RF FRONT-END CONSIDERATIONS FOR FUTURE MULTI-

BAND MOBILE TERMINALS

Hiroshi Okazaki, Takayuki Furuta, Kunihiro Kawai, Yuta Takagi and Shoichi Narahashi

Research Laboratories, NTT DOCOMO, INC., 3-6, Hikarinooka, Yokosuka, JAPAN

{okazakih, furutat, kawaikun, takagiyu, narahashi}@nttdocomo.co.jp

Keywords: Low noise amplifier, Multi-mode multi-band terminal, Multi-band power amplifier, RF filter, RF front-end.

Abstract: A Radio Frequency (RF) front-end (RF-FE) is one of the keys in multi-mode and multi-band mode mobile

terminals. In conventional cellular phones, multiple RF-FEs are installed for multi-band use. There have

been a lot of studies of achieving multi-band or broadband circuits that comprise a compact and cost-

effective multi-band RF-FE with adequate RF performance. Reconfigurable or tunable RF-FEs are a way to

provide multi-band function in future mobile terminals, which takes into consideration the capability to

support a large number of wireless systems that use different frequency bands. However, the performance

degradation should be considered if adaptive circuits are installed in the RF-FE. This paper presents

considerations for the issues that might occur in a multi-band RF-FE for future mobile terminals. As an

example, a concept of collaboration between a top RF filter and low-noise amplifier (LNA) is presented.

1 INTRODUCTION

After explosive growth of smart-phones, connecting

to the Internet and/or someone else anywhere

anytime is becoming necessary in modern lifestyle.

To be able to connect from anywhere with adequate

bit rate, mobile terminals must have multi-mode and

multi-band operation capabilities. In other words,

mobile terminals will be expected to function in all

the required wireless systems and frequency bands.

With regard to frequency bands for cellular systems,

some reports indicate that the number of frequency

bands required for a global mobile terminal is 3-5

for the third generation (3G) cellular system and it

will be increased to 7-9 in the Long Term Evolution

(LTE) era. A Radio frequency (RF) front-end (RF-

FE) is one of the key components for the global

mobile terminal, which should cover a vast number

of frequency bands. In this paper, the RF-FE

consists of power amplifier(s) (PA(s)), RF filters as

a part of duplexers, antenna switches and low noise

amplifier(s) (LNA(s)). In conventional cellular

phones, multiple PAs, filters, and LNAs are installed

for multi-band use, as shown in Figure 1. RFIC

generally includes most of RF functions except PA,

duplexer, band-pass filter (BPF) and antenna switch,

and may have transceivers for wireless local area

network (WLAN), Bluetooth and a receiver for the

Global Positioning System (GPS).

Figure 1: A conventional RF-FE configuration for

frequency-division duplexing (FDD) systems.

Okazaki H., Furuta T., Kawai K., Takagi Y. and Narahashi S.

RF FRONT-END CONSIDERATIONS FOR FUTURE MULTIBAND MOBILE TERMINALS.

DOI: 10.5220/0004785300580061

In Proceedings of the Second International Conference on Telecommunications and Remote Sensing (ICTRS 2013), pages 58-61

ISBN: 978-989-8565-57-0

The RF-FE configuration shown in Figure 1 is

based on a conventional scheme, which is selecting

the most suitable circuit combination from among

several built-in circuits for each frequency band. The

RF-FE in the figure can process 5 different

frequency bands, and also have a diversity or 2-

stream multiple-input and multiple-output (MIMO)

receiving capability at 3 out of the 5 bands by using

a sub receiver. However, the conventional

configuration will reach impasse, and the terminal

employing the configuration will be bulky and

expensive. This is because the terminal handling m

bands and n spatial streams for MIMO should have

(m x n) transceivers in it. Therefore, there have been

a lot of studies of achieving multi-band or

broadband circuits that comprise a compact and

cost-effective multi-band RF-FE with adequate RF

performance (Hueber and Staszewski (Ed.), 2011),

(Bezooijen, Mahmouji, and Roermund, 2011). A

reconfigurable or tunable technique seems to be a

way to provide a multi-band RF-FE that supports a

large number of frequency bands. An example is

shown in Figure 2. However, the performance

degradation should be considered if adaptive circuits

are installed in the RF-FE. This paper presents

considerations for the issues that might occur in a

multi-band RF-FE for future mobile terminals. As an

example, a concept of collaboration between a top

RF BPF and LNA is presented.

Figure 2: An example of RF-FE configuration employing

multi-band circuits for FDD systems.

2 CONSIDERATIONS ON MULTI-

BAND CIRCUITS

In this section, some considerations on each

component in RF-FE are presented.

2.1 PA

In conventional configuration shown in Figure 1,

highly sophisticated PAs are employed. Each of

those PA is optimized for a specific frequency band

and modulation scheme (mode) operation, thus, it is

called as a single-band PA. The main concern of the

PA for the mobile terminal is efficiency at required

output power levels and spurious specifications at an

operating frequency. Generally, efficiency versus

frequency characteristics show a peak. That means

frequency range where the PA works at high

efficiency is limited. There are trade-off relationship

between efficiency and frequency range. Figure 3

shows a rough comparison of the efficiency

characteristics with PA design schemes.

A single-band PA is usually designed by a

narrow-band matching scheme. A multi-band PA

based on wide-band matching (wide-band PA) can

cover closely-located frequency bands, such as 800,

850 and 900 MHz bands, in compensation for some

efficiency degradation from the single-band PA. A

wide-band PA is becoming popular because it can

merge several PAs which should be required for the

conventional RF-FE in a multi-band terminal.

However, the wide-band PA is not practical to

merge the PAs for widely spread frequency bands,

such as 800 and 1900 MHz bands. The PAs based on

broadband or distributed matching generally achieve

lower efficiency than the wide-band PA, and they

are not suitable to use in the mobile terminal.

Figure 3: A comparison of design schemes of PA on

efficiency versus frequency characteristics.

RF Front-End Considerations for Future Multiband Mobile Terminals

Figure 4: Circuit diagram of reconfigurable amplifier with

switches in its matching networks.

On the other hand, a PA with reconfigurable or

tunable matching network is expected to achieve

high efficiency at each frequency band even if the

bands are widely spread. The PAs have variable

devices such as switches or varactors in their

matching networks to change their operating

frequency. Figure 4 shows a circuit diagram

example of a reconfigurable amplifier. The status or

parameters of the variable devices are set to

optimized values for high efficiency based on

narrow-band matching at each target frequency.

However, efficiency degradation should be

considered because the variable components have

losses. In the case of the reconfigurable PA with

switches, detailed evaluations were presented and it

is reported that the degradation can be mitigated

(Fukuda, et al., 2011).

2.2 Filters and LNAs

Figure 5 shows an example of a receiver RF-FE

configuration, which is one generation earlier than

that shown in Figure 1. With the progress of CMOS

process and efforts on circuit technologies such as

“SAW-less” technique (Darabi, 2007), it becomes

natural that the LNA is integrated into an RFIC, and

the BPF between LNA and RFIC shown in Figure 5

is removed. The main concerns of a LNA for the

mobile terminal are sensitivity (gain and noise

figure) and linearity, which includes immunity from

out-of-band signals. Broadband or wide-band

operation of the LNA itself seems to be easier than

that of an efficient PA.

Figure 5: One generation earlier receiver RF-FE

configuration example of receiver side.

Figure 6: Inter-band CA.

Comparing the receiver configurations shown in

Figures 1 and 2, only tunable BPF is required to

configure the RF-FE shown in Figure 2. However, it

is a considerable challenge to attain a tunable BPF

with low-loss at pass-band, high isolation at

suppression band, and a wide-tuning frequency

range.

Carrier aggregation (CA) technology that

employs several bands aggregately and concurrently

will be utilized to obtain a wide operating band-

width in the LTE-advanced era. Figure 6 shows an

example of spectrum usage in inter-band CA. One

technical issue for the CA is configuring a duplexer

and BPF. Characteristics at the combination band

should be considered in the duplexer design in

addition to a conventional duplexer design scheme

which mainly considers characteristics at an original

frequency band for isolation between transmitter and

receiver. Because so many combinations of pair-

bands are considered for the inter-band CA, a

conventional duplexer-bank scheme will make a

global terminal more bulky, and using tunable BPFs

and tunable band-elimination filers (BEFs) will

become more valuable. Otherwise localized mobile

terminals for a specific carrier, county, or region will

revest.

Considering a tunable filter as a part of the

tunable duplexer, one of the serious problems for

receiver chain is to generate gain and phase

modulation caused by strong out-of-band signals.

The LNA will be required to enhance its frequency

selectivity in order to prevent the performance

degradation from undesired out-of-band signals in a

multi-band receiver that yields non-optimum RF

filter performance. Frequency response adjustment

of the LNA will be a solution.

Figure 7 shows small signal frequency responses

of the reconfigurable LNA that has a same circuit

topology shown in Figure 4. The responses were

obtained in two different switch states (0 and 1) at a

class A bias condition. Figure 8 shows the results of

the gain suppression measurement at 3.1 GHz with

out-of-band signals of 2 GHz. From the results

above, the input power of 3.1 GHz for each state is

set to -20.3 dBm, which lies in a linear region. In the

figure, “gain difference” and “gain diff. at 3.1-GHz,”

correspond to the difference in gain at 3.1 GHz

Second International Conference on Telecommunications and Remote Sensing

between with and without the 2.0-GHz signal input

power indicated on the horizontal axis, respectively.

The gain of the 2.0-GHz signal without the 3.1-GHz

input is also shown for comparison. The gain

suppression is observed at a higher level of 2.0-GHz

input power, and is mitigated in state 1. Thus, the

frequency response adjustment of the LNA is

effective in the gain suppression problem caused by

out-of-band signals as well as filter response

improvement. (Okazaki, et al., 2010)

Figure 7: Frequency response of reconfigurable LNA.

Figure 8: Results of the gain suppression measurement.

3 CONSIDERATIONS ON

OVERALL RF-FE

One of the main issues for the overall RF-FE is

known collectively as RF coexistence, which is a hot

topic (Sahota, 2012). A modern mobile phone has

different kind of RF systems such as cellular,

WLAN, Bluetooth, GPS, etc., which will be

activated at the same time. Each of them may

become a source of interference and also a victim of

that. Moreover, the MIMO and intra-band CA

require also simultaneous operation within multiple

transceivers for cellular systems. It seems to be

difficult to work all the RF transceivers and

receivers installed in a multi-band mobile terminal

with expected performance in RF quiet environment.

4 CONCLUSIONS

Several RF-FE considerations for future multi-band

mobile terminals are described. Some of them will

be solved but others such as ideally tunable filters

are remained as issues to be solved and the problems

of RF coexistence are still growing. Continuous

efforts and innovations are really needed.

REFERENCES

Bezooijen, A., Mahmouji, R., Roermund, A., 2011.

Adaptive RF Front-Ends for Hand-held Applications,

Springer. Dordrecht/Heidelberg/London/New York.

Hueber, G., Staszewski, R.B., (Ed.), 2011. Multi-

Mode/Multi-Band RF Transceivers for Wireless

Communications: Advanced Techniques, Architectures,

and Trends, John Wiley & Sons, Inc. Hoboken, New

Jersey.

Fukuda, A., Furuta, T., Okazaki, H., Narahashi, S., Nojima,

T., 2012. Low-Loss Matching Network Design for

Band-Switchable Multi-Band Power Amplifier. IEICE

Transactions on Electronics E95-C: 1172-1181.

Darabi, H., 2007. A Blocker Filtering Technique for

SAW-Less Wireless Receivers. IEEE Journal of Solid-

State Circuits, 42: 2766-2773.

Okazaki, H., Fukuda, A., Kawai, K., Furuta, T.,

Narahashi, S., 2010. Reconfigurable amplifier towards

enhanced selectivity of future multi-band mobile

terminals. In International Microwave Workshop

Series on RF Front-ends for Software Defined and

Cognitive Radio Solutions, IEEE.

Sahota, K., 2012. Coexistence Issues and Mitigation in

Multi-Mode Band Cellular Radios, In International

Microwave Symposium, Workshop WSG, IEEE.

RF Front-End Considerations for Future Multiband Mobile Terminals