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If you look at the allocation requirements, all of the allocation resources belong to PDSCH. Needless to say then that the allocation signaling is done over PDCCH. Below diagram shows a subframe (6 RB bandwidth, 3 control symbols) looked from allocation point of view. PDCCH reside in shaded portion and PDSCH in the un-shaded part.
Fig 1.6.6
The idea is quite simple: Check PDCCH for PDSCH/PUSCH allocation parameters. The pattern is same for every subframe, so we have flexibility of assigning every resource block of every subframe. That means control till the extent of 1 ms/180 kHz.
Next goal is to devise allocation signaling with least overhead. In above example, say we want to assign one RB to one UE. Of course, only 6 UEs can get allocation in one subframe. How do we do this signaling ? ... simple approach would be “UE ID” plus its “assigned RB#” plus “number of RBs assigned”. Assigned RB# can be accomodated in 3 bits and so the number of RBs assigned. How do we improve this ? Instead of one RB assignment, we can assign two RBs at a time. This way we gain 2 bits (one bit each for RB# and number of RB groups assigned),but then we lose flexibility ! We can certainly find out some more methods to signal allocations with varying degrees of flexibility and overhead. To achieve optimum results, we can as well use separate (and best suited) method for each UE in a subframe. Like more number of RBs are to be allocated, we can use RB group method otherwise first method of RB# and number of RBs.
3GPP adopts the same approach, that is to have different methods of informing allocations depending on the allocation; the methods are called “resource allocation types”. The resource allocation type is part of physical downlink signaling done through DCI (Downlink Control Information). There are number of formats for DCI and not all would have resource allocation as we will see later.
Another parameter which we must consider is UE identification. More the number of UEs we want to support, more number of bits would be needed to identify them. So how do we save here ? LTE uses a method of scrambling CRC with UE ID. If UE is able to successfully decode PDCCH/PDSCH with CRC scrambled with its ID, it means the transmission is meant for that UE.
Next is specifying modulation and coding. Here LTE uses method of table lookup. Table index is specified in DCI and with tables specified in [36.213:7.1.7], UE can determine modulation and coding used.
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