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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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Bluetooth uses 16 to 32 channels as advertising channels (which are used for broadcasting data) and the rest of channels are used as data channels ( used for the transmission of application data). In contrast, BT-LE defines only 3 channels as advertising channels, while the other 37 channels are defined as data channels. The small number of advertising channels is the key to a significant improvement in terms of energy consumption in BT-LE. When a device wants to connect with another device, the latter must be announcing that it is discoverable via the advertising channels. Hence, the first device needs to scan the advertising channels before connecting to the second one. While in Bluetooth scanning the advertising channels requires 22.5 ms, in BT-LE it takes only up to 1.2 ms. Hence, in BT-LE, the radio must be turned on for a length of time that is one order of magnitude shorter than that of Bluetooth.

Table 3.7 shows the frequency assignment for each channel.

Note that advertising channels operate in frequencies that minimize overlapping with the channels typically used by IEEE 802.11.

Hereafter, traditional Bluetooth will be referred to as ‘Bluetooth’.

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As Bluetooth, BT-LE uses the Gaussian FSK (GFSK) modulation, which is an FSK modulation where a Gaussian filter is applied. This filter allows the transitions between frequencies to be smoothed, which would otherwise lead to a large out-of-band spectrum. In BT-LE, however, a parameter of the modulation, which is the modulation index, is increased. This reduces the peak power consumption, and, as a side effect, provides better range and robustness.

The Data channels are dynamic and use adaptive FHSS, which precludes the use of crowded frequencies in the hopping sequence. For these channels, each carrier selected by the FHSS algorithm is used as the centre frequency for the GFSK modulator (see Fig. 3.15). In contrast, the Advertising channels are fixed.

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3.7. Coexistence Since ISM bands are unlicensed, many systems operate with these bands and may indeed share them. In particular, the 2.4 GHz band is quite crowded. Some technologies operating in this band are Bluetooth, IEEE 802.11b/g, IEEE 802.15.4 and some types of cordless phones. In addition, microwave ovens use 2.45 GHz irradiation [5]. Each of these systems is a potential source of interference for the others. Coexistence requires the use of appropriate mechanisms for reducing the interference of one system on another, and for reducing the impact of interference on a given system.

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3.7.1. Coexistence of IEEE 802.15.4-2006 with IEEE 802.11b/g IEEE 802.11b has a maximum data rate of 11 Mbps and uses DSSS for spreading the signal bandwidth. IEEE 802.11g allows up to 54 Mbps and uses OFDM. Both standards specify the use of 14 overlapping channels in the 2.4 GHz band and spacing between consecutive centre frequencies equal to 5 MHz, while the bandwidth of each channel is 22 MHz.

An IEEE 802.15.4-2006 signal may appear as a narrowband interferer to an IEEE 802.11b/g device, while an IEEE 802.11b/g signal may constitute a wideband noise for an IEEE 802.15.4-2006 node. The typical power output of an IEEE 802.11b/g transmitter is between 12 dBm and 18 dBm, which is greater than the typical power output of an IEEE 802.15.4-2006, which is up to 0 dBm [3].

The centre frequencies in Megahertz in IEEE 802.11b/g are assigned

according to the following equation:

Fc = 2412 + 5·(channel number -1) (6)

Where channel numbers are from 1 to 11, mainly in America, and from 1 to 13, mainly in Europe. Non-overlapping channels are commonly used in IEEE 802.11b/g systems. These channels are 1/6/11 and 1/7/13, in America and Europe, respectively.

As shown in Fig. 3.16, IEEE 802.15.4-2006 can take advantage of the bands between non-overlapping channels for minimizing inter-system interference; e.g., the channels 11 (2405 MHz), 15 (2425 MHz), 20 (2450 MHz), 25 (2475 MHz) and 26 (2480 MHz) are not overlapped to IEEE

802.11 channels in America frequency arrangement, although in practice the energy of IEEE 802.11b/g channels is not zero in those regions of the spectrum.

–  –  –

Fig. 3.16. (top) IEEE 802.15.4 channels, (medium) non-overlapping IEEE

802.11 channels in America and (bottom) non-overlapping IEEE 802.11 channels in Europe. NOTE: PSD stands for Power Spectrum Density The distance and centre frequency offset between IEEE 802.15.4 and IEEE

802.11b nodes is important for coexistence. The packet error ratio (PER) of an IEEE 802.15.

4-2006 device is smaller than 10-5 if the distance between IEEE 802.15.4-2006 and IEEE 802.11b devices is greater than 8 m. The PER of an IEEE 802.11b device is smaller than 10-5 for a distance greater than 4 m.

Furthermore, IEEE 802.15.4-2006 and IEEE 802.11b channels suffer nearly ignorable interference for centre frequency offsets greater than 7 MHz [20].

–  –  –

where the channel number is between 0 and 78.

Since the IEEE 802.15.4-2006 signal in the 2.4 GHz band is 2 MHz, it may interfere three Bluetooth channels. Since Bluetooth uses FHSS over 79

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channels, the maximum probability of interference between two proximal Bluetooth and IEEE 802.15.4-2006 devices would be 3 out of 79 channel hops. Since Bluetooth supports adaptive frequency hopping (AFH), it may detect the channels that offer bad performance and avoid their use, thus allowing a graceful coexistence between Bluetooth and other networks.

However, this may not occur if a bad channel is not detected, e.g. due to the low power or low duty cycle of an IEEE 802.15.4 device [3]. A similar reasoning applies to BT-LE, which also supports AFH. The lower duty cycle of BT-LE is an additional contributor to its coexistence with other networks.

REFERENCES

[1] T.K. Sarkar, Z. Ji, K. Kim, A. Medouri, M. Salazar-Palma, “A Survey of Various Propagation Models for Mobile Communication”, IEEE Antennas and Propagation Magazine, Vol. 45, No. 3, pp. 51-82, June 2003.

[2] J. Pottie, W. J. Kalser, “Wireless Integrated Network Sensors”, Communications ACM, Vol. 43, No 5, pp. 551-558, May 2000.

[3] S. Farahani, “ZigBee Wireless Networks and Transceivers”, Elsevier, 2008.

[4] T.S. Rappaport, “Wireless Communications: Principles & Practice”, Upper Saddle River, NJ, Prentice Hall PTR, 1996.

[5] P.E. Gawthrop et al., “Radio Spectrum Measurements of Individual Microwave Ovens”, NTIA Report 94-303-1, March 1994.

[6] E. Faussurier, “Generic Regulation for Ultra-Wideband (UWB) Applications in Europe”, 2nd Congress of Portuguese Committee of URSI, November 2008. Available at http://www.anacom.pt/streaming/emmanuel_faussurier.pdf? contentId=760018&field=ATTACHED_FILE.

[7] H. Schwetlick et al., “PSSS - Parallel Sequence Spread Spectrum: A Physical Layer for RF Communication”, IEEE International Symposium on Consumer Electronics, 2004, pp. 262-265.

Chapter 3. Physical layer

[8] J. Cramer, M. Z. Win, R. A. Scholtz, “Impulse Radio Multipath Characteristics and Diversity Reception”, in Proceedings of ICC’98, 1998.

[9] IEEE 802.15.4-2006, “Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs)”, September 2006.

[10] Atmel “Low-Power Transceiver for ZigBee Applications”. AT86RF230 Target Specification. 5131A-ZIGB-08/15/05.

[11] IEEE 802.15.4, “Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs)”, May 2003 [12] IEEE 802.15.4a, “Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs), Amendment 1: Add Alternate PHYs”, March 2007.

[13] DecaWave website: http://www.decawave.com [14] H. Arslan, Z. N. Chen, M, Di Benedetto, “Ultra Wideband Wireless Communication”, Wiley 2006.

[15] IMEC website: http://www2.imec.be [16] Nanotron Technologies: http://www.nanotron.com [17] Ultra Wideband Communications - Past, Present, and Future http://bul.ece.ubc.ca/UWB_UBC.pdf.

[18] R. Heidon, K. Kerai, “Bluetooth Low Energy Training”, April 2009.

[19] S.Y. Seidel, T.S. Rappaport, “914 MHz Path-Loss Prediction Models for Indoor Wireless Communications in Multifloored Buildings”, IEEE Transactions on Antennas and Propagation, pp. 207-217, February 1992.

[20] S. Y. Shin, H. S. Park, W. H. Kwon, “Mutual Interference Analysis of IEEE 802.15.4 and IEEE 802.11b”, Computer Networks 51, pp. 3338–3353, 2007.

[21] R. Prasad, “OFDM for Wireless Communications Systems”, Artech House, 2004.

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4. MAC layer The Medium Access Control (MAC) layer is responsible for managing access to a channel shared by several nodes. The principles of MAC layer design for WSNs differ from those of traditional wireless networks mainly in two aspects: i) energy conservation is the key design concern, and ii) multihop communication schemes are commonly used.

A MAC protocol for WSNs must deal with several phenomena in order to achieve energy efficiency. Idle listening has been identified as the problem that leads to the largest energy waste in many traditional MAC protocols [1].

Since a node does not know a priori when it can receive a message from a neighbour, its radio must be on in order to listen to the medium. However, the channel may be idle for most of the time. In order to save energy, many MAC proposals keep the radio in sleep mode (i.e. switched off) during some periods of time, trading off energy conservation for latency. Furthermore, collisions contribute to energy inefficiency, since energy is consumed for the transmission of a data unit that is not received successfully. In addition, control overhead must be kept reasonably low. Finally, because a multi-hop path requires the transmission of a data unit in several links, the nodes must be appropriately organized to achieve good performance in terms of end-toend latency and energy consumption.

This chapter is devoted to MAC protocols for WSNs. Sections 4.1 to 4.4 present several types of MAC protocols, including proposals for WSNs that have been conceived in research circles. These are classified into scheduling-based, contention-based, duty-cycle-based and other protocols.

Section 4.5 describes the MAC layer of IEEE 802.

15.4 MAC, which is

–  –  –

currently the de facto standard and commercial-off-the-shelf solution for WSNs. Finally, Section 4.6 introduces the MAC functionality of Bluetooth Low Energy, which has recently appeared on the market.

4.1. Scheduling-based MAC protocols Scheduling-based MAC protocols make use of dedicated resources for node transmissions. In WSNs, the protocols in this category select specific time intervals for the transmission of each node. This approach assumes that nodes are synchronized, which can be performed by using timestamps [13] or even GPS. Since nodes do not compete for the medium and have reserved resources, scheduling-based MAC protocols are collision-free.

4.1.1.TDMA Time Division Multiple Access (TDMA) is a MAC mechanism in which time is divided into structures called frames, which are composed of time intervals called slots. In general, slots have a fixed size. Each slot is reserved for the data transmission of a specific node (e.g. the n-th node uses the n-th slot of each frame). Hence, a node has a slot for transmitting data without contention - every frame period (see Fig. 4.1). Frames of a TDMA scheme for N devices comprise N slots.

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A drawback of TDMA is that when nodes have no data to transmit, their slots remain unused. However, in a WSN, a node can turn off the radio for energy conservation during the slots of the remaining nodes. In addition, TDMA is a simple and easy to implement approach.

–  –  –

4.1.2. LEACH Low-Energy Adaptive Clustering Hierarchy (LEACH) [2] is a protocol that organizes a sensor network into clusters. In each cluster there is a node called a cluster head, which is responsible for several tasks within its cluster.

With regard to MAC functionality, channel access of a LEACH node is based on a TDMA schedule, which is established by the cluster head for all the sensors within the cluster. The details about the cluster formation and cluster head election algorithms in LEACH are shown in Chapter 6.

4.1.3. TRAMA

The TRaffic-Adaptive Medium Access (TRAMA) protocol [14] is a conflict-free, scheduling-based MAC protocol. TRAMA was designed for energy efficiency. This feature is achieved by transmission schedules, which prevent collisions between data packets (and their retransmissions), and by allowing nodes to switch the radio to a low power mode when they are not involved in communications.

TRAMA uses a single and time-slotted channel for data and signalling transmissions. Time is divided into scheduled- and random-access periods.

Every node disseminates one-hop neighbour information to all its onehop neighbours. In this way, all nodes obtain two-hop neighbour information. The related messages are transmitted during the random-access period. Because the random access is based on contention, these messages are prone to collisions.

A node announces its schedule before starting the transmission of its data. A schedule contains the slots that will be used by a node and the intended receivers. This information is periodically broadcast to the one-hop neighbourhood of a node in a scheduled-access period. The time between schedules is based on the rate at which upper layer data is generated. When a sender has no more packets to send in its MAC queue, the vacant slots are announced and potential senders are evaluated for reuse of those slots.

TRAMA achieves a higher percentage of sleep time and smaller collision probability than contention-based protocols. However, these latter provide better performance in terms of delay.

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