18mpd pilots

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New Measurements with CDMA Field Test Equipment

by Ray ChadwickBerkeley Varitronics Systems, Inc.

Introduction

Berkeley Varitronics Systems has developed three CDMA Pilot PN scanners that arecurrently being used to diagnose and remediate problems in CDMA networks. This

article presents some of the problems that have been found and diagnosed with pilotscanners. Other potential problems in CDMA networks are addressed, and CDMA Field

tools capable of diagnosing these potential problems are introduced. These tools areunder development, have capabilities in addition to pilot scanning and are suitable for

both real time analysis or post-processing.

Review of the Pilot Channel

Each CDMA Base Station (BS) transmits a pilot code to aid the handset withdemodulation and to measure BS strength. The pilot code is a pseudo random sequence,

the same for each BS, but aPN sequence offsetis assigned to each BS. Remember that aPN sequence with an offset in time is nearly orthogonal to the original PN sequence [1].

This is the principle that allows a CDMA mobile to distinguish between multiple BSs thattransmit on the same frequency. This is the same principle that PN scanners use to scan

for offsets of the pilot PN code. Each offset detected is a different time shift of the PNcode and is the result of a direct transmission path or a multipath. Figure 1 depicts the

relationship between BS PN offset and GPS time.

Figure 1: CDMA Time Offsets

Ec/Io is the CDMA specific measurement of choice for the forward link. This is the ratioof the energy of the pilot signal being measured to the total power in the channel. The

total power in the channel includes the pilot channel being measured, all channels fromall BSs and noise. Figure 2 is a screen shot of a Super Eagle PN Scanner. Note that the

PN scanner is measuring two different RF frequencies. In this CDMA network, there are2 CDMA channels 1.25 MHz apart. The Y-axis is the strength of the pilot in dB and theX-axis is the PN Offset.


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Figure 2: Super Eagle Display of Ec/Io vs. PN Offset

A Pilot Scanner in the CDMA Network

The primary function of the pilot scanner is to measure and report the Ec/Io and PNOffset of all pilots detected. This may sound like a trivial task, but requires a bit of

processing. For the Super Eagle example, shown in figure 2, the PN scanner searchesfor: 512 BS PN Offsets X 64 chips per Offset X 2 Oversamples per chip or 65,535 pilots

for each scan of each frequency! Fortunately, the pilot signals represent a lot ofinformation about the CDMA system. The pilots can be analyzed to detect:

Areas with no pilots or only weak pilots will have no dominant server.

Pilot coverage from neighboring BSs should overlap in fringe areas to accommodate

hand-off. However, the overlap should not be excessive or have a large number of BS inthe same overlap. Each BS that has significant power in the overlap area will raise Io,

decrease Ec/Io and capacity and/or quality will suffer. This phenomenon is known aspilot pollution.

Searching for a pilot signal requires a significant amount of processing. To reduce the

processing time that a mobile requires to find and maintain a list of neighboring BSs, theBS sends the mobile a neighbor list message. Pilot scanners can verify that a neighbor

list of a particular BS does indeed contain its neighbors. This may seem trivial, butgeography and RF shadowing can make distant BS signals stronger or close BS signals

weak.


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PN scanners excel at measuring pilot signals in areas where the pilots change rapidly.Hills or buildings create RF shadows. Consider a mobile traveling from a valley up a hill

and then down to another valley. The mobile may see three completely different BSpilot sets. A set in the first valley, another at the top of the hill and finally a third in the

second valley. The mobile may be required to conduct two hand-offs quickly. Carefulselection and verification of neighbor lists may be necessary in these areas.

Carriers are adding additional CDMA capacity at a second or even third RF frequency. A

single calibrated Super Eagle pilot scanner (figure 2) contains enough processing powerto measure multiple CDMA RF channels several times per second.

Sync and Page Channels

The pilot channel is the lowest level channel in CDMA. It is not modulated by any

data. The next step-up is the Sync Channel. This channel is modulated with a Walsh

code; this keeps it orthogonal to the pilot and other channels. It is then modulated withsync channel data. The sync channel message provides the mobile with moresynchronization information and parameters. The message also contains the PN Offset of

the transmitting BS.

CDMA mobiles rely on relative timing to search and locate other BSs. A mobile doesnot have a GPS receiver and derives its timing reference from a BS. For example, if a

mobile s active BS is 10 and the mobile is searching for BS 11, the mobile will delay itssearch by 64 chips relative to when the pilot for BS 10 was received and center its search

for BS 11 there. A search window defines the size of the search.

The time at which a mobile receives a pilot signal is very important. Consider figure 4.BS1 and BS2 have the same PN offset. Since they have the same PN offset, they would

typically be located distant from one another in the reuse pattern of the CDMA system.The mobile is located close to BS2, but is between BS1 and BS2. If the difference

between the propagation times T1 and T2 is less than the half the search window, themobile could confuse BS1 s pilot as multipath from BS2. Normally BS1 s pilot signal

would be very low and not create a problem, but geography and RF shadowing canattenuate BS2 s signal so that it is near the level of BS1 s. This is a simple example of

pilot aliasing [2]. Pilot aliasing does not necessarily have to occur with BSs of the samePN offset.


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Figure 3: PN alias

A CDMA tool that can demodulate any BS sync channel with significant Ec/Io and

measure pilot Ec/Io can easily detect PN aliasing. This capability has been developed in

the Hummingbird hand-held PN scanner.

Page Channel

This channel is modulated with another Walsh code; which keeps it orthogonal to the

pilot, sync and other channels. It is then modulated with Page channel data. The pagechannel message provides the mobile with system parameters and paging information.

The system parameters of interest are listed below:

SRCH_WIN_A The search window (in chips) that the mobile uses to track Active andCandidate Pilots. The window is centered around the earliest arriving usable signal. The

window should be large enough to find multipaths, but small enough to not degradesearcher performance.

SRCH_WIN_Nis the search window that the mobile uses to track the neighbor pilots.

The window is usually larger than SRCH_WIN_A because of the greater distancebetween the mobile and the neighboring BSs.

Neighbor Listcontains a list of neighboring PN Offsets.

These parameters effect the following Pilot Sets maintained by the mobile:

Active Set Contains the pilots of those sectors that are actively communicating with themobile on traffic channels.

Candidate Set Contains pilots whose Ec/Io is above the pilot detection threshold(T_ADD).

BS1 BS2T1 T2

Search Window Cen tered on P2

T1-T2

P2

P1

T1-T2 > 1/ 2 SRCH_WIN_A or P1's Ec/Io must be very low.


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Neighbor Set Contains those pilots that are in the mobile s current serving sector. The

neighbor set is initialized by the serving base station. Pilots may also enter this set whenthey are dropped from the active or candidate set.

Pilot scanners that search forallpossible pilots excel at verifying search window sizes

and neighbor lists. The pilot scanner can verify that pilots with significant power do notfall outside the search windows and verify that the search window is not too small. It can

also detect that there are pilots with significant power near the extremes of searchwindows verifying that the search windows are not too big. Figure 4 illustrates the

concept of a properly sized search window. SRCH_WIN_A should be large enough forthe mobile to capture useable multipath and as small as possible to minimize the mobile s

search processing.

1,000m / (244m/chip) = 4.1 chips

2,000m / (244 m/chip) = 8.2 chips

Figure 4: SRCH_WIN_A is large enough to capture path 1 and 2.

Conclusion

Pilot PN scanning is conceptually simple, but very powerful when measuring or verifying

CDMA coverage. Future CDMA tools will maintain the powerful PN scanningcapability, and be integrated with many of the features described here to enable real-time

detection and debugging or to reduce the amount of number crunching and processeswhen post-processing drive data. Powerful PN scanning is being integrated with: multi-

carrier support, sync and page channel demodulation, real-time search window

verification, neighbor list verification andpilot aliasing detection.

BS 1km

1km 1km

P1 P2

P2

P1

SRCH_WIN_A

t

4.1 chips


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References

[1] Chadwick, Ray and Groome, W.C. Andrew, CDMA Forward Link Coverage Field

Testing (Part 1 and Part 2), Microwave Product Digest, July/August, 1998.

[2] Yang, Samuel C., CDMA RF System Engineering, Artech House, Inc., 1998.

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