Diversity [Under LTE]
Literally "Diversity" mean "Variety". In wireless communication, there are various ways by which transmitted signal reach receiver or is received by receiver. Another way to look at it is that transmitted signal gets subjected to various (undesirable) radio conditions before it is received by receiver. Result is faded, corrupted signal. Challenge is to get back the original information - as much as possible - from this received signal.
Diversity techniques
- either make use of these various
possible radio conditions to achieve better signal strength (or
signal-to-noise ratio)
- or condition the signal before
transmission or transmission method so that even though received
signal is faded/corrupted, original information can be extracted
with good quality (low error rate).
As diversity techniques attack undesirable radio conditions, it helps improving both peak data rates and coverage. Due to this, diversity techniques are of high importance in HSPA+/LTE.
In this article, we will not talk about undesirable radio conditions, but rather look at how diversity techniques help in tackling these radio conditions.
Let us take an example of Multipath diversity.
Multipath diversity
When there is no direct line of sight between base station antenna and mobile device, signal is still received by mobile, mostly reflected from various obstacles (like walls, buildings etc.). These signal consists of "multiple paths", all carrying original information, but with varying delays and differences in phase and power. Multipath diversity techniques focuses on catching and combining these multipaths to get back the original signal. Rake receiver is good example of it; it has ability to identify and separate these paths (that is possible within certain time gap). The paths (called fingers) can then be combined using MRC (Maximal Ratio Combining) to get (back) much better signal strength (and so better signal-to-noise ratio).
Interleaving (Time diversity technique)
We know that in a stream of bits sent, it is likely that few successive bits will get lost/corrupted (mostly due to deep fading that may happen any time). If these bits all belong to one coded packet, error correction (used in coding) will fail to retrieve the bits and the packet will be useless. Interleaving splits the bits from coded packets and sends them in different transmissions. So now even if stream of bits is subjected to successive bit errors, once reverse interleaving (i.e. combining of bits for each packet) happen, successive bit errors becomes random errors in different coded packets (that were split and sent by transmitter). These random errors can then easily be corrected by error correction (used in coding). Note that for interleaving to work, the period of time for which successive bit errors happen should be shorter compared to time required for certain number of coded packets (on which interleaving works) to transmit.
As coding and interleaving takes care of successive bit errors that may happen any time, it is called Time diversity.
Note that Rake receiver works directly on actual multi-paths on shorter time duration and it is located in PHY whereas Interleaving tries to overcome successive bit errors on longer time duration and located more up in stack.
Wideband transmission (Frequency diversity technique)
It is possible that certain frequencies will suffer more loss than rest of it due to frequency seletive radio conditions. The frequencies will not remain same, but will vary over time. To overcome this problem, we can split the signal and send it over multiple (preferably non-consecutive/random) frequencies so that even if few frequencies suffer loss, error correction can still get back the original information.
Transmissions like CDMA, OFDM do not face this problem as signals are spread over number of frequencies. More wideband the transmission, better will be protection against frequency selective losses.
In next article, we will look at some more diversity techniques that try to make efficient use of possible radio conditions.
References: UMTS by Sanchez and Thioune, WCDMA for UMTS by Holma and Toskala, 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming, and WiMAX handbook by Ahson and Ilyas.
| Copyright © Samir Amberkar 2010-11 | § |
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