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LTE (3G till Rel 6 - HSDPA) - 8

3G till Rel 6 - HSDPA [Under LTE]
» HSDPA overall description - 25.308, Rel 6 «

By the time LTE standardisation inaugurated in 2004, 3G Rel 6 was ready with HSDPA and HSUPA (collectively HSPA). Some of the radio access techniques that we discussed in earlier articles on Requirements and challenges are already present in 3G Rel 6 WCDMA radio access. We will look into 3G radio access enhancements done till Rel 6 starting with HSDPA.

How did we reach HSDPA ?

In 3G WCDMA PHY before HSDPA, appropriate spreading factor is used to realise required data rates. In downlink, OVSF code remain allocated till end of call (network initiated or UE initiated) or reconfirguation. The approach has scope for improvement.

1) Above approach is best suited for circuit switched or constant bit rate services. But for variable bit rate or bursty traffic, channel is not needed all the time with full capacity, making the approach in-efficient.

2) In field scenario, the quality of transmission varies in time (e.g. due to fading as shown in diagram below). So even though channelisation code remain allocated, not all the time the channel is of "full use" due to varying radio conditions. This adds to in-efficiency.

One way to solve these in-efficienciesis is "Shared (and radio condition dependent) allocation of channelisation code".

In this approach, channelisation code(s) can be used for a UE for certain TTI(s). Two immediate questions are:

1) What would be value of TTI ?
2) On what basis, channelisation code can be allocated to a paritcular UE (scheduling) ?

Smaller the TTI value, more granular allocation that can be made, giving more control to base station and improving adaptability to varying radio conditions. Also, latency in retransmissions improve with smaller TTI (due to shorter/smaller chunks of information). Yes, too small TTI value will result in more overhead (related to signaling required for allocation) and lesser flexibility of choosing appropriate coding/error correction mechanism. R99 use 10 ms frame structure (transport format article), so possible value that we can look for would be from 1 ms to 10 ms.

Next question: under which conditions or criteria, channelisation codes are to be used for paritcular UE ? To make full use of channelisation code at a paritcular instant (i.e. during particular TTI), it need to be used for UE which has best radio conditions among. It is possible that some UEs may never get best radio conditions among. Scheduling should take care of it by regularly scheduling these UEs once their individual best radio conditions are seen.

Further improvement can be made by choosing best modulation and coding scheme based on radio conditions (check eariler article on Requirements and challenges).

Needless to say, we will require a way to estimate instantanous radio conditions as seen by individual UEs (may be separate uplink channel to convey estimated quality !).

There are other considerations as well (like the amount of instantaneous power that can be used for shared transmission) which scheduling need to take care.

Above approach has been adopted by for 3G under HSDPA (High Speed Downlink Packet Access).

References: 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming, UMTS by Sanchez and Thioune, and Release 5 and 6 documents at 3gpp.

Copyright © Samir Amberkar 2010-11§

So we choose ... « LTE Index » 3G till Rel 6 - HSDPA cont.