硅基射频前端：超宽带无线电技术

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"Aminghasem Safarian 和 Payam Heydari 合著的《Silicon-based RF Front-Ends for Ultra Wideband Radios》是Springer出版社在2008年出版的一本专业书籍，专注于超宽带无线电的硅基射频前端设计。该书属于Analog Circuits and Signal Processing系列，由Ohio State University的Mohammed Ismail担任咨询编辑。书中涵盖了与超宽带(UWB)射频微波无线电相关的模拟电路和信号处理技术，探讨了在无线多吉赫兹应用中的单芯片快速跳频合成器、超低功耗电容传感器接口、0.5V下的模拟电路设计技巧、CMOS射频收发器的IQ校准技术、软件定义无线电的基带模拟电路以及标准CMOS中的宽带光电接收器等主题。" 这本书详细阐述了硅基射频前端在超宽带无线电中的关键角色。超宽带无线电技术因其在通信、雷达、定位等多个领域的广泛应用而备受关注。射频前端作为超宽带系统的核心部分，其设计和优化直接影响到系统的性能，包括带宽、功率效率、选择性、灵敏度等。书中详细讨论了模拟电路的设计，这是射频前端性能的基础。模拟电路的设计涉及到放大器、滤波器、混频器、振荡器等组件，这些组件必须在超宽带的宽频率范围内工作，同时保持高性能和低功耗。作者可能详细介绍了如何利用硅基半导体工艺实现这些挑战性的设计，特别是在微米和纳米尺度下的集成技术。此外，书中可能涵盖了信号处理策略，这对于超宽带信号的检测和解调至关重要。信号处理包括预失真、匹配网络设计、同步算法等，这些都是为了确保在宽频率范围内的高效信号传输和接收。 IQ校准技术对于CMOS射频收发器而言是必不可少的，因为这种技术可以纠正非理想相位和幅度失配，从而提高系统的线性和整体性能。软件定义无线电的基带模拟电路部分可能探讨了如何实现灵活且可编程的射频系统，以适应不同的通信标准和频率范围。最后，书中提及的宽带光电接收器在标准CMOS工艺中的实现，展示了如何在单一芯片上集成光电子功能，这为未来无线通信系统提供了新的可能性，尤其是在高速数据传输和光无线融合网络方面。《Silicon-based RF Front-Ends for Ultra Wideband Radios》是关于超宽带无线电领域射频前端设计的一本深入研究专著，适合于研究人员、工程师以及对超宽带技术感兴趣的高级学习者。通过阅读本书，读者将能够理解和掌握硅基射频前端设计的关键技术和挑战，以及如何克服这些挑战以实现高性能的超宽带通信系统。

8 2 Ultra Wideband Systems

implies that narrow time duration pulses can directly be transmitted to the antenna.

Moreover, in a multipath-dominated environment, larger transmission bandwidths

result in increasingly ﬁne resolution of multipath arrivals, thereby leading to reduced

fading per resolved path. In the time-domain, the impulsive nature of transmitted

UWB waveforms prevents signiﬁcant overlap, and hence, reduces possibility of

destructive combining. Nonetheless, processing such narrow time-duration pulses

in an IR-UWB receiver requires high-speed wideband RF building blocks with high

sensitivity.

Alternatively, in a MB-OFDM UWB system the 3.1–10.6-GHz band is split into

narrow sub-bands. Sophisticated time-frequency codes are used to interleave data

sequences in different sub-bands. Each sub-band occupies an absolute bandwidth

of 500MHz. A UWB system incorporating MB–OFDM approach exhibits several

useful properties, which include:

Less interfering coexistence and interoperability with existing communication

standards

Less intersymbol interference (ISI) and inter-carrier interference (ICI)

Easier adoption to different worldwide regulatory environments

Future scalability and backward compatibility

Possibility of designing circuits in standard CMOS process to take advantage of

all the beneﬁts resulting from technology scaling

Excellent robustness in multipath environments

As will be seen in Section 2.5.2, the main drawback of MB-OFDM systems is

the use of transceiver architectures with complex (de)modulation schemes and so-

phisticated signal processing units. Another problem is that the peak-to-average

power-ratio (PAPR) of OFDM signals tends to be higher than that of narrow

pulses in pulse-based UWB. This, in turn, imposes more stringent requirement

on MB-OFDM transceiver design, particularly on the power ampliﬁer (PA) in the

transmitter.

2.4 Advantages of Ultra Wideband over Narrow-band Systems

UWB technology represent few advantages over the commonly-used narrowband

communication systems, such as simpler transceiver architecture, lower cost, lower

power consumption, and the capability to resolve multipath fading.

The lower complexity and cost is mainly due to simple transceiver architec-

ture which is different from the conventional narrowband communication sys-

tems. In a conventional communication system, a baseband signal is mixed with

a higher frequency carrier, producing a radio frequency signal which is trans-

mitted in the desired wireless channel. A power-hungry PA then ampliﬁes this

RF signal and delivers required amount of power to the transmit antenna. In

the UWB transmitter, the pulse generator can be designed in such a way as to

generate UWB-compliance short-duration pulses, which obviates the need for an

2.5 UWB Transceiver Architecture 9

up-conversion mixer or a high-power PA. Similarly, in the receiver path of a

UWB system, an analog correlator as a matched ﬁlter is used to detect the sig-

nal, which essentially substitutes the conventional down-conversion mixers and

de-modulators.

Because of the pseudo-random nature of UWB pulse signals with low duty cy-

cles, the signal power is concentrated in a short interval within one repetition period.

Hence, the power spectral density of transmitted pulses in the channel looks like

that of thermal noise. This indicates that UWB transceivers are capable of com-

municating undetectable noise-like signals at low energy levels [4, 6]. Accordingly,

the impact of UWB interference on the existing wireless networks, such as 802.11a

WLAN is negligible [7].

2.5 UWB Transceiver Architecture

2.5.1 Impulse Radio (IR) UWB

IR-UWB is often known as carrier-less technology, wherein the modulated base-

band signal is directly transmitted through the antenna into the air, as shown in

Fig. 2.4. This attribute greatly reduces the complexity of transceiver architecture,

and speciﬁcally the RF front-end, compared to narrowband transceivers which

employ complex frequency conversion architecture. The low-power emission re-

quirement in the IR-UWB transceiver relaxes the design of antenna pre-drivers

on the transmitter side. This is quite opposite to narrowband transceivers where

high power PAs are needed to launch the signal with sufﬁcient power to the

antenna.

In a receiver structure of an IR-UWB system one could insert the analog-to-digital

converter (ADC) just after a low-noise ampliﬁer (LNA) and variable-gain

ampliﬁer (VGA), to carry out much of the signal processing in the digital do-

main [8].

Although the system architecture is seemingly easy to implement, it entails a se-

rious design challenge. The ADC for such system should operate at extremely high

sampling rate of at least 15Gsamples/sec with resolution of 4 to 6 bits. Designing

such an ADC in currently available nanoscale CMOS process is impossible. Despite

encountering serious practical issues when used for 3.1–10.6GHz UWB band, this

solution may be suitable for 0–900MHz UWB band dedicated for sensor networks.

Sequence

Generator

Pulse

Generator

Modulator

(e.g., PAM, PPM)

PLL-Based

Synchronizer

Fig. 2.4 Block diagram of an Impulse Radio-UWB transmitter [8]

10 2 Ultra Wideband Systems

Clearly, the ADC can be moved down the analog chain so that some of the signal

processing is carried out in the analog domain, as shown in Fig. 2.5. This is achieved

by using a time-integrating analog correlator after LNA/VGA and before the ADC.

The analog correlator consists of a wideband multiplier followed by an integrator

[9]. The two inputs of the correlator, or the multiplier, are the input signal and the

template generated by the pulse generator. The product of these two input signals at

the output of the multiplier is further integrated to produce a robust signal level with

relatively low frequency content. This signal is fed to the ADC.

The UWB receiver of Fig. 2.5 needs a precise pulse generator, which provides

periodic timing signals at the receiver. The local template pulse is triggered by

the coded timing signal and produces a series of template signal pulses ideally

synchronized with the sequence of all the possible transmitted signals. The cor-

relator then converts the received RF signal to baseband for detection [10]. In

a typical spread spectrum receiver, the correlator slides the received signal and

correlates it with a reference template pulse. When the received signal and the

reference pulse are synchronized in phase, a peak emerges at the output of the

correlator.

2.5.2 MB-OFDM UWB

Multi-carrier modulation, also known as OFDM, widely used in the implementa-

tion of IEEE 802.11.a, HIPERLAN II, DVB, DAB standards and xDSL technol-

ogy, performs well in dispersive channels, and enables high rate communication

with inexpensive receivers. When combined with guard interval and cyclic preﬁx,

OFDM eliminates ISI/ICI, and overcomes fading by using forward error correction

(FEC) coding [11]. The channel equalizer employs the frequency-domain equal-

ization technique that is easily performed using training sequences. Furthermore,

the channel estimate can be incorporated into a soft decision Viterbi decoder in a

bit-interleaved coded modulation (BICM) fashion to get even more coding gain,

ADC

Template

Generator

Sequence

Generator

Clock Recovery and Synchronization

LNA VGA

Time-domain Correlator

Fig. 2.5 Block diagram of an IR-UWB receiver with a time-domain correlator [8]

12 2 Ultra Wideband Systems

downconvert the received wideband signal to either DC or low-IF. The lowpass ﬁlter

in companion with VGA ﬁlter the out-of-band interferes and set the required ampli-

tude level of the signal for proper analog-to-digital conversion. Finally, the digital

signal processing (DSP) unit performs the inverse of the transmitter’s baseband op-

eration, which is, taking the FFT from the received baseband signals.

2.6 Challenges

Clearly, designing an RF front-end circuit in UWB transceivers, either IR or MB-

OFDM UWB, will face a great challenge in the implementation using low cost

CMOS device due to stringent requirements of the UWB technology. Since the

bandwidth of UWB licensed by FCC is from 3.1 ∼ 10.6GHZ, this implies that

RF front-end of UWB transceiver including LNA, down conversion mixers, corre-

lators which are conceptually mixers, should be able to process very wide bandwidth

signals spread over the wide range of frequencies. Such a front-end has to exhibit

wideband characteristics including gain, noise ﬁgure and linearity. This is the key

challenge for the RF front-end circuit design besides the low power constraint.

2.7 Link Budget

The link budget for UWB systems is restricted by the FCC rules, signal bandwidth,

and the device technology. However, in general the following steps are necessary

to calculate the link budget. The average transmit power, Avg_TX_Power, is readily

expressed as [12]:

Avg_TX_Power = EIRP. BW . TX

(2.2)

where TX

denotes the percentage of the time that the transmitter is “ON” in a

time duplex (TD) multiplexing system. The EIRP stands for equivalent isotropically

radiated power. It is the amount of power that would have to be emitted by an

isotropic antenna that evenly distributes power in all directions to produce the peak

power density in the direction of maximum antenna gain.

The path loss between transmitter and receiver is obtained using the well-known

Friis equation, i.e,

Path Loss (dB) = 20 log (4␲rf/c) (2.3)

where c is the light speed, r is the distance between transmitter and receiver, and f

is the carrier frequency.

The receiver sensitivity depends on the thermal noise level (−174dBm/Hz ), bit

rate (r

), noise ﬁgure (NF) of the receiver, and the minimum signal-to-noise ratio

(SNR

min

) which is the required E

to achieve a given BER for a certain modu-

lation such as QPSK. Therefore, the receiver sensitivity, RX_Sensitivity,is:

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