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IMPLICATIONS OF PULSE ENERGY AND PULSE REPETITION RATE ON THE UWB RECEIVER

Technical articles

IMPACT ON THE SIGNAL-TO-NOISE RATIO AT THE RECEIVER

In the first technical article, we explained the LRP and HRP UWB flavors and how UWB technology overcomes the limitations of the emitted transmit power to achieve high SNR and communication range similar to other short range radio technologies. In this article we show the impact of the pulse energy and the pulse repetition rate on the UWB receiver in terms of signal-to-noise ratio, power consumption and silicon area. We observe significant differences between the two flavors.

The reader can recall from the first article that the total energy radiated per millisecond in UWB is 37µW (for a 500 MHz channel bandwidth). This energy can either be split into to a few high energy pulses (LRP) or into many low energy pulses (HRP). The individual pulse energy (Ep) in HRP is typically about 10 times lower than the case of LRP, while the pulse repetition rate is about 10 times higher. This means that a data symbol in HRP is typically encoded with multiple pulses as opposed to LRP where each pulse or only a few pulses encode a data symbol.

The implications of the above are important: 1) in terms of Signal-to-Noise Ratio (SNR) at the Receiver for a given link attenuation (or path loss), proportional to 20log10(distance) and 2) in terms of complexity of the Receiver. We have learned that the pulse energy for LRP can be sent 10 times stronger than for HRP (still compliant with the regulation). It means that at the same distance, the SNR of LRP is 10 times better (assuming same receiver noise figure). This clearly sets a much higher constraint on HRP receivers to extract the incoming UWB signal out of receiver noise close to the maximum distance of operation or in the case of non-line-of-sight (NLOS) conditions.

Figure 1: For LRP and HRP the range between TX signal peak power and Noise Floor is only shifted. The link budget remains similar and theoretically depends on the modulation scheme and its demodulation method.

  • For the same link attenuation, the instantaneous SNR at the receiver is better for LRP. Given the same energy used, the sensitivity and maximum achievable link budget depends on the modulation scheme and its demodulation method;
  • The complexity of receivers operating at very low SNR such as HRP drastically increases the power consumption and may introduce additional losses in link budget and other key performance indicators.

 

IMPACT ON THE POWER CONSUMPTION AND SILICON AREA

An additional benefit from the low pulse repetition rate of the LRP is the robustness to propagation multipath. A low pulse rate allows for reflections to fade out before the next pulse arrives at the receiver. This enables direct and extremely efficient digital processing of the incoming pulses to derive the Channel Impulse Response (CIR) needed for time-of-arrival in ranging and data decoding.

In contrast, the high repetition rate of the HRP pulse transmission creates an Inter-Pulse Interference (IPI) at the receiver. HRP uses specific communication codes with special correlation properties in order to allow searching of the pulses during the preamble and extract the CIR. To resolve IPI, digital signal processing and post-processing has to be performed on the received signal including both preamble and data decoding. This digital processing becomes challenging under low signal-to-noise conditions, multipath, clock offset and other receiver imperfections.  Figure 2.a) and b) illustrate the problem of preamble search.

Figure 2: Illustration of the problem of preamble search and extraction of the channel impulse response (CIR) a) LRP periodic pulse preamble enables direct integration b) HRP preamble requires cross-correlation on a periodic ternary code c) HRP STS makes the noise floor increase and needs further post-processing

The current commercially available UWB chips confirm well the implications on the UWB receiver in terms of energy consumption and digital silicon area required for signal processing [B2-B5].

 

Energy consumption in TX and RX for a packet payload of 12 bytes

Feature

3DB (LRP)

DW1000 (HRP)

Energy consumption RX

       8 uJ

61 uJ1

Energy consumption TX

            3.6 uJ

43 uJ1

Wake-up Radio

Partially

No (needs other radio)

1 DW1000 in Mode 10 (best mode in terms of energy consumption) and Channel 5 (6.5 GHz)

 

Digital silicon area

Feature

3DB (LRP)

DW (HRP)

APPLE U1

Digital area

~1.6 mm2 (65 nm)

~6 mm2 (90 nm)

~7.8 mm2 (16 nm) 1

Est. Digital area

~0.4 mm2 (16 nm)

~1.2 mm2 (16 nm)

~7.8 mm2 (16 nm)1

1 We note that Apple U1 has at least 2 front-ends for precision angle of arrival, therefore the digital area may be increased to provide support. The other commercially available ICs are with single front-end.

 

In addition to the problem of preamble search in HRP, the new IEEE 802.15.4z has also introduced a sequence of randomly generated pulses referred to as Scrambled Timestamp Sequence (STS) for enhanced ranging in HRP mode [B1]. Such sequences do not have good correlation properties and therefore increase the noise floor (see Figure 2c). As a result, the UWB receiver needs more digital signal processing for early path extraction. We will show in a next article the limitations of such sequences in terms of early path detection dynamic range and level of integrity (manipulations).

 

  • LRP can be easily powered by a CR2032 coin cell battery and it is attractive for wearables, locks, car key fobs, trackers and devices which operate with very low energy.
  • HRP can be used in less energy-constrained devices like mobile phones. The energy efficiency of HRP can be improved by combining it with other low-power short range radio technologies such as Bluetooth.

 

      

 

REFERENCES

[B1] IEEE P802.15.4z, IEEE Draft Standard for Low-Rate Wireless Networks Amendment: Enhanced High Rate Pulse (HRP) and Low Rate Pulse (LRP) Ultra Wide-Band (UWB) Physical Layers (PHYs) and Associated Ranging Techniques, URL: https://www.techstreet.com/ieee/standards/ieee-p802-15-4z?gateway_code=ieee & vendor_id=7291&product_id=2087572

[B2] Datasheet and evaluation kit, DW1000, www.decawave.com          

[B3] Datasheet and evaluation kit, 3DB6830, www.3db-access.com

[B4] Decawave DW1000 UWB radio IC basic floorplan analysis. Report BFR-1902-801, TechInsights, 2019

[B5] Apple U1 TMKA75 Ultra Wideband die (USI RF Module of the iPhone 11) Circuit Analysis: CAR 1910 801 https://50.16.45.81/blog/apple-u1-tmka75-ultra-wideband-uwb-chip-analysis

 

14-Jan-2020