Guide to CDMA2000 Network PN offset Planning (V1.0)Internally Unclassified

Huawei Technologies Co., Ltd.

Radio Network Planning DepartmentDocument No.Product VersionSecret Level

Product Name:18 Pages in Total

Guide to PN Offset Planning of CDMA Networks (V1.0)

(For Internal Use Only)

Prepared by:Wireless Network Planning DepartmentDate:2002-07-10

Reviewed by:Date:yyyy/mm/dd

Reviewed by:Date:yyyy/mm/dd

Approved by:Date:yyyy/mm/dd

Huawei Technologies Co., Ltd.

All Rights ReservedRevision Record

DateRevised versionDescriptionAuthor

2002/07/101.00 betaFirst Draft finishedSun Jintao

2002/08/061.00ModifiedSun Jintao

Table of Contents51 General Description

52 PN Offset Planning Related Knowledge

52.1 Code Types Used in a CDMA System

52.1.1 Orthogonal Code- Walsh Function

62.1.2 Pseudo-Noise Code, M-Sequence

62.2 Search Windows

73 Purposes of PN Offset Planning

94 PN Offset Planning Analysis

94.1 Determining on PILOT_INC

104.1.1 PN Offset Separation between Pilots

104.1.2 PN Offset Reuse Distance between two Co-Offset Pilots

104.1.3 Calculating the Lower Limit of PILOT_INC

124.1.4 Calculating the Lower Limit of the PN Offset Reuse Distance between two Co-Offset Pilots

134.2 Engineering Approaches to PN Offset Planning

144.2.1 Approach 1 to PN Offset Planning

164.2.2 Approach 2 to PN Offset Planning

195 Sum-Up

Guide to PN Offset Planning of CDMA2000 Networks

Key words: CDMA, PN offset planning

Abstract: This document describes the PN offset planning principles to be followed during the network planning from the point of view of PN offset reuse distance and adjacent offset separation, as well as engineering approaches to PN offset planning.

List of Abbreviations: Describe the abbreviations used in the document. It is required to list both the full English form and the Chinese form of each abbreviation.

List of References: Please list the name, author, title, number, release date and publishing unit etc. of each reference used in the document in the table below.Table of References

TitleAuthorDocument No.

Release DateLook-up Place or Channel

Publishing Unit (Fill in this field if the document is not published by our company.)

PN Phase Offset Planning of CDMA2000 NetworkLi Yunzhi

Technologies of CDMA (Code Division Multiple Access) Mobile Communications SystemsSun Lixin, Xing NingxiaPeoples Post and Telecom Publishing House

Design and Optimization of CDMA SystemKyoung I1 KimPeoples Post and Telecom Publishing House

1 General DescriptionIn a CDMA system, a pair of m-sequences with length 215 are used as the spectrum spreading codes for forward and backward links, which are referred to as I and Q PN sequences. This pair of m-sequences are also the pilot codes for pilot channels, and different sectors are assigned different pilot code phases.

Different sectors employ different phases of the m-sequence with length 215 for modulation, with the phase difference between adjacent phases required to be at least 64 bits. Hence, the maximum number of available phases is: 215/64=512.

Although all sectors employ different PN offsets, at a MS, some undesired pilot signal may act as strong interference due to either a propagation delay (adjacent offset interference) or not long enough reuse distance between two pilots that are assigned the same PN offset (co-offset interference). There would be a propagation delay before a pilot signal arrives at a MS, and if the propagation delay difference between two pilots of two different sectors just compensate for the time shift from one another, the MS would err in synchronizing to the intended pilot signal and may switch to a wrong sector, or even drop calls. Therefore, a detailed PN offset plan should be developed for a CDMA system.

2 PN Offset Planning Related Knowledge

2.1 Code Types Used in a CDMA System

Two types of codes are used in a CDMA2000 system, i.e. orthogonal code and pseudo-noise code.

2.1.1 Orthogonal Code- Walsh Function

An orthogonal code used in a CDMA2000 system is a Walsh function, named after Walsh, a mathematician, who proved it in 1923 to be an orthogonal function. It is represented in the form of Wal (n, t), where n is the sequence number.

The following recursion formula can be used to obtain 2N-order Walsh functions:

, where N is the power of 2.In a CDMA system, the 1.2288Mbit/s 64-order Walsh function is used to spread the spectrum of each code multiplexed forward channel so as to make code multiplexed forward channels orthogonal to each other. Each forward channel in a sector is assigned a Walsh code. A code multiplexed forward channel spread by using 64-order Walsh function n (n=0-63) is defined as code multiplexed channel n, wherein Walsh function n refers to row n+1 in the above Walsh function matrix. The sequence number in the Walsh function of the pilot channel is 0, namely, Wal (64, 0).

2.1.2 Pseudo-Noise Code, M-Sequence

A pseudo-noise code (called PN code in short) has properties similar to a noise sequence. It is actually a regular periodical binary sequence though looking like a noise one. M-sequences are the most important and fundamental among all pseudo-noise codes. M-sequence is the abbreviation for Maximal Length Sequence of Linear Feedback Shift Register . The definition of such a sequence is as follows:

If the period of an output sequence of a N-stage linear feedback shift register is P =2N -1, this sequence is called a m-sequence.

In a CDMA system, usually Galois generators are used to produce m-sequences each of which is comprised of the sequence of period 2N-1 (not all-0 state) and a mask. M-sequences with different masks have different output phases.

Pseudo-noise codes used in a CDMA2000 system are of two types, namely, m-sequence with length 215-1 and that with length 242-1.

On the forward link, the m-sequence with length 242-1 is used to scramble fundamental channels while the m-sequence of length 215-1 is used for quadrature modulation of the forward link (the period of the m-sequence is 215 with an all-0 state being added into it). Different sectors use different phases of the m-sequence for modulation, with the phase difference between adjacent phases required to be at least 64 bits. Hence, the maximum number of available phases is 512.

On the backward link, the m-sequence with length 242-1 is used for direct spreading. Each MS is assigned a m-sequence phase calculated by the ESN of the MS. Backward channels for these MSs are basically orthogonal to each other for the two-valued self-pertinency of m-sequence.

2.2 Search Windows

A CDMA system employs the synchronized detection technique on the forward link. In order to successfully demodulate the intended signal, a MS must be able to estimate the system time with precision. The MS is receiving a pilot signal and will get the estimation from this reference pilot channel. Using this reference pilot as the reference signal, the MS can send and receive signals with any PN code by modulating and demodulating the carrier wave.

However, the pilot intended for a MS may not arrive precisely at the anticipated time because the system time estimated by the MS also includes the propagation delay of the reference pilot, and besides, the time shifts of other pilots arriving at it are also based on their propagation delays. The MS cannot tell the propagation delay of a random given pilot, and therefore it must search the pilot within an appropriate delay window till it detects the actual time shift of the reference pilot, and this window is referred to as the search window.

A MS would search the pilot using the following 3 different types of search windows.

SRCH_WIN_A: used to search the activated and candidate pilot set;

SRCH_WIN_N: used to search the adjacent pilot set;

SRCH_WIN_R: used to search the remaining pilot set.

The measurement unit for each window is chip. As illustrated in the figure below, the center of a SRCH_WIN_A window is located near the peak of the first arrived multipath component.

With the SRCH_WIN_A as the reference, the center of a SRCH_WIN_N or SRCH_WIN_R is located about at the PN offset from the moment when the intended pilot arrives. As illustrated in the figure below, suppose a sector is assigned PN offset 4, and an adjacent sector is assigned PN offset 20.

With a search window, a MS can detect multipath components either before or after the arrival of the intended pilot, and therefore, it can maintain a coherent detection regardless of the propagation delay. The size of a search window must be set to the effect that the best quality pilot signal should fall into it. If a search window is too small, the important pilot signal will be lost, thus resulting in interference; if it is too large, the search efficiency will decrease, and the conversation quality will be compromised.

Sizes of search windows are listed in the following table:

SRCH_WIN_A

SRCH_WIN_N

SRCH_WIN_RSearch Window Size (chips)SRCH_WIN_A

SRCH_WIN_N

SRCH_WIN_RSearch Window Size (chips)

04860

16980

2810100

31011130

41412160

52013226

62814320

74015452

3 Purposes of PN Offset Planning

As we know, sector separation for a CDMA2000 system is performed by two m-sequences with length 215, which are phase shifted by PN-offset for different sectors. Because there is a limited number of PN offsets, a maximum of 512, a PN offset plan must be developed to avoid any PN confusion.

As described above, although all sectors employ different PN offsets, at a MS, some undesired pilot signal may act as strong interference due to either a propagation delay (adjacent offset interference) or not long enough reuse distance between two pilots that are assigned the same PN offset (co-offset interference). There would be a propagation delay before a pilot signal arrives at a MS, and if the propagation delay difference between two pilots of two different sectors just compensate for the time shift from one another, the MS would err in synchronizing to the intended pilot signal and may switch to a wrong sector, or even drop calls.

We will illustrate it with an example.

If we use C(0)(t) to represent a pilot signal with PN offset 0, the pilot signal with PN offset i can be then represented by:

Suppose there are two cells with two different PN offsets that are respectively and ; and are time delays respectively from the base stations to the MS; and are the radiuses of coverage areas of the two cells; represents one half of search window SRCH_WIN_A while represents one half of search window SRCH_WIN_N; represents the chip width. The measurement unit for all the above values is chip.

Transmit powers of the two cells are p1 and p2 (dB). See the figure below.

Figure 1 Analysis of interference between pilot signals of two cells

Then, pilot signal of cell 1 is represented as:

Now, is the time delay. Suppose is the propagation loss, then is the propagation loss in dB.

Then, the propagation attenuation slope of the pilot signal received at the MS from cell 1 is:

Likewise, the propagation attenuation slope of the pilot signal received at the MS from cell 2 is:

When arriving at the MS, the two pilot signals may have the same PN offset if

In other words, if , the two pilot signals with different offsets would have the same PN offset when arriving at the MS, thus causing interference.

Hence, a detailed PN offset plan should be developed for a CDMA system.

The PN offsets of pilot signals are usually represented by offset indexes ranging from 0 to 511.

4 PN Offset Planning Analysis

In a CDMA system, two adjacent PN offsets must be separated from each other at an interval of at least 64 chips.

1 chip=3108/1.2288M=244.14(m)

64 chips=64244.14=15.6(km)

In reality, due to complicated radio propagation environments and limited sizes of MS search windows, the 15.6km separation is not enough for distinguishing between two adjacent PN offsets in an actual PN offset planning. For this reason, we use parameter PILOT_INC to set the number of available PN offsets.

Available number of PN offsets=512/PILOT_INC.

4.1 Determining on PILOT_INC

The value of PILOT_INC determines the phase difference between pilots of different cells. The less the PILOT_INC is, the more the available PN offsets, and the further the PN offset reuse distance between two co-offset pilots. Though it can reduce co-offset interference, it can also reduce the phase difference between adjacent pilots as well, thus being likely to result in a pilot confusion. Therefore, we can analyze this issue to get the lower limit of the PILOT_INC.

The greater the PILOT_INC is, the less the available PN offsets, the less the remaining pilots, and accordingly, the shorter time a MS would spend on pilot searching. Hence, in practice, the probability of losing a strong pilot signal decreases. But this improvement is only to a limited degree, because the priority for the remaining pilot set is the lowest in pilot searching. Moreover, the greater the PILOT_INC is, the less the available PN offsets, thus resulting in a decrease in the PN offset reuse distance between co-offset pilots and an increase in co-offset interference. Therefore, the reuse distance between two co-offset pilots must be appropriate.

In reality, we can compare the PN offset separation issue to the adjacent frequency isolation issue in GSM systems, and the PN offset reuse distance issue to the frequency reuse issue in GSM systems.

According to the above analyses, how to select an optimal PILOT_INC value is the key to PN offset planning.

Here below is how we should calculate the lower limit of PILOT_INC.

4.1.1 PN Offset Separation between Pilots

According to the above analyses, the minimum PN offset separation between two pilots determines the lower limit of PILOT_INC. So, the restriction on the minimum PN offset separation between two pilots should be taken into consideration first.

PN offset separation between two pilots should be mainly based on the following principle:

When a pilot of another sector with a different PN offset is present in search window SRCH_WIN_A of the current PN offset, the interference of the current pilot by it should be below a certain lower limit.

4.1.2 PN Offset Reuse Distance between two Co-Offset Pilots

Likewise, the PN offset reuse distance between two co-offset pilots of different cells should be based on the following principle:

The interference of the current pilot by a co-offset pilot of another cell should be below a certain lower limit.

4.1.3 Calculating the Lower Limit of PILOT_INC

As shown in figure 1, suppose that the strength of the pilot signal of cell 2 received at a MS in cell 1 is T (dB) lower than that of the pilot signal of cell 1. If the pilot signal of cell 2 is required not to jam the pilot signal of cell 1, then:

; where Ls are propagation losses.Suppose P1 is equal to P2, then:

Considering the propagation loss to be:

Where d is the distance between the base station and MS in question.

Then, we have:

In the case of a direct radiation signal, To make the above inequality equation true under any circumstances, this inequality equation must hold true even if assumes the maximum value and assumes the minimum value at the same time.

Consider the worst case in which interference is most likely to occur: a MS is located at the boundary of cell 1, in which case the pilot signal of cell 1 arriving at it is the weakest, and therefore, the interference likelihood is the highest, namely, in this case. In addition, the MS search window is just large enough to let the pilot signal of cell 2 enter into it,

Namely:

According to the above equation, we have:

Then:

Because:

Then:

; where is the propagation attenuation slope of a radio wave. Generally speaking, =4.3 in a densely populated downtown area; =3.84 in a suburb. is related to such system parameters as T_ADD and T_DROP, and =24dB in most cases.

According to the above inequality equation, the lower limit of PILOT_INC is related to the coverage area radius, r, propagation attenuation slope as well as size of the MS search window. The less the coverage area radius, r, is, or the greater the attenuation slope is, or the smaller the search window ARCH_WIN_A is, the less the lower limit of PILOT_INC would be.

Suppose that radiuses of coverage areas of all cells are identical. We then have:

In a densely populated downtown area: In a suburb: ; where r is the coverage area radius of the cells, and S1A represents one half of search window SRCH_WIN_A (in chips).

For a densely populated downtown area, if r is 500m (about two chips), we can draw the following conclusions from the above inequality equation:

Coverage area Radius r (m)r (chips)SRCH_WIN_A

SRCH_WIN_A Size (chips)S1APILOT_INCLower Limit of PILOT_INC

Number of Available PN Offsets

5002.0480420.178938321512

5002.0481630.194563321512

5002.0482840.210188321512

5002.04831050.225813321512

5002.04841470.257063321512

5002.048520100.303938321512

5002.048628140.366438321512

5002.048740200.460188321512

5002.048860300.616438321512

5002.048980400.772688321512

5002.04810100500.928938321512

5002.04811130651.163313322256

5002.04812160801.397688322256

5002.048132261131.913313322256

5002.048143201602.647688323170

5002.048154522263.678938324128

For an open field, if r is 10Km (about 41 chips), we can draw the following conclusions from the above inequality equation:

Coverage area Radius r (m)r (chips)SRCH_WIN_A

SRCH_WIN_A Size (chips)S1APILOT_INCLower Limit of PILOT_INC

Number of Available PN Offsets

1040.960423.3701076224128

1040.961633.3857326224128

1040.962843.4013576224128

1040.9631053.4169826224128

1040.9641473.4482326224128

1040.96520103.4951076224128

1040.96628143.5576076224128

1040.96740203.6513576224128

1040.96860303.8076076224128

1040.96980403.9638576224128

1040.9610100504.1201076225102

1040.9611130654.3544826225102

1040.9612160804.5888576225102

1040.96132261135.104482622685

1040.96143201605.838857622685

1040.96154522266.870107622773

According to the above statistics, if r=10km, SRCH_WIN_A=6 (the size of SRCH_WIN_A is 28 chips), and PILOT_INC3.5576, namely, PILOT_INC=4.

4.1.4 Calculating the Lower Limit of the PN Offset Reuse Distance between two Co-Offset Pilots

According to the said principle, i.e. the interference of the current pilot by a co-offset pilot of another cell should be below a certain lower limit. In other words, the PN offset reuse distance between co-offset pilots is required to be long enough.

As illustrated below in Figure 2, cells 1 and 3 use the pilots assigned the same PN offset; D (A, N) represents the distance between point A and cell N; D represents the distance between cells 1 and 3; is the coverage area radius of cell i; is the relative offset of cell 1; is the time delay from base station i to the MS; represents one half of search window SRCH_WIN_A while represents one half of search window SRCH_WIN_N; represents the chip width. The measurement unit for all the above values is chip.

is the transmit power of cell i; is the propagation loss attenuation slope.

Figure 2 Calculation of the PN offset reuse distance between co-offset pilots

Here below is how we should calculate the lower limit of D.

II. To prevent a MS from being unable to distinguish between two co-offset pilots within search window SRCH_WIN_A

Consider the worst case in which interference is most likely to occur: a MS is located at the boundary of cell 1 and aligned with cell 3. Now the MS is about to be handed over for cell 2 to cell 1, in which case the pilot signal of cell 1 arriving at it is the weakest and the interfering signal from cell 3 is the strongest; and therefore, the interference likelihood is the highest, namely, D (A, 1)=r1 and D (A, 3)=D-r1. In addition, search window SRCH_WIN_A is just large enough to let the pilot signal of cell 3 enter into it. Then, we have:

Namely,

(4.1.41)

III. To prevent a MS from being unable to distinguish between two co-offset pilots within search window SRCH_WIN_N

To prevent co-offset pilot signals from both falling into search window SRCH_WIN_N of the MS at point A in cell 2, one of pilot signals of cells 1 and 3 should fall out of search window SCH_WIN_N. In other words, one of distances from cells 1 and 3 to cell 2 should be:

Consider the worst case, namely, cells 1, 2 and 3 are aligned, and suppose r1=r3=r, then:

(4.1.4--2)

Compare two inequality equations (4.1.4-1) and (4.1.4-2); obviously, (4.1.4-2) should be used to calculate the lower limit of PN offset reuse distance.

4.2 Engineering Approaches to PN Offset Planning

In PN offset planning, we should first of all settle on PILOT_INC.

According to the previous analyses of the PILOT_INC lower limit, the PILOT_INC is usually set to 4 in practical applications, in which case the number of available PN offsets is 128 and the adjacent offset separation is 256 chips, equivalent to 62km; in other words, a pilot signal must travel at least 62Km in order to fall into search window SRCH_WIN_A for another pilot. This can work in most of network applications. Moreover, to avoid a networking complication, two adjacent networks had better employ the same PILOT _INC so as to prevent MSs from searching the remaining pilot set (a MS shall search the remaining pilot signals by integral multiples of PILOT_INC).

Having determined the value of PILOT_INC, we can then proceed to the PN offset reuse model. If PILOT_INC=4, the number of available PN offsets is 128. When three cells are taken into consideration, there can be a maximum of 42 cells in each reuse cluster.

4.2.1 Approach 1 to PN Offset Planning

If N is the number of cells in a reuse cluster, it should be

; where i and j are positive integers. We must follow the procedure below to find out the adjacent co-offset cells nearest to a certain cell: 1, move across i number of cells from each link on the regular hexagon (each borderline of the hexagon); 2, turn 60 counterclockwise and then move across j number of cells. See figure below, wherein i=3 and j=2(N=19)

Figure 3 Locating co-offset cells in a reuse cluster

If i=4 and j=3, N=37.

Figure 4 Approach 1 to PN offset planning

In this case, the number of PN offsets required is 111. Because the number of available PN offsets is 128, the remaining PN offsets are reserved for future use. In practical applications, some PN offsets should be reserved for use in future capacity expansions or of microcells. Or, if some antennas must be mounted on higher levels because of environmental restrictions, to avoid interference of other cells by them, PN offsets not used by cells in the reuse cluster can be used by them. PN offsets are typically grouped as follows:

128 PN offsets are divided into two groups, with 111 of them being used by cells in the reuse cluster while the remaining 17 of them being reserved PN offsets. The 111 PN offsets are further put into three groups. Based on the principle that adjacent PN offsets of sectors should be separated as far from each other as possible, and to standardize data settings, PN offsets are assigned as in the following table. Numbers in a column for a sector represent the PN offset serial numbers assigned to different sectors in the cell in question.

Cell No.Sector 1(Group A)Sector 2 (Group B)Sector 3 (Group C)

Cell 14172340

Cell 28176344

Cell 312180348

............

Cell nn*4(n+42)*4(n+84)*4

............

Cell 37148316484

In approach 1, there are 37 cells in each reuse cluster. Since unified cell numbering is complicated, the rule below should be followed to number cells.

As illustrated below, cell n+1 is on the southwest (S60W) of a given cell n with two cells on the south skipped. If cell n+1 is not within the reuse cluster, the same rule should be followed to number it.

Figure 5 Cell numbering rule

The above cellular distribution is an ideal one that does not exist in reality; therefore, it is not easy to practice this approach to PN offset planning. Here below is another approach, used more often in engineering, to PN offset planning.

4.2.2 Approach 2 to PN Offset Planning

We put 42 cells in one reuse cluster, and divide this cluster into several sub-clusters. There is no absolute rule on the number of sub-clusters in each cluster (or, the number of cells in each sub-cluster). Usually, in a densely populated downtown area, there are relatively more cells in each sub-cluster. In network planning, 42 cells are usually put into 4 sub-clusters with each of them being comprised of 10~11 cells.

Sub-cluster 1

Sector 142036526884100116132148164

Sector 2172188204220236252268284300316332

Sector 3340356372388404420436452468484500

Sub-cluster 2

Sector 182440567288104120136152168

Sector 2176192208224240256272288304320336

Sector 3344360376392408424440456472488504

Sub-cluster 3

Sector 1122844607692108124140156

Sector 2180196212228244260276292308324

Sector 3348364380396412428444460476492

Sub-cluster 4

Sector 1163248648096112128144160

Sector 2184200216232248264280296312328

Sector 3352368384400416432448464480496

The distribution of cells within each sub-cluster is illustrated in the following figure (sector 1 is used as the example):

Figure 6 Distribution of cells in a sub-cluster

As shown above, cells in a sub-cluster are distributed in a spiral (please refer to the application case at the end of this section).

As shown below in the figure, several kinds of reuse cluster layouts are available (sector 1 is used as the example):

Figure 7 Comparison between reuse cluster layouts

By comparing the above three reuse cluster layouts, we can conclude that the PN offset reuse distance between co-offset pilots in the second layout can reach a maximum of 15.2r (where r is the cell coverage area radius). Hence, this layout is usually adopted in engineering. See figure below (sector 1 is used as the example):

Figure 8 Frequently used cluster layout

Here below is an application case of PN offset planning:

Figure 9 Approach 2 to PN offset planning

5 Sum-Up

An appropriate PN offset plan should be made at the initial stage of the system design. An inappropriate PN offset plan may result in network interference and modifications of a large number of network data in future network optimization, thus making network maintenance a difficult job.

Before an IS-95 system is upgraded (moved or expanded in capacity) to a CDMA 1X system, the existing PN offset planning data should be collected. It is recommended that the same principles be followed for re-planning to avoid confusions caused by PN offsets within search windows in adjacent cells, which may result in interference between a large number of signals within the upgraded network, and even drop calls.

A description on pages appended to a document or revisions of it due to version upgrades, which is usually included in a design document.

2002-9-3

2002-08-06All Rights ReservedPage 18 of 18

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