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Above procedure seem all OK, but how does UE/PHY sends Scheduling Request (SR) without UL allocation ? The answer is in the way PUCCH (PHY signaling) is transmitted, so let us look at PUCCH (and with that UCI - Uplink Control Information as well).
As apparent from example of earlier section, PHY UL signaling is needed even though there is no explicit UL allocation. This is true not only for scheduling request but also for sending HARQ acknowledgements, channel quality indications. So actually there are two problems here: first, how to multiplex UEs and second, how to multiplex various possible PHY signaling for a UE. Let us see how these problems are tackled.
a) Corner radio blocks of subframes are given to PUCCH as shown in UL PHY structure in Annex A.2 (and simplified in below diagram). Ref: [36.211:5.4.3].
| 6 |
m = 1
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m = 0
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| 5 |
H
| H
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| 4 |
H
| H
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| 3 |
H
| H
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| 2 |
H
| H
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| nRB = 1 |
m = 0
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m = 1
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b) PHY UL signaling is divided in two formats, UCI 0 and UCI 1 as shown below:
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1
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Scheduling Request
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1a/1b
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HARQ-ACK or with SR
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2
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CQI
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2a/2b
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CQI with HARQ-ACK
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Each "m" carry one PUCCH format. In above example, m=1 carry PUCCH format 1/1a/1b and m=0, PUCCH 2/2a/2b.
c) UEs are assigned orthogonal sequences (Zadoff-Chu sequence with a cyclic shift). This allows multiple UEs to be multiplexed on one PUCCH RB. Zadoff-Chu sequence is cell specific i.e. same for all UEs within a cell. This sequence has speciality of being orthogonal (zero correlation) to its cyclically shifted version. So UEs are actually provided with "cyclic shifts" to be used on 12-bit cell specific sequence.
d) Orthogonal sequence is one part of UE multiplexing. Not all UEs can be multiplexed on one PUCCH RB - smaller the cyclic shift, more the interference in practice (even though theoretically nil).
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