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On the specification of restricted set of cyclic shifts of root Zadoff-Chu sequences


TSG RAN WG1 Meeting #49 Kobe, Japan, May 7-11, 2007 Agenda Item: Source: Title: 7.4 Huawei, Panasonic, LGE, Nokia, Nokia Siemens Networks

R1-072626

On the specification

of restricted set of cyclic shifts of root Zadoff-Chu sequences

Document for: Discussion and decision

1

Introduction

It has been decided by RAN1 to use a restricted set of cyclic shifts of root Zadoff-Chu sequences to generate the random access preambles in cells with high Doppler shifts. However, it remains to specify the restricted set in TS 36.211. In this contribution we discuss two options for specification of the restricted set.

2

Specification Options for Restricted Set of Cyclic Shifts

Two explicit formulas of the restricted set are presented in [1] and [2] and may be used for the specification. With Formula 1 the restricted set of cyclic shifts is obtained as multiples of the cyclic shift increment NCS [1] [2, Section 5], With Formula 2 [2, Section 4], the size of the restricted set of cyclic shifts is somewhat larger than for Formula 1.The minimum difference between two cyclic shifts is still NCS but the cyclic shifts are not necessarily multiples of NCS. This gives up to 11% more preambles [2] generated from the whole set of root sequences with Formula 2 than with Formula 1. The set of cyclic shifts over all root sequences becomes larger than if the cyclic shifts are multiples of NCS, which may imply more test effort. Implicit definition of the restricted set of cyclic shifts in [4] leads to the same number of preamble sequences than formula 2, if small inconsistency in formula 2 is fixed (alias distance range 2 defined as (N/3) ≤ du ≤ (N-Ncs)/2). Some root sequences should not be used in cells with high Doppler shifts [1-4]. High Doppler shifts induce correlation peaks in the receiver’s bank of correlators corresponding to cyclic shift coff or N- coff when the uth root sequence of length N is transmitted. Both Formula 1 and Formula 2 exclude root sequences with coff in the range coff < NCS, coff > N-NCS, or (N-NCS)/2< coff < (N+NCS)/2 that prevent correct time-of-arrival estimation.

3

Conclusion

The restricted set of cyclic shifts for cells with high Doppler shifts should be specified explicitly by one of the formulas either based on cyclic shifts multiple of Ncs as given in [1] [2, Section 5] or not multiple of NCS as given in [2] and modified as proposed above. Which formula to select is a trade-off between complexity and the total number of preambles. Root sequences that induce cyclic offsets coff in the range coff < NCS, coff > N-NCS, or (N-NCS)/2< coff < (N+NCS)/2 are not used for preambles in cells with restricted sets of cyclic shifts.

References
[1] R1-072325, “Specification of restricted set of cyclic shifts of root Zadoff-Chu sequences,” Huawei, Panasonic, May 2007. [2] R1-072331, “Formula for restricted cyclic shift set,” LG Electronics, May 2007. [3] R1-072080, “Limitation of RACH sequence allocation for high mobility cell,” Panasonic, May 2007.

[4] R1-072268, “On construction and signalling of RACH preambles,” Nokia Siemens Networks, Nokia, May 2007.


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