Chapter 1 Introduction
1.2 Thesis Organization
The organization of this thesis is overviewed as follows:
In Chapter 2, we will introduce the overview of ultra-wideband system. Two possible approaches are introduced in brief at the section, and we pay attention to impulse-radio UWB system. Some relations about definition, features, pulse shaping, modulation, and several receiver architectures will be presented in the chapter.
In Chapter 3, we start to design circuits used in UWB systems, concluding a low-power LNA with the transformer-feedback matching network and an analog correlator with variable gain control. Individual design concepts, simulation and measured results would be presented in detail. Eventually we also make short discussion and conclusion in each circuit.
In Chapter 4, based on the previous circuits, we propose a novel front-end receiver circuit design of impulse-radio UWB system. In this chapter, the architecture of the proposed receiver and principles of circuit design and even layout consideration will be presented. Furthermore, the integrated circuit has been fabricated and the measured results are exhibited in the chapter.
In Chapter 5, conclusions and future work are presented.
Chapter 2
Ultra-Wideband Communication System
2.1 Introduction
The FCC issued in February 2002, allocated 7500MHz of spectrum for unlicensed use of UWB devices in the 3.1 to 10.6 GHz frequency band. The FCC defines UWB as any signal that occupies more than 500MHz bandwidth in the 3.1 to 10.6 GHz band and that meets the spectrum mask shown in Fig.2-1. This is by far the largest spectrum allocation for unlicensed use the FCC has ever granted. It is even more relevant that the radiation power is relatively low. A comparison with the other unlicensed bands currently available and used in United States is shown in Table 2-1. This allocation opens up new possibilities to develop UWB technologies different from older approaches based on impulse radios. This novel wireless short range communicative specification is deserved to expect.
Two proposals detailing the operation of UWB devices are being considered. One is multi-band orthogonal frequency division multiplexing (MB-OFDM) [3] and the other is direct sequence spread spectrum (DSSS) [4]. We will make a brief introduction in each approach as following.
MB-OFDM UWB
The Multiband OFDM Alliance (MBOA) standard for UWB communications draws heavily upon prior research in wireless local area network (WLAN) systems. In a manner similar to IEEE 802.11a/g, MBOA partitions the spectrum from 3 to 10 GHz into several 528-MHz bands and employs OFDM in each band to transmit data rates as high as 480Mb/s. A significant departure from the original principle of “carrier-free”
Radiation limits
Figure 2-1 UWB spectrum mask for indoor communication systems
Table 2-1 U.S. spectrum allocation for unlicensed use Unlicensed bands Frequency of operation Bandwidth
ISM at 2.4 GHz 2.4000-2.4835 GHz 83.5 MHz U-NII at 5 GHz 5.15-5.35 GHz
5,75-5.85 GHz
300 MHz
UWB 3.1-10.6 GHz 7500 MHz
signaling, the multiband operation is chosen to both simplify the generation and detection of signals and achieve well-established OFDM solutions from WLAN systems.
To ensure negligible interference with other existing standards, the FCC has limited the output power level of UWB transmitters to 41.3 dBm/MHz.
Fig.2-2 shows the plan of the MB-OFDM bands and the channelization within each band. The 14 bands span the range of 3168 MHz to 10560 MHz. In contrast to IEEE 802.11a/g, MB-OFDM employs only QPSK modulation in each sub channel to allow low resolution in the baseband analog-to-digital (A/D) and digital-to-analog (D/A) converters. Usually, bands 1-3 constitute “Mode 1” and are mandatory for operation
wherreas the remmaining bandds are envissioned for hhigh-end prooducts [5].
Figure 2-2 Multi-baand OFDM frequency bband plan
UWB (DSSSS)
Furthermore, DS-UWB provides four key advantages over other wireless technologies: quality of service; high data rates that scale to 1 Gbit/sec or more; lower cost; and longer battery life. That the technology reduces implementation complexity while allows increased scalability, makes it ideal for applications such as high-rate data transmission and power-constrained handheld devices. These attributes mean DS-UWB is well suited to be the physical layer for PANs.
Figure 2-3 DS-UWB operating band (a) low band (b) high band
2.2 Impulse-Radio UWB Communication System 2.2.1 Definition
FCC provides the following definitions for UWB operation [6], and the illustration is shown in Fig.2-4.
(a) UWB bandwidth
The UWB bandwidth is the frequency band bounded by the points that are 10 dB below the highest radiated emission, as based on the complete transmission system including the antenna. The upper boundary is designated fH and the lower
boundary is designated fL. (b) Center frequency
The center frequency, fc, equals (fH + fL)/ 2. (c) Fractional bandwidth
The fractional bandwidth equals 2
(
fH − fL)
/(fH + fL). (d) Ultra-wideband (UWB) transmitterAn intentional radiator that, at any point in time, has a fractional bandwidth equal to or greater than 20% or has a UWB bandwidth equal to or greater than 500 MHz, regardless of the fractional bandwidth.
(e) Equivalent isotropically radiated power (EIRP)
The product of the power supplied to the antenna and the antenna gain in a given direction relative to an isotropic antenna. At UWB specification, the signal of 3.1 to 10.6 GHz frequency allocation cannot exceed -41.3dBm/MHz of power spectral density (PSD).
fL fc fH
Figure 2-4 Illustration of UWB
Generally speaking, IR-UWB specification is similar to DS-UWB. The reason is that they both transmit data by pulses. However they still have slight difference in sub-channel allocation. DS-UWB system is divided into high band and low band for avoiding the interference with narrow band communication systems. Therefore, IR-UWB use very short pulses which occupy the almost entire band (3~10 GHz) for
high data rate and average power degradation. In this thesis we focus on IR-UWB and design receiver block circuits suited in IR-UWB system.
2.2.2 Features and Advantages
UWB has a lot of advantages that make it attractive for consumer communications applications [7,8 ]. In particular, IR-UWB systems owns the advantages of
(1) High-data transmission rate
According to channel capacity formula, (i.e. log2(1 ) N B S
C = × + ), the data capacity (C) is proportional to channel bandwidth (B). In terms of 7.5 GHz of UWB bandwidth, the transmission rate can reach in 110Mbit/s if the range is less than 10m and up to 480Mbit/s if less than 2m. It is an ideal technology for realizing wireless personal area network (WPAN) systems. In addition, UWB systems coexist with other communication systems band instead of occupying crowded and expansive frequency bands arbitrarily.
(2) Low power consumption
UWB system delivers data by discrete-impulse instead of continuous sinewave, and the hold time of impulse is very short. Compare with continuous-signal of conventional communication system, impulse radio system can achieve ultra low power consumption.
The consumption of UWB is only 1/100 of traditional mobile phone, and 1/20 of Bluetooth equipments. Therefore, impulse radio UWB system is superior to typical wireless system concerning battery life and electromagnetic radiation.
(3) Low cost and low complex architecture
Unlike conventional radio system, the UWB transmitter produces a very short time domain pulse, which is able to propagate without the need for an additional RF mixing stage. The signal will propagate well without any additional up-conversion. The reverse
process of down-conversion is also not required in the UWB receiver. In other words, one of the greatest benefits of UWB architecture compared to continuous-wave one is that there is no need for complex circuits such as the power amplifier and frequency synthesizer, and these are the most complex components of conventional architecture. In addition, UWB will have potential of low cost if circuits are integrated in single chip by CMOS process.
(4) High security
Due to the low energy density and the pseudo-random (PR) characteristics of the transmitted signal, the UWB signal is noiselike, which makes unintended detection quite difficult. If the transmitted signal collocates with pseudo-random noise sequence for coding, the receiver must have the accurate transmitter’s pulse sequence. As a result, it can get the right signal from the transmitter, and the impulse radio therefore has high security in data transmission.
(5) High positioning resolution
Due to the characteristic of very narrow impulse, IR-UWB is easy to combine with positioning system and communication system. Moreover, the narrow-impulse has robust penetration, which can apply in precision position in door or below ground.
Compared to general global position system (GPS) mechanism which only works in visible range of the positioning satellite, impulse radio can achieve an accurate scale of centimeter.
(6) Multi-path capacity
The radio signal of the conventional wireless communication mostly adopts continuous-wave, and the sustained time is far longer than multi-path propagation time.
So multipath effect limits the quality of service and data rate and causes channel distortion. Besides, traditional receivers deal with multi-path signal by rake receivers.
When multipath effect becomes more serious, the circuits’ complexity, power, and
storage requirements are potentially higher. However, the emission of impulse-radio UWB is narrow impulse, which can directly be decomposed in multipath without interference. It is suitable in multi-path signal processing.
2.3 Pulse Shaping
In impulse-radio UWB, it delivers data by very short pulses. Different kinds of pulse would affect the spectrum distribution [9]. In this section we will introduce some pulses and each spectrum used in UWB system.
(1) Gaussian pulse
As shown in Fig.2-5, Gaussian pulse is the common waveform in impulse communication. The mathematic equation is
[( ) / ]2
( )
t Tc Tauw t = Ae
− − (2-1) where A is amplitude, is the pulse width, and is the parameter about pulse forming, or called pulse shape parameter, which is mainly in the adjustment of central frequency and bandwidth of signal. This spectrum allocation has large DC component and the central frequency is close to low band.Tc Tau
Figure 2-5 Gaussian pulse and spectrum allocation
(2) Gaussian monocycle
As shown in Fig.2-6, Gaussian monocycle is the 1st derivative form of Gaussian
pulse. It can be observed that the central frequency moves toward higher band. However, it still cannot satisfy the spectrum mask of UWB. The mathematic equation is
2 [( )/ ]2
Figure 2-6 Gaussian monocycle and spectrum allocation
(3) Scholtz’s monocycle
As shown in Fig.2-7, the waveform was presented initially in [10] by Dr. Scholtz.
This pulse approximates the 2nd derivative form of Gaussian pulse, and the mathematic equation is
We can observe the spectrums of these three kinds of pulse above. Gaussian pulse has more DC components, which would degrade the efficiency of antenna radiation. The 2nd derivative of Gaussian pulse (or Scholtz’s monocycle) has wider 3dB bandwidth and fewer components in low band. This pulse occupies much bandwidth under UWB specific mask and be appropriate in the waveform of impulse radio UWB.
Figure 2-7 Scholtz’s monocycle and spectrum allocation
(4) Higher-order derivatives of Gaussian pulse
Higher-order derivatives of Gaussian pulse indicate that the derivative order is higher than three. Although more derivative orders cause the central frequency toward high frequency [11], as shown in Fig.2-8, but the circuit architecture becomes more complex to realize the high-order pulse. Furthermore, signal processing in receiver is very difficult. In practical, we select 2nd-order derivative Gaussian pulse as the signal of the receiver design in the thesis.
Figure 2-8 Spectrum allocations of higher-order derivatives of Gaussian pulse
2.4 Pulse Modulation
Information can be encoded in a UWB signal in a variety of methods. The most popular modulation schemes developed to date for UWB are pulse-position modulation (PPM), on-off keying (OOK), pulse-amplitude modulation (PAM) and binary phase-shift keying (BPSK), which also is called biphase. Other schemes have been selected by various groups to meet the different design parameters for different applications. We will make a brief explanation in each modulation method as follows.
PAM (or OOK)
PAM is based on the principle of encoding information with the amplitude of the impulses, as shown in Fig.2-9. The picture shows a two-level modulation, respectively.
The difference is that bit 0 is presented by zero and lower amplitude, where one bit is encoded in one impulse. As with pulse position, more amplitude levels can be used to encode more than one bit per symbol.
Figure 2-9 (a) OOK (b) PAM
BPSK (Biphase)
In biphase modulation, information is encoded with the polarity of the impulses, as shown in Fig.2-10. The polarity of the impulses is switched to encode a 0 or a 1. In this
case, only one bit per impulse can be encoded because there are only two polarities available to choose from.
Figure 2-10 BPSK
PPM
PPM is based on the principle of encoding information with two or more positions in time, referred to the nominal pulse position, as shown in Fig.2-11. A pulse transmitted at the nominal position represents a 0, and a pulse transmitted after the nominal position represents a 1. The picture shows a two-position modulation, where one bit is encoded in one impulse. Additional positions can be used to provide more bits per symbol. The time delay between positions is typically a fraction of a nanosecond, while the time between nominal positions is typically much longer to avoid interference between impulses.
Figure 2-11 PPM
Additionally, combining with PPM and random-time hopping code can flat the spectrum of signals. It also mitigates multi-path effect and interference between clients.
As a result, it accomplishes UWB time-hopping system for multiple-user case. The
transmitted signal for single user is coding by pseudo random code (PR code) to control the pulse position in each frame. The equation of time-hopping PPM can be presented as [7]
Tf is pulse repetition time, which also be called time frame width. For fixed Tf , symbol rate (Rs) can be represented as Rs =1/N Ts f . It can be observed that if time frame width keeps fixed, the transmission rate would decrease. c( )jk represents the
time-hopping sequence of k-th user’s own, which is decided by PR code. is the chip rate. δ is the parameter of pulse position modulation, which can be changed for optimal modulated efficiency. indicates the binary data.
Tc
Figure 2-12 Graphic form of time-hopping pulse position modulation
2.5 IR-UWB Receiver
The main characteristic of UWB impulse radios is that very low emission power density can be achieved by spreading the energy of short-time pulses in wideband.
These radios present the advantage of not requiring up/down conversion of frequency,
which results in reduced complexity and low cost of manufacturing. According to the demodulated method, IR-UWB receivers can be divided into two architectures: coherent receiver and non-coherent receiver. Coherent receivers, as shown in Fig.2-13, rely on the correlation of the received pulses and a local template demand complex implementations [12,13]. Because of short pulse adoption and known position, it has some advantages of high data rate and high SNR. Unfortunately, it needs precision timing synchronization between transmitter and receiver ends. Generally the requirement is realized by delay lock loop (DLL) circuits to achieve synchronization.
On the other hand, the non-coherent receiver shown as Fig.2-14 requires neither pulse synchronization nor estimation of the shape of the incoming pulses. Instead, it recovers the energy of the pulses during a symbol time and compares it to the noise level in order to determine the presence or absence of a symbol [14]. The main drawback of using this kind of detector is that the UWB pulses cannot be detected when the signal-to noise ratio (SNR) is very low, and hence, it cannot make use of the processing gain that spread spectrum systems have. Accordingly, the non-coherent impulse receiver will only work properly when the SNR is above a threshold which is close to the noise level.
Due to the very limited power that is allowed in the UWB transmitter, this only occurs at very short distances.
As to sensitivity, the BER vs. SNR curves of a non-coherent receiver is simulated and compared to that of a coherent receiver using BPSK modulation [14]. The simulation results are presented in Fig.2-15. Both simulations are executed for 10, 25, 50, and 100 Mbps using a pulse rate of 100MHz. The monocycles pulses are shaped to use the low part of the UWB spectrum 3.1GHz-5GHz. As expected, the coherent receiver presents processing gain and hence requires less SNR than the non-coherent receiver for a fixed BER. In addition, the simulation shows that the non-coherent receiver can still detect the UWB pulses for SNR close 0 dB.
Figure 2-13 Architecture of the coherent receiver
Figure 2-14 Architecture of the non-coherent receiver
Figure 2-15 Performance curves of a non-coherent receiver and a coherent receiver using BPSK modulation
Chapter 3
Design of the Ultra-wideband LNA and Correlator
3.1 Overview
In this chapter we propose two circuits, which can apply in the front-end receiver of impulse-radio UWB system. The first one is a UWB LNA with transformer-feedback matching network. It employs the characteristic of the transformer to achieve good input matching and noise performance. Subsequently, the correlator with dynamic gain control has presented. In addition, the wide bandwidth is achieved by canceling the dominant pole at the internal node with the zero introduced by the shunt inductor at the loading stage. These circuits have been fabricated by TSMC 0.18μm 1P6M CMOS technology. We also exhibit the simulation and measurement results of each circuit.
3.2 A UWB LNA with Transformer Feedback Matching Network
In this section we propose another UWB LNA design method. It utilizes transformer feedback for wideband matching and noise degradation. First, the general monolithic transformer prototype is introduced. Then we analysis different kinds of transformer and list each advantages and disadvantages. Then we demonstrate that the design concepts in the proposed UWB LNA in detail. Subsequently, we show the simulation and measured results. Finally, the differences between the simulation and measured results are discussed.
3.2.1 Introduction of Monolithic Transformers
Transformers have been used in radio frequency (RF) circuits since the early days of telegraphy. Recent works have shown that it is possible to integrate passive
transformers in silicon IC technologies because of useful performance characteristics [15,16,17]. In general, the operation of a passive transformer is based upon the mutual inductance between two conductors. Basically, the transformers are used for the following three different functions.
1. Impedance matching: depending on the number of windings, the transformer has the property to change the impedance of the primary or secondary when measuring from the opposite port.
2. Balun: balanced to unbalanced conversion and vice versa
3. DC isolation: obtained with the magnetic (nonelectric) connection between the primary and secondary
Fig.3-1(a) summarizes the basis of an ideal transformer where the primary self-inductance and the secondary self-inductance are characterized with ideal inductors. The mutual inductance is represented by
L1 L2
M , the primary and secondary currents and voltages are , , , and , and the primary and secondary winding numbers are and , respectively. The coupling factor k is defined by
i1 i2 v1 v2
N1 N2 M , ,
and and represents the energy transmitted from the primary port to the secondary port [18,19].
L1
L2
The behavior of the ideal transformer in Fig.3-1(a) is ruled by its characteristic equations
When transformers are applied in silicon ICs, the ideal model cannot be easily establish
because of substrate loss and parasitic effects. Thus, the electrical model of an integrated transformer must be redefined, and the equivalent model of an integrated transformer is shown in Fig.3-1(b).
(a) (b)
Figure 3-1 (a) Electrical model for an ideal transformer (b) Equivalent circuit of an integrated transformer
where and represent the ohmic losses due to the resistivity of the inductor
where and represent the ohmic losses due to the resistivity of the inductor