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5 6 9 , 6 5 9
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LTE (Requirements and challenges - 1) - 3

Requirements and challenges - 1 [Under LTE]
» E-UTRA/EUTRAN requirements - 25.913, Rel 8 «

As noted in article, "Background", WCDMA based 3G technology is being enhanced and in parallel OFDM/SC-FDMA based 3.99G technology also being developed. Targets for both are kept at similar levels: the idea here is to strive to rip more benefits out of existing 3G deployments and at the same time, make way for newer and better OFDM based technology. By the time LTE standardisation inaugurated in 2004, 3G Rel 6 was ready with HSDPA and HSUPA (collectively HSPA). Note that some of the radio access techniques that we will discuss for HSPA+/LTE are already present in 3G Rel 6 for WCDMA radio access, but discussed anyway for fuller understanding.

That was about access network. Core network also need improvement, but this should be in such a direction that we will have common core network elements for HSPA+ and LTE, making migration to LTE easier. This is of course possible as during 3G design we took an approach of making access network protocols independent from core network protocols (refer initial 3G articles). HSPA+/LTE takes next step with SAE (System Architecture Evolution). In later articles, we will study more about SAE.

Coming back to access: we know that both 3G and LTE PHY technologies are likely to face same or similar challenges and hurdles. First we will have a quick look at requirements for HSPA+/LTE and then we will see the challenges and how HSPA+/LTE cope up to it.

Here are few important parameters and their rough targets for LTE:

- Higher data rates (5-7 times of Rel 6) e.g. DL > 100 Mbps,UL > 50 Mbps
- Better spectral efficiency (2-4 times of Rel 6) e.g. Peak > 5 bps/Hz
- Higher data rates or better spectral efficiency at cell edges (2-4 times of Rel 6) e.g. 0.4 bps/Hz
- Significantly reduced latencies in both control and user planes (almost 1/5th of Rel 6) e.g. Less than 50 ms between Cell_DCH and Cell_PCH states, and less than 5 ms delay for smallest IP packet transfer from UE to/from RAN node.

As can be seen, after Rel 6, focus is not only at getting more data rates/spectral efficiencies but also at improving performance in terms cell edge bit rate, C/U plane latencies at RAN.

Before we take a look at challenges/hurdles faced by HSPA+/LTE in meeting the requirements, one difference need to be mentioned:

Mandatory requirement for HSPA+ is to be compatible with existing 3G radio access. LTE can choose different access parameters and in fact can go for different radio access technology altogether. As an example, an important access parameter is bandwidth. 3G/WCDMA use fixed BW of 5 MHz. Higher (than 5MHz) and scalable bandwidth is desirable to get better results. LTE can design radio access with better bandwidth profile to achieve the targets.

With this background, let us look at challenges (mainly radio access) faced by and solutions used by HSPA+/LTE in next article.

References: 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming, LTE - From Theory to Practice, Edited by Sesia, Toufik, and Baker, and Release 8 document at 3gpp.

Copyright © Samir Amberkar 2010-11§ §

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