A variety of wireless communications standards provide indoor positioning systems, such as Wi-Fi, Bluetooth®, RFID, and HRP UWB. They are using different positioning methods for indoor ranging, including received signal strength indication (RSSI), angle of arrival (AoA), angle of departure (AoD), remote coupling, and time of flight (ToF), also called the time of arrival. Table 1 shows key attributes and techniques for each wireless standard.

Table 1. Common indoor positioning technologies
Wireless systems	Wi-Fi	Bluetooth	RFID	HRP UWB
Standard	IEEE 802.11az	Bluetooth 4.0 / 5.1	EPC / ISO	IEEE 802.15.4/4z
Technique	RSSI	AoA or AoD	Remote coupling	AoA or ToF
Frequency	2.4, 5, 7 GHz	2.4 GHz	12.5 kHz to 5.8 GHz	Sub-GHz; 3.1 to 4.8 GHz; 6 to 10.6 GHz
Modulation	OFDM	GFSK	ASK, 2-FSK, OOK	BPM and BPSK
Range	< 150 meters	< 75 meters	< 1 meter	< 100 meters
Accuracy	<15 meters	< 1 meter (BT 5.1); < 8 meters (BT 4.0)	Presence detection only	< 30 cm
Power consumption	Medium	Low	Passive powered	Low / medium

Among these standards, the HRP UWB offers fine-ranging and security capabilities and has been adopted by major mobile phone makers. It enables new applications such as real-time spatial context to mobile devices, advanced ranging, location-based services, and seamless and secure point-to-point (peer-to-peer) services.

The IEEE 802.15.4-2020 standard has updated the physical layer (PHY) and medium access control (MAC) sublayers of the technology for high precision, secure-ranging applications. The HRP UWB ranging system is based on ToF and AoA measurements to provide reliable and robust ranging timestamps to accurately measure the distance and direction between devices.

Two-Way Ranging

The measurements for ranging are using a data frame acknowledge frame sequence. The HRP UWB uses the ToF measurements for single-sided two-way ranging (SS-TWR). Figure 1 shows the SS-TWR scheme. The ranging marker (RMARKER) is the location of the first chip after the start of the frame delimiter and is a timestamp used for calculating ToF. Device A sends a frame to Device B and receives a reply from Device B. Device B needs to communicate its reply time T_reply to Device A. Then, Device A can calculate the estimated time of flight T_prop by the following formula:

T_prop = (T_round- T_reply) / 2

The distance or range between devices is based on estimating the propagation time (T_prop).

**Figure 1. Single-sided two-way ranging**

Similarly, the double-sided two-way ranging (DS-TWR) requires Device B to initiate the same process. Figure 2 illustrates the DS-TWR procedure. It improves accuracy when clock synchronization between the two devices is not available. Both devices measure the time between packets. With all four times measured the propagation time can be estimated by the equation below.

T_prop = (T_round1 * T_round2 - T_reply1 * T_reply2) / (T_round1 + T_round2 - T_reply1 - T_reply2)

**Figure 2. Double-sided two-way ranging**

Figure 3 illustrates the test setups for ToF measurements and the test procedures for DS-TWR as follows.

The device under test (DUT) initiates a ranging procedure.
Keysight M9415A VXT PXI vector transceiver receives the packet from the DUT, and PathWave 89600 vector signal analysis (VSA) software starts recording.
The M9415A VXT PXI vector transceiver will transmit a reply packet back to the DUT. Both the DUT and M9415A VXT’s input will receive the reply packet. Once the DUT receives the reply packet, it will send the second packet.
The DUT will send the last packet that contains the data payload and the measured timing measurement parameters.
The PathWave 89600 VSA stops the recording and makes timing measurements of the first three packets (two from the DUT and one from the M9415A VXT) and decodes the data from the last packet.

**Figure 3. Test setups using the M9415A VXT vector transceiver for ToF measurements**

Angle of Arrival (AOA)

AoA is related to the position of the target node. Each angle translates into a time difference between the arrival of the signals to array antennas. For example, Figure 4 shows that the simplest antenna array is the uniform linear array.

The time delay between the arrival of the signals to the consecutive antenna is given by

t = d * sinø / c

where t is the time delay, d is the distance between antennas, ø is AoA, and c is the speed of light.

**Figure 4. Uniform linear array of antennas**

Figure 5 shows using a high-precision delay line to simulate propagation delay for one of the test channels to validate the AoA for an HRP UWB device.

**Figure 5. Test setups with the M9415A VXT vector transceiver for AoA measurements**

Please be aware that chipset makers may use different ranging algorithms and measurement setups to calibrate and validate both ToF and AoA.

Enable Accurate Ranging Measurements

The accurate position is one of the key features of the HRP UWB technology. UWB uses up to 500 MHz channel bandwidth with a pulse period of about 2 ns that enables centimeter-ranging accuracy. However, it also increases test challenges to generate and analyze the wide bandwidth signals. . Keysight offers a flexible suite of signal generation tools that reduce the time you spend on signal simulation and powerful analysis tools to understand the structure and quality of the transmitted HRP UWB signal as shown in Table 2. To learn more about IEEE 802.15.4 HRP UWB tests, download the white paper: An Overview of IEEE 802.15.4 HRP UWB Test Requirements.

	Signal analyzer				Signal generator	Vector transceiver¹
Model	N9042B UXA	N9040B UXA	N9032B UXA	N9030B PXA	M9384B VXG	M9415A VXT
Frequency (GHz)	26.5, 44, 50	13.6, 26.5, 44, 50	13.6, 26.5	13.6, 26.5, 44, 50	14, 20, 31.8, 44	12
RF bandwidth (GHz)	1, 1.5, 2, 4	0.51, 1	1, 1.5, 2	0.51	0.5, 1, 2	0.8, 1.2
Software	PathWave vector signal analysis for IoT modulation analysis (89601BHTC) PathWave signal generation for IoT waveform signal creation (N7610C)

¹ Vector transceiver integrates a vector signal generator and a vector signal analyzer in a three-slot PXIe module.

Further Reading

Tag: Signal Analyzers

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