Paper Review

A Study on Low Rx-Band Noise Power Amplifier for Reconfigurable RF Front-End Circuit

Takaya Wada

Title on original presentation: A Study on Low Rx-Band Noise Power Amplifier for Reconfigurable RF Front-End Circuit
Presentation venue: International Microwave Symposium (IMS), IEEE MTT-S

Reference

3GPP standardization http://www.3gpp.org/
J.Mueller, T.Bruder, P.Herrero, N.Norholm, P.Olesen, J.Rizk and L.Schumacher, “Requirements for reconfigurable 4G front-ends,” 2013 IEEE MTT-S Int. Microwave Symp. Digest, session WE1C-1, June 2013.
M.Pourakbar, L.Linton, M.Tormanen and M.Faulkner, “Tunable duplex filter for adaptive duplexers of advanced LTE handsets.” 2013 IEEE MTT-S Int. Microwave Symp. Digest, session WE2B-6, June 2013.
A.S.Morris III and V.Steel, “Integrated tunable systems for scalable 4G Radios”, 2013 IEEE MTT-S Int. Microwave Symp. Digest, session WE1C-6, June 2013.
H. Obiya, T. Wada, H. Hayafuji, T. Ogami, M. Tani, M. Koshino, M. Kawashima and N. Nakajima, “A New Tunable RF Front-End Circuit for Advanced 4G Handsets”, 2014 IEEE MTT-S Int. Microwave Symp. Digest, session WEP-54, June 2014.
T.Ogami, M.Tani, K.Ikada, H.Kando, T.Wada, H.Obiya, M.Koshino, M.Kawashima and N.Nakajima,”A New Tunable Filter Using Love Wave Resonators for Reconfigurable RF”, 2014 IEEE MTT-S Int.Microwave Symp. Digest, session TU3A-2, June 2014.
T. Wada, R. Nakajima, H. Obiya, T. Ogami, M. Koshino, M. Kawashima and N. Nakajima, ”A Miniaturized Broadband Lumped Element Circulator for Reconfigurable Front-end System”, 2014 IEEE MTT-S Int. Microwave Symp. Digest, session WEP-28, June 2014.
N. Khlat, M. Granger-Jones, R. Bauder, A.Folkman, ”Radio Front End for Enhanced Data Rate at Cell Edges”, 2010 IEEE MTT-S Int. Microwave Symp. Digest, session WSK-1, May 2010.

Abstract — RF paths in mobile phone have been increasing and the configuration has been becoming a complex year by year. A new tunable RF front-end circuit for a single RF chain architecture was proposed. That solution needs the tunable low RX-band noise power amplifier(LNPA) in order to reduce the noise at a RX input port of a transceiver IC in cooperation with the tunable duplexer in FDD system handsets. In this paper, a principle of LNPA is described and it is shown that an interstage filter is effective compared with other circuit configulations. The required characteristics for the interstage filter in LNPA are studied and the RX band attenuation of the interstage filter is calculated with employing equations using noise figure and gain parametaers. It is found that the RX-band noise floor of LNPA is determined by the noise level of a back stage amplifirer Q2, when the interstage filter RX band attenuation is more than 25dB. LNPA is fabricated to clarify the operation and less than -140dBm/Hz which is the target value of the system can be obtained. This results show the promising possibility of Tunable LNPA for simple RF front-end for future mobile handsets.

I. INTRODUCTION

In the cellular systems, the number of frequency bands which are specified in the 3rd Generation Partnership Project (3GPP) standard has been increasing (1). To satisfy the rapid growth of mobile data traffic, the coverage area of Long Term Evolution (LTE) service, which needs Multi Input Multi Output (MIMO) technology, has been spreading, and carrier aggregation (CA) service has started. Current RF front-ends of general cellular handsets are including more than 10 RF chains and the number of chains will be twice in near future. As a result, the extreme complexity of the front-end circuit configuration has been required continually.

Fig.1 Proposed tunable solution using circulators, tunable filters and tunable LNPA

In the present 3G and 4G systems, main frequency bands are employing frequency division duplexing (FDD) system. Generally, RF front-ends for FDD system in cellular handsets consist of SAW/BAW duplexers, power amplifiers (PAs), antenna switches and some matching components. With the expand of operating frequency bands, those devices and components have been increasing in number and the complicated wiring arrangements between RF devices and transceiver IC ports make the circuit design harder work. A reconfigurable circuit structure and/or tunable devices are one of the solution against these issues (2). A number of approaches to simplify the RF front-end have been studied using tunable devices (3, 4). However, sufficient solutions to meet the 3GPP specifications could not be founded at the moment. The new tunable RF front-end solution (5) which is the SAW based single chain tunable RF front-ends (6) for three paths each (low bands, medium bands and high bands), using wideband circulators (7) , tunable filters and tunable LNPA was proposed as shown in Fig.1. This paper focuses on LNPA and shows a principle of the reducing RX-band noise in LNPA. A Low RX noise PA for the purpose of improvement of RX sensiticity in the cell edge was reported in the middle band (8). However, in the low band case, the closest frequency band gap between TX and RX is 30MHz, hence LNPA adopts SAW based interstage filters to reduce RX band noise in narrower frequency gap.

II. CIRCUIT CONFIGURATIONS AND ANALYSIS

A. Tunable solution target value of RX band noise from TX to RX

RX band noise which comes into the RX port of the transceiver RXIC deteriorates the receiver sensitivity. In the case of FDD systems, RX band noise generated from PA and transceiver TXIC. In current solution, SAW or BAW duplexers are commonly employed because of their small size and high isolation characteristics such as over 50dB. However, Tunable Duplexer Isolation is not achieved the target characteristic as of today. Hence the noise level of PA output needs to be improved. RX noise level of the conventional PA output is -130dBm/Hz. The noise level and isolation value at each stage of the developed tunable solution have been set as follows, conventional level is shown in parentheses.

Fig.2 RX noise Level diagram of proposed solution

RX band noise level of LNPA: -140dBm/Hz (-130dBm/Hz)
Duplexer isolation value: 40dB (50dB)
Target level of Rx band noise: -180dBm/Hz (-180dBm/Hz)

B. Concept of LNPA

Tunable LNPA which block diaglam is shown in Fig.3 has an interstage tunable filter to reduce RX band noise. Frequency coverage is 699MHz-960MHz. Although, in order to reduce RF chain, tunable filters which operate adequate variable frequency ranges are very useful means, they have a tradeoff between variable frequency range and attenuation characteristics in general. Therefore the best suited RX band attenuation characteristics of tunable filters have been investigated.

Fig.3 LNPA diagram

The parameters of LNPA characteristics is shown in Fig.3. The insertion loss of interstage SAW filter is 2.5dB. And the RX band noise is simulated by each parameter. LNPA total noise figure (NF), PA factor and Total Noise are calculated by equation(1), (2) and (3).

Fig.4 shows the simulation results of modifying filter attenuation. The main factor of PA output RX band noise is the output noise from TXIC when RX band attenuation is 0dB. Basically, TX SAW filter used to be put in between TXIC output and PA input to reduce it. However it is not possible to reduce the noise generated in Q1. The other hand additional filter uses in PA out(Q2 out) is not good in terms of power hundling since PA output power is the highest point in RF system. At the result of this investigation, the most suitable location of interstage filter is between Q1 and Q2. And RX band attenuation requirement for interstage filter is more than 25dB to eliminate the noise from Q1 factor and RFIC factor.

Fig.4 Simulation results of filter attenuation

III. EXPERIMENTAL RESULT

Prototype of LNPA has been developed using SAW interstage filter to conduct verification. The developed LNPA, interstage filters and double stage GaAs PAs have been mounted on 9.5x10mm2 PCB.

A. Evaluation results of the interstage SAW filter

The purpose of this experimental is validation of filter RX band noise attenuation. Hence in this study, the interstage filter is using a fixed Band13 SAW filters which characteristic is shown in Fig.5. Band13 frequency is Tx band:777MHz-787MHz and Rx band:746MHz-756MHz. RX band attenuation is more than 30dB. I.L. is less than 2.5dB. RX band attenuation is good enough to eliminate RX band noise from TX chain. Theses characteristics meet the target spec.

Fig.5 RX-band attenuation characteristics of tunable filters

B. Evaluation results of LNPA

The evalutaion results of Q1, Q2 RX band Noise are shown in Fig.6. Q1 output power (X axis) is shifted according to Q2 output power level. Q1 output noise increases to -134dB by Q2 Gain at Q2 output, Hence Q1 noise factor is the main cause of PA noise factor (Fig.4).

Fig.6 Q1, Q2 evaluation results

Fig.7 RX noise evaluation results

Fig.7 shows the evaluation results of LNPA and conventional PA for Band13, respectively. Dot line is the simulation results of LNPA to modify ATT value of the inter stage filter. LNPA RX band noise level (Fig.7) has been obtained as same value of Q2 noise level (Fig.6) to occupied with the interstage filter. And the evaluation results of LNPA RX noise level correspond with the simulation results of SIM ATT25dB and ATT40dB. This results indicate 25dB attenuation is enough to eliminate Q1 and RFIC noise factors. RX noise level of LNPA is achieved less than the target value of -140dBm/Hz.

IV. CONCLUSION

This paper shows principle of LNPA. The requirement of RX band attenuation for the interstage filter is derived. It is found that the RX band noise floor of LNPA is determinded by the noise level of a back stage amplifirer Q2. Since LNPA noise floor asymptotic to Q2 noise level when RX band attenation of the interstage filter is more than 25dB.

This study has clarified the level diagram to analyze LNPA prototype. And it has been yielded targeted characteristics in band13 experiment. The authers have been challenging the size shrinking to use tunable filter and improving the Q2 noise level. Those results indicate that tunable LNPA is strongly promising for a single chain tunable RF front-end of cellular terminals.

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