International Journal of Electrical & Computer Science IJECS-IJENS Vol:14 No:03
36
Low Power QPSK RF Transmitter for
405-406 MHZ MEDS Band
Heba A. Shawkey, Ghada H. Ibrahim, Mostafa A. Elmala and Dalia A. El-Dib

Abstract—
This paper presents a low power QPSK
transmitter for MEDS band operating in 405-406MHz frequency
range with 10 channels, with 100KHz channel bandwidth. RF
carrier is generated using an injection locked ring oscillator. A
digitally controlled programmable charge pump is used for
channel selection. The four quadrant phase shift signals are
generated by two RC-CR phase shifters. The transmitter uses a
Gilbert cell mixer for QPSK data modulation followed by a
nonlinear class E power amplifier, as it is compatible with the
constant envelope of the QPSK signal. The transmitter is
designed in a UMC 130nm CMOS process with 1.2V power
supply. Simulation shows that the proposed transmitter has
2.8mW total power dissipation for an output power of 250uW
achieving 5.7% EVM and it can modulate data rates up to
60Mbps.
Index Term— four quadrant phase shift, injection locked
oscillator, low power transmitter, MEDS band, QPSK
modulation and RF transmitter.
I. INTRODUCTION
In recent years, the tremendous progress in biomedical and
healthcare electronic systems as well as the significant
improvements in wireless communication and semiconductor
technologies, led to the rapid development in the wireless
body area networks (WBAN) technologies [1].
Most of reported RF transceivers for biomedical applications
targeted the frequency bands allocated in the 400 MHz
frequency range, especially, after the official adoption of the
400MHz frequency bands in the recently released IEEE
802.15.6 standard for Body Area Networks (BAN)
applications [2]. The 400-MHz MedRadio frequency band is
preferred primarily because the signal propagation
characteristic inside or around human bodies at this frequency
range is more suitable for wireless communication compared
to that at other ISM bands [3,4]. Within the 400 MHz
frequency range, several sub-bands are defined, which are all
This paragraph of the first footnote will contain the date on which you
submitted your paper for review. It will also contain support information,
including sponsor and financial support acknowledgment. For example, “This
work was supported in part by the U.S. Department of Commerce under Grant
BS123456”.
Heba A. Shawkey. Author is with the Elecrtonics Research Institute (ERI).
National research center, physics buildings- ElBohous st, Dokki, Giza, Egypt :
heba_shawkey@eri.sci.eg
Ghada H.Ibrahim. Author is with the Elecrtonics Research Institute (ERI).
ghadahamdy@eri.sci.eg
Mostafa A. Elmala. Author is with the Elecrtonics Research Institute (ERI).
mos_elmala@eri.sci.eg
Dalia F.Eldib. Author is with the Elecrtonics Research Institute (ERI).
dafeldib@eri.sci.eg
named Medical Device Radio communications Service
(MedRadio), as defined by FCC [4]. The MedRadio is defined
in the 401 – 406, 413 – 419, 426 – 432, 438 – 444, and 451 –
457 MHz frequency ranges.
First, for the 401-406 MHz frequency band, it is a recent
extension to the original 402-405 MHz frequency range called
the Medical Implant Communication Service (MICS), which
dates back to 1999 when the FCC established this naming
convention. This is the adopted sub-band in the IEEE 802.15.6
from the FCC defined sub-bands, where maximum channel
bandwidth is defined to be 300 kHz.
The extension of the MICS band to be 401-406 MHz was done
in 2009 [5], where the maximum channel bandwidths for 401402 and 405-406 MHz ranges was set to 100 kHz, which suits
biomedical applications involving the exchange of low data
rate signals like glucose monitoring [6]. The 401-402, and
405-406 MHz sub-bands are denoted by Medical Data Service
(MEDS) in the paper context, which is a notation defined in
[6]. Different modulation schemes have been reported for
400MHz biomedical bands.
This paper proposes an energy-efficient QPSK RF transmitter
for medical applications. The proposed transmitter operates in
the MEDS band from 405-406MHz with 10 channels, with
100KHz channel bandwidth. The paper is organized as
follows. Section II explains proposed system architecture. In
Section III, circuit implementation for the proposed
transmitter is described. Section IV presents complete
simulation performance for each block. Finally, section V
shows the conclusions.
II. PROPOSED QPSK TRANSMITTER ARCHITECTURE
Most of realized transmitters targeting MedRadio bands use
either amplitude shift-keying (ASK) or frequency shift-keying
(FSK) modulations for simpler realization and lower power
consumption [7-9]. However, ASK modulation has a poor
immunity to noise [10], while FSK and phase shift keying
(PSK) are more robust modulation schemes. PSK modulation,
when compared to FSK modulation, offers the advantage of
lower bit error rate (BER) at the same Eb/N0 [11].
Furthermore, multiple phase shift- keying (M-PSK) offers
better bandwidth efficiency than multiple frequency shiftkeying. However, the conventional realization of PSK and
quadrature PSK (QPSK) modulation mandates the use of two
digital to analog converters (DAC) and quadrature upconversion mixers while using a frequency synthesizer, mostly
a phase locked loop, to achieve frequency tuning and channel
selection. This system complexity constructs the main obstacle
that makes low power transmitters avoids the adoption of PSK
and QPSK modulations. Liu et al. illustrated the phase-MUXbased transmitter shown in Fig.1 (a) [12]. In this architecture,
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International Journal of Electrical & Computer Science IJECS-IJENS Vol:14 No:03
a Gilbert cell phase-Mux directly implements the quadrature
phase-shift keying operation, and generates the desired OQPSK/QPSK modulation signals in a hardware efficient way.
The RF carrier is generated by a PLL based frequency
synthesizer followed by a frequency divider to obtain the four
quadrature phases signals. In this work, we propose a power
efficient QPSK transmitter considering phase_Mux instead of
traditional up-conversion mixers. In the proposed transmitter,
the PLL frequency synthesizer – illustrated in Fig.1(a) - is
replaced by an injection locked oscillator followed by a 4
quadrant phase shift block. Fig.1 (b) shows the block diagram
of the proposed transmitter. The RF carrier -405 to 406 MHz is generated by a ring oscillator (RO) while phase stability is
obtained by a crystal oscillator 405MHz frequency range
injected to the ring oscillator. The 10 channels are swept by a
programmable charge pump that injects 10 different current
levels to the RO, while the charge pump is controlled by a 10
bit control word B<0:9>. The outputs of RO ( fo and f )
0
are input to a 4 quadrant phase shift block to generate four
quadrature RF carriers P0, P90, P180 and P270. The four
carriers are then input to the Gilbert phase_MUX for QPSK
data modulation. Finally the RF modulated signal is input to
the power amplifier.
1
N tp t p
2
V
t p  C dd I av
2
f 
37
(1)
(2)
To improve RO performance, injection locked technique is
used. By applying a crystal oscillator - f xtal - reference signal
to the RO with frequency ωo , equal or close to the free
running oscillator frequency, jitter performance of the RO is
dramatically improved. The reference signal can be injected at
a single node or multiple nodes to the ring oscillator. Fig. 2
shows the circuit model of a single node injection for 3 stages
RO [14].
Fig. 2. Injection locked ring oscillator
III. TRANSMITTER CIRCUIT IMPLEMENTATION
In this part we demonstrate the circuit implementation of
different blocks of the proposed transmitter.
A. RF Carrier Generation Circuit
The RF carrier is generated by the RO. For N stage ring
oscillator, the frequency of oscillation can be calculated
as[13]:
(a)
B. Four Quadrant Phase Shift Circuit
Fig.4 (a) shows the basic RC-CR 90O phase shift circuit,
where the two outputs of the RO are 90O phase shifted [15].
By applying the signals fo and f
to 2- RC-CR phase
0
Channel selection
B<0:9>
fxtal
RF Carrier
generation
fo, fo
4 Quadrature
phase shift
P0
P90
P180
P270
TX Data
Digital QPSK
MOD.
D<1:0>
Phase_MUX
In the proposed transmitter, a DC current is added to the RF
signal which is injected to the RO. According to Equation (2);
by changing this DC current, the RF carrier output from RO
can be changed to obtain required 100KHz channel. Fig.3 (a)
shows the block diagram of the injection locked RO used for
RF carrier generation. A crystal oscillator with 405 MHz is
injected to the RO to ensure frequency stability across PVT
variations. Channel selection occurs by a programmable
charge pump that injects different Iinj current levels to RO to
obtain different channels for the MEDS band. The
programmable charge pump shown in Fig.3 (b) consists of 10
parallel current sources each converts the crystal oscillator
carrier signal to a current signal, which is injected to the RO.
The charge pump is controlled with 10 bit control word
B<0:9> for channel selection, in thermometric code. The
outputs of RO fo and f are input to a four quadrant phase
0
shift block to generate four quadrature RF modulation
signals.
PA
PMout
Trout
(b)
Fig. 1. (a) phase-MUX-based QPSK transmitter.
(b) Proposed QPSK transmitter architecture.
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International Journal of Electrical & Computer Science IJECS-IJENS Vol:14 No:03
38
C. Phase_Mux
The four quadrant carrier signals are input to the phase_MUX
shown in Fig.5. The phase_MUX is a Gilbert cell mixer that
selects a pair of complementary phases based on the data
symbol D <1:0>. This architecture is selected as it is less
sensitive to PVT variations compared to conventional mixers
[12].
(a)
(b)
Fig. 3. (a)QPSK transmitter injection locked RO
(b) Programmable charge pump
.
Fig. 5. Gilbert cell phase_MUX
shift blocks we obtain the four quadrature phase carrier
signals. Fig.4(b) shows the circuit diagram of the phase shift
block. Although PVT variations can affect the R and C values,
no degradation occurs for the output as the variation will
change the identical components by same value which leads to
same phase shift. The four phase shifted signals are then input
to buffers to restore the amplitude distorted by RC stages.
D. Class E PA
Nonlinear class E PA has been used due to its compatibility
with the constant envelope of the QPSK signal, beside its high
efficiency compared with other topologies. A zero current
switching PA is designed [16] with two cascaded inverters.
Fig.6 shows the circuit diagram of the PA with the first
inverter - M1 and M2- acts as a buffer for the modulated
signal output from the phase_MUX while the second inverter M3 and M4- drives the matching network.
(a)
Fig. 6. Class E PA
IV. SIMULATION RESULTS
(b)
Fig.4. (a) RC-CR 90O phase shift circuit diagram.
(b) RC-CR Phase shifter
The QPSK transmitter has been designed using UMC 130nm
CMOS technology. Simulation results for each block are
presented in this part.
Fig.7 (a) shows the output performance of the injection locked
RO. The control word is generated by applying a control
signal ctrl to an ideal ADC with a 10 bits output, representing
the control word B <0:9>. The control word B <0:9> changes
in thermometric code. The 10 bits control the charge pump
PMOSFET switches shown in Fig.3 (b) where each current
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International Journal of Electrical & Computer Science IJECS-IJENS Vol:14 No:03
branch delivers 30uA to the RO injection node. For ctrl
changes from 1 to 10, the control word changes from
"000...00" - injects highest current level to RO- to "111...11" injects lowest current level to RO- corresponding to changing
the output frequency fo from 405.95MHz to 405.05MHz.
Table1 shows the value of the output frequency fo of the RO
for each control word. The phase noise of the designed
injection locked RO is shown in Fig.7(b).
Fig.8 shows the four signals output from the four quadrant
phase shift circuit P0, P90, P180 and P270, where all the
signals are π/2 phase shifted.
39
Fig.9- shows the output spectrum from the PA at 405 MHz.
The PA dissipates 0.78 mA from 1.2V voltage supply. The PA
has an efficiency of 30% with ACPR -36 for both the upper
and lower channel.
Fig.10 shows the sinusoidal output signals from the transmitter
for different data symbols where D<1:0> have" 00", "01",
"10" and "11" states.
p90
p270
1.2
1.0
p270, V
p90, V
406.0
freq[1], MHz
405.8
405.6
0.8
0.6
0.4
0.2
405.4
0.0
25.000
405.2
25.833
26.667
27.500
28.333
29.167
30.000
29.167
30.000
time, nsec
405.0
1
2
3
4
5
6
7
8
9
10
ctrl
p180
(a)
p0
1.4
1.2
1.0
p0, V
p180, V
0
-40
0.8
0.6
0.4
0.2
-60
0.0
-80
-0.2
-100
25.000
25.833
26.667
-120
27.500
28.333
time, nsec
-140
0.0
0.2
0.4
0.6
0.8
1.0
Fig. 8. Four quadrant phase shift output signals
noisefreq, MHz
Fig. 7. (a) RO output frequency for different control word
Transmitted Spectrum
(b) Injection locked RO phase noise (dBc/Hz)
TABLE I
PCP INJECTION CURRENT
Ctrl
B0-B9
1
2
3
4
5
6
7
8
9
10
0000000001
0000000011
0000000111
0000001111
0000001111
0000111111
0001111111
0011111111
0111111111
1111111111
fout (MHz)
405.95
405.85
405.75
405.65
405.55
405.45
405.35
405.25
405.15
405.05
-20
Spectrum_out
v1.pnmx, dBc
-20
-40
-60
-80
-100
-100
-75
-50
-25
0
25
50
75
100
freq, KHz
Fig. 9. PA output spectrum
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International Journal of Electrical & Computer Science IJECS-IJENS Vol:14 No:03
D00_out
D01_out
1.0
D01_out, V
D00_out, V
0.5
0.0
-0.5
-1.0
25.000
25.833
26.667
27.500
28.333
29.167
30.000
time, nsec
40
Reduction of supply voltage can improve power dissipation.
Simulation shows that reducing supply from 1.2V to 0.9V
saves up to 0.87mW of power consumption. More supply
reduction degrades the output signal.
Different data rates have been applied to the transmitter. It is
found that the transmitter maximum data rate is 60Mbps.
Table 3 shows the transmitter power dissipation and output
power for different data rates. By increasing data rate, the
efficiency is reduced which shows that the proposed
architecture is more power efficient in low data rates
applications.
TABLE III
- FOR DIFFERENT DATA RATES
PERFORMANCE COMPARISON
Data rate
(Mbps)
D10_out
D11_out
1.0
Transmitted
power (uW)
efficiency
1.1
1.1
1.05
1.02
0.98
0.95
0.834
0.77
250
240
231
221
198
188
136
118
23%
22.8%
22%
21.5%
20%
19.7%
16.3%
15.3%
0.5
2
5
10
20
25
50
60
0.5
D11_out, V
D10_out, V
Power dissipation
(mW)
0.0
-0.5
TABLE 4
-1.0
25.000
25.833
26.667
27.500
28.333
29.167
30.000
PERFORMANCE COMPARISON WITH DIFFERENT ARCHITECURES
time, nsec
Fig. 10. QPSK Transmitter output signal for different data sequences
The transmitter performance has been simulated using 1.2V
supply voltage with fixed data input to the phase_MUX (0Hz
data rate). Transmitter performance is summarized in Table 2.
The power dissipated by both phase_MUX and PA is 1.1mW
for 250uW output power. These values are calculated for a
phase_MUX bias current 50uA. Power dissipation can be
reduced by reduction of phase_MUX bias current, however
this would reduce the output power and maximum data rate
dramatically.
TABLE II
SUMMARY OF QPSK TRANSITTER PERFORMANCE
Process
Supply voltage
Output frequency
Channel spacing
Data rate
Output power
Power dissipation
(Phase_MUX and PA)
0.13- um CMOS
0.9-1.2V
405-406MHz
100 KHz
Up o 60Mbps
250uW (1.2V Supply voltage)
128uW (0.9V Supply voltage)
1.1mW(1.2V Supply voltage)
0.87mW (0.9V Supply voltage)
Power dissipation
(RF carier generator and 4
quadrant phase shift block)
1.7mW
Total Power dissipation
EVMrms
2.8mW
5.7%
Process
Modulation scheme
Supply voltage
Operating frequency (MHz)
Max. Data rate (Mb/s)
Output power
Power dissipation
This
work
[12]
[17]
0.13um
QPSK
0.9-1.2V
405-406
60
-6dBm
(250uW)
2.8mW
0.18um
O_QPSK
1.2-1.8V
355-440
17.5
-15dBm
0.18um
O_QPSK
1.2-1.4V
900
50
-3.3dBm
3.5mW
3mW
Table IV shows a comparison between the proposed
.
transmitter and other related works. It is obvious that the
proposed architecture has low power consumption compared
with other types, beside its simple technique for RF carrier
generation and channel control which makes it easy to extend
this technique for multiband operation with slight variations in
the injection locked RO. Although the proposed transmitter is
designed for low data rate medical bands, it can be used in
high data rate applications.
V. CONCLUSIONS
.
In this
paper, a MEDS band 405- 406MHz low-power QPSK
transmitter has been proposed. The reported transmitter does
not use traditional frequency synthesizers and uses an
injection locked ring oscillator followed by two RC-CR phase
shift circuit for four quadrant phase carrier generation. A
programmable charge pump is used to generate the 10
channels required for MEDS band with 100KHz channel
bandwidth. A phase_MUX and a nonlinear power amplifier
are used for data modulation. The proposed transmitter power
147503-8686-IJECS-IJENS © June 2014 IJENS
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International Journal of Electrical & Computer Science IJECS-IJENS Vol:14 No:03
41
dissipation is 2.8mW for an output power of 250uW and
achieves 5.7% EVM.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
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