GB2446227A

GB2446227A - Timing advance commands

Info

Publication number: GB2446227A
Application number: GB0702151A
Authority: GB
Inventors: Mieszko Chmiel; Przemyslaw Jan Czerepinski
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2007-02-05
Filing date: 2007-02-05
Publication date: 2008-08-06
Also published as: WO2008095938A1; GB0702151D0

Abstract

A method of co-ordinating a timing advance command between a base station and a terminal in a mobile communications system by determining the timing advance command format at the base station using a pre-configured rule applied to a cell range dependent parameter and determining the timing advance command format at the terminals using the pre-configured rule applied to a cell range dependent parameter. A base station and terminal configured to perform the method and computer program are also disclosed.

Description

TIMING ADVANCE COMMANDS

This invention relates to the problem of transmitting uplink (UL) timing advance (TA) commands in the downlink (DL) of a mobile communication system. A method for selecting the right command format by the base station and for unambiguous interpretation of the command by the terminal is proposed. The method minimizes the number of bits used to adjust the terminal's initial timing inaccuracy and eliminates the need for explicit signalling of the command format from the base station to the mobile terminal. The method is particularly applicable in evolved universal mobile telecommunications system (UMTS) terrestrial radio access (E-UTRA), when the Ut single carrier frequency division multiple access (SC-FDMA) signals are time controlled by TA commands transmitted by the base station to maintain time alignment of the uplink signals at the base station receiver.

One of the E-UTRA requirements is to support cells with cell ranges up to 100km (R), given that the agreed granularity of timing advance corrections (Z) is in the range of 0.52us (micro seconds), this results in the maximum round trip time advance (RTTA) of around 666.(6)us (RTD=2*RJc, where c is the speed of light), this gives 1283 levels (max RA/Z) of TA adjustments to support all base-to-mobile distances (D, DE <O,R>), which requires 1 ibits (log2 maxRTTAJZ) to signal a TA value from zero (terminal at the base station) to max RT1'A (terminal at the cell range).

Transmitting 1 ibits TA in each non-synchronized Random Access Response in response to a Random Access burst is sub-optimal e.g. in a cell of 1km range where only 4bits are sufficient to signal all possible TA values.

Conventionally, in global system for mobile communication (GSM), the UL TA commands have a fixed number of bits dimensioned for the worst case cell range (i.e. the worst case base-to-mobile distance). In E-UTRA, the number of bits in an UL TA command could be reduced depending on the cell range. However, the current understanding is that the format of TA commands, or the number of bits in a TA command, that is used in a given cell should be signalled on the downlink Broadcast Channel (BCH), either on primary-BCH or secondary-BCH. The TA command is transmitted in response to a non-synchronized Random Access Preamble (random access channel (RACH) burst) in a Random Access Response. At the reception of a command the terminal adjusts its transmit timing by the TA value relative to its previous transmit timing, or relative to the DL timing. a,

To optimize the number of bits in UL TA command depending on the cell range, without explicit signalling of the command format, the following method is proposed. The exact UL TA command format, or the number of bits thereof, used in a cell during initial access, or cell reselection, or handover, or time alignment re-establishment, can be derived by the terminal without explicit TA command format signalling, but on a basis of pre-configured rules. The rules couple the TA command format and a certain cell range dependent parameter, or a set of parameters. which are available at the ten-ninal to configure the non-synchronized Random Access procedure.

The cell range dependent parameters may include Random Access burst format, i.e. there might be a one-to-one mapping between the RACH burst format, or field(s) thereo configured in a cell and the TA command format. An example of such mapping is shown in Figure 1. In particular, the duration of the guard time is a cell range dependant parameter. The length of the guard time must be equal to. or greater than the max RTTA in the cell (while max RTT'A = 2*max R/c). Therefore, to cover larger cell ranges the GT duration must scale up with the range. as considered in E-UTRA. Although the CP length and the preamble length are not directly coupled with the cell range, but rather with required coverage and receiver operation, they might also depend on the cell range. The currently considered baseline non-synchronized RACH burst includes a cyclic prefix, a preamble and a guard time as shown in Figure 2. The structure is lengthened (or at least the GT) to support cell ranges beyond the baseline, see examples in Figure 1.

The cell range dependent parameters may included Random Access preamble sequence parameters i.e. there might he one-to-one mapping between the configured number of cyclic shifts of a root Zadoff-Chu Zero-Correlation-Zone sequence (ZC-ZCZ) and/or the configured number of root sequences in a cell. Examples of such mapping are shown in Table 1 and 2 and described below for the current E-UTRA RACH numerology.

In E-UTRA. it is assumed that (a maximum of) 64 preamble sequences are available per cell and it is also agreed that the sequences are ZC-ZCZ sequences The sequences are generated eitlr by cyclic shits of a root ZC-ZCZ sequence and/or by using additional ZC-ZCZ root sequences. The maximum number of cyclic shifts (and the length of a shift determining the length of ZCZ) of a root sequence depends on the cell range, for example and as shown in more detail in Table I and 2: In very small cells the 64 sequences can be generated as 64 cyclic shifts (CS) of one root sequence (RS) (RSxCS = 1x64); in large cells the 64 sequences can be generated as 64 different root sequences (identified by an index) without any cyclic shits (64x1); and in medium cells, the 64 sequences can be generated from 4 root sequences each with 16 cyclic shifts (4x16) etc (for example 2x32, 8x8, 16x4. 32x2).

The number of cyclic shifts of a root ZC-ZCZ sequence can give a finer granularity of implicit cell range signalling than the guard time duration (or the RACH burst format). Furthermore, a combination of the parameters can be used to derive the UL TA command format, since the RACH burst format and preamble parameters are to some extent coupled. For example, according to Figure 1 and Table 1, it is reasonable to use 4 cyclic shifts per root sequence resulting in ZCZ =199us with a burst format with the GT of 200us.

Furthermore, it may be also possible to derive the number of TA bits on the basis of the GT duration when it is above a certain threshold (e.g. 200us) and on the basis of the number of cyclic shifts andIor root sequences when the GT duration is less than or equal to certain threshold (e.g. 200us).

A ZC sequence is defined by n(ri+) a(n) exp-jpu)

LC

where u denotes the index of the root sequence, Nzc denotes the length of the sequence, and n=0. 1, ... NG-1 is the index of the samples, must be a pnme integer. While a((n-d) m,ir'1) refers to the d:th cyclic shift of the ZC sequence. For the baseline E-UTRA 800us preamble it is assumed that Nzc=863, and the total number of root sequences is limited byNz.

The base station and the terminal must be configured with the same set of rules of mapping between configured RACH parameters and the number of hits in a TA command transmitted in a Random Access Response. The configuration of RACH parameters and the number of TA command bits is coupled by the rules.

The Random Access parameters used by the terminal to implicitly derive the number of bits in UL TA command can be signalled to the terminal as system information via the Broadcast Channel.

There are at least two different formats of TA commands each characterized by the number of bits used to encode the terminal timing correction, which should not exclude the case of zero TA bits, or the case when a TA command is not present in a Random Access Response.

In the case when the number of sequences (root sequences and/or cyclic shifts per root sequence) used in a cell is restricted e.g. to reduce the processing at the base station, the implicit signalling method is not excluded. The method is also applicable when the number of cyclic shifts per root ZC-ZCZ sequence is restricted in a cell supporting high velocity. The rules may take into account these restrictions.

To simplify the terminal's reception and processing of TA commands of different lengths. it may he beneficial to transmit the TA bits in the order from the least significant bit (LSB) to the most significant bit (MSB).

Advantages of the present invention include that the number of bits in a TA command is reduced depending on the cell range and DL throughput is increased; and that there is no need for additional signalling to inform the terminal about the cell's TA command format, instead available and broadcast by the base station cell range dependent (RACH) parameters are re-used.

Table 1 illustrates an example of relation between ZC-ZCZ preamble parameters and # of TA command bits. It is assumed that ZCZ covers only RTTA.

# of root ZC # of cyclic cell range max # of levels of # of bits in sequences shifts per (R) RITA in TA TA (RS) root supported the cell adjustments command sequence by the range [us] to support (CS) duration of max RITA the CS [km] 1 64 1.8 12 24 5 2 32 3.6 24 47 6 4 16 7.3 49 95 7 8 8 14.8 99 191 8 16 4 29.8 199 383 9 32 2 59.8 399 768 10 64 none 100 666. 6 1283 11

Table 1

Table 2 shows an examp'e of relation between ZC-ZCZ preamble parameters and # of TA command bits. It is assumed that ZCZ covers RTTA and 6.6us delay spread.

# of root ZC # of cyclic cell range max # of levels of # of bits in sequences shifts per supported RTTA in TA TA (RS) root by the the cell adjustments command sequence duration of range [us] to support (CS) the CS [km] max RTTA 1 64 0.8 5.4 11 4 2 32 2.6 17.4 34 6 4 16 6.3 42.4 82 7 8 8 13.8 92.4 178 8 16 4 28.8 192.4 371 9 32 2 58.8 392.4 755 10 64 none 100 666.6 1283 II

Table 2

Claims

1. A method of co-ordinating a timing advance command between a base station and a terminal in a mobile communications system, including: i) determining the timing advance command format at the base station using a pre-configured rule applied to a cell range dependent parameter; and ii) determining the timing advance command format at the terminal using the pie-configured rule applied to a cell range dependent parameter.

2. A method as claimed in claim 1, wherein the cell range dependent parameter is selected from a set of cell range dependent parameters.

3. A method as claimed in claim 2, wherein the cell range dependent parameter is selected from a set of cell range dependent parameters in dependent on a threshold.

4. A method as claimed in claim 3, wherein the threshold is a timing threshold.

5. A method as claimed in any one of the preceding claims, wherein the cell range dependent parameter is also used to configure a non-synchronised random access procedure.

6. A method as claimed in claim 5, wherein a random access burst format for the non-synchronised random access procedure includes a cyclic I...

prefix, a preamble, and a guard time. S. *

:: 30

7. A method as claimed in claim 6, wherein the cell range parameter is a * cyclic shift and/or a configured number of root sequences based on the : *. cyclic prefix. S., *

S S..

S

8. A method as claimed in claim 6, wherein the cell range parameter is the guard time.

9. A method as claimed in claim 6 when dependent on claim 2, wherein the set of cell range dependent parameters includes a cyclic shift, a configured number of root sequences based on the cyclic prefix and the guard time.

10. A method as claimed in claim 9 when dependent on claim 4, wherein if the cell range of the base station is above the threshold then the guard time is selected as the cell range dependent parameter otherwise the cyclic shift and/or configured number of root sequences is selected.

11. A method as claimed in any one of the preceding claims, wherein the number of bits in the timing advance command format is determined in both determining steps.

12. A method as claimed in any one of the preceding claims when dependent on claim 5, wherein the cyclic prefix is based on a root Zadoff-Chu Zero-Correlation-Zone sequence.

13. A method as claimed in any one of the preceding claims wherein the mobile communications system is a universal mobile telecommunications system (UMTS).

14. A method as claimed in claim 13 wherein the UMTS is evolved UMTS terrestrial radio access (E-UTRA). S... * S S...

. :

15. A method as claimed in any one of the preceding claims wherein the * 30 terminal is a mobile terminal. e*

16. A base station for a mobile communications system, including: S..

a determination processor configured to determine a timing advance command format using a pre-configured rule applied to a cell range dependent parameter.

17. A terminal for a mobile communications system, including: a determination processor configured to determine a timing advance command format using a pre-configured rule applied to a cell range dependent parameter.

18. A mobile communications system, including: a base station as claimed in claim 16; and a terminal as claimed in claim 17.

19. A computer program product configured to determine a timing advance command format for a mobile communications system using a pre-configured rule applied to a cell range dependent parameter * ** * * * * I. * ** * * S S.. S *S S * S S * ..

S

S * .* * S S * S

S IS.

S