LINT MODULATOR
Date
2002
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Masters
Abstract
As the radio spectrum in the lower microwave frequency bands is becoming more crowded, it is almost impossible to support new broadband systems, which use larger portions of the radio spectrum. Thus, new and emerging wireless systems available for broadband services turn towards the high microwave and millimeter-wave spectrum range. Linear modulation schemes like Quadrature Amplitude Modulation (QAM) are preferred in broadband wireless communication systems due to their high spectrum efficiency, which makes them utilize the limited channel bandwidth more efficiently. The problem is that the non-constant envelope of linear modulation schemes requires linear amplification. Linear amplification can be obtained with a Class A amplifier solution, which suffers from very low power efficiency. Poor power efficiency is not suitable for highly integrated or portable equipment, because large batteries are needed to supply extra power, and large transistors and heat sinks are required to dissipate this power. Therefore, a linear modulator architecture that can achieve high power efficiency and implementation at upper microwave and millimeter-wave frequencies is very attractive.
The novel LInear modulation with Nonlinear Translation (LINT) modulator architecture proposed in this thesis is based on the direct modulation method and the LInear amplification with Nonlinear Components (LINC) technique. It involves decomposing
arbitrary baseband signals into two constant envelope signals. Then each constant envelope signal is modulated at a subharmonic carrier using vector modulation followed by xN frequency multiplication to achieve frequency translation to the desired output frequency. Highly efficient power amplifiers can be employed to prepare the signal for transmission at a required power level. Finally, two amplified signals are passively combined to produce a high frequency and amplified replica of the input signal. Although frequency multiplication and power amplification are nonlinear processes, the overall input to output transfer function of the LINT modulator is linear. As opposed to the more convensional method of modulation at an IF and upconversion to the desired transmit frequency, the direct modulation method removes the requirement for IF, upconversion, and filtering circuitry,
resulting in cost and complexity reduction of the hardware implementation.
The main part of the modulator is a x12 two-stage microwave frequency/phase multiplier chain. The multiplier chain consists of x3 and x4 multipliers connected in cascade, and translates a modulated subharmonic carrier at 2.33 GHz to 28 GHz. The circuitry is simulated assuming soft substrate implementation, which simplifies the fabrication process. The multiplier chain itself shows good performance, and can be used to generate stable source signals at high microwave frequencies. Performance of the LINT modulator is investigated using realistic multiplier chains. The effect of amplitude imbalance and phase noise on the 16-QAM modulated signal is simulated.
The result of this research presents a generic modulator architecture that is very attractive for broadband wireless applications at upper microwave and millimeter-wave frequencies.
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Degree
Master of Science (M.Sc.)
Department
Electrical Engineering