Reader Forum: But what about LTE-Advanced?

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In the Red Queen’s Race which is technology, it is always true that you must run as fast as you can to keep position, and that the “next big thing” is always there to be chased after. In wireless, that means that every few years the new generation needs to be developed, engineers work late to design the new products.
For consumers, LTE is nowhere near a concern (even the nerdiest early adopter cannot yet buy anything), for operators it is a current issue – but increasingly for OEMs and development organizations, LTE is “last year’s headache” and their attention is moving on to LTE-Advanced (“next year’s headache”). This is being standardized and managed by 3GPP as LTE Release 10 and beyond.
An interesting thing is how with each generation, the technology problem to be solved changed radically. With 1G to 2G we moved from analog to digital (but the fundamental architecture and logic of cellular had been addressed); then we moved from narrowband to wideband, and enabled fast data services with 3G; as we go from 3G to LTE we move to all-IP networks, to OFDM and to MIMO.
But what about LTE-Advanced?
Strictly speaking, LTE-Advanced (and its counterpart WiMAX 2 or 802.16m) are the first 4G air-interfaces, and they meet the ITU requirement of 4G: 1 Gbps stationary and 100 Mbps mobile. Personally, I think that is incorrect: the ‘G’ should refer to generation, to architecture change. 1G was analog; 2G was TDMA narrowband; 3G was wideband CDMA – and 4G is OFDMA, with very wide bands (20 MHz and up), and a ‘flat architecture’ with end-to-end IP. With that perspective, LTE is 4G and LTE-Advanced is perhaps 4.5G.
One of the interesting things is that this time the changes to the air-interface are relatively minor: we seem to have reached a plateau where Turbo codes, OFMDA and MIMO are “as good as it gets” as architectures. This reflects the reality that improvements to efficiency (“bits-per-second-per-Hz”) have essentially flatlined: we are now so close to the Shannon limit that opportunities are minimal. Indeed, that perhaps has already happened with HSPA+ and Release 8. The enhancements that are happening are mostly to use wider channels and more antennas: eg MIMO is being enhanced for LTE release 10 for support of 8 antennas at eNBs, and uplink transmit diversity/SU-MIMO.
Picochip has a number of customers using our platform for LTE-Advanced development. The attraction of a fully programmable platform, with extremely high performance, a rich set of development tools and a library of certified air-interfaces is an excellent fit for a software-defined radio development or a cognitive radio program for 4G and LTE-Advanced.
So, with what seems to be close the limit on the modulation aspects of the air-interface where do we go next?
Many of the developments come from topology: indeed, the CEO of Qualcomm said “the impact of topology will be as significant as the change from analog to digital”. One of the most obvious, but most important, of these changes is the role of small cells. To get high data rates, we need good SNR – and that inevitably implies small cells. Not only is it true that above a km or so there is no advantage in LTE over 3G, it soon becomes the case that as range increases there is little advantage over GSM. … Of course too, the bigger the cell, the greater the number of users – and the lower the shared (per user) throughput. Then there is the impact of walls: particularly at high frequencies, walls and metallised windows are excellent at blocking RF. But in a nice piece of technology ju-jitsu these apparent drawbacks can be turned to good use, by creating a network as a dense mesh of small cells, with short distances and few users, all of which benefit from excellent data-rates. For those users indoors, which, of course, is where the majority of users will be, then the walls serve to keep that data where you need it and reduce interference from others outside.
These technologies have become viable with 3G (Picochip supplies most of the HSPA femtocells today) but become even more so with LTE and LTE-Advanced. Indeed, while femtocell was a late addition to the 3GPP standard for WCDMA, it is a fundamental feature of LTE, with the Home eNodeB as a standard architecture from Release 8.
With LTE-Advanced these ideas become even more sophisticated, with techniques like collaborative MIMO, multi-Node B routing, mesh and multi-hop relay all coming to the fore. In 3GPP relays are being specified for LTE release 10, others such as collaborative MIMO will be in later releases.
The other areas where there is scope for innovation is in “using resources optimally” – especially spectrum. Some of these are “white space” technologies, or the “spectrum sharing” ideas, but there is a lot research around cognitive radio, assigning different traffic to different streams and optimally using the bandwidth and SNR available. In 3GPP Carrier Aggregation (including cross-carrier scheduling) is being specified for LTE release 10.
Finally (although far less Picochip’s domain), there is protocol stack optimization.
So it is all about 3D improvement: up the stack, across the network in space (topology & small cells), across the network in frequency (cognitive radio), all optimized and “smarter.” But there is far less emphasis on dramatic changes to the specific link of the air-interface, with OFDMA and MIMO staying as the essential foundations.