**1. Introduction**

92 Wireless Communications and Networks – Recent Advances

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Modern multimedia applications demand higher data rates and the trend towards wireless is evident, not only in telephony but also in home and office networking and customer electronics. This has been recently proven by the accelerating sales of IEEE 802.11 family WLAN hardware. Current WLANs are, however, capable of delivering only 30-100 Mb/s connection speeds, which is insufficient for future applications like wireless high-quality video conferencing, multiple simultaneous wireless IEEE 1394 (Firewire) connections or wireless LAN bridges across network segments. For these and many other purposes, more capacity — wirelessly — is needed. Service provided by IEEE 802.11 WLANs fulfills casual internet users and office workers actual needs. But, bandwidth demands are still rising. Millimetre-wave technology is one solution to provide up to multi-Gbps wireless connectivity for short distances between electronic devices. The data rate is expected to be 40-100 times faster than today's wireless LAN systems, transmitting an entire DVD's data in roughly 15 seconds. 60 GHz is ideally suited for personal area network (PAN) applications. A 60 GHz link can replace various cables used today in the office or in home by wireless link as shown in Fig.1, including gigabit Ethernet (1000Mbps), USB 2.0 (480Mbps), or IEEE 1394 (~800Mbps). Currently, the data rates of these connections have precluded wireless links, since they require so much bandwidth. While other standards are evolving to address this market (802.11n and UWB), 60 GHz is another viable candidate. In such a context, 60 GHz millimeter wave (MMW) systems constitute a very attractive solution due to the fact that there is a several GHz unlicensed frequencies range available around 60 GHz, almost worldwide. In Europe, the frequency ranges 62 - 63 GHz and 65 - 66 GHz are reserved for wideband mobile networks (MBS, Mobile Broadband System), whereas 59 - 62 GHz range is reserved for wideband wireless local area networks (WLAN). In the USA and South Korea, the frequency range 57 - 64 GHz is generally an unlicensed range. In Japan, 59 - 66 GHz is reserved for wireless communications (Nesic et al., 2001). This massive spectral space enables densely situated, non-interfering wireless networks to be used in the most bandwidth-starving applications of the future, in all kinds of short-range (< 1 km) wireless communication. Also in this band, the oxygen absorption reaches its maximum value (10-15 dB/km), which gives an additional benefits of reduced co-channel interference. Hence, it is a promising candidate for fulfilling the future needs for very high bandwidth wireless connections. It enables up to gigabit-scale connection speeds to be used in indoor WLAN networks or fixed wireless connections in metropolitan areas.

Fig. 1. Short range communication.

These new systems will need compact and high efficient millimeter wave front-ends including antennas. For antennas, printed solutions are often demanding for the researchers because of its low profile, lightweight and ease of integration with active components (Zhang et al., 2006). High gain and high efficient antennas are needed for 60 GHz communication due to high path losses at this range of frequencies. Conventional antenna arrays are used for high gain applications. But in these cases for achieving high gain, a large number of elements are needed, which not only increases the size of the antenna but also decreases its efficiency (Lafond et al., 2001), (Kärnfelt et al., 2006) & ( Soon-soo oh et al., 2004). It has been reported that for high gain, a superstrate layer can be added at a particular height of 0.5 λ0 above the ground plane (Choi et al., 2003) , (Menudier et al., 2007) & (Meriah et al., 2008).
