5GHz MIMO System Power Amplifier design with Adaptive Feedforward Linearization technique

5GHz MIMO System Power

Amplifier design with Adaptive

Feedforward Linearization

technique ENG. / AHMED NASSER AHMED

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Introduction

Recently, MIMO systems that can achieve transmission speed of several

hundreds of Mbps and above have attracted interest due to the demand

for high throughput data transmission in a wireless communication

network.

In the case of wireless LAN, IEEE 802.11n and 802.11VHT provides methods

to realize such a high throughput.

Direct conversion architecture is often adopted for such application due

to its possibility of 1-chip implementation in standard CMOS process.

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Contents

Power Amplifier Design

• Introduction

• Operating class

• Push–pull configuration

• Output power requirement for MIMO

• Chip layout

Adaptive Feedforward Amplifier Linearization

• Introduction

• linearization techniques

• Feedforward Amplifier Linearizer

• Adaptive Feedforward Amplifier Mode

• Simulation results l

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1) Introduction:

In such transceiver system, we used power amplifier

stage in transmitter section and polyphase filter (PPF)

in local oscillator (LO) section

Less linearity of power amplifier causes higher order

intermodulation and consequently destroys

orthogonality between subcarriers in OFDM signals.

Phase error in quadrature LO signal causes crosstalk

between I and Q signals and results unavoidable

demodulation errors.

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2) Operating class:

only class A amplifier (θ = π) has capability of linear

amplification.

the other operation classes, even in the class AB, have

the term proportional to the square of input amplitude in

its output fundamental component.

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3) Push–pull configuration:

only class A operation can be used as a linear amplifier. However, there

exists 2nd order harmonic in the drain current. so, we adopt push–pull

configuration shown to cancel 2nd order harmonic.

Drain efficiency, defined as

Where:

PRF and PDC are output power and DC supply power,

η: is a kind of figure of merit in power amplifier. Well known 50% efficiency value in class A amplifier

But the new efficiency with Push–pull configuration is 66%

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Power amplifier design

4) Output power requirement for MIMO :

Output power requirement for our MIMO design is 100[mW] at 5[GHz] band.

We designed a push-pull amplifier with output transformer, however, simulated

output is only 20[mW] due to insufficient transfer coefficient at the output

transformer.

To overcome this drawback, we adopt series-combining transformer (SCT)

technique As shown , we connected 3 power amplifier sections in parallel and

combined their output in series.

Simulated output power (saturation) is greater than 100[mW] and 1[dB]

compression output is 19[dBm].

Calculated drain efficiency is 24.7[%], however, it is better than conventional

class A bias case (20.6[%]).

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5) Chip layout:

power amplifier in TSMC 90nm process. The following Figure shows a microscope

photograph of the test chip. Occupied areas are 1.2[mm2] for power amplifier

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Contents


• Introduction

• Operating class

• Push–pull configuration

• Output power requirement for MIMO

• Chip layout


• Introduction

• linearization techniques

• Feedforward Amplifier Linearizer

• Adaptive Feedforward Amplifier Mode

• Simulation results l

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1) Introduction:

First the power amplifier (PA) presents amplitude and phase distortion

it will be assumed that its out-put is made up of an amplified version of the input

signal plus certain intermodulation (IMD) products.

These IMD products occupy the same frequency band as the input, the in-band

distortion and a spectrum of frequencies outside of the band of interest, the out-

of-band distortion.

The linearization technique of the amplifier aims to eliminate completely the

distortions present in the PA output signal

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Adaptive Feedforward Amplifier

Linearization

2) linearization techniques:

Several linearization approaches have so far been developed:

1. Predistortion technique: has the advantage of unconditional stability but, it has

limited accuracy when implemented with the analog technology. With digital technology, its bandwidth is limited to one or two cellular channels by the computational rate.

2. Feedback linearization: is simple but, it reduces the gain and the stability

considerations limit its bandwidth and accuracy.

3. Feed-forward linearization : has some distinct advantages. Since the signals are

manipulated by inherently wideband analog technology it can handle multicarrier signals at a mobile base station. Also, it is nonparametric; that is, it does not rely on any knowledge of the signal structure or any family of curves, such as polynomials, to represent the amplifier characteristic

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Linearization

3) Feedforward Amplifier Linearizer:

Feedforward is the most effective and broadly used linearization

technique employed in modern multicarrier and digital

communication systems

The architecture of the well-known feedforward is consists of two

loops.

1. The first loop is a carrier cancellation loop, which is used to cancel the carrier and obtain the IMD products of the main amplifier. The error signal is adjusted to be equal in amplitude but 180 out of phase with the IMD products of the main amplifier.

2. The second loop is the IMD cancellation loop, which used to reduce the output IMD products with the error signal.

The amount of correction is limited by the ability of the two loops to

match gain and phase between the main signal and error paths.

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Linearization

4) Adaptive Feedforward Amplifier Model:

Input signal can be assumed as:

the nonlinear PA introduces amplitude and phase distortion so The PA output is:

where

G = G e jα : The linear gain of PA

1 in G v : Amplified version of input signal plus a certain phase shift α

IMD v : Intermodulation products

In the signal cancellation circuit a fraction of the PA output signal va/k1, with k1 real, is compared with a sample of the input avin, with a complex, resulting in an error signal vei given by

If a is adjusted in such way that

The error signal vei will contain only the intermodulation products.

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Linearization

4) Adaptive Feedforward Amplifier Model:

In a similar way, in the error canceling circuit, the error signal is amplified in a second(error) to obtain

The gain and phase of the signal va will be adjusted by means of the complex parameter b, before comparing it to the signal vo free of distortions

the output voltage can be formed as follows;

If b is adjusted such that,

The equation for vo becomes

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Linearization

5) Simulation results

It is clearly seen that 3rd order IMD is about -55dBc and 25dB improvement is

achieved in IMD performance of amplifier.

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Conclusion

Only class–A amplifiers have capability of linear amplification and output

saturation is caused by operation shift from saturation region to triode one

Test chip design results show that the output power of 100[mW] is obtained

by using series-combining transformer technique.

a feedforward linearizer was possible, using digital processing of the

corresponding signals. A basic adaptive structure based on the LMS

algorithm was analyzed and simulated

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References

[1] Palaskas, Yorgos, et al. "A 5-GHz 108-Mb/s 2 2 MIMO Transceiver RFIC

With Fully Integrated 20.5-dBm Power Amplifiers in 90-nm CMOS." Solid-State Circuits, IEEE Journal of 41.12 (2006): 2746-2756.

[2] Kurt, Engin, and Osman Palamutçuoğullari. "An adaptive feedforward

amplifier application for 5.8 GHz." Turkish Journal of Electrical Engineering & Computer Sciences 14.3 (2007): 437-443.

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Thank You18

Contact me:

Web site:

www.ahmed_nasser_eng.staff.scuegypt.edu.eg

Email:

[email protected]

[email protected]

http://www.ahmed_nasser_eng.staff.scuegypt.edu.eg/

mailto:[email protected]

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