Anritsu TDOA simulation tool (Vers. 0.9)

Welcome to Anritsu TDOA simulation tool. The purpose of the tool is to visualize the multilateration of an unknown transmitter using Time Difference On Arrival (TDOA). The unknown signal is emitted from a transmitter (TX) and it will generally arrive at slightly different times at two or more spatially separated receiver sites (e.g. RSM1 and RSM2). The locations of a constant Time Difference On Arrival being due to the different distances of each receiver from the RSM locations are forming a hyperbola. The intersection of different hyperbolas from different pairs of RSMs are equal to the calculated TX location. Somebody can imagine that the geometrical arrangement of a TDOA bearing base will have an important impact on the intersecting angle of the hyperbolas and thus the accuracy of the calculated emitter location. This tool is intuitively visualizing the results of a deploymenr of up to four Remote Spectrum Monitor in the field.

The user can select between a default TDOA bearing base with 3 RSMs, or use up to 4 spectrum sensors. By drag /drop it is possible to change the location of the RSMs thus changing the geometry. It is also possible to change the TX location dragging green TX point to another location on the screen. The RSM deployment geometry together with the absolute RSM location precision is leading in addition of the inherent TDOA inaccuracies to additional error terms called Geometrical Dilution Of Precision (GDOP).

By clicking the check boxes of the RSM paires (Red, Green, Purple and Light Blue), the user can activate the hyperbola calculation between a pair of RSMs. For instance, the first check box is activating the red hyperbola between RSM1 and RSM2. Green is used between RSM1 and RSM3, purple between RSM1 and RSM4. Finally Light Blue is representing the hyperbola between RSM2 and RSM3.

The check boxes "Show TX" and "Show base line to TX" are revealing the bearing result. Clicking "RSM Distances" is displaying the distance in kilometers between the deployed sensors.In the next section clicking to "RSM n" is displaying the azimut against north, the distance to the TX location and Hata Model based radio pathloss from the RSM to the bearing target and in addition the max. pathloss that is based on receiver sensitivity calculation. The RX sensitivity based pathloss estimation is very useful in order to estimate if the RSM is able to "see" the target signal with a sufficient SNR or not.

The selection of the signal parameters like radio frequency, signal bandwidth, CW-power and RSM as well as TX deployment heights are used as input parameters for the Hata Model pathloss calculation.

• Open Terrain,
• Suburban Terrain,
• Small Cities and
• Large Cities areas

• Frequency: 150 – 1500 MHz
• Mobile Station Antenna height: 1 – 10 m (in our case the TX height)
• Base Station Antenna height: 30 – 200 m (in our case the RSM height)
• Link distance: 1 – 20 km

In the following we will re-calculate the maximum possible pathloss between a Remote Spectrum Monitor (RSM) and the transmitter under investigation. The theory behind is based on AXONN RF Path Loss & Transmission Distance Caluculations. But before discussing path loss relevant distances, we need to define the receive sensitivity of a Remote Spectrum Monitor. A seven step process will guide you through the idea behind.

1. Estimate or measure signal bandwidth of signal under interest (Bw [kHz])
2. Calculation of typical RSM noise floor for observed signal E1: P_RXNoise = -174 dBm/Hz + 10log₁₀(BwSignal [Hz]) Example: A signal with an equivalent noise bandwidth of 25 kHz will result in a -130 dBm RSM channel noise floor
3. The minimum noise floor of a Remote Spectrum Monitor is equal to E2: E2: RX_min = -174 dBm/Hz + 10 log₁₀(RBW [Hz]) + NF_RX + Cable Loss Example: For a typical RBW of 3 kHz RXmin = -139 dBm + NFRX + Cable Loss For successful TDOA it is necessary to "see" the signal under interest with some margin above the min. receiver noise floor. In this simulation tool the Signal-To-Noise Ratio (SNR) is set by default to 6 dB . E3: RX’min = RXmin + 6 dB + Cable Loss = -139 dBm + NFRX + 6 dB + Cable Loss Example: With SNR 6 dB the above min. noise floor and a Cable Loss of -1 dB RX’min rises up to -132 + NFRX
4. In a spectrum analyzer (SPA) the min. noise floor is influenced by the input attenuation, the use of a pre-amplifier and the resolution bandwith (RBW) setting. With below given parameters some-body can now conclude on a real figure for RX’min. • RSM RX Sensitivity (DANL) = -150 dBm/Hz • PreAmp Gain = 12 dB (be aware that the PreAmp gain is frequency band dependent) @ 100 MHz 6 dB @ 400 MHz 8 dB @ 800 MHz 9 dB @ 1000 MHz 9 dB @ 1500 MHz 11 dB @ 2000 MHz 13 dB @ 3000 MHz 13 dB • example of typical sensitivity performance with 1 Hz VBW and RMS detector DANL @ 400 MHz - without PreAmp with 10 Hz RBW = 137,5 dBm ↦ 147 dBm/Hz - with PreAmp with 10 Hz RBW = 155,0 dBm ↦ 165,0 dBm/Hz DANL @ 1000 MHz - without PreAmp with 10 Hz RBW = 136 dBm ↦ 146 dBm/Hz - with PreAmp with 10 Hz RBW = 154 dBm ↦ 164 dBm/Hz • From above somebody can estimate the inherent Noise Figure of the RX frontend - RSM Noise Figure without PreAmp = 17 dB - RSM Noise Figure with PreAmp = 9 dB • In our above example receiver sensitivity at e.g. 400 MHz, RBW 3 kHz, SNR 6 dB, RX Cable Loss 1 dB and no antenna gains is resulting - without PreAmp in -115 dBm - with PreAmp in -123 dBm
5. The ideal line of sight based pathloss (L’P) between a receiver (RX) and a transmitter (TX) is based on the TX output power (PT), the total of all antenna gains (G) and cable losses (CL) on both sides and the min. noise floor of the receiver RX’min. In other words, by knowing the receive threshold it is possible to calculate the max. pathloss and to recalculate the corresponding distance. E4: L’P = P’T + GTOT – RX'min with PT = CW TX power; user can select 0,5 W (27 dBm) or 5 W (37dBm) P’T = PT – 10log10(BwSignal) (as the power goes down with the bandwidth) GTOT = total of all antenna gains, minus total of all cable losses - 1 dB GRSM = 0 dB     RSM cable loss = -1 dB GTX = 0 dB        TX cable loss = 0 dB Example: L’P = 27 dBm – 10log10(25000) + (-1 dB) -(-115 dBm) = 97 dB Using E5 it is now possible to recalculate the corresponding distance. In our case 4,4 km. E5: L'P Free Space = 32,45 20log10(d [km]) + 20log10(f [MHz])
6. As the view of point 5. is just based on ideal free space performance, we will now apply the rules of the Hata Model in order to come to a more realistic estimation. Input parameters for the Hata Propagation Model are: • RF = radio Frequency • PTX CW = CW TX output power • GTOT = total of all antenna gains, minus total of all cable losses • hRSM = RSM antenna height above ground • hTX = TX antenna height above ground • RX’min = min. RX sensitivity Now it is possible to • re-calculate the distance d'i [km] that corresponds with the above parameters, and then • calculate the pathlosses di [km] based on given distances between RSMn and TX location that was calculated by the TDOA hyperbola intersections
7. Comparison of d’i in relation to di is a measure if the max. possible pathloss that is based on the system parameters is violated due to a too large distance between RSM and TX. d > d’ ↦ critical, because the TX location is more far away as allowed by above calculations d < d’ ↦ good use case, resulting SNR might even be higher