Using impulse radios as an example, we'll examine the advantages of ultra-wideband (UWB) technology compared to other short-range wireless communication technologies.
Ultra-wideband (UWB) is a short-range wireless communication technology - like Wi-Fi or Bluetooth - that uses a very large relative and/or absolute frequency band to send and receive information. According to the FCC regulations, a UWB device can be operated on an unlicensed basis in the 3.1-10.6 GHz band (PDF).
In this article, we'll take a look at some of the important characteristics of UWB technology.
UWB Shares the Radio Spectrum
Parts of the frequency range allocated for UWB are already used by existing communication systems. For example, as shown in Figure 1, both 802.11ac - a high-throughput WLAN communication protocol - and UWB are allowed to use the frequency band around 5 GHz.
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| Figure 1. | Diagram of the UWB working below the "noise floor." | |
UWB attempts to more efficiently utilize the scarce spectrum resources.
How can the UWB technology use the same spectrum as existing wireless systems without causing interference? This is achieved by restricting the power spectral density of the electromagnetic signal emitted by a UWB transmitter.
According to the FCC (the U.S. frequency regulator), the power spectral density of an indoor UWB transmitter should be below -41.3 dBm/MHz between 3.1 and 10.6 GHz. This limits the interference caused by a UWB device.
Figure 2 shows the spectral mask mandated by the FCC for an indoor UWB transmitter.
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| Figure 2. | The spectral mask for an indoor UWB transmitter. | |
UWB offers advantages with respect to data transfer rate, immunity to the multipath effect, high ranging accuracy, low power consumption, and the simplicity of implementation. Let's consider a class of UWB systems called impulse radios to gain a better insight into the key features of this technology.
Impulse Radio
While conventional narrow-band communication systems transmit a continuous waveform, an impulse radio transmits ultra-short duration pulses (less than 1 ns) to communicate information, shown in Figure 3.
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| Figure 3. | A typical pulse sequence transmitted by an impulse radio | |
After each pulse, the transmitter remains "silent" for a relatively long period of time. For example, an impulse radio might transmit only a single 1-ns pulse during every 100-ns time intervals. In this case, we say that the duty cycle is 1% (the pulse is present only 1% of the transmission time).
These pulses can be modulated in different ways to convey information. Figure 4 below shows how pulse position modulation and bi-phase modulation change an unmodulated sequence.
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| Figure 4. | Pulse position (b) and bi-phase modulations (c) change an unmodulated sequence (a). |
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Note that a short time duration corresponds to a wide bandwidth in the frequency domain (Fig. 5). Therefore, depending on the signal duration, a wideband signal will be emitted by the UWB transmitter antenna.
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| Figure 5. | The signals transmitted by an impulse radio occupy a large frequency band. | |
Both the center frequency and the bandwidth of the transmitted signals completely depend on the width of the pulse.
Low Power Consumption
Since the pulses are transmitted only during a small percentage of the transmission time, the average power emitted by the transmitter is very low. Having a transmission power on the order of microwatts, a UWB device can extend the battery lifetime.
High Data Rate
Although the emitted power is restricted, UWB enables unlicensed usage of an extremely wideband spectrum. This allows us to have high data rates (>100 Mbit/s). However, this high data rate can be achieved only over a relatively short transmission distance of 10 m. This is because only very low power is emitted for each bit of information.
At lower data rates (<1 Mbit/s), we can employ a large spreading factor to support longer distances. The Table 1 compares the data rate and range of UWB with other indoor wireless communication technologies.
| Table 1. | UWB vs. comparable indoor wireless communication technologies |
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Robustness to the Multipath Effect
UWB signals exhibit more robustness to the multipath effect than conventional wireless technologies. Assume that in addition to a direct path for the electromagnetic wave propagation from the transmitter to the receiver, there is another path caused by the reflections from an object (Fig. 6).
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| Figure 6. | Depiction of a multipath effect | |
The time (t) it takes for the transmitted signal to travel the total distance (d) of a given path can be obtained by the following equation:
d = c × t,
where c denotes the speed of the electromagnetic wave that is approximately 3×108 m/s. Hence, for every pulse that we transmit, two pulses appear at the receiver input. This is illustrated in Figure 7 in which the transmitted and the received pulses are shown in one diagram.
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| Figure 7. | For every pulse transmitted, two pulses appear at the receiver input. | |
In Figure 7, the two received pulses are easily recognizable because they do not overlap with each other. However, this is not the case in general. Examining the above figure, we can see that the pulses will not interfere - only if the delay difference between the two paths (t1-t0) is larger than the pulse width (PW).
Since UWB pulses have a very short duration, the pulses coming from the different paths are more likely to not interfere with our desired pulse. Hence, we can easily extract the desired signal from those originating from unwanted reflections. This gives a UWB system more immunity to the multipath effect. Alternatively, the energy can be summed together by a rake receiver.
High Ranging Accuracy
As discussed above, the sharp time resolution of UWB signals allows us to have a system that can resolve multipath components without resorting to complex algorithms. This makes UWB suited for time-of-arrival (ToA)-based range estimation applications.
It is worthwhile to mention that although these time-base ranging schemes benefit from the high time resolution of UWB signals, they have their own limitations. For example, because the UWB pulses having a very short duration, clock jitter becomes a limiting factor.
Conclusion
As we've seen with impulse radios, UWB can be a beneficial short-range communication technology because of its data transfer rate, immunity to the multipath effect, high ranging accuracy, low power consumption, and ease of implementation. For these reasons, many commercial developers are turning to UWB instead of near-field communication (NFC) options to enhance design implementation and security.
