GB2534928A

GB2534928A - Telecommunications network planning

Info

Publication number: GB2534928A
Application number: GB1502013.4A
Authority: GB
Inventors: Miguel Martins Caetano Peralta Gaspar Bruno
Original assignee: Vodafone Ltd
Current assignee: Vodafone Ltd
Priority date: 2015-02-06
Filing date: 2015-02-06
Publication date: 2016-08-10
Anticipated expiration: 2035-02-06
Also published as: GB2534928B; GB201502013D0

Abstract

A mobile telecommunications network includes a mobile telecommunications terminal (UE) 1 and a plurality of transceiver nodes (eNB) 5 providing a plurality of cells, each cell having an allocated identifier, e.g. a Physical-layer Cell Identifier (PCI). The telecommunications terminal 1 transmits preambles to access one of the cells, wherein the telecommunications network includes preamble generation means 25 operable to generate the preambles in dependence upon the cell identifier and the cell range (coverage area). The preambles may be generated from one or more cyclically shifted sequences of values, preferably Zadoff-Chu sequences, and the preamble generation means 25 may determine a number of the cyclically shifted sequences of values in dependence upon the cell range.

Description

Telecommunications Network Planning

TECHNICAL FIELD

The invention relates to a mobile telecommunications network including a telecommunications terminal; a plurality of transceiver nodes providing a plurality of cells, each cell having an identifier allocated thereto; the telecommunications terminal being operable to transmit preambles to access a one of the cells.

BACKGROUND TO THE INVENTION

The User Equipment (UE) operating in a Long Term Evolution (LTE) system needs to decode the Physical-layer Cell Identifier (PCI) to be able to camp on any cell. The PCI enables the UE to read system information blocks from a particular network. In LTE one PCI is allocated for each cell from a total of 504 (0-503) PCIs allowed in a same carrier.

PCI planning is performed to avoid UE synchronisation clashes between different cells. Neighbouring cells are given different PCIs.

Another parameter that needs to be defined is the Random Access Channel (RACH) Root Sequence used by each UE to access the network and to perform handovers.

During initial access, the UE attempts to communicate with the network to register and commence services. Random access (RA) is performed as an uplink control procedure to enable the UE to access the network. As the initial access cannot be scheduled by the network, the RA procedure is contention based. Collisions may occur between communications from different UEs and an appropriate contention-resolution scheme is therefore implemented.

A UE transmits a random access preamble, allowing the eNode B providing the cell to estimate the transmission timing of the UE. Uplink synchronisation allows the UE to subsequently transmit uplink data.

Prior to sending the preamble, the UE synchronises to the downlink transmissions and reads the Broadcast Control Channel (BCCH). The BCCH indicates where the RA time slots are located, which frequency bands may be used and which preambles (sequences) are available.

At the next available RA slot, the UE sends a preamble sequence. The preamble sequence includes a random ID which identifies the UE. LTE provides for each cell 64 such random IDs and thus 64 preambles.

The eNode B estimates the Uplink timing of the UE based on the timing of the received preamble.

The preamble sequence should have good auto-correlation function (ACF) properties and good cross-correlation function (CCF) properties. A sequence that has suitable (periodic) ACF and CCF properties is the Zadoff-Chu sequence. The periodic ACF of Zadoff-Chu (Z-C) sequence is only non-zero at time-lag zero (and periodic extensions) and the magnitude of the CCF is equal to the square-root of the sequence length Nzc. Due to special properties of Zadoff-Chu sequences the number of sequences is maximised if Nzc is chosen prime. This together with the requirement that the effective RA bandwidth Nzc1250 Hz (for preamble format 0-3, see 3GPP36.211 Table 5.7.3-1) should fit into 1.08 MHz (RA occupies 6*PRB (Physical Resource Blocks), which one PRB has 180KHz bandwidth -giving a total 1.08MHz), leads to Zadoff-Chu root sequence length Nzc=839. Thus, there are 838 sequencies with optimal cross-correlation properties where Zadoff-Chu Logical root sequence number are provided, numbered 0 to 837 (see 3GPP36.211 Table 5.7.2-4: Root Zadoff-Chu sequence order for preamble formats 0 -3).

From one Zadoff-Chu sequence (referred to as "root sequence" in LTE) multiple preamble sequences can be derived by cyclic shifting. Due to the ideal ACF of a Zadoff-Chu sequence, multiple mutually orthogonal preambles can be derived from a single root sequence by cyclic shifting one root sequence multiple times. The amount of each shift depends on the maximum allowed round trip time plus delay spread in the time-domain. The correlation of such a cyclic shifted sequence and the underlying root sequence has its peak no longer at zero but at the cyclic shift. If the received signal has a valid round trip delay -i.e. not larger than the maximum assumed round trip time -the correlation peak occurs at the cyclic shift plus the round trip delay which is still in the correct correlation zone. For small cells up to 1.5 km radii all 64 preambles can be derived from a single root sequence and are therefore orthogonal to each other. In larger cells not all preambles can be derived from a single root sequence and multiple root sequences must be allocated to a cell. Preambles derived from different root sequences are not orthogonal to each other.

The 3GPP standards do not specify a methodology for assigning the RACH root sequences to cells. The current approach is for the system operator to plan and assign them manually which leads to an additional complexity and time for any LTE network deployment.

As mentioned above, the RACH Root Sequence uses a finite pool of 838 Zadoff-Chu root sequences, identified by the numbers from 0 to 837 for a Ncs (Cyclic shift value used for random access preamble generation) "unrestricted set" and for non-high speed UEs -see Standard 3GPP 36.211. The numbers of RACH Root Sequences required for a cell depends on the defined cell range of the cell.

RACH Root Sequence number(s) should be set to different values in adjacent cells in order to avoid false preamble detections from UEs in neighbouring cells.

3GPP -36.211 provides the table below, Table A, which gives the Ncs (Cyclic 30 shift value used for random access preamble generation) for each of 16 possible input values (zeroCorrelationZoneConfig): Table 5.7.2-2: N, for preamble cgeneration (preamble formats 0-3) zeroCorrelaricnZoneConfig value Unrestr cted set Rest eci set t1 0 15 2 15 22 3 18 26 4 22 32 26 38 6 32 7 38 55 46 68 9 5g 82 76 1.730 11 93 128 12 119 158 13 167 202 14 279 237 419

TABLE A

The following table B is derived from 3GPP -36.211 relrenn-.1 Cn eiconfiF Ne.511nr.F.frintee. sc4 Tne MN, ne!aySfo ii - ,C.f.:I range ire; NPrralah:ecM111 ar..Nns Ncnnheynenr.e=ai4,INpreamnic 3501 32; le..2,1 55 1 930 523 2.5 1725 55 2 SX: 239 2.5 22259 4: 3 2'71 3,.4 '154i 32 KC' 523 1202 26 33 330 5 i 23 5045 22 3 tl.4.b 5253 5435 3.3 or.. 35 4 9 55 Ix, 239 3 grsca /I 76 40C. 133:0295 13 5 Y3 Y1P S53 eA 12'92,2 S 97:: nn 75 tai.;; 7 AY ri 13/ g.333 0357 95 235/0 279 5D3 229 a 33515 a 22 413 40C. 133 5553 2 42

TABLE B

Where: * Tseq is the length of the preamble in microseconds * Nzc is Zadoff-Chu root sequence length. For preamble format 0-3 the Nzc = 839.

* Cell Range is the radius in meters of the expected cell coverage * Npreamble is the number of preambles that can be generated by one root sequence, and is dependent on the Ncs value * Nrootsequence is the number of root sequences required for the cell to generate 64 preambles.

Thus, Nrootsequence represents the number of RACH Root Sequences required, and which is related with the cell range according the above table B for "unrestricted" set and preamble format type [0 to 3] (non-high speed UEs).

The value Ncs is related to cell range (size). The Ncs for a cell is dependent on Round Trip Time (RTT) in the cell, i.e. on the cell radius, and sequence length (800ps), and can be calculated using the following formula: Presently only RACH Root Sequence manual planning or static formulas linked to PCI are known, which is limited to a fixed cell range and does not guarantee a unique RACH Root Sequence for all possible PC's.

SUMMARY OF THE INVENTION

In one aspect the present invention provides a mobile telecommunications network including: a telecommunications terminal; a plurality of transceiver nodes providing a plurality of cells, each cell having an identifier allocated thereto; the telecommunications terminal being operable to transmit preambles to access a one of the cells; wherein the telecommunications network includes preamble generation means operable to generate the preambles (automatically) in dependence upon the cell identifier and the cell range (coverage area).

In the embodiment the cell identifier is a Physical-layer Cell Identifier, PCI, although this is not essential.

In the embodiment the preambles are Random Access preambles, although this is not essential.

The preambles may identify the telecommunications terminal to the cell.

The preambles may be generated from one or more cyclically shifted sequences of values, preferably Zadoff-Chu sequences.

The preamble generation means may be operable to determine a number of the cyclically shifted sequences of values for generating the preambles in dependence upon the cell range (coverage area).

In the embodiment the preamble generation means is operable to generate 20 the preambles in dependence upon an index value (RachRootSequence_index) that is calculated as follows: RachRootSequence_index = (PCI -INT (PCI / INT(MaxRRS / Nrrs)) * INT(MaxRRS / Nrrs)) * Nrrs + INT (PCI / INT(MaxRRS / Nrrs)) where: MaxRRS = a maximum (logical) number of available sequences of values (e.g. the maximum logical RachRootSequence (RRS) number available, which is 837 for Preamble type 0-3 and 137 for preamble 4 type, see 3GPP36.211), Nrrs = the determined the number of cyclically shifted sequences of values for generating the preambles, and INT is an operation that rounds a number down to the nearest integer.

In the embodiment the preamble generation means is operable to generate 5 the preambles by using the index value (RachRootSequence_index) to determine which of the cyclically shifted sequences of values to use for generating the preambles.

The present invention also provides a corresponding method.

The embodiment allows a simple approach for RACH Root sequence planning required e.g., on integration of new eNodeB on an LTE network, saving time and improving network performance.

According to the embodiment to be described, a method of facilitating planning of a telecommunication network including a plurality of nodes (e.g., eNodeB cells) comprises: - obtaining information about a first planning scheme (e.g., PCI planning), said information including a set of first identifiers (e.g., PCI) for assignment to the plurality of nodes; - determining a (coverage) range for a node of the plurality of nodes; - obtaining, based on the determined range, a plurality of subsets (e.g., RACH Root Sequences) from a set of second identifiers (e.g., RACH Root Sequence numbers); and -generating a second planning scheme (e.g., RACH Root Sequence index planning) based on the obtained information about the first planning scheme, said second planning scheme including the plurality of subsets for assignment to the plurality of nodes.

The generating step may further include: associating each of the plurality of subsets with a different one of the identifiers from the first set of identifiers (e.g., one-to-one mapping). The first identifiers are of a first type (e.g., PCI), the second identifiers are of a second type (e.g., RACH Root Sequence numbers).

The obtaining step may further include: determining the plurality of subsets so that each of the plurality of subsets is at least in part different from any of the remaining subsets.

The determination may include determining a size for each of the plurality of subsets (e.g., RACH Root Sequence length). The size may be based on the determined range. The determination may further include: selecting a first number of identifiers from the second set for inclusion in a first subset; and selecting a second number of identifiers from the second set for inclusion in a second subset. The first and the second number may correspond to the determined size. The selected identifiers may be consecutive identifiers. The first and second subset may be different or partially overlapped (e.g. a shifted version of the other one). For example, a shifting window could be used to select the subsets. Each subset may be identified by an index (e.g., RACH Root Sequence), said index corresponding to the identifier with the lowest cardinality in said subset.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention an embodiment will now be described by way of example, with reference to the accompanying drawing, where Figure 1 shows the main elements of an SAE/LTE 4G network in accordance with the embodiment.

DETAILED DESCRIPTION OF EMBODIMENT OF THE INVENTION

Overview of SAE/LTE Network Figure 1 shows schematically the logical elements of a SAE/LTE cellular telecommunications network. Mobile terminal (UE) 1 is registered with mobile telecommunications network core 3. The mobile terminal 1 may be a handheld mobile telephone, a personal digital assistant (PDA) or a laptop or desktop personal computer -for example, equipped with a wireless datacard. The device 1 communicates wirelessly with the mobile telecommunications network core 3 via the radio access network (RAN) of the mobile telecommunications network core 3 over radio interface 2. The RAN comprises a transceiver node (base station), an eNodeB (or eNB) 5 in this example. An eNodeB 5 performs functions generally similar to those performed by the NodeB and the radio network controller (RNC) of a 3G network. In practice there will be a multiplicity of eNodeBs 5, each serving a particular area or "cells". Each eNodeB is coupled to one or more antenna devices. The eNodeB and antenna device form a cell site. The cell site provides radio coverage to a plurality of cells, e.g. three.

Signalling in a mobile telecommunications network can be considered to be separated into "control plane" signalling and "user plane signalling". The control plane performs the required signalling, and includes the relevant application protocol and signalling bearer for transporting the application protocol messages. Among other things, the application protocol is used for setting up the radio access bearer and the radio network layer. The user plane transmits data traffic and includes data streams and data bearers for the data streams. The data streams are characterised by one or more frame protocols specific for a particular interface. Generally speaking, the user plane carries data for use by a receiving terminal -such as data that allow a voice or picture to be reproduced -and the control plane controls how data are transmitted. A Packet Data Network Gateway (PDN-GW) terminates the user plane within the core 3.

A PDP (packet data protocol) context defines parameters that support the flow of data traffic to and from a mobile terminal. Among the parameters that are set are the identifier of the external packet data network with which the terminal wishes to communicate, a PDP address recognised in that network (for example, the IP address allocated to the mobile terminal), the address of the network gateway, quality of service (QoS) parameters etc. A mobility management entity (MME) 7 provides equivalent functions to the control plane functions of the SGSN and GGSN from the 3G architecture (Release -6). The MME handles security key management. The MME also provides control plane function for mobility between LTE and GSM/UMTS networks. Communications between the eNodeB 5 are transmitted to the MME 7 via the Si-c Interface 4.

A user plane entity (UPE) 9 handles the user plane traffic functions from the terminal 1 which includes the IP header and payload compression and ciphering. This UPE 9 provides the equivalent functions to the user plane part of the 3 RNC and the user plane part of the 3G GGSN. Communications between the eNodeB 5 are transmitted to the UPE 7 via the S1-u Interface 6. The known 3GPP authentication procedure may be re-used in the SAE/LTE architecture shown, between the terminal 1/UE and the MME 7.

It should be noted that, although in Figure 1 the MME 7 and UPE 9 are shown as separate logical entities they may exist as a single physical node of the telecommunications network in gateway aGW 8.

Data are transmitted between the eNodeB 5 and the MME 7 and UPE 9 via the IP transport network 11.

The backhaul from the eNB 5 may comprise any or all of the links providing 20 the S1-c Interface 4, the S1-u Interface 6 and the connection between the gateway aGW 8 and the mobile telecommunications network core 3.

Although only one mobile terminal 1 is shown, there will in practice be a multiplicity of mobile terminals, each of which is registered with the network core 3. Each mobile terminal (including mobile terminal 1) is provided with a respective subscriber identity module (SIM) 15. During the manufacturing process of each SIM, authentication information is stored thereon under the control of the mobile telecommunications network core 3. The mobile telecommunications network core 3 itself stores details of each of the SIMs issued under its control. In operation of the mobile telecommunications network core 3, a terminal 1 is authenticated by a SIM 15.

The network also performs O&M (Operations & Maintenance). This term refers to the processes and functions used in managing a network or element within a network.

Mobile telecommunications networks have an active state of communication with their mobile terminals and an inactive/idle state of communication with their terminals. When in the active state, as the mobile terminals move between different transceiver nodes of the network, the communication session is maintained by performing a "handover" operation between the transceiver nodes. In the inactive/idle state, as a mobile terminal moves between different transceiver nodes of the network the mobile terminal performs "cell reselection" to select the most appropriate transceiver node on which to "camp" in order that the mobile terminal can be paged by the network when mobile terminating data is destined for that mobile terminal.

RACH Route Index Calculation The embodiment provides a planning scheme that allows the RACH root sequence to be linked with the known PCI planning mentioned above.

The proposed formula automates the RACH Root Sequence planning and provides a unique RACH Root Sequence index for each of the individual available PC's, optimised for the indicated cell size. The RACH Root Sequence index indicates the first root sequence number that is to be used by a UE for preamble generation.

This formula is applicable when Ncs unrestricted set and preamble format type [0 -3] are applied. The following parameters are taken into account: * MaxRRS = 837 ->Maximum logical RachRootSequence (RRS) number available (which is 837 for Preamble type [0-3] and 137 for preamble 4 type, see 3GPP36.211).

* Nrrs -> Number of RachRootSequence required according to the Cell Range to which the RACH sequence should be applied. This corresponds to Nrootsequence in table B above.

* Ng = INT(MaxRRS / Nrrs) ->Number of RACH groups available.

* Gn = INT (PCI / Ng)-> RACH group number.

* Gindex = PCI -Gn * Ng -> Index within the RACH Group.

* RachRootSequencelndex = Gindex * Nrrs + Gn The Global formula can be expressed as follows: RachRootSequence_index = (PC1 -INT (PCI / INT(MaxRRS / Nrrs) ) * INT(MaxRRS / Nrrs)) "Nrrs + INT (PCI / INT(MaxRRS / Nrrs)) (INT(number) is an operation that rounds a number down to the nearest integer.) The allocation of RACH Root Sequence index is based on PCI planning, and the number of RACH Root Sequences required (Nrrs) is retrieved by the Cell Range table relation from table B above -or by any other suitable method.

If Nrrs =1, then the RACH Root Sequence Index will indicate the number of the single root sequence that is used by the UE to generate preambles.

If Nrrs > 1, then the RACH Root Sequence Index indicates the first (lowest) number n of root sequence numbers that is used by the UE. In the 25 embodiment the succeeding root sequence number(s) (n+1, n+2, etc depending on the value of Nrrs) are used by the UE to generate preambles.

In this way the UE is able to generate 64 preambles for any cell.

By applying this algorithm the conventional manual planning of RACH Root Sequence becomes unnecessary and it also minimises the false preamble detection from UE in neighbouring cells. On Multi Operator Radio Access Network (MORAN) environment it has the additional benefit of avoiding the setup of an additional rule to manage RACH Root Sequence on the MORAN borders.

The global formula may be implemented by an algorithm performed 5 automatically on an O&M element 25 of a telecommunications network, by the mobile terminal or by any other data processing apparatus. The value RachRootSequence_index may be communicated to the mobile terminal and used by a processor of the terminal to generate the preambles. The steps of the algorithm may be performed by a computer program. The computer 10 program may be stored on a computer readable medium or hardware.

Examples of RACH Root Sequence allocation based on the global formula As discussed above, for a conventional LTE system the following values apply: RACH Root Sequence numbers = 0,1,2,...,837 In these examples, cell Range = 10km -> RACH Root sequence length (Nrrs) = 6; For PCI = 0 the global formula gives RACH Root Sequence index = 0. Therefore, the number n of the first root sequence used by the UE is 0. For the cell in question 5, additional root sequences are required, and so 5 successive root sequence numbers are also used by the UE. The 6 root sequence numbers used to generate preambles are 0,1,2,3,4,5.

For PCI = 1 the global formula gives RACH Root Sequence index = 6. Therefore, the number n of the first root sequence used by the UE is 6. For the cell in question 5, additional root sequences are required, and so 5 successive root sequence numbers are also used by the UE. The 6 root sequence numbers used to generate preambles are 6,7,8,9,10,11.

For PCI = 10 the global formula gives RACH Root Sequence index = 60. Therefore, the number n of the first root sequence used by the UE is 60. For the cell in question 5, additional root sequences are required, and so 5 successive root sequence numbers are also used by the UE. The 6 root sequence numbers used to generate preambles are 60,61,62,63,64,65.

For PCI = 139 the global formula gives RACH Root Sequence index = 1. Therefore, the number n of the first root sequence used by the UE is 1. For the cell in question 5, additional root sequences are required, and so 5 successive root sequence numbers are also used by the UE. The 6 root sequence numbers used to generate preambles are 1,2,3,4,5,6.

The examples above are for Preamble type 0-3. The invention is also applicable to Preamble 4 type, where MaxRRS = 137 -see 3GPP36.211.

Claims

CLAIMS1. A mobile telecommunications network including: a telecommunications terminal; a plurality of transceiver nodes providing a plurality of cells, each cell having an identifier allocated thereto; the telecommunications terminal being operable to transmit preambles to access a one of the cells; wherein the telecommunications network includes preamble generation means operable to generate the preambles in dependence upon the cell identifier and the cell range.
The mobile telecommunications network of claim 1, wherein the cell identifier is a Physical-layer Cell Identifier, PCI.
The mobile telecommunications network of claim 1 or 2, wherein the preambles are Random Access preambles.
4. The mobile telecommunications network of claim 1, 2 or 3, wherein the preambles identify the telecommunications terminal to the cell.
The mobile telecommunications network of claim 1, 2, 3 or 4, wherein the preambles are generated from one or more cyclically shifted sequences of values, preferably Zadoff-Chu sequences.
The mobile telecommunications network of claim 5, wherein the preamble generation means is operable to determine a number of the cyclically shifted sequences of values for generating the preambles in dependence upon the cell range.
The mobile telecommunications network of claim 6, wherein the preamble generation means is operable to generate the preambles in dependence upon an index value (RachRootSequence_index) that is calculated as follows: RachRootSequence_index = (PCI -INT (PCI / INT(MaxRRS / Nrrs)) " INT(MaxRRS / Nrrs)) * Nrrs + INT (PCI / INT(MaxRRS / Nrrs)) where: MaxRRS = a maximum number of available sequences of values, Nrrs = the determined the number of cyclically shifted sequences of values for generating the preambles, and INT is an operation that rounds a number down to the nearest integer.
8. The mobile telecommunications network of claims 7, wherein the preamble generation means is operable to generate the preambles by using the index value (RachRootSequence_index) to determine which of the cyclically shifted sequences of values to use for generating the preambles.
9. A method of operating a telecommunications terminal in a mobile telecommunications network including a plurality of transceiver nodes providing a plurality of cells, each cell having an identifier allocated thereto, the method including: the telecommunications terminal transmitting preambles to access a one of the cells; wherein the telecommunications network includes preamble generation means that generates the preambles in dependence upon the cell identifier and the cell range.
10. The method of claim 9, wherein the cell identifier is a Physical-layer
11. The method of claim 9 or 10, wherein the preambles are Random Access preambles.
12. The method of claim 9, 10 or 11, wherein the preambles identify the telecommunications terminal to the cell.
13. The method of claim 9, 10, 11 or 12, wherein the preambles are generated from one or more cyclically shifted sequences of values, preferably Zadoff-Chu sequences.
14. The method of claim 13, wherein the preamble generation means determines a number of the cyclically shifted sequences of values for generating the preambles in dependence upon the cell range.
15. The method of claim 14, wherein the preamble generation means generates the preambles in dependence upon an index value (RachRootSequence_index) that is calculated as follows: RachRootSequence_index = (PCI -INT (PCI / INT(MaxRRS / Nrrs)) * INT(MaxRRS / Nrrs)) * Nrrs + INT (PCI / INT(MaxRRS / Nrrs)) where: MaxRRS = a maximum number of available sequences of values, and Nrrs = the determined the number of cyclically shifted sequences of values for generating the preambles, and INT is an operation that rounds a number down to the nearest integer.
16. The method of claim 15, wherein the preamble generation means generates the preambles by using the index value (RachRootSequence_index) to determine which of the cyclically shifted sequences of values to use for generating the preambles.
17. A mobile telecommunications network substantially as hereinbefore described with reference to and/or substantially as illustrated in the accompanying drawing.
18. A method of operating a telecommunications terminal in a mobile telecommunications network, substantially as hereinbefore described with reference to and/or substantially as illustrated in f the accompanying drawing.