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Chapter 4 Conclusions and future works

4.2 Future works

4.2.1 UWB receiver

In the design of UWB LNA and multiband frequency synthesizer, there are several directions for future study. First, for higher frequency applications, more accurate RF CMOS component models such as large size MIM capacitors and the inductance spiral inductors with higher Q-value should be built up for exactly matching network design in the future. All parasitic effects including parasitic capacitance, resistance and inductance must be considered more carefully. A more accurate and efficient EDA tool for extracting parasitic effects are quietly important. The UWB LNA may be improved as gain-controllable one for higher dynamic linearity application and lower noise figure to depress the average noise figure of the receiver.

Besides, the multiband frequency synthesizer with 12 selective LO bands is presented. By combining PLL architecture, the frequency synthesizer can offer more stable LO frequency, but the settling time increases because of the PLL structure, as shown in Fig. 4.2.1. For example, the fractional-N divider has fast setting time performance than integer-N divider.

High reference clock speeds up to lock frequency but induces more in-band noise. The more charge current in the charge pump accelerates the variation change of Vctrl signal, but induces more power consumption. Therefore, it must be trade-off to achieve the whole PLL structure for multiband frequency synthesizer.

Fig. 4.2.1 A basic PLL structure

As for the MB-OFDM UWB receiver, it can be achieved by combining the UWB LNA, the multiband synthesizer, and an additional UWB mixer. The fully integrated UWB transceiver, including receiver front-end, power amplifier, up-mixer, quadrature VCO, multi-modulus frequency synthesizer, and IF Gm-C filters may be realized for future system-on-chip (SOC) design.

4.2.2 Cognitive communications

Wideband communication is a trend of the future communication. Based on MB-OFDM UWB technique, Cognitive radio is viewed as a novel approach for improving the utilization of a precious natural resource: the radio electromagnetic spectrum, the use of which by transmitters and receivers is licensed by governments. But, Federal Communications Commission (FCC) has pointed out that, in many bands, spectrum access is a more significant problem than physical scarcity of spectrum, in large part due to legacy command-and-control regulation that limits the ability of potential spectrum users to obtain such access [35]. Less than 20% of the licensed spectrum is in use at any given time.

The cognitive radio, built on a software-defined radio, is defined as an intelligent wireless communication system that is aware of its environment and uses the methodology of understanding-by-building to learn from the environment and adapt to statistical variations in the input stimuli. The underutilization of the electromagnetic spectrum leads us to think in terms of spectrum holes. A spectrum hole is a band of frequencies assigned to a primary user, but, at a particular time and specific geographic location, the band is not being utilized by that user [36]. Spectrum utilization can be improved significantly by making it possible for a secondary user (who is not being serviced) to access a spectrum hole unoccupied by the primary user at the right location and the time in question.

As time evolves and spectrum holes come and go, the bandwidth-carrier frequency

picture in Fig. 4.2.2 for the case of seven carrier frequencies, and the way in which the spectrum manager allocates the requisite channel bandwidths for three instant time, depending on the availability of spectrum holes.

Fig. 4.2.2 Illustrating the notion of dynamic spectrum-sharing for OFDM based on seven channels

The front-end of the transceiver used in the cognitive communication is designed with wideband LNA, wideband mixer, wideband PA, and multiband frequency synthesizer, as shown in Fig 4.2.3. When the spectrum sensor detects an unused spectrum hole, baseband processor offer control signals to adjust multiband frequency synthesizer. Therefore, the carrier frequency can be shifted to the spectrum hole for secondary users. When the spectrum sensor detects that a primary user will use the spectrum covering the spectrum hole. The carrier frequency of the secondary users will be shifted to another spectrum hole or stop transmission. In the cognitive communication, UWB LNA and the frequency synthesizer with wider tuning range are needed. Unlike the MB-OFDM UWB specifications, the number of LO bands produced by frequency synthesizer must be extended to more possible spectrum holes to tens of GHz.

Therefore, it is widely recognized that the use of a MIMO antenna architecture can provide for a spectacular increase in the spectral efficiency of wireless communications [37].With

improved spectrum utilization as one of the primary objectives of cognitive radio, it seems logical to explore building the MIMO antenna architecture into the design of cognitive radio.

The end-result is a cognitive MIMO radio that offers the ultimate in flexibility, which is exemplified by four degrees of freedom: carrier frequency, channel bandwidth, transmits power, and multiplexing gain for future communications.

Fig. 4.2.3 The transceiver architecture of the cognitive communication

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Publication Remarks

International conference papers:

1. Bo-Yang Chang and Christina F. Jou, “Design of a 3.1-10.6GHz Low-Voltage, Low-Power CMOS Low-Noise Amplifier for Ultra-wideband Receivers”, IEEE Asia-Pacific Microwave Conference (APMC 2005), December 4-7, 2005, Suzhou, China.

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