LTE-U: U for unwanted or underappreciated?

Confused about LTE-U, LAA? And now MuLTEfire?

Don’t worry–you are certainly not the only one. Even 3GPP had struggled for a while before they finally reached consensus on these terminologies. Simply put, LTE-U is the general term for a modified version of LTE to be deployed in the unlicensed band (ISM band in 5GHz in particular), while License Assisted Access (LAA) is one specific way to deploy LTE-U.

Ericsson has introduced the concept of LAA. It enables the operators to use their licensed spectrum to transmit control data and the unlicensed band only for downlink data transmission. Thus it allows the operator to retain certain level of control on the quality of service (QoS). The catch is that you still need a piece of licensed spectrum to employ LAA.

Then what about MuLTEfire? It is the name Qualcomm branded on their idea of standalone LTE-U. Standalone LTE-U is another way to deploy LTE in the unlicensed band, where both control and uplink/downlink data are transmitted over the unlicensed band without any requirement for licensed spectrum. Everybody could deploy it as a standalone system just like Wi-Fi, hence the name. Unlike LAA, MuLTEfire is not a standard terminology included in 3GPP.

LTE-U vs WiFi

The original idea of LTE-U is fairly straightforward. Operators need more offloading capacity, especially indoors. So 3GPP engineers asked themselves why should we let WiFi have all the fun when we could also deploy LTE in the unlicensed band? The claim is, after all, WiFi is so inefficient in utilizing the spectrum, and LTE could provide a much higher capacity if given access to the unlicensed band.

Technically, LTE, being a centralized system, is indeed much more spectral efficient than the distributed/best-effort WiFi system. However, regulations in many regions require LTE protocol to be modified considerably to adhere to certain etiquette rules, such as ‘listen-before-talk’. This measure is to ensure everyone could still have a fair access to the unlicensed band. Therefore LTE-U would not be as efficient as standard LTE and its performance is only moderately higher than WiFi. For standalone LTE-U, without the benefit of a dedicated control channel, its advantage in throughput would be even less significant.

Where 3GPP is heading?

LTE-U was discussed as early as in the preparations for Release 11. Since then, some technical feasibility studies have been done during Release 12. Gradually the focus shifts more towards regulation requirements and detailed protocol implementations. The LAA version of LTE-U is currently being standardized in Release 13, which is planned to come out by the end of this year.

Despite of these progresses pushed mainly by the vendors, the standardization process for LTE-U is far from a smooth journey. One of the major reasons is the lack of interest, and sometimes the open hostilities, from many operators. One would assume a technology coming from 3GPP would have the best interests of the operators at heart, then why are they not buying it?

Why is LTE-U not popular among operators?

I see two main reasons behind their oppositions: licensed spectrum and WiFi hotspots.

It may seem counter-intuitive that the operators are against deploying LTE in unlicensed bands while at the same time they are asking for more capacity and spectrum. But in fact what they want is not just any spectrum, but only the licensed spectrum—an exclusive strategic asset that has always been the main entry barrier to the mobile service industry. The idea that anyone could deploy standalone LTE-U without a licensed spectrum and offer competitive mobile services is horrifying for these operators. It would substantially dilute the strategic value of the licensed spectrum that they have spent so much to get hold of.

Some operators consider License Assisted Access as a good compromise that allows them to enjoy the advantages of LTE technology in the unlicensed band while keeping the entry barrier intact. However, a few ardent antagonists of LTE-U are worried that the standardization of LAA would place 3GPP onto a dangerous slippery slope leading towards the eventual inclusion of standalone LTE-U.

The latter group of operators shares a common trait: heavy investment in WiFi hotspots comparing to the operators who supported LAA. Why should the dominant players endorse a new technology that would give the underdog a potential edge in the quality of service? Concerns about its coexistence with WiFi have also been raised as a main argument against LTE-U. Because LTE is more resilient to interference, it might overwhelm its WiFi neighbors with more aggressive transmissions. The vendors are trying to demonstrate that LTE-U can be a good neighbor to existing WiFi deployments, which is the main study item in Release 13.

Where the vendors stand?

Both Ericsson and Huawei are mentioning LAA only, since the 3GPP community has agreed that LAA is the politically correct term. They keep themselves a good distance from the standalone LTE-U to avoid touching the operators’ sensitive nerves.

Qualcomm, on the other hand, is persistently pushing its standalone LTE-U idea. Apparently frustrated by the tough resistance within 3GPP, it has set up a so-called ‘LTE-U Forum’ outside 3GPP to gain a wider publicity. So far Verizon has been the only major operator on board. In June, Qualcomm also unilaterally announced its own version of standalone LTE-U, so-called MuLTEfire.

Being the leading LTE chipset manufacture, Qualcomm stands at an advantageous position for pushing its favorite technology in the handset market. But I am not sure whether or how MuLTEfire could be employed independently outside the 3GPP framework, considering the fact that its core is still 3GPP’s LTE standard.

Who wants standalone LTE-U or MuLTEfire?

If the operators are not interested, Qualcomm has made it perfectly clear that standalone LTE-U can also be adopted by broadband ISPs, enterprises, venue owners, and indeed any others lacking licensed spectrum. Sounds like a logical and attractive offer, right?

Curiously enough, Google, which I had assumed to be a potential customer of MuLTEfire, responded negatively. It is alarmed by the development of LTE-U, which might convert the unlicensed 5GHz band into a de-facto licensed spectrum band and drive out other unlicensed users. Cablelabs, representing the cable operators, also voiced their suspicion over LTE-U, fearing its deployment might damage their own WiFi services. The rationale behind these concerns becomes clearer: when the existing solution is cheap and sufficiently good, why pay more for a slightly better but yet unproven alternative?

In conclusion: from a pure technical point of view, LTE-U seems to be a natural step forward to extend the scope of LTE, but in the business reality, it is a tough sale. It’s a classic technology push rather than a product driven by market demand. LAA may gain some traction after it becomes standardized later this year, but it still has to face the property ownership issue that frustrated many Femtocell deployment attempts. The prospect for standalone LTE-U hasn’t yet lightened up after Qualcomm’s announcement of MuLTEfire. Maybe MuLTEfire would become the corner stone of Qualcomm’s next killer product or it might turn out to be another ‘MediaFlo’. The jury is still out…


Lei is a Consultant at Northstream

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One Response to LTE-U: U for unwanted or underappreciated?

  1. jzander says:

    Good summary of the situation, Lei.

    Regarding the performance of LTE-U vs WiFi, the two technologies have radically different approaches for co-existence. LTE is a cellular technology design for simultaneous transmissions in many radio links since as each link is capable of tolerating significant interference from other links. The price paid for being able to transmit almost all the time is a lower instantaneous data rate (due to channel coding). WiFi with is “listen-before-talk” protocol, on the other hand strives for a “clear channel”, where a very high data rate can be used. This means that each station has to wait for its turn but then can “blast away” at maximum speed without suffering interference. Which approach is best ? Well from theory we now that the LTE/cellular approach with simultaneous transmissions is best for noisy conditions, i.e. for large cells, where as in short-range, low noise conditions (e.g. indoor) it is better for the stations to “time-share” a clear channel. What complicates the comparison in practice, are the practical limitations of WiFi – the listen-before-talk mechanism is not capable of perfectly detecting a clear channel reliable.

    What we on the other hand can say, is that if LTE-U is to strictly obey the “listen-before-talk” co-existences rules, it will be stuck with the worst of both worlds: It can (just as WiFi) not perfectly detect the “clear channel”, and at the same time it has a lot of fancy interference mitigation mechanisms that will be rarely used (but will still consume power) as there is little interference to deal with in a clear channel. It is in this case hard to see the really significant performance gains that could motivate the added complexity in LTE compared to WiFi.

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