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«Carles Gómez Josep Paradells José E. Caballero Edita: Fundación Vodafone España Autores: Carles Gómez Montenegro* Universitat Politècnica de ...»

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When a signal is incident to an object, the characteristics of the object material, the signal frequency and the angle at which the signal impinges the surface of the object determine the attenuation (i.e. the loss of signal) that the signal will undergo. A material can be characterized by its attenuation constant, which can be expressed in dB/m. For instance, the water attenuation constant at a typical home temperature for a 2.4 GHz signal is around 330 dB/m. This means that the human body attenuates the signal significantly [3].

3.1.3. Reflection, diffraction and scattering

Reflection, diffraction and scattering are the three basic propagation mechanisms that affect wireless propagation [4].

When a signal reaches a large object in comparison with the signal wavelength, a part of the signal or the whole signal will be reflected. The ground, walls and furniture cause reflection (see Fig. 3.1). The amount of reflected signal will depend on the properties of the reflecting materials, in addition to the properties of the incident signal.

Sensors Everywhere

Diffraction (see Fig 3.2) takes place when an obstacle with sharp edges is present in the path between the radio sender and receiver. As a result, the waves bend around the obstacle and may reach the receiver, even when there is no LOS path between them.

Scattering (See Fig. 3.2) occurs when the signal is incident to small obstacles (compared to the signal wavelength) or rough surfaces. In such a case, the signal is propagated in many directions.

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3.1.4. Multipath In view of the propagation phenomena already presented, the same signal can reach the receiver via several different paths. For instance, a signal can be directly propagated from the transmitter to the receiver through an LOS path. A different version of the same signal can reach the receiver as well;

for example, due to the reflection of the signal on the ground (see Fig. 3.1). The amplitude, phase and delay of the first and second versions of the same signal may not be the same. Therefore, the sum of these different versions will be a distorted signal. A signal strength measurement may provide greater values than in the case of a single path transmission. However, the quality of the signal may not be greater. Even the different versions of the same signal may lead to signal nulls. For instance, in the two-path example, if the two versions of the same signal have 180º phase difference and their amplitudes are quite similar, the signal power will be significantly reduced.

For all the aforementioned reasons, Received Signal Strength Indicator (RSSI) measurements, which are often offered by radio chips used in sensor platforms, have to be taken carefully as an indication of good quality signal reception.

3.2. Frequency bands

The frequency band used by a wireless communications system is a key factor that influences the performance of the system. In fact, the frequency band is related with the symbol rate that the system is able to achieve. On the other hand, as already seen in the previous section, the quantitative effect of several propagation phenomena depends on the frequency of the signals transmitted. Another important consideration is interference, since transmissions at a given frequency may interfere the communications of other systems operating at the same frequency..

3.2.1. Regulation Many wireless communications systems, including WSNs, exploit bands originally intended for industrial, scientific and medical (ISM) applications.

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These bands, which are available worldwide or in particular regions of the world, are defined by the International Telecommunication Union (ITU) and are reserved for private and unlicensed use, subject to certain restrictions that may depend on regional or national regulation (e.g. regarding transmit power and duty cycle). Further regulation may apply in certain regions. For this reason, some parts of the spectrum are license-free in some countries or geographical areas.

Table 3.1 shows a subset of the ISM bands and license-free regional bands of interest to low power applications.

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A particular case is the regulation for Ultra Wideband (UWB) that applies in the United States and Europe, which covers frequencies up to 14 GHz (see Fig. 3.3). In some regions, it is mandatory to implement a mechanism called Detect and Avoid (DAA) to decrease UWB signal levels if the spectrum is being used by any other service.

Fig. 3.3. Emission limits for handheld UWB systems allowed by the European Telecommunications Standards Institute (ETSI), blue curve, and the Federal Communications Commission (FCC, USA), red curve (adapted from [6])

3.3. Modulations and spreading techniques A modulation is a method used for the transmission of a signal that contains information over another signal, which has the purpose of carrying the information signal. The modulation used in a communications system influences several properties of the signal transmitted, such as the bandwidth required, robustness against interference and the data rate.

3.3.1. Classical modulation schemes The classical types of modulations used in radio applications are Amplitude Modulation (AM), Frequency Modulation (FM) and Phase

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Modulation (PM). In AM, the information signal modulates the amplitude of the carrier signal; in FM, the information signal modulates the frequency of the carrier signal; finally, in PM, the information signal modulates the phase of the carrier signal. When the information signal is digital, these modulations are called Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK), respectively. Incoming bits can be grouped so that the resulting signal defines a sequence of states, each state corresponding to a physical waveform called symbol. Each symbol, which codifies one or more bits, is transmitted at a certain rate.

The most simple ASK modulation uses a sinusoid with a certain amplitude to encode the symbol “1”, and another with null amplitude (i.e.

absence of signal) for the symbol “0 ” (see Fig. 3.4). In Binary PSK (BPSK), the symbols “1” and “0” are codified by adding 180º and 0º, respectively, to the phase of a sinusoid (see Fig. 3.5). In Quadrature PSK (QPSK), the symbols “11”, “01”, “00” and “10” (which correspond to the four combinations of a group of two bits) are codified by adding 45º, 135º, 225º and 315º, respectively, to the phase of a sinusoid. A simple FSK modulation transmits a sinusoid with frequency f1 or f0 during a symbol period for the transmission of the symbols “1” and “0”, respectively (see Fig. 3.6).

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Fig. 3.5. An example: use of the BPSK modulation for the transmission of the sequence 101101. Every “1”introduces a 180º phase shift Fig. 3.6. Example: use of the FSK modulation for the transmission of the sequence 101101. In contrast to this example, the transmission of two different consecutive symbols may lead to an abrupt change in the signal, which enlarges the signal spectrum 3.3.2. Spread spectrum techniques Many radio systems also use spread spectrum techniques for transmitting data in order to increase their resistance against interference and multi

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path, and in some cases even to prevent detection. These techniques expand the bandwidth of a signal by distributing the energy of the signal into a range larger than the original one in the frequency domain [7].

In Frequency Hopping Spread Spectrum (FHSS), which is the oldest spread spectrum technique, the carrier frequency of the signal is quickly changed according to a pseudo-random pattern of changes known to both the sender and the receiver. Direct Sequence Spread Spectrum (DSSS) is based on multiplying a sequence of binary data by a pseudo-random sequence composed of the values “-1” and “1”. The result is a signal that looks like noise. However, the original signal can be obtained at the receiver by multiplying the received signal by the same pseudo-random sequence used at the transmitter and then summating the result. Parallel Sequence Spread Spectrum (PSSS) extends the concept of DSSS by parallelizing the binary data so that N data bits are the input of a scheme formed by N branches. In each branch, the input bits are multiplied by a sequence composed of the “-1” and “1” values. The N sequences are near-orthogonal, which allows recovery of the original data at the receiver. The result is summated for every group of N input bits, so that every N input bits form a single symbol.

3.3.3. Ultra wideband (UWB)

The original definition of an Ultra Wideband (UWB) system is based on the usage of very narrow pulses5 (in the order of hundreds of picoseconds to nanoseconds) that generate a very wide spectrum. This scheme, which has been used in Radar applications, allows precision ranging applications and has good properties in terms of interference. In fact, since the energy is distributed over a very wide band, it neither interferes nor is interfered by typical narrowband communication systems. In addition, because the pulses are also very short in space, reflections cannot overlap the original signal. Hence, this scheme is resilient to multipath [8].

The United States Federal Communication Commission (FCC) has extended the original definition to a general term for a radio technology that In many contexts, these pulses are also referred to as ‘chips’.

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uses pulses of energy with spread energy over more than 0.5 GHz or exceeding 20% of the central frequency. The FCC ruled the UWB to be in the frequency range of 3.1 GHz and 10.6 GHz in 2001. In other countries, other regulations apply.

The two main UWB techniques are Direct-Sequence UWB (DS-UWB) and Multiband – Orthogonal Frequency Division Multiplex (MB-OFDM). DS-UWB consists in transmitting a set of pulses that can transport information by changing their polarity or their amplitude, or by using an orthogonally shaped set of pulses. One advantage of DS-UWB is that it can be implemented by using a small number of circuit gates. MB-OFDM divides the spectrum into several bands, and data are transmitted in each band using OFDM [21].

In OFDM, data is divided into several parallel data streams. The available bandwidth is divided into several orthogonal6 sub-bands, and each stream is transmitted over each sub-band. OFDM allows the power used for each subband to be chosen adaptively. This feature avoids the use of frequencies that may undergo problems (e.g. interference).

UWB has attracted the attention of different communication proposals, such as the Wireless Personal Area Networks (WPANs) for low and high bit rates, and also from the Bluetooth Special Interest Group (SIG).

3.4. Transmission range

The transmission range of a radio transmitter can be defined as the maximum distance from the antenna of the transmitter that leads to a Signal Interference and Noise Ratio (SINR) above the minimum required by the receiver. In consequence, the transmission range depends on the transmission power, the environment in which the signal is propagated, the frequency band and the modulation and bit rate.

A common transmission range figure for a transmitted power of 1 mW at

2.4 GHz is 30 m indoors and 90 m when transmitting at 100 mW. For outdoors, the range reaches 100 m for 1 mW and a further 10 km for the 100 mW.

The sub-bands are considered ‘orthogonal’ because there is no cross-talk between them.

Sensors Everywhere

In addition, in general terms, when using a channel in the 800-900 MHz band, the range is almost 2.5 times that which can be obtained at the

2.4 GHz band.

A relevant parameter influencing the transmission range is receiver sensitivity, which specifies the minimum received signal level to enable a sufficient reception quality. For example, IEEE 802.15.4 requires a receiver sensitivity of -85 dBm in the 2.4 GHz band and -92 dBm in the 868/915 MHz band in order to yield a Packet Error Rate (PER) of less than 1% [9]. Other values for the receiver sensitivity can be found in the market, such as

-101 dBm for 1% PER [10].

3.5. Physical layers of IEEE 802.15.4 This section presents the physical layers (PHYs) of the several versions and amendments to the IEEE 802.15.4 standard.

3.5.1. IEEE 802.15.4-2003 PHYs The physical layer (PHY) of IEEE 802.15.4-2003 is responsible for a number of tasks including data transmission and reception as well as carrying out measurements that are needed or can be exploited by upper layer services.

3.5.1.1. Main features of IEEE 802.15.4-2003 PHY The original IEEE 802.15.4 version [11] specifies the use of radio channels in three different bands: 1 channel in the 868 MHz band, 10 channels in the 915 MHz band and 16 channels in the 2.4 GHz band. Recall that the first band is available in Europe, the second one is available in the Americas region and the third is available worldwide. The spacing between consecutive channels is 2 MHz in the 915 MHz band and 5 MHz in the 2.4 GHz band.

Note that the actual effective bandwidth of the signal may be smaller than the spacing between channels.

In all cases, DSSS is used for spreading the spectrum of the signal. The standard mandates the use of Binary PSK (BPSK) modulation for the

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868/915 MHz bands and Offset-Quadrature PSK (O-QPSK) in the 2.4 GHz band. Table 3.2 summarizes the main physical layer features of IEEE 802.15.4-2003.

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Table 3.2.

Physical layer features of IEEE 802.15.4-2003 3.5.1.2. Centre frequencies The centre frequency of each channel, denoted by Fc, can be obtained as

follows:

Fc (MHz) = 868.3 (for channel number 0)

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3.5.1.3. PHY protocol data unit (PPDU) The IEEE 802.15.4 PHY protocol data unit (PPDU) is the data unit that carries the upper layer data unit (i.e. MAC frame), and is transmitted and received by the IEEE 802.15.4 PHY. The PPDU is composed of three components: the Synchronization header (SHR), the PHY header (PHR) and the PHY payload (see Fig. 3.7).



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