Phase coherency of CDMA caller location processing based on TCXO frequency reference with intermittent
GPS correction
Dingchen Lu, Alfredo Lopez, Surendran K. Shanmugam, John Nielsen and Gerard Lachapelle
University of Calgary
1. Introduction
Positioning system based on CDMA cellular network has acquired great attention over the past few years since the Federal Communications Commission (FCC) passed a series of regulations in 1996. The mandate is to provide the Enhanced 911 (E-911) service. At present the fixed line phone user’s information will be sent to the control centers when the subscriber calls 911. However the mobile cell phone user’s location information is not known when the user calls 911. The essential requirement is to provide the cellular phone user’s information. There are two types of wireless location systems. One is network-based in which the computation of the position is carried out in base stations (BSs) based on the received reverse link signals from the cell phone. Another type is handset- based in which the computation of the position is done by the cell phone based on the received forward link pilot signals from CDMA BSs. In this paper a GPS- assisted handset-based wireless location system is assumed in which the GPS
provides a reference frequency suitable for phase synchronization of the CDMA receiver. This allows better accuracy and sensitivity through longer coherent integration epochs.
2. Wireless Location System and Location Methods
There are two fundamental methods to locate a subscriber: angle of arrival (AOA), time of arrival (TOA) and time difference of arrival (TDOA) [1]. Our present research project [2] is based on TOA method. So this research is considered to be applied in the TOA technique. TOA method requires that all BSs be precisely synchronized to each other and that the mobile station (MS) to be located be synchronized to the network too. In CDMA system the BSs are all synchronized to the GPS time by connecting each BS to one GPS receiver as shown in Figure 1 after [3]. However the MS is not synchronized to the GPS time. In our project, the time of the MS is controlled by the local free running TCXO. In order to make the MS synchronized to the GPS time, there are two ways: one way is to extract the time information from the pilot signals and let the MS synchronized to the GPS time;
the other way is to combine the CDMA receiver with an inexpensive GPS receiver which may have intermittent performance due to limited sensitivity.
In this research, a GPS receiver is used
as a reference to get the TCXO
frequency correction value for the MS
synchronization.
Figure 1: CDMA Network
3. Mobile Station Synchronization
Because the time of the mobile station with CDMA receiver is controlled by the free running TCXO 10 MHz clock [4], in order to be synchronized to the GPS synchronized CDMA base stations, the mobile station is integrated with a GPS reference source as shown in Figure 2.
The two inputs to the FPGA board are the valid NovAtel OEM4 10 MHz clock [5] and free running TCXO 10 MHz clock. Two 16-bit counters implemented by Altera Flex 10K70 FPGA chip [6] are used to calculate the TCXO frequency according to the accumulated counter value which can be expressed by equation 1.
f TCXO / f GPS = N TCXO / N GPS (1)
where f TCXO is the frequency of TCXO;
f GPS is the frequency of GPS; N TCXO is the value of the counter for TCXO clock; N GPS is the value of the counter for GPS clock.
When K periods of the 16-bit counter values of GPS 10 MHz clock are recorded, N GPS = K * 65535, so the measured frequency will be:
f TCXO = (total accumulated N TCXO in k periods / k*65535) * 10 (MHz) (2) If 1000 second data from the counter recording the GPS 10 MHz clock are recorded, the frequency measurement resolution is approximately 0.001 Hz.
The more time of the data has been collected, the higher measurement resolution it has. The collected data is transferred to the computer via National Instrument Data Acquisition (NI DAQ) card [7] together with the sampled CDMA signal as correction information for post processing of the signal shown in Figure 3.
Figure 2: TCXO 10 MHz Frequency Measurement with respect to the GPS
10 MHz
D-latch RE Q
D-latch
PC 16-count
(1)
16-count (2) CLK
CLK
16 8 8
16
16 Data 16 D-latch
Locked GPS 10MHz
TCXO 10MHz
NI DAQ
FPGA implementation of TCXO frequency measurement
4. Experiment Analysis
There are also some other methods to measure the TCXO frequency [8] [9], however they are not so flexible to be applied in other circuit. Our experimental apparatus is being constructed to investigate phase synchronization with an intermittent GPS reference source combined with the data multiplexer as shown in Figure3.
Figure 3: Experiment Diagram In the associated CDMA receiver, the received pilot signal is sampled based on a free running TCXO 10 MHz clock. To achieve the synchronization, the phase difference between the 10 MHz TCXO and GPS 10 MHz is continuously measured and stored, along with the CDMA data collected. CDMA pilot samples are then phased corrected based on the Kalman filtered version of the phase difference between the TCXO and GPS clocks. Figure 4 is the simulation result of the correlation function for 3 IS-95 pilot PN periods under TCXO frequency offset of +5 Hz. It results in the correlation peak bias of +100 ns which has TOA measurement error of 30 M. The longer the integration time, the
bigger the bias is. Longer integration is necessary for weak signal detection. In addition, the shape of the correlation function has been smoothed which makes it hard to determine the position of the correlation peak and the SNR has also been reduced. The TOA measurement error due to the local frequency offset can be cancelled out when TDOA technique is applied.
However if the available number of the signals from the BSs is only three not enough for 3-diensinal positioning which needs four observation equations, this frequency correction information is important to estimate the present time based on the previous time value to finally get the solution of the 3-D position based on three observation equations. The accurate TOA can also help the study of the wireless channel.
Figure 4: Correlation Functions with Local PN Code Generated by Accurate TCXO Frequency (dash
line) and with Local PN Code Generated by + 5 Hz TCXO Frequency Offset (solid line) Figure 5 is the experiment result of the measured TCXO 10 MHz offset. It can be seen that the average offset is +0.03
Frequency measurement
GPS RX (NovAtel OEM4)
10MHz
10 MHz data
REQ Trig
Multiplexer front end 16 bit RF
16 bit Sampling
clock
Signal post mission
(PC)
data TCXO
(Altera Flex 10K70) DAQ NI
Hz. The existing error sources are: GPS receiver noise, the frequency variations of the oscillator and counter quantization error in FPGA, etc.
Figure5: Measurement Result of the TCXO 10 MHz Offset
Figure 6 is the measured Allan deviation [10] of the TCXO based on the circuit in Figure 2. The Allan variance can be expressed by equation 3.
−
=
−
− +
= 1
1
2
2 [ ( 1 ) ( )]
) 1 ( 2 ) 1
( M
i
y y i y i
τ M σ
(3) where M is the number of fractional frequency values and y is the (i ) fractional frequency value averaged over measurement interval ττττ which can be described by equation 4.
τ
)]
( ) 1 ( ) [
( i x i x i
y = + − (4) where x is called the phase data. It can (i ) be generated from the counter value of FPGA in Figure 2 because the counter values change periodically.
It can be seen from Figure 6 that the Allan deviation of the TCXO reaches the
order of 10 -10 . The typical Allan deviation when τ = 1 is 1× 10 -10 , where τ is the time interval between the two successive data measured.
Figure 6: Experiment Result of the TCXO Allan Deviation
The relationship between the receiver clock bias and the Allan variance of the oscillator is given in [11] as equation 5.
2 clock (t) = 2 clock (t 0 ) + (t-t 0 ) 2 * 2 y (t-t 0 ) where clock (t 0 ) is the clock uncertainty (5)
at time t 0 , 2 y (t-t 0 ) is the Allan variance for time interval τ = t - t 0 .
Suppose at time t 0 the clock uncertainty is zero, that is 2 clock (t 0 ) = 0, given Allan Deviation y (t-t 0 ) = y (1s) = 10 -10 , the clock bias after 1 second is 10 -10 (s) and the range uncertainty is 3 cm. So the TCXO has very good short term stability.
The number of collected data f(Hz)
Allan Deviation of TCXO TC-140, Warm up 14 hrs.
Interval time ττττ (seconds)
y
