**5. DS-UWB scheme and RF transceiver**

Direct-sequence spread-spectrum (DSSS) technique is a powerful multiple access (MA) technique that could be combined with UWB modulation to provide robustness against interference. In DS-UWB, the data to be transmitted is modulated using bipolar modulation, based upon a certain spreading code. Modulation is either phase-shift keying (PSK) or PPM. DS-UWB transmitters are super simple and use very low power, but the receiver and its complex correlation recovery circuits are somewhat more of a challenge. DS-UWB has many attractive properties, including low peak-to-average power ratio and robustness to multiple access interference (MAI) [Win et al., 1997].

The basic transmitted CDMA waveform of user *k* is given by

$$\alpha\_k(t) = \sum\_{j=0}^{N-1} \mathbf{C}\_j^k w\left(t - jT\_c\right) \tag{3}$$

Where, *w (t)* represents the transmitted monocycle and *<sup>k</sup> Cj* denotes jth spreading chip of the pseudo-random noise (PN) Sequence. *N* is the number of pulses of the PN sequences to be used for each user.

The transmission signal format is shown in Fig. 3. The encoded data of each user are considered as a data symbol, which is multiplied by the transmitted CDMA code.

**Figure 3.** Transmission signal format

Let, *<sup>f</sup> T* be the symbol period and *<sup>c</sup> T* be the chip period such that *<sup>f</sup> T* = *N <sup>c</sup> T* . Hence, a typical DS format of the *k*th impulse radio transmitter output signal is given by

$$\mathcal{S}\_k\left(t\right) = \sqrt{p\_k} \sum\_m d\_m^k \ge\_k \left(t - mT\_f\right) \tag{4}$$

**Figure 4.** DS-UWB transceiver

**5.3. Bandpass filter** 

A combined mixer is proposed for both RF down-conversion in RX and for the RF upconversion in the TX. In the receiver, it needs to synchronize the received pulse with local controlling signals to down-converted first. For superheterodyne transceivers, it is further down converted to baseband signal by a quadrature mixer. Because of the two stage frequency translation, local oscillator leakage does not have a significant impact on the receiver. In case of direct conversion transceiver, the RF signal is directly down-converted to baseband signal without any intermediate frequency. Therefore, the cost and size of the overall transceiver are reduced. The double-balanced Gilbert-type mixer topology has been widely used due to its

The UWB filters are required to have a specified a small bandpass lter (BPF) with a notched band in the UWB passband (for DS- UWB) in order to avoid being interfered by the

low oscillator leakage and low even-order distortion products at the output.

**5.2. Mixer** 

Ultra-Wideband RF Transceiver 7

Where *<sup>k</sup> md* represents the data symbols and *<sup>k</sup> p* is the transmitted power corresponding to the *k*th user. It is important to note that even an ideal channel and antenna system modify the shape of the transmitted monocycle *w(t)* to *wrec(t)* at the output of the receiving antenna, where *wrec(t)* is the derivatives of a Gaussian function.

As indicated in [Ge et al., 2002, Wu et al., 2002, Wang et al., 2007], the DS-UWB system performance is severely downgraded by inter-symbol and multiple access interferences. Hence, researches on reducing effects of inter-symbol interference (ISI) and MAI is of great importance in designing of the transceiver for DS-UWB [Nassar et al. 2003]. The transceiver architecture of DS-UWB is shown in Fig.2 and the building blocks have bee presented in the following subsections.

#### **5.1. Low noise amplifier**

The primary factors in choosing a low noise amplifier (LNA) scheme are noise figure, dynamic range, linearity and power consumption. LNA is not the first block of a receiver circuit. It is followed by a band pass filter, a switch, or a duplexer that has to be implemented as a first block of the receiver chain in front of Low noise amplifier (LNA). As the band-pass filter is constructed with LC-tank in the first stage mixer to perform filtering of out-of-band interference and this block has signal loss characteristics instead of signal amplification, LNA requires providing a reasonable noise figure (NF) and impedance matching. By using a darlington topology a high gain can be achieved over the entire operating band. The design of a UWB LNA is more challenging than a traditional narrowband LNA. Detailed design and consideration of the LNA can be found in [Hu et al., 2010; Lee H-J, 2006 ].

**Figure 4.** DS-UWB transceiver

#### **5.2. Mixer**

6 Ultra Wideband – Current Status and Future Trends

**Figure 3.** Transmission signal format

where *wrec(t)* is the derivatives of a Gaussian function.

Where *<sup>k</sup>*

following subsections.

Lee H-J, 2006 ].

**5.1. Low noise amplifier** 

Let, *<sup>f</sup> T* be the symbol period and *<sup>c</sup> T* be the chip period such that *<sup>f</sup> T* = *N <sup>c</sup> T* . Hence, a

Data Symbols DS format signal

CDMA Code

 ( ) *<sup>k</sup> k k mk f m*

the *k*th user. It is important to note that even an ideal channel and antenna system modify the shape of the transmitted monocycle *w(t)* to *wrec(t)* at the output of the receiving antenna,

As indicated in [Ge et al., 2002, Wu et al., 2002, Wang et al., 2007], the DS-UWB system performance is severely downgraded by inter-symbol and multiple access interferences. Hence, researches on reducing effects of inter-symbol interference (ISI) and MAI is of great importance in designing of the transceiver for DS-UWB [Nassar et al. 2003]. The transceiver architecture of DS-UWB is shown in Fig.2 and the building blocks have bee presented in the

The primary factors in choosing a low noise amplifier (LNA) scheme are noise figure, dynamic range, linearity and power consumption. LNA is not the first block of a receiver circuit. It is followed by a band pass filter, a switch, or a duplexer that has to be implemented as a first block of the receiver chain in front of Low noise amplifier (LNA). As the band-pass filter is constructed with LC-tank in the first stage mixer to perform filtering of out-of-band interference and this block has signal loss characteristics instead of signal amplification, LNA requires providing a reasonable noise figure (NF) and impedance matching. By using a darlington topology a high gain can be achieved over the entire operating band. The design of a UWB LNA is more challenging than a traditional narrowband LNA. Detailed design and consideration of the LNA can be found in [Hu et al., 2010;

*md* represents the data symbols and *<sup>k</sup> p* is the transmitted power corresponding to

*S t p d x t mT* (4)

typical DS format of the *k*th impulse radio transmitter output signal is given by

A combined mixer is proposed for both RF down-conversion in RX and for the RF upconversion in the TX. In the receiver, it needs to synchronize the received pulse with local controlling signals to down-converted first. For superheterodyne transceivers, it is further down converted to baseband signal by a quadrature mixer. Because of the two stage frequency translation, local oscillator leakage does not have a significant impact on the receiver. In case of direct conversion transceiver, the RF signal is directly down-converted to baseband signal without any intermediate frequency. Therefore, the cost and size of the overall transceiver are reduced. The double-balanced Gilbert-type mixer topology has been widely used due to its low oscillator leakage and low even-order distortion products at the output.

#### **5.3. Bandpass filter**

The UWB filters are required to have a specified a small bandpass lter (BPF) with a notched band in the UWB passband (for DS- UWB) in order to avoid being interfered by the

5–6 GHz for IEEE 802.11a wireless local area networks (WLANs). To avoid the frequency use of WLAN radio signals, the direct sequence ultra-wideband (DS–UWB) specifications for wireless personal area networks (WPANs) need further to divide into a low band of 3.1– 4.9 GHz and a high band of 6.2–9.7 GHz [IEEE.15 Working Group].

Ultra-Wideband RF Transceiver 9
