What's so great about LTE Advanced? (Lots, actually)

Tools

Initial deployments of LTE Advanced are expected to begin in late 2013, but it will take years for operators to explore and use the myriad of features offered by this rich, yet complex, technology.

Standardization work on LTE Advanced, otherwise known as 3GPP Release 10, was frozen in April 2011, and the technology was officially designated in January 2012 by the International Telecommunication Union as one of two approved IMT-Advanced technologies.  (The other is WirelessMAN-Advanced, also known as WiMAX 2.)

While LTE's peak downlink speed is theoretically 300 Mbps, LTE Advanced can ostensibly reach 1 Gbps but realistically would get only 100 Mbps in everyday use. LTE's uplink peak rates of 75 Mbps pale in comparison to LTE Advanced's 500 Mbps.

 

LTE vs. LTE-Advanced Key Performance Indicators

 

Rel. 8 LTE

Rel. 10 LTE-Advanced

Peak data rate

Downlink – 300 Mbps
Uplink – 75 Mbps

Downlink – 1 Gbps
Uplink – 500 Mbps

Peak Spectrum Efficiency

Downlink – 15 [bps/Hz]
Uplink – 3.75 [bps/Hz]

Downlink – 30 [bps/Hz]
Uplink – 15 [bps/Hz]

Scalable Bandwidth Support

Up to 20 MHz

Up to 20 MHz, with band
aggregation up to 100 MHz

Capacity

200 active users per cell in 5 MHz

3 times higher than LTE

 

Yet aside from the improved user experience promised by LTE Advanced, there is the fact that it also offers profound economic benefit for wireless operators because it can reduce the cost per bit delivered. However, LTE Advanced's benefits accrue only if the technology's features are fully exploited, a likelihood not expected to come to fruition anytime soon.

"The best way to describe LTE Advanced is to say it's a long roadmap of capabilities after you launch the first set," said Iyad Tarazi, vice president of network development and engineering for Sprint Nextel (NYSE:S).

LTE Advanced's top features can be boiled down to:

  • RF/carrier aggregation
  • heterogeneous networks (HetNets)
  • and high-order MIMO

But individual operators will have different needs for these functionalities.


Source: Ericsson

Aggregation is a priority

Carrier aggregation covers three main scenarios: intra-band adjacent, intra-band non-adjacent and inter-band. Sprint, which is launching LTE in mid-2012 and plans to upgrade to LTE Advanced in early 2013, is keenly focused on carrier aggregation because the operator aims to exploit frequencies in 1.9 GHz, 800 MHz and its partner Clearwire's 2.5 GHz spectrum. Sprint's initial FD-LTE deployment will use two 5 MHz channels in the 1.9 GHz G band, adjacent to the PCS band, and it will gain another two 5 MHz channels toward the end of 2013 when it shutters its iDEN network and repurposes that 800 MHz spectrum for LTE.

Over time, Sprint will migrate its existing CDMA traffic in the 1.9 GHz PCS band to LTE Advanced, which is where its aggregation capability specifically comes into play because that will enable the operator to eventually use two 10 MHz channels in the band.

"Carrier aggregation lets us stitch those newly available bands together with the bands we launch on day one for LTE and give customers the performance of having a contiguous two channels of 10 MHz. It allows you to repurpose spectrum in a much more meaningful and impactful way than if you were just managing separate bands that don't connect together," said Tarazi.

CTO John Saw clearwire

Saw

Spectrum-rich Clearwire is also eyeing aggregation for its planned TD-LTE network. CTO John Saw said the operator will start offering LTE with a 20 MHz channel and will use intra-band aggregation to combine that with another 20 MHz, eventually creating a 40 MHz pipe at every sector.

Tarazi and Saw both believe inter-band carrier aggregation, which many operators will need to exploit based on their spectrum holdings, will take a longer time to implement than the intra-band varieties.

Experts say inter-band aggregation is feasible despite intermodulation and interference challenges, which are more problematic in terminals than base stations. The bottom line is that some bands will be easier to combine than others. In the U.S., considerable focus will be on aggregating 700 MHz with the AWS bands.

Of course, Sprint and Clearwire aren't the only carriers eyeing LTE Advanced; for example, AT&T Mobility (NYSE:T) has hinted at an LTE Advanced launch for 2013. Click here for the latest on U.S. wireless carriers' rollout strategies.

HetNets, MIMO on deck

LTE Release 8 already supports the use of macro-, pico- and femtocells in a network. But LTE Advanced's HetNet capabilities--such as inter-cell interference coordination (eICIC) and Coordinated Multipoint (CoMP)--enable more spectral efficiency, which will become crucial once networks fill up.

Peter Carson, senior director of product management at Qualcomm

Carson

"You don't typically spend a lot of effort on that topic until you start to see your spectrum resources getting tapped out," said Peter Carson, senior director of product management at Qualcomm. Most operators launching LTE Advanced "have a lot of bandwidth that can be put to bear to support data traffic growth, but right after carrier aggregation, we would expect the next highest priority to be HetNet enhancements in the form of eICIC," he said.

Transmission Mode 9, included in Release 10, is an enabler of high-capacity HetNets. "TM9 gives a lot of flexibility to the number of antennas and streams and whether you use spatial multiplexing or open-loop architecture. Small cells in the coverage area of large cells benefit from new reference signals, for example, that enhance the way the mobile can report the radio environment statistics back to the base station, so the base station can make very advanced decisions on how to transmit the data best to the user terminal," said Hanna Maurer Sibley, LTE product manager at Ericsson (NASDAQ:ERIC).

Also featured in LTE Advanced is support for 8x8 MIMO downlink antennas, as opposed to the present LTE architecture, which supports up to 4x4 MIMO for the downlink. In addition, LTE Advanced supports 4x4 MIMO in the uplink. "Every wireless device is uplink-limited. So if you can do something to help the uplink, you've just extended your range or you've just improved your customers' experience," said Saw, noting Clearwire is pushing its device vendors to exploit high-order MIMO uplink capability.

The benefits of high-order MIMO will be limited by reality in many cases, however. On the base station side of the equation, site acquisition for multiple antennas can be problematic, while adding antennas to a terminal increases its size, cost and complexity. Larger form factors, such as tablet computers, are more conducive to LTE Advanced MIMO schemes that require multiple antennas, said Maurer Sibley, noting: "That will be helpful since those devices are the ones most likely to be hungry for data. HD video is more interesting on a larger screen."