3.5G operators race to counter WiMAX threat

Though the actual rollout of 4G technologies is a long ways away; the selection of which technologies to be used is not.  Because of the head start by WiMAX, proponents of Long Term Evolution (LTE) and Ultra Mobile Broadband (UMB) are scrambling to catch up.  The good news is that LTE and UMB will use the same air interface (OFDMA) as WiMAX.  One benefit is that test companies have a head start in supporting LTE and UMB because of their WiMAX testing capabilities and experience.

It's not expected that the ITU will release an official definition of wireless 4G technology until the 2008/2009 timeframe, but there are already clear contenders of what those technologies should be:  LTE, UMB and WiMAX.

'Companies are extremely uncomfortable talking about '4G' technologies, since the ITU has not defined 4G yet,' says Gemma Tedesco, In-Stat senior analyst.  Thus, 4G technology roll-outs won't likely start until the 2010-2012 timeframe, not with the dominant worldwide technology currently being GSM/EDGE, and HSPA and EV-DO handsets not expected to become dominant until 2012.  In-Stat's guess is that mobile operators will initially deploy 4G very slowly, relying on their EV-DO or HSPA networks to provide for more ubiquitous coverage.

So, if ITU is not going to have an official definition of wireless 4G until 2008/2009, then what's the rush to get the technologies developed today‾  It seems like the industry is getting the cart before the horse.  In a sense, it is.  That's because WiMAX is well along its development path.  The underlying technology that will be used in mobile WiMAX applications is already being incorporated in fixed WiMAX applications. 

Why the rush‾

And to mix metaphors, WiMAX is the 'dark horse' in this race of competing wireless technologies.

'It is generally accepted that, beyond the increasing deployment of important technology enhancements such as High Speed Packet Access (HSPA), both on the uplink and the downlink, significant, further 3G evolution is required if it is to continue to dominate the global cellular market,' according to Phil Windred of Aeroflex Test Solutions, Wireless Division.  '3G will need to compete head-on with DSL in order to win the fixed/mobile substitution battle as well as compete with rapidly developing alternative technologies such as WiMAX for broadband wireless access and DVB-H for broadcast.'

To put it succinctly:  WiMAX-service providers will be viable contenders in the near future to provide not only much faster broadband services but voice services as well.

Technically, in UMTS networks HSDPA (R6) maxes out at 14.4-Mbps download speeds under absolutely ideal conditions, though, this will be increased to 42 Mbps with the Evolved HSPA release (R7).  On the other hand, WiMAX has the theoretical limit of 70 Mbps. Therefore, 3.5G service providers need a fix, and they don't want to wait until the 4G technology standards are finalized by the ITU. 

ABI Research forecasts that 2009 will be an important year as service providers are expected to evaluate the performance of LTE and make choices between it and other broadband technologies.  However, in reviewing the timetable for release of LTE's technology standard by 3GPP, Othmar Kyas, strategic marketing director at Tektronix Network Diagnostics, stated earlier this year that LTE 'is roughly two years behind where WiMAX is today.'  There's a lot of catching up to do.

 

Consequently, LTE development is moving very fast, even faster than any previous new technologies.

Though the bad news for LTE and UMB is that WiMAX had a 2-year lead earlier this year, the good news is that many of the underlying technologies used in WiMAX will also be used in LTE and UMB systems as well.  Therefore, much of the development work 'proving out' the underlying technologies for LTE and UMB has been started (if not already completed) for WiMAX:  To this end, so has some of the needed testing requirements and test equipment.
The goal for LTE is to achieve peak download speeds of 100 Mbps.  This can't be achieved with W-CDMA technology.  Thus, the 3G LTE specification will be based on a change from W-CDMA to OFDMA (orthogonal frequency division multiplexing access) as the air interface.  This is a significant change at the very lowest level of the radio communications - as significant a change as the change from TDMA in 2G systems to W-CDMA in 3G systems.

Key test challenges

LTE is faced with at least three key test challenges:  OFDMA testing, MIMO testing and handover testing.

The switch from W-CDMA to OFDMA is a significant change at the very lowest level of the radio communications link and achieving synchronization between the base station and the mobile device will be a major challenge.  Test products like Aeroflex's TM500 LTE and 6401 are needed to provide complete visibility into the very lowest layers of the radio modem, allowing users to diagnose the actual cause of a synchronization problem rather than just knowing that synchronization has failed.

Not having the higher layer protocol will compound the testing.  It will be necessary to completely configure the physical layer using test scripts.  As a consequence, many early test failures may not be the result of real problems but rather by a mismatch in the setup between the prototype under test and the test equipment.

A further implication of physical layer testing when the higher layer protocol is not available is that test automation is essential to ensure extensive and complete testing.

On the positive side many test equipment companies including Aeroflex, Agilent Technologies, Anritsu, Rohde & Schwarz and Tektronix have test equipment and the capability because of their WiMAX experience to quickly come up to speed with the OFDMA issues now being addressed.
Another essential technology for LTE's success is MIMO.  MIMO is the use of multiple antennas by both the transmitter and the receiver sides of a connection (multi-input, multi-output).  For the LTE downlink, a 2X2 configuration is assumed as the baseline configuration, for example, two transmit antennas at the base station and two receive antennas on the mobile device.  Configurations with four antennas are also being considered.

The dynamics are much higher in networks using MIMO.  The antennas are smart, being able to dynamically react to application service profiles and the environment.  But that makes the network difficult to measure, to assess trends and to isolate problems.  For test equipment vendors the complexity is high with the need to adapt to new RF standards and find new ways for identifying faults.  Work is needed, defining which KPIs are relevant and what is to be displayed to show what is really going on in the network.

 

Test vendors are developing test features especially for MIMO to ensure that both the network and mobile devices are able to get the signaling right and then transmit and receive in full synchronization with the signaling.

Because LTE is an evolution of existing UMTS systems based on W-CDMA and will also be fully integrated with existing GSM/GPRS/EDGE networks as well, seamless handovers will be critical to the gradual rollout of the first LTE networks.  The handovers may simply be inter-cell between neighboring LTE cells or they could be handovers to W-CDMA or GSM/GPRS/EDGE cells as a user moves in or out of LTE coverage.  (The 3G/2G handover problems are only too fresh in our minds.)

'Historically, new technology rollouts have been subjected to delays, often due to problems  experienced when the new technology mobile devices are tested against the new technology networks,' concluded Windred at Aeroflex. '"&brkbar; experience shows that the earlier problems are detected, the lower the cost of correcting them.'

'Earlier' is none too soon for 3.5G service providers racing to counter the threat of WiMAX.



Why the switch from W-CDMA to OFDMA


The basic problem that service providers needed to have solved is how to get more data to users, quicker and cheaper.  However, W-CDMA just won't allow more data to be transmitted - or pushed onto a high-speed data stream - and sent to the user.  OFDM resolves the problem by splitting the high-speed data stream into several lower speed data streams and sending the lower speed streams on individual frequency channels (subcarriers) to the user.  The user's device, then recombines the lower speed streams and effectively sees the information as if transmitted on a single, very high-speed data stream.

An OFDMA handset must be capable of receiving all 2,048 subcarriers defined in the LTE standard that a base station could transmit.  But a base station need only support transmitting 72 subcarriers.  So depending on a number of environmental factors, user data requirements, QoS, etc., a base station determines how many subcarriers are assigned to a user for a particular call.

The 'O' in OFDMA stands for orthogonal.  By using orthogonal carrier waves, the 2,048 subcarrier frequencies can be spaced - in fact overlapped to maximize efficiency - without interfering with each other.  The orthogonal carrier waves (just like the spreading codes in CDMA) must be carefully calculated to cancel each other out as far as possible.  This requires very precisely tuned antennas and sophisticated processing, which prevented OFDMA from becoming available in portable devices before now.



Migration options spur LTE and UMB development


Not only are LTE, UMB and WiMAX based on OFDMA technology, all three will be based on IP-only services with no backward compatibility for circuit-switched services.  To this end, there is a real break - a discrete change - in technologies when moving to 4G; therefore, service providers don't necessarily have to select the evolution path of the 2G or 3G standard that they are currently using when they move up to 4G.

 

For example, it's true that UMTS service providers would most likely move up to LTE.  Handsets will be backward compatible to their existing 3G networks.  The LTE radio network should connect seamlessly to their existing core network infrastructure and services.  But there may be some providers that are willing to make a break and switch to WiMAX if they think they can gain a competitive advantage.

Some EVDO service providers don't see their evolutionary move - UMB (EVDO rev. C) - coming soon enough.  To illustrate, a number of providers in Taiwan and Australia decided to move from their current 3.5G EVDO networks to 3.5G UMTS/HSDPA networks.  Sprint, late last year, announced that instead of evolving to UMB that they have chosen WiMAX as its 4G technology and will start rollout of the network in this year.

Consequently, backers of all three technologies have reason to push harder on their respective standards bodies, technology providers and test-solution vendors to have answers ready for service providers in 2009.

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