**3. UWB transceivers**

Both MB-OFDM (Ranjan & Larson, 2006; Zheng H et al., 2007; Bergervoet et al., 2007; Beek et al., 2008) and DS-UWB (Zheng Y. et al., 2007, 2008) are carrier-modulated systems, where a mixer is used to up/down convert the radio frequency (RF) signal, therefore it requires local oscillator (LO) synthesis. On the other hand, IR-UWB (Yang, C. et al., 2005 ; Xia L. et al., 2011) is a carrier-less pulse-based system, therefore, we can eliminate the fast hopping LO synthesis, thus reducing the complexity and power consumption of the entire radio. Furthermore, since the signal of a pulse-based UWB system is duty-cycled, the circuits can be shut down between pulses intervals which would lead to an even lower power design.

There are a number of different fabrication options for UWB transceivers; CMOS is mainly compelling due to its low cost, low power consumption and single chip transceiver architecture with few external components. Poor passive components and lower operating voltages associated with process scaling pose significant problems for the radio architect and designer. Moreover, the design of UWB transceivers faces the following issues such as - 1) broadband circuits and matching; 2) the low-noise amplifier (LNA) with reasonable noise figure (NF) and impedance matching 3) broadband transmit/receive switch. Narrowband interference imposes some extra issues- the linearity and dynamic range. Even though some important issues that impact the receiver design are given above, there are many other factors that affect the receiver design and choice. For example, the modulation that is used at the transmitter impacts the receiver design. If the transceiver complexity and cost are the primary concerns, a scheme that enables noncoherent demodulation (OOK, positive PAM, PPM, and M-ary PPM) can be considered. On the other hand, some other modulations like BPSK, M-ary PAM, and QAM have the potential to provide better performance and require coherent demodulation since the information is embedded in the polarities of the pulses.

#### **3.1. UWB transmitter/Pulse generator**

2 Ultra Wideband – Current Status and Future Trends

architectures, e.g., noncoherent receivers.

embedded in the polarities of the pulses.

**3. UWB transceivers** 

power design.

continuous ultra-wideband modulated signals while DS-UWB systems are transmitting discrete short pulses which cover ultra-wide bandwidth. On the other hand, IR-UWB is a carrier-less pulse-based system which means IR-UWB and DS-UWB are the two different categorizes of pulse based UWB. Pulse modulation scheme includes OOK (On Off Keying), BPSK (Binary Phase Shift Keying) and PPM (Pulse Position Modulation). OOK modulation is performed by generating transmitted pulses only while transmitting '1' symbols. BPSK modulation generates 180° phase-shifted pulses while transmitting baseband symbols '1' and '0'. PPM modulation is performed by generating pulses where each pulse is delayed or sent in advance of a regular time scale. Thus a binary communication system can be established with a forward and backward shift in time. By specifying specific time delays for each pulse, an M-ary system can be created. BPSK has an advantage over other modulation types due to an inherent 3 dB increase in separation between constellation points (Wentzloff & Chandrakasan, 2006); however, BPSK modulation is not suitable for some receiver

Both MB-OFDM (Ranjan & Larson, 2006; Zheng H et al., 2007; Bergervoet et al., 2007; Beek et al., 2008) and DS-UWB (Zheng Y. et al., 2007, 2008) are carrier-modulated systems, where a mixer is used to up/down convert the radio frequency (RF) signal, therefore it requires local oscillator (LO) synthesis. On the other hand, IR-UWB (Yang, C. et al., 2005 ; Xia L. et al., 2011) is a carrier-less pulse-based system, therefore, we can eliminate the fast hopping LO synthesis, thus reducing the complexity and power consumption of the entire radio. Furthermore, since the signal of a pulse-based UWB system is duty-cycled, the circuits can be shut down between pulses intervals which would lead to an even lower

There are a number of different fabrication options for UWB transceivers; CMOS is mainly compelling due to its low cost, low power consumption and single chip transceiver architecture with few external components. Poor passive components and lower operating voltages associated with process scaling pose significant problems for the radio architect and designer. Moreover, the design of UWB transceivers faces the following issues such as - 1) broadband circuits and matching; 2) the low-noise amplifier (LNA) with reasonable noise figure (NF) and impedance matching 3) broadband transmit/receive switch. Narrowband interference imposes some extra issues- the linearity and dynamic range. Even though some important issues that impact the receiver design are given above, there are many other factors that affect the receiver design and choice. For example, the modulation that is used at the transmitter impacts the receiver design. If the transceiver complexity and cost are the primary concerns, a scheme that enables noncoherent demodulation (OOK, positive PAM, PPM, and M-ary PPM) can be considered. On the other hand, some other modulations like BPSK, M-ary PAM, and QAM have the potential to provide better performance and require coherent demodulation since the information is In principle, all the pulses with the spectra (≥ 500 MHz) falling into the UWB band can be used as signals. However, for practical purposes, the pulses which are simple to generate, controlled, and have low power-consumption (no direct component), are selected to generate UWB signals. The proper selection of the source pulse can maximize the radiated power within the UWB band and meet the required emission limits without filters before the transmitting antennas while minimizing anticipated inter-symbol (and in the case of DS-UWB, inter-chip) interference and providing spectral flexibility as a method to coexist with other radio systems.

In the transmitter, the binary information stream from devices such as PC, PDA or DVD player is passed to the front end of the transmitter and mapped from bits to symbols if higher order modulation schemes are to be used. Each symbol representing multiple bits is then mapped to analog pulse shape which is generated by pulse generator. The mapping of information into waveforms is referred to data mapping or modulation. The generated pulse then can be optionally amplified before being passed to transmitting antenna. Typical IR-UWB use transition generators with edge rates designed to occupy 3 GHz of bandwidth or more while other systems use various forms of gated frequency generators, where the edge rates are selected to spread the energy around the fundamental frequency of the generator.

### **3.2. UWB receiver**

It is necessary to have an optimal receiving system same as generating signal with the desired spectral characteristics. The optimal receiving technique often used in UWB is a correlation receiver. The correlator in the receiver multiplies the received signal with the template waveform. It is critical to note that the mean value of the correlator is zero. Thus, for in-band noise signals received by a UWB radio, the correlator's output has an average value of zero. Moreover, the standard deviation or rms of the correlator output is related to the power of those in-band noise signals. The level of hardware implementation and computational complexity plays an important role in determining which modulation to be used in what application.

The receiver sensitivity is generally defined by the signal level required to gain the given signal-to-noise (S/N) ratio. This means sensitivity is increased when there is less noise. The following formula shows the factors used to define receiver sensitivity.

$$\text{LS (dBm)} = -174 + \text{NF} + 10 \log \text{B} + 10 \log \text{(S/N)} \tag{2}$$

Where, S (dBm) is the receiver sensitivity, NF is the noise figure, B is the bandwidth and S/N is the signal to noise ratio.

If communication is established by QPSK with 8 dB of S/N ratio and 6 dB of total circuit NF, receiver sensitivity with MBOFDM receiver will become –73dBm, when the bandwidth, B= 528 MHz for the data rate 480 Mb/s, To raise the data rate from 54 to 480 Mb/s, the channel bandwidth B need to increase from 20 to 178 MHz. MB-OFDM derives the receiver sensitivity requirements ranging from -80.8 dBm (for 54 Mb/s) to -73 dBm ( for 480 Mb/s) at different data rates. If the required SNR is 2.4 dB, the receiver noise figure is 11.7 dB and the channel bandwidth is 1.32 GHz, the receiver sensitivity with a DS-UWB receiver will become –76.5 dBm at 220 Mb/s.

Ultra-Wideband RF Transceiver 5

LNA & Balun

PGA

Sync

Comparator

Buffer Correlator

In fact, most companies are diving head-on into DS-CDMA and MB-OFDM to form the foundation for most of the coming UWB products though the impulse approach is the hot

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

1

*k k jc j x t C w t jT* 

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

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.

0

( ) *N*

(3)

Pulse Generator

Output

**Figure 2.** Microphotograph of IR-UWB transceiver

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

multiple access interference (MAI) [Win et al., 1997].

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

research area in academia.

used for each user.
