TD-SCDMA Service
A high Performance/Price
Accurate Locating Technology
China PUTIAN Institute
of Technology CTO GuoLiang Qian

Location Service
and Locating Technology

Location
service plays a very important role in mobile communication system.
Some studies indicate that location oriented services, such as urgency
succor, information service, traffic navigation, team management, cell
system optimization and design etc, have nice future.
Recently,
in kinds of mobile communication system, such as GSM, CDMA, WCDMA, and
etc, some locating technologies have been developed. Among these technologies,
some are realized based on terminal, or network, or both terminal and
network.

Locating technology
based on terminal

In
this case, a UE calculates its present position. Now it mainly consists
of GPS, A-GPS(Assisted GPS), E-OTD(Enhanced Observed Time Difference),
and so on.

GPS:

GPS
uses “man-made satellites” as reference points to calculate positions
accurate to a matter of meters, whose positions including longitude,
latitude and altitude. Raw data received from satellites can be dealt
with not only by terminal, but also by network side. Usually, GPS locating
needs 4 satellites to finish.

A－GPS：

For
a first location, a UE may takes about 10 minutes to finish if directly
using GPS, which delay is not fit to urgency services such as E911,
and so on. However, A-GPS (Assisted GPS) is a relatively new system
that improves the functionality and performance of GPS by integrating
the classic GPS information with mobile/cellular network information.
If a UE uses A-GPS or Assisted GPS to locate, it will remarkably reduces
both the window of code searching and frequency searching. As a result,
the delay of location will decreases to several seconds.

E－OTD：

The
E-OTD method is based on measurement in the UE of the time difference
of arrival of bursts from nearby pairs of Base Stations, It is employed
to calculate the position of the UE relative to Base Station.
In
this technology, the UE’s position can be determined by time difference
that electric wave reaches the same receiver from different transmitter.
Original measurement is realized by calculating arrival time difference
among different public pilot channels from BSs. Three Base Stations
make a plane, if the distance between one point and two immovably points
is the fixed value, then the point must be in one hyperbola. The transmit
delay from UE to several base stations is measured, Then the position
of the UE can be fixed according to the focus of two hyperbolas.

Locating technology
based Network

Locating
technologies based network is that network calculates the position of
UE based on the measurement data, which mainly including Cell_ID, UL-TOA,
TDOA and AOA.

Cell-ID：

Cell-ID
is global identifier of cell and a cell holds one and only a Cell-ID,
which consists of Location Area Identity (LAI) and Cell Identity(CI),
that is LAI＋CI＝MCC＋MNC＋LAC＋CI.

UL TOA：

TOA
locating method of Uplink arrival time is similar to EOTD method, but
in UL TOA method, base station measures the arrival time of terminal
data.

AOA

Angle
of Arrival(AOA) method is a measurement to estimate UE position that
two or more base stations measure arrival angle of received signals.
The AOA method can determine uniquely a two-dimension location point.
But
AOA method is realized only based on SA(smart antenna), so present BTS
antenna have to be replaced by smart antenna if AOA method is employed
to locate UE.

Mixed locating
technology

Mixed
locating technology is a location method that integrates two or more
two methods introduced above. For example, a locating system integrates
AOA and TDOA methods simultaneous.
Location
technologies described above are deficient in actual application, because
there are some defects inside those methods. For instance, some methods
have no high accuracy although it is simple to realize, some methods
have high cost to realize although it have high accuracy.

TD-SCDMA Accurate
Locating Technology

As
a third generation mobile communication international standard whose
main copyrights belong to China, TD-SCDMA adopts many new technologies
such as smart antenna, time and code division, uplink synchronization,
asymmetric loads. Through those technologies, TD-SCDMA special applications
have outstanding advantages. For example, accurate location technology
is a valued application based on TD-SCDMA, which has high Performance/Price
and can provide low price and excellent service for public users.
2.1 A New
Locating Technology
It
is well known that TD-SCDMA has two most important technologies named
uplink synchronization technology and smart antenna technology:

Uplink Synchronization

In
TD-SCDMA system, the process of Uplink Synchronization is compellingly
supported by UE. In this process, BS should know the exact position
of UE in cell in order to estimating TOA, which purpose is to synchronize
all signals transmitted by UE at BS and reduce interference of uplink
signals. Therefore, the process of uplink synchronization shall be finished
by UE and BS to cooperate.

Establishment of
uplink synchronization

When
a UE is powered on, it first needs to establish the downlink synchronization
with the cell. Only after the UE has established the downlink
synchronization, it shall start the uplink synchronization procedure.
The establishment of uplink synchronization is done during the random
access procedure and involves the UpPCH and the PRACH. The procedure
described blow:
Although
the UE can receive the downlink signal from the Node B, the distance
to Node B is still uncertain. This would lead to unsynchronized uplink
transmission. Therefore, the first transmission in the uplink direction
is performed in a special time-slot UpPTS to reduce interference in
the traffic time-slots.
The
timing used for the UpPCH is set e.g., according to the received power
level of DwPCH and/or P-CCPCH.
After
the detection of the SYNC-UL sequence in the searching window, the Node
B will evaluate the timing, and reply by sending the adjustment information
to the UE to modify its timing for next transmission. This is done with
the FPACH within the following 4 subframes. After sending the PRACH
the uplink synchronization is established. In 4 subframes, if UE have
not received response from Node B, UE thinks synchronization request
failed. After a time, UE will try to synchronization send.
The
uplink synchronization procedure shall also be used for the re-establishment
of the uplink synchronization when uplink is out of synchronization.

Maintenance of uplink
synchronization

The
distance between UE and Node B is always changing because of UE always
moving. According to this, Node B needs uninterrupted detects the distance
of UE and adjusts properly TOA. This procedure is named as maintenance
of uplink synchronization, describing below:
In
each uplink time slot, the midamble for each UE is different. The Node
B may estimate the timing by evaluating the channel impulse response
of each UE in the same time slot. Then, in the next available downlink
time slot, the Node B will signal Synchronization Shift (SS) commands
to enable the UE to properly adjust its Tx timing.
To
keep stability of uplink synchronization, the detection process of uplink
synchronization shall take place in each TDD subframe and the value
of the “Uplink synchronization frequency” is configured
or re configured by higher layers, usually at lease 1/8 chips.
The
uplink synchronization procedure shall also be used for the re-establishment
of the uplink synchronization when uplink is out of synchronization.
To
keep synchronization, as we know, base station must know transmitting
TOA of UE. Through the TOA, the distance between base station and UE
can be calculated. But for an omni antenna, it is only determined a
ring around base station and accurate position of UE still can not determined.

Smart antenna

Smart
antenna technology requires that base station can estimate the orientation
of UE around it for exactly beam forming, based on main path of uplink
signals transmitted by each UE.
So
it is well known that base station must know azimuth parameter of UE
oriented own.
From
these two parameters, exact position of UE in base station can be accurately
determined. Associated with geography coordinates of BS, the UE’S
exactly geography coordinate position can be determined.
This
locating technology is realized based on present technologies of TD-SCDMA
and needs no more additional methods. Furthermore, relatively accurate
locating is finished in a single base station.
2.2 Location
Accuracy Analysis
In
TD-SCDMA system, the step length of synchronization adjusting is 1/8
chip. Namely, uplink synchronization may have 1/8 chip error. TD-SCDMA
’s chip rate is 1.28Mcps, so distance error is about 28m. The accuracy of azimuth adjusting
depends on the material realization way and step length of azimuth adjusting
is usually 1 degree. Accordingly it is concluded that the location accuracy
of this technique is a zonal area whose radical length is 56m and angle
is 1 degree, which is shown in figure 1:

Figure
1
2.3 Subjects
to be studied
1． The accuracy of uplink synchronization
Because
of some problem about realization, present TD-SCDMA equipments can only
realize 3/8 chip accuracy. Accordingly the location accuracy can only
reach about 89m. This accuracy mayn’t be adequate to some application
service. It can be said that it is the accuracy of uplink synchronization
who directly affects the location accuracy. How to enhance the accuracy
of uplink synchronization is problem to be resolved.
2．The azimuth of smart antenna
The
azimuth accuracy is only 1 degree and also limited by the material realization
way. How to enhance the accuracy of beam forming is also a subject to
be studied.
3．The effect on location accuracy by multi-path
In
the complicated application environment, there is severe multi-path
effect, which arises from electromagnetic wave’s diffraction and multiple
reflections.
Figure
2
Under
the multi-path condition, main path isn’t the line-of-sight between
UE and BS possibly. For example, in figure2, path 1 is a main path but
not line-of-sight.
For
uplink synchronization and beam forming, this phenomenon can’t effect
on system’s performance but on location accuracy. Generally, this
phenomenon is named as Imagining Effect, which can be detailed as follows:

The effect on distance
location accuracy by multi-path

The
time calculated by BS is the amount of time along path 1. Calculated
by location algorithm, this distance is treated as line distance between
UE and BS. Obviously it is overrated.

The effect on azimuth
accuracy by multi-path

According
to the uplink direction of electromagnetic wave, BS concludes the mobile
direction is the line along path 1 to the base station, which is obviously
misestimated.
In
the same way, multi-path can effect on some other locating technologies,
for example: ETO and so on.
4．Location accuracy affected by UE’s movement.
It
is well known that the step length of synchronization adjusting is 1/8
chip, and synchronization detection and control is repeated for every
subframe (5ms). Generally, BS detects the azimuth of UE ’s uplink
signal in every subframe (5ms) for beam forming. So BS can offer the
location information in at least 5ms.
That
is to say, BS can’t detect the movement of UE in a subframe: For a
mobile object with 300km/s speed, it can move 0.42m in 5ms, which distance
is in the error range of TD-SCDMA location technique. So a mobile UE
has no effect on location accuracy.

Conclusion

Although
present mobile communication system can support more than one location
technologies, there still are some defects in practice. For example,
some of those are realized easily but with low location accuracy, such
as CELL ID; some of those has higher location accuracy need some additional
functions, such as A-GPS that needs both UE and network to support GPS
functions.
TD-SCDMA
can support almost present location technology, such as CELL ID, A-GPS,
E-OTD, etc. Furthermore, because of uplink synchronization and smart
antenna being adopted in TD-SCDMA system, it can provide a new high
Performance/Price location service in a single base station without
adding or modifying any equipment.
Based
on some technologies used in TD-SCDMA, the article describes the principle
of the new location technology in TD-SCDMA. And it analyses the factors
that affect accuracy and some other existing problems that need to be
further studied.
As
a result, location service in TD-SCDMA system may provide for public
users as a basic service.

Reference
[1] TD-SCDMA-Third
Generation Mobile Communication System
Standard, ShiHe
li
[2] 3G Services
Market Requirements Analysis, Rong Qin, Institute of Communication Standards
Research
[3] 3GPP TS 23.271
V5.11.0 (2004-06)，Functional stage
2 description of Location Services (LCS)