**2. UWB Technology**

UWB systems historically have been based on impulse radio signals; therefore they can communicate at very high data rates by sending pulses rather than using narrow band frequency carriers. The pulses normally have an ultra-wide frequency spectrum caused by short pulse durations which are about nanoseconds. The concept of impulse radios initially was introduced by Marconi, in the 1900s [1], but since 1960s, impulse radio technologies started being developed for radar and military applications.

In February, 2002, the FCC allocated a bandwidth of 7.5GHz, i.e. from 3.1GHz to 10.6GHz for UWB applications [1]. It was the largest spectrum allocation for unlicensed applications that the FCC has ever permitted. According to the FCC's report, any signal that contains at least 500MHz spectrum can be used in UWB systems. It means that UWB applies to any technology that uses 500MHz spectrum and complies with all other requirements for UWB.

Shannon-Nyquist criterion in Equation (1) shows the relation between channel capacity, bandwidth and signal-to-noise ratio (SNR), when channel is assumed to be ideal bandlimited with Additive White Gaussian Noise (AWGN):

$$\mathbf{C} = \text{BW}\log 2 \left( \mathbf{1} + \text{SNR} \right) \tag{1}$$

Where C is the maximum data rate and BW is the channel bandwidth. Equation (1) indicates that by increasing the SNR (which is directly related to transmission power) or bandwidth, transmitting data rate can be increased. Because of power limitations, increasing the SNR is not a general solution [2, 3, 4 and 5]. Therefore to increase channel capacity and achieve high data rate, a large frequency bandwidth is needed. Considering Shannon-Nyquist Equation indicates that channel capacity can be increased more rapidly by enhancing the channel bandwidth than the SNR. Thus, the wider frequency range can lead to the greater channel capacity. This is more applicable for WPAN which works over short distances and SNR is more satisfactory there.

#### **2.1. UWB benefits**

186 Ultra Wideband – Current Status and Future Trends

be discussed in this part.

passive antenna.

**2. UWB Technology** 

operational band. Various methods have been employed to enhancement antenna

In part 4, planner spiral antenna characteristics and features will be reviewed as a frequency independent antenna. Since without optimization, spiral antenna has some limitations for UWB applications, these limitations will be improved by using some optimization techniques. One of new methods is using active circuit in antenna structure. So in the fifth part, improving history of active antenna technology will be reviewed. Integration of active circuit into passive antenna gives a lot of advantages such as increasing the effective length of short antennas, increasing bandwidth, improving noise factor, impedance matching and sensitivity of receiver antennas and some applications such as utilizing active antenna arrays in mobile communications and beam control, solving channel capacity limitation problems by increasing data rate and improving smart antenna technologies [3] and many other advantages. Overall active antenna structure and different types and applications will

A review of distributed amplifiers characteristics will be done in the sixth part as the active part of active antenna structure. Here the aim is to design a UWB distributed amplifier with uniform and acceptable parameters such as Gain and VSWR in the 3.1- 10.6 GHz band. Calculation of the optimum load resistance and the number of amplifier stages, and then design, optimization and analysis of the circuit must be done for active circuit design completion. Adding antenna element to the active circuit and combined circuit analysis will be explained in this part too. Finally a brief analysis of design and simulation results of UWB active antennas will be shown in the seventh part and it will be favorable that active antenna parameters such as VSWR and Gain are appreciably optimized rather than

UWB systems historically have been based on impulse radio signals; therefore they can communicate at very high data rates by sending pulses rather than using narrow band frequency carriers. The pulses normally have an ultra-wide frequency spectrum caused by short pulse durations which are about nanoseconds. The concept of impulse radios initially was introduced by Marconi, in the 1900s [1], but since 1960s, impulse radio technologies

In February, 2002, the FCC allocated a bandwidth of 7.5GHz, i.e. from 3.1GHz to 10.6GHz for UWB applications [1]. It was the largest spectrum allocation for unlicensed applications that the FCC has ever permitted. According to the FCC's report, any signal that contains at least 500MHz spectrum can be used in UWB systems. It means that UWB applies to any technology that uses 500MHz spectrum and complies with all other requirements for UWB. Shannon-Nyquist criterion in Equation (1) shows the relation between channel capacity, bandwidth and signal-to-noise ratio (SNR), when channel is assumed to be ideal band-

started being developed for radar and military applications.

limited with Additive White Gaussian Noise (AWGN):

bandwidth. In this part, frequency independent antennas will be studied for instance.

UWB has many satisfactory advantages which make it an interesting technology for wireless systems. It is probably the most promising technology for new wireless systems because of some advantages such as low complexity, low power consumption, low cost, high data rate and short-distance wireless connectivity. From circuit point of view, accurate power transfer between transmitter and receiver is the major challenge of UWB system design to obtain a flat received power with minimum ripple.

Here some other benefits are reviewed:


**Figure 1.** Ultra wideband communications spread transmitting energy

• Because of low energy density of the UWB signal, it is a noise-like signal and therefore its undesirable detection is unlikely. Since it is a noise-like signal which has a particular shape, it can be detected in related receiver. In contrast, real noise has no shape, thus interference cannot distort the pulse shape completely and it can still be recovered to restore primary signal. Hence UWB communications are very secure and reliable means.

Active Integrated Antenna Design for UWB Applications 189

Abw f / f H L = (2)

<sup>c</sup> Fbw Abw / f = (3)

H L BW f / f = (4)

U= (5)

G e D rad = (6)

( ) c HL f f f /2 = +

Where fH and fL express the high and low frequencies of the bandwidth respectively and fc shows the center frequency. Although sometimes the bandwidth is expressed as the ratio of

Radiation Pattern; is the representation of the radiation properties of the antenna as a function of space coordinates. Usually radiation Pattern is determined in the farfield region

The radiation pattern can be expressed in two or three-dimensional spatial distribution and it is usually in normalized form with respect to the maximum values. Radiation properties

• Directional - An antenna with the radiation pattern in some directions significantly

• Omni-directional - An antenna which have a non-directional radiation pattern in one

Gain and Directivity; directivity is calculated as the ratio of the radiation intensity in a given direction over an isotropic source radiation intensity, to describe the directional radiation properties of an antenna. The directivity is expressed by D letter and can be calculated by

<sup>U</sup> <sup>D</sup>

0

P rad <sup>0</sup> <sup>4</sup> <sup>U</sup> *<sup>π</sup>* <sup>=</sup>

Where U0 is the radiation intensity for an isotropic source, U is radiation intensity of

Antenna gain is related to the directivity and radiation efficiency and it can be calculated by

the high to low frequencies of operational bandwidth for broadband antennas:

to avoid effects of the distance on the spatial distribution of the radiated power [1].

• Isotropic - An ideal lossless antenna with equal radiation in all directions.

of an antenna can be described by three types of radiation patterns:

plane and a directional pattern in other orthogonal planes.

greater than the others.

antenna and rad P is radiated power.

Which G is antenna gain and erad is radiation efficiency.

equation (5):

equation (6):

• Baseband nature of the UWB signal which is based on impulse radios causes low cost and low complexity of operation systems. Because it does not require system components such as mixers, filters, amplifiers and local oscillators which are necessary for modulation and demodulation units.

Some of the other UWB benefits and advantages are briefly listed below:


And so many other interesting advantages which cannot be explained here. For more information see references [4 and 5].
