.e. 802.16e and 802.16m mobile devices can be served by the same base station on the same carrier. The 802.16e device, however, does not see the channel bundling and continues to use only one carrier. To be backwards compatible, high speed zones are introduced in a frame, which are only available for 802.16m devices. If the carriers used for transmission are adjacent, guard bands that are normally in place to separate the carriers can be used for transferring data.
Self Organization and Inter Base Station Coordination
Interference from neighboring base stations and mobile devices is undesired in wireless systems, as it reduces the overall system throughput. The new version of the standard introduces methods and procedures to request mobile devices to perform interference measurements at their location and send them back to the base station. The base station can then use information gathered from different devices to adjust its power settings and potentially also to organize themselves with neighboring base stations using the same frequency.
New Frame Structure
In practice, it has been observed that the 802.16e frame structure, with frame lengths of up to 20 milliseconds is too inflexible. The downside of such long frames is a slow network access and a slow repetition of faulty data blocks, as devices only have one transmission opportunity per frame. 802.16m uses a new frame structure which consists of super-frames (20 ms) which are further divided into frames (5 ms) and again divided into eight sub-frames (0.617 ms). Within each frame of 5 milliseconds, the transmission direction can be changed once. Since eight sub-frames fit into a frame, downlink uplink time allocations of 6/2, 5/3, etc. can be achieved. By switching the transmission direction at least every 5 milliseconds,  foresees that HARQ retransmission delays are cut by ¾, the idle to active state transmission time is reduced from above 400 milliseconds down to less than 100 milliseconds and the one way access delay is reduced from almost 20 milliseconds down to less than 5 milliseconds.
What I haven't seen in the SSD so far is to go beyond 2×2 MIMO to further increase data rates. That's a bit strange since LTE is already at this point!? For the moment, I don't see anything that would push the data rates by an order of a magnitude, which I think would be necessary to comply with IMT-Advanced. Unless, however, the ITU is thinking about downgrading their requirements. Thoughts, anyone?