**1. Introduction**

50 Will-be-set-by-IN-TECH

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*systems*, PhD thesis, Centre for Wireless Communications, Oulu, Finland.

50 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications

*dem Institut für Nachrichtentechnik der Universität Karlsruhe (TH), Band 13* .

*IEEE Digest of Papers Ultra Wideband Systems and Technologies*.

University Press.

26(No, 4): 48 – 66.

9): 1684 – 1691.

55(4): 523–531.

The growing request for low-to-medium data rate low cost networks is raising the interest in wireless sensor networking. For instance in the field of industrial and logistic applications, in order to improve the processes' efficiency the tight monitoring of goods, tools, and machinery -as to their state and position- is required. In response to this interest, IEEE approved in 2003 the IEEE 802.15.4 standard, this being the first one for low data rate, low complexity, and low power wireless networks.

The market success of wireless sensor networks (WSN) requires inexpensive devices with low power consumption. In order to satisfy this requirement, transmission technology, protocol as well as hardware design must give a common answer. UWB radio, particularly with impulse radio transmission (IR), is especially suitable for the development of WSN. IR-UWB is expected to allow low power, low complexity and low cost implementation as well as centimeter accuracy in ranging. The low complexity and low cost characteristics arise from the essentially baseband nature of the signal transmission. The high ranging accuracy results from the large absolute bandwidth, which must be at least 500 MHz. Indeed, the introduction of ranging functionality in low data rate networks was one of the main reasons for the IEEE 802.15.4a (2007) amendment, which added an IR-UWB physical layer to the original standard.

IEEE 802.15.4a allows for the use of non-coherent receivers, and defines ALOHA<sup>1</sup> as the mandatory medium access control (MAC) protocol. The use of a non-coherent receiver, such as an energy detector, helps to minimise power consumption and reduces implementation complexity. The choice of ALOHA is justified by the potential robustness of IR-UWB to multi-user interference (MUI) and by the low data rate nature of the applications envisioned.

In fact, the design of the MAC layer plays a very important role in order to materialize the benefits of IR-UWB in sensor networks. From a networking perspective, one potential

<sup>1</sup> Random medium access scheme that does not check whether the shared medium is already busy before transmission.

©2013 Pérez Guirao, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ©2013 Pérez Guirao, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### 2 Will-be-set-by-IN-TECH 52 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications Pulse Rate Control for Low Power and Low Data Rate Ultra Wideband Networks <sup>3</sup>

benefit of IR-UWB over narrowband radio technologies is the possibility to allow concurrent transmissions by using different pseudo-random, time hopping codes (THCs) as a multiple access (MA) method. However, TH codes are not perfectly orthogonal, and even if, Multi User Interference (MUI) is still a challenge due to the presence of multipath fading and the asynchronicity between sources. Beyond it, non-coherent receivers are less robust to MUI than coherent receivers; particularly, interference coming from close-by interferers can be very harmful. Thus, and specially if non-coherent receivers are used, additional interference mitigation features at the MAC layer are required.

This chapter assumes a generic Time-Hopping IR-UWB (TH-UWB) physical layer as described in [21]. Time is divided into frames of length *Tf* and each user transmits one pulse of length *Tp* per frame. Furthermore, by dividing the frames into non-overlapping chips of length *Tc*, multi-access capability is provided. Each user transmits its pulse in a randomly chosen chip, according to a pseudo-random TH sequence (THS). Data modulation follows a pulse position modulation (PPM) scheme. Thus, the signal emitted by the *k*-th TH-PPM transmitter consists

(*k*) <sup>−</sup> *<sup>c</sup>*(*k*)

A typical expression is given in 1, where *wtr*(*t*) represents the transmitted pulse waveform and *Tf* is the average frame time, which is also denoted as the mean pulse repetition period. The inverse of the mean pulse repetition period is referred to as the mean pulse repetition

transmitted by *Ns* identical pulses to enhance the quality of reception. The symbol duration equals then *Ts* = *NsTf* . The TH code value for pulse *j* is given by *c*(*k*)[*j*]. The constant term *η* represents the time shift step introduced by the PPM modulator. Usually, this shift is much smaller than the one due to the TH code (*Tc*). The time shift *τ*(*k*) represents the relative delay time between the instants at which user *k* and a reference user *i* start their transmission; it can be considered as a realization of a random process determined by the actions of the users.

Figure 1 illustrates some of the mentioned parameters; in the example: *Ns* = 1, *c*(*k*) = (2, 3, 4),

T(k) f

For more detailed information about UWB technology, the author recommends the book [3], which offers an easy-to-read, but complete introduction to the field. Concerning IR-UWB, see

The design space for the MAC layer is large; it embraces several dimensions such as multiple access (MA), interference management, resource allocation and power saving. This section summarises the most relevant research findings concerning the design of the MAC layer for

**2.2. MAC layer design for low power low data rate IR-UWB networks**

(*k*)

[*j*]*Tc* − *ηb*

(*k*)

Pulse Rate Control for Low Power and Low Data Rate Ultra Wideband Networks 53

�*j*|*Ns*� <sup>−</sup> *<sup>τ</sup>*(*k*)

�*j*|*Ns*� represents that each data symbol *<sup>b</sup>*(*k*) can be

¢¢¢

t

), (1)

of a sequence of pulse waveforms shifted to different times.

∞ ∑ *j*=−∞

*pr f* . The expression *b*

¿ (k)

low power, low data rate (LDR) IR-UWB networks.

¢¢¢

s(k) (t)

**Figure 1.** TH-PPM signal structure.

Tc

*wtr*(*t* − *jTf*

*s* (*k*) (*t*) =

frequency, *Tf* = <sup>1</sup>

*b*(*k*) = (1, 1, 0).

[21].

This work is motivated by the fact that interference management at the MAC layer has not been extensively explored in the context of IR-UWB autonomous networks yet. The chapter is organized as follows. A general introduction into the field of IR-UWB radio technology and its relevant technical fundamentals is given in section 2. Additionally, a short overview into current research activities and basic principles of MAC protocol design for low to medium data rate IR-UWB networks is given. It follows a discussion about the use of game theory as a tool to model and analyse distributed MAC algorithms in wireless networks. The section ends with the description of the investigated scenario and the simulation model.

Section 3 introduces distributed Pulse Rate Control (PRC) as a novel approach for interference mitigation in autonomous IR-UWB networks. PRC enables concurrent transmissions at full power, allowing each source to independently adapt its pulse rate - measured in pulses per second (Pps)- to control the impact of pulse collisions at nearby receivers. This section shows that it is possible to incite autonomous users to decrease their impulsive emissions and thus, prevent network resource break-down. Finally, section 4 summarizes the achievements of the work presented and gives directions for future research.
