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LTE (Requirements and challenges - 2) - 4

Requirements and challenges - 2 [Under LTE]
» E-UTRA/EUTRAN requirements - 25.913, Rel 8 «

As noted in earlier article, LTE need not stick to WCDMA and can actually go for non-CDMA radio access if advantages are enormous (especially from future LTE+ enhancements point of view).

So our aim is to find improvements to WCDMA for HSPA+ and (possibly new) radio access for LTE that will give better data rates, better spectral efficiency, with good consistency even at cell edges.

As we know (and in fact shown theoretically by Shannon's equation published in 1948), peak data rates are proportional to bandwidth and signal to noise ratio. So our first few challenges are:

1) How to go for higher bandwidth ?

Frequency spectrum is limited (NLOS frequencies) and shared (among technologies, operators etc.) Also, the hardwares need to support wider bandwidths with consistent response. We can not go for as much bandwidth as possible. But yes, what we can do is go for certain discrete values of bandwidths, starting from certain minimum value and scalable till certain maximum value (values to be determined after other considerations). WCDMA bandwidth is already been chosen at 5 MHz, so may be we can not do much here for WCDMA. Also, there are certain disadvantages when higher bandwidth is used for CDMA based systems, so we may not choose CDMA based solution for LTE.

There is another consideration regarding frequency spectrum which is not apparent in theoretical analysis: in practical licensing done by governments, operator may not have access to continuous chunk of frequency spectrum, but rather it may have smaller bands located at different places in frequency spectrum. It would be desirable that radio access we choose can somehow be able to work with distributed frequencies at ease, may be something like Multi-carrier transmission !

2) How to improve signal to noise ratio ?

An obvious answer is to pump-in more signal power. It is equally obvious that putting more power will in fact create more interference to other users, thus defeating its own purpose. What we need is improved the ratio of signal to noise ratio ! Note that more signal power can still be used for special purposes (say while trying to get initial random access).

When we talk about Signal power, we should not miss out on uplink and downlink considerations. In uplink, the peak power that mobile device can pump-in is limited, but in downlink more power can be pumped-in by base station. Then why not go for smaller cell size ? That way, both in uplink and downlink, better rates can be achieved keeping power levels at same level as for bigger cell size. Coming out with appropriate cell size has more to do proper radio planning depending on region, demand etc., so we will not go into more detail there, but yes, cell size will play a role in determining peak data rates.

In downlink, another way to improve received power is to go for advanced technique called "beam forming". In simple words, signals are "focussed" (sort of beam) to particular point so that interference is minimal to farther points and at the same time, we get better SNR at the particular point.

In uplink, received signal power can be improved if base station is capable of catching same signal received via various paths ! What we are referring here is MIMO - wherein multiple antennas are located at receiver (and/or transmitter) to catch (and/or transmit) the multipaths. Beam forming too would require multiple antennas at transmitter.

When we say noise, information for one becomes noise for other. In other words, noise does include interference. But interferences are calculable and so can be reduced to optimum and controllable level.

Above two techniques, beam-forming and MIMO, can be considered as part of "Diversity" techniques. We will take a look at Diversity in later article. In brief, diversity techniques makes use "diversity" available and/or possible in signal power reception to achieve better signal strength. In addition to improving data rates (due to better received signal power), diversity techniques improve coverage.

3) Information rate loss in retransmissions

When we talked about signal power, we did not mention about amount of information contained therein. If signal power is limited, can we put in more information there ? That way we will have better information rate anyway !

Let us take a little more close look at this consideration 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§ §

Requirements and challenges - 1 « LTE Index » Requirements and challenges - 3