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, [34] 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.
Summary
What I haven't seen in the SSD so far is to go beyond 2x2 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?
You hit major changes being made in 802.16m.
The development of both WiMAXm and LTE-Advanced target IMT-Advanced which pursues a more unified approach to spectrum and applications that is made feasible by evolution of IC, antenna, RFIC, networking, and markets. To achieve goals for scalability that addresses a range of markets and applications from basic connectivity to multi-service platforms, the industry must evolve to make better use of spectrum through the advances in smart antenna and system architectures. The wireless link technologies are the core enablers needed to reach the goals of IMT-Advanced but are only capable of attaining them when capitalized with a multi-carrier approach.
The impact of the development trends puts the recent U.S. 700 MHz auction into a focus:
Here are the top seven successful bidders, compiled by Goldman Sachs:
1. Cellco Partnership d/b/a Verizon Wireless: $9,363,160,000
2. AT&T Mobility Spectrum, LLC: $6,636,658,000
3. Frontier Wireless: $711,871,000
4. QUALCOMM: $558,142,000
5. King Street Wireless (backed by U.S. Cellular): $400,638,000
6. MetroPCS 700 MHz $313,267,000
7. Cox Wireless, Inc.: $304,633,000
The majority of the spectrum was acquired by operators with intent to deploy LTE rather than WiMAX. The WiMAX Forum on recently established a program for a 700 MHz system profile, far too late to have had an influence on the auction.
Sprint-Nextel and Clearwire expressed confidence that 2.5 GHz was a much better band for wireless broadband. Regardless, neither Sprint or Clearwire had the capital available to compete effectively against Verizon and AT&T.
The auction came in about where I had expected, about double the expectations of the FCC. We had advocated to members of the Forum and potential financial backers that $10-$15 billion would be needed to mount effective competition for the spectrum.
Several analysts have questioned the outcome, suggesting that the parties paid too much. But they and the 'WiMAX camp' missed the significance of making use of sub 1 GHz in combination with higher bands and staking out the path for IMT-Advnaced.
I call multi-carrier development under WiMAXm and LTE-Advanced (IMT-Advanced systems), the Purpose Use of Multiple Spectra, or PUMS.
The impact of sub 1 GHz is certainly to take advantage of desirable long range and penetration properties, particularly useful in rural area connectivity, multicast, messaging, VoIP and low bandwidth web applications. But the real power will come out of a combination of lower and higher frequencies in which the overal network takes better advantage of the signal propagation characteristics of each to deliver a combination of reach & penetration, reliability/continuity of service, and the ability to utilize the spatial-architectural domain made feasible at higher frequencies.
WiMAX faces an uphill battle to gain access to spectrum and develop market momentum. A problem for both WiMAX and LTE is that the majority of spectrum is already spoken for. The vision of IMT-Advanced is to eventually recast IMT-2000 in combination with sub 1 GHz and other spectra that becomes available to develop the multi-carrier, PUMS type system approach. WiMAX might have seized on the 700 MHz auction as a way to gain a jump on the market. Although this owuld have been limited to Clearwire-Sprint and other operators that have uncommitted available spectrum, the move would have gotten WiMAX on a clearer tract for being an evolutionary pathway to IMT-Advanced.
The SDR platforms, multi-band antenna components, RFICs, and common WiMAX (or LTE) ICs now make it practical to pursue multi-carrier for first deployments (with further development down the road to WiMAXm, LTE-Advanced IMT-Advanced) in 2-4 years.
Posted by: Robert Syputa | May 26, 2008 at 08:21 PM
Martin, What we see is that higher order MIMO is likely to be used in BS & RS as options before it is incorporated into MS or as a requirement of the standard of system profiles. At least one chip set is now sampling to provide up to 4X4 MIMO. the use would be within a local area in-building extension, multi-hop link or grid, and similar situations where the complexity is limited to the localized network topology. I think that will reduce issues of complexity for mobile station devices.
Since this and similar enhancements can occur as options or supersets of standard requirements it can avoid conflicts with the standard, provide for innovation and allow rapid progress.
Of course, optional features can become standard requirements and thus venturing too far afield may result in incompatibilities down the road.
But so far I don't see that as a major issue.
This touches on a key differentiation (theory) between WiMAX and LTE: WiMAX supposes to be very open to innovation both within the development of the wireless standards process and system development: somewhat like creating the PC development environment for wireless Ethernet. That extends to being an open access platform which welcomes in efforts such as Android and open device development.
This can be criticized as being too open of an approach both from a context of developing sufficient capital to build out multi-billion dollar networks and from the non-orchestrated way developments might occur. s
Posted by: Bob Syputa | May 30, 2008 at 05:55 AM