OutlineofchangesinDraftCRonLTETDDPRACHtime-frequencymapping.doc

TSG-RAN WG1 #53 R1-08xxxx
Kansas City, USA, 5 – 9 May, 2008

Source: Ericsson
Title: Outline of changes in Draft CR on LTE TDD PRACH time/frequency mapping
Agenda Item: 7.1.3
Document for: Discussion and Decision

1 Introduction
This contribution outlines the changes done in the attached companion draft CR, for which changes are based on the agreed CR [1] and the contributions [2] and [3].
2 Proposed changes to 36.211
The proposed changes to the subclause 5.7.1 can be divided into two parts.
The first part refer to include the agreed CR [1] into the baseline specification [4], but where the PRACH density and versions notation and , respectively, have been replaced by and in order to avoid notation collisions in the specification. This means that [1] can be replaced with the proposed draft CR if agreed by RAN1. Otherwise, a new revision of [1] should be prepared to resolve this collision issue.
The second part represents a text proposal to capture the suggested mapping in [2][3] into the specification, which defines the PRACH resource mapping in time and frequency for each preamble format. We here propose to capture this mapping with a table with entries specifying the location of a specific PRACH resource. Each PRACH resource location is here specified by the quadruple, where
is a non-negative frequency resource index, and is used to map PRACH resource blocks in the frequency domain for configurations with densities that require frequency multiplexing (see below).
· is a radio frame number related index, and indicates whether the PRACH resource is in every even frame (0), in every odd frame (1), or in both odd and even frames (2), respectively The definition of this index differs here slightly from [2], but it is used in the same way.
. Thus, for PRACH densities larger than 0.5, this index is always set to 2.
· is a half frame index, and indicates whether the PRACH resource is located in the first half frame (0) or in the second half frame (1) of the radio frame
· is an uplink subframe index, and indicates where the preamble starts such that 0 indicates at the first uplink subframe in each half frame, etc. Preamble resources in UpPTS (preamble format 4 only) are indicated by an asterisk in this field.
The benefits of this mapping as well as the design principles behind it are stated in [2]. The mapping algorithm used to generate the table is summarized in the Appendix below, and is the same as the one proposed in [2] but with slightly changed notations.
In [2], two alternative resource mappings in frequency were discussed in which we here propose for preamble format 0-3 the following mapping

whereis the first resource block allocated to a PRACH opportunity and where indicates the first resource block available for PRACH. As in [4],above represents the uplink bandwidth configuration expressed in multiples of resource blocks in the frequency domain. With this mapping, the location of the PRACH opportunities with consecutive values of will alternate between lower and upper frequencies within the configured uplink frequency band. Furthermore, the above mapping corresponds to the preferred one in [2]. For preamble format 4, we propose the same mapping as proposed in

whereis the system frame number and whereis the number of DL to UL switch points within the radio frame. For UL-DL allocations with a single DL-to-UL switch per frame, PRACH opportunities with even system frame number will be mapped to a frequency band on the low side whereas PRACH opportunities with odd system frame number will be mapped to a frequency band on the high side. For UL-DL allocations with two DL-to-UL switches per frame, the PRACH resources will instead alternate between low and high bands on a half frame basis. By doing alternating preamble format 4 PRACH in this manner, frequency diversity between random access attempts can be provided at the same time as the whole system bandwidth can be sounded over two consecutive UpPTS.
3 Summary
The companion draft CR provides a text proposal for TS 36.211 by capturing the agreed TDD PRACH configurations in [1] together with the mapping proposed in [2][3], but with a slight change of notation in order to better harmonize with the notation in [4]. The mapping of PRACH opportunities is on a time first basis and frequency multiplexed only if time multiplexing is not sufficient. When frequency multiplexing is employed frequency diversity between random access attempts can be achieved.
4 References
[1] R1-081691, 36.211 CR0011 (Rel-8, F) “TDD PRACH Configurations”, ZTE
[2] R1-081571, “Configurations and Location Mapping for TDD PRACH”, ZTE, CHTTL, CMCC, Nokia, Nokia Siemens Networks, Huawei, RITT
[3] R1-081766, “Time & Frequency Location Mapping for TDD PRACH”, ZTE
[4] TS 36.211 v.8.2.0
5 Appendix
Let be the average density of PRACH opportunities per frame within a configuration and let index the RACH opportunities so that Letbe the number of configuration versions with the same density and the same preamble format and index these versions Furthermore, let index all the PRACH opportunities for all the configurations. Then
.
Moreover, let L be the number of consecutive subframes that the PRACH preamble occupies ( = 1, 2, 2, 3, and 1 for preambles 0, 1, 2, 3, and 4, respectively), let
,
and let be the number of DL to UL switch points per frame (= 2 for UL/DL allocation configuration 0,1,2 and 6, and = 1 for Uplink-Downlink allocation configurations, see Table 4.2-2 [4]). Further, let
,
where the number of PRACH:es that can be held by half frame without overlapping in time can be found as
(for preamble format 4,),
where is the number of UL subframes in half frame i. Then,
and .
The frequency index is then for each timing triplet and version, in order of ascending
,
where denoted the number of PRACH opportunities within the considered PRACH configuration that has the timing given by the triplet .