A 110W 10Mb/s eTextiles transceiver for body area
networks with remote battery power
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Citation
Mercier, P.P., and A.P. Chandrakasan. “A 110µW 10Mb/s
Etextiles Transceiver for Body Area Networks with Remote
Battery Power.” Solid-State Circuits Conference Digest of
Technical Papers (ISSCC), 2010 IEEE International. 2010. 496497. Copyright © 2010, IEEE
As Published
http://dx.doi.org/10.1109/ISSCC.2010.5433868
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ISSCC 2010 / SESSION 27 / DIRECTIONS IN HEALTH, ENERGY & RF / 27.6
27.6
A 110µW 10Mb/s eTextiles Transceiver for Body
Area Networks with Remote Battery Power
Patrick P. Mercier, Anantha P. Chandrakasan
Massachusetts Institute of Technology, Cambridge, MA
Emerging sensor technologies are enabling low-cost ambulatory medical
devices for remote patient monitoring. In order to replace traditional bulky wired
links used to communicate data around and away from the body, recent work
has proposed the use of wireless body area networks (BANs) and body-coupled
communication (BCC) systems [1-3]. Although such links typically communicate over short distances, the human body presents significant path loss, requiring high TX output power or large RX amplification. Additionally, these inherently single-ended systems must tolerate significant interference from external
sources and nearby users.
An emerging technique for conveying information around the human body uses
electronics textiles (eTextiles) as a communication medium [4-5]. Figure 27.6.1
shows the implemented eTextiles system, where the medium consists of two
electrically separate grids of conductive yarn. Sensor nodes physically connect
to the shared medium using metallic button-snaps, and communicate via an
eTextiles transceiver chip. Using a pair of physical low-impedance connections
has the distinct advantage over wireless and/or BCC systems to be able to: 1)
signal differentially, permitting energy-efficient amplitude-modulation schemes
that tolerate coupled interference, and 2) power sensor nodes remotely from a
local basestation (BS) at extremely high efficiency, minimizing the energy storage requirements on each node. In the proposed system, each sensor node is
pre-programmed with a unique 5b ID code, enabling the BS to administer medium access using a TDMA scheme. New nodes added to the eTextiles network are
dynamically recognized and added to the BS queue. Additionally, energy-expensive multi-user access and encryption operations can be delegated to the BS
since the wired communication link is inherently secure.
A block diagram of the proposed eTextiles transceiver SoC, used in both sensor
nodes and the BS, is shown in Fig. 27.6.2. The two main inputs, v+ and v-, feed
the RX front end (FE), and are the outputs of differential transmitters. Between
packets, a fixed amount of time is allocated to activating transistors M1 and M2
on all sensor nodes and the BS, directly connecting v+ and v- to the supply terminals of each chip. This permits the BS battery to remotely charge each node’s
external super capacitor, which functions as each node’s energy supply.
Time-sharing of the eTextiles medium with remote charging circuitry forces the
DC voltages on v+ and v- to be at opposite rails at the beginning of packet communication. To save the energy otherwise required to completely charge and discharge the primarily capacitive medium, the DC voltages are held constant by
high impedance resistors, and transmitted signals are AC coupled onto the
medium with supply-rail-coupled (SRC) differential transmitters (Fig. 27.6.3).
This approach is advantageous, as capacitive-driving reduces output voltage
swing and driver load [6,7], irrespective of the network DC potential.
Additionally, by using dual capacitors C1 and C2 that are nominally discharged
and charged, respectively, a ternary signaling scheme can be used, simplifying
RX synchronization algorithms. To illustrate, asserting pa[0] in TX+ charges C1,
producing a negative voltage swing on the output that is proportional to the
capacitive divider ratio of C1 and CL. Asserting pa[1] would instead discharge C2,
generating a positive voltage swing. The opposite effects are arranged for TX-,
making the signaling scheme differential, yet operating at different DC levels.
Both TX+ and TX- consist of 7 pairs of binary-weighted tri-state inverters and
capacitive DACs to provide voltage swing configurability.
The RX FE samples and digitizes the SRC differential voltage across v+ and vusing 4 time-offset acquisition (AQ) blocks (Fig. 27.6.4). An SRC common-mode
independent sampling structure is implemented, exploiting the fact that v+ and
v- have DC potentials centered at opposite rails. Before packet reception, the
sampling capacitors are purged. During the preset phase, the capacitors are
charged to the supply rails; since the top plates are floating, their potentials settle to mid-rail. During sampling, the bottom capacitor plates are connected to the
eTextiles network. As the inputs are already centered at opposite DC potentials
and the top plates remain floating, only differential charge is sampled on top of
the existing mid-rail charge residing on each capacitor. As a result, during the
496
• 2010 IEEE International Solid-State Circuits Conference
hold phase, the inputs to the comparators are differentially centered at mid-rail,
requiring no additional biasing and reducing the CMRR requirements of the
ensuing comparators.
Samples are converted to ternary digits (trits) by two clocked comparators sized
for a 3σ offset under 25mV. Each comparator has 8 bits of differential pair and
current source weighting, providing offsets that vary by ±60mV. The comparators are configured to have equal and opposite non-zero offsets, such that any
differential samples above or below the absolute offset level convert to trits ‘+1’
or ‘-1’, respectively; samples residing between the offset levels convert to ‘0’.
The conversion is performed by an offset orientation-independent ternary
encoder, permitting the comparator pair to swap roles. After calibration, this
form of comparator configuration-redundancy improves the σ of offset errors,
measured as the difference between the desired and attained offset for each
comparator, by 1.5-2.5X.
Each sample and conversion operation completes in two clock cycles, requiring
two interleaved AQ blocks to demodulate data at full rate. Synchronization is
achieved in the RX back end (BE) by correlating incoming data using two additional AQ blocks to ensure sampling occurs every half clock period (Fig. 27.6.5).
A custom multiplier is implemented for the correlator ternary arithmetic, saving
2 bits in each adder stage over a traditional 2’s complement topology. If a correlator output crosses a programmable threshold, synchronization is achieved,
and the two unused AQ blocks are clock gated. Alternatively, the RX BE can be
configured in an auto-correlation mode for a CSMA MAC.
The transceiver is fabricated in 0.18µm CMOS, occupies a core area of 0.83mm2,
and operates at 0.9V. Although healthcare applications typically only require 10100kb/s per sensor, the transceiver communicates at a raw data rate of 10Mb/s
to accommodate up to 30 time-multiplexed sensor nodes on the shared medium and to provide margin for remote charging duty-cycling and coding overhead. The RX FE consumes 2pJ/bit, which is at least 20X lower than wireless and
BCC systems operating at similar distances, and is comparable to wireless
eTextiles systems operating over much shorter distances (Fig. 27.6.6). Over 1m,
the TX FE consumes 0.7-to-18pJ/bit for output voltage swings from 6-to290mV. At 100% receive-mode duty cycle, the chip consumes 110µW, including
RX, digital baseband, and I/O power. The remote battery scheme achieves 95%
power transfer efficiency from BS to sensor node, compared to 54.9% for wireless power transfer efficiency [8]. Figure 27.6.6 shows measured transmitted
and received waveforms, and summarizes the chip results. A die photo is shown
in Fig. 27.6.7.
Acknowledgements:
This work was funded in part by the FCRP Focus Center for Circuit & System
Solutions (C2S2), under contract 2003-CT-888.
References:
[1] M. Verhelst et al., “A reconfigurable, 0.13µm CMOS 110pJ/pulse, fully integrated IR-UWB receiver for communication and sub-cm ranging,” ISSCC Dig.
Tech. Papers, pp. 250-251, Feb. 2009.
[2] A. Fazzi et al., “A 2.75mW wideband correlation-based transceiver for bodycoupled communication,” ISSCC Dig. Tech. Papers, pp. 204-205, Feb. 2009.
[3] N. Cho et al., “A 60kb/s-to-10Mb/s 0.37nJ/b Adaptive-Frequency-Hopping
Transceiver for Body-Area Network,” ISSCC Dig. Tech. Papers, pp. 132-133, Feb.
2008.
[4] J. Yoo et al., “A 1.12pJ/b resonance compensated inductive transceiver with
a fault-tolerant network controller for wearable body sensor networks,” Proc. ASSCC Tech. Papers, pp. 313-316, Nov. 2008.
[5] S. Lee et al., “A Dynamic Real-time Capacitor Compensated Inductive
Coupling Transceiver for Wearable Body Sensor Network,” Proc. Symp. VLSI
Circuits, pp. 42-43, Jun. 2009.
[6] R. Ho et al., “High-Speed and Low-Energy Capacitively-Driven On-Chip
Wires,” ISSCC Dig. Tech. Papers, pp. 412-413, Feb. 2007.
[7] P. Mercier et al., “An Energy-Efficient All-Digital UWB Transmitter Employing
Dual Capacitively-Coupled Pulse-Shaping Drivers,” IEEE J. Solid-State Circuits,
vol. 44, no. 6, pp. 1679-1688, Jun. 2009.
[8] J. Yoo et al., “A 5.2mW Self-Configured Wearable Body Sensor Network
Controller and a 12µW 54.9% Efficiency Wirelessly Powered Sensor for
Continuous Health Monitoring System,” ISSCC Dig. Tech. Papers, pp. 290-291,
Feb. 2009.
978-1-4244-6034-2/10/$26.00 ©2010 IEEE
ISSCC 2010 / February 10, 2010 / 3:45 PM
Shared eTextiles
communication and
power delivery medium
v-
ON-CHIP
VDD
v+ (inside shirt)
M2
Charging
Logic
Rlarge
TX+
V+
RX FE
AQ{3}
eTextile
Network
Snap-button
interface
RX BE
(corr. &
demod.)
AQ{2}
AQ{1}
AQ{0}
Basestation prototype
(eTextiles transceiver
& wireless relay)
V-
5
3-16
Payload
5
M1
= No RX/TX
Figure 27.6.2: eTextiles transceiver block diagram used for sensor nodes. The
BS uses the same chip, but replaces the super capacitor with a battery.
TX+ {6:0}
trit[1:0] 00
pa[1]
Nominally
Charged
01
11
00
+1
-1
0
V+
VDD
0
AQ{3:0}
Φ1{0}
V-
en
trit[1]
GND
= BS RX / Node TX
Charge
1-256
Figure 27.6.1: Implemented eTextiles system with packet diagram shown.
trit[0]
Charge
Injection
Cancellation
= BS TX / Node RX
Beacon Req. PKT Beacon Node ID
3-16 trits
Csuper
TXRlarge
Packet Diagram:
Digital Baseband
& MAC
eTextile nodes
Φ1{0}
V+
GND
ProgrammableOffset
Comparators
Cs
{6
Purge
Offset-OrientationIndependent
Ternary Encoder
:0
}
C2
pa[0]
Rlarge
V+
trits{3:0}
Φ2{0}
Φ2{0}
clk{0}
Cs
clk{0}
Boost
C1
en
eTextiles
Network
{6
:0
Differential
Conversion
}
Nominally
Discharged
pa[1]
pa[0]
en{6:0}
CL
V-
Purge
V-
Φ1{0}
preset sample
TX- {6:0}
Φ1{0}
Rlarge
SRC Differential Sampling
AC
2x8
AQ{2}
5
clk{0}
clk{1}
5
en[2]
AC
AQ
Selection
Logic
2x8
AQ{1}
Parallel
Correlator
en[1]
AQ{0}
en[0]
a[1:0] b[1:0]
sysclk
0
2
4
6
8
10
0
2
4
6
8
10
2
4
6
8
1
0.9
1
0.5
0
0
en[3:0]
Ternary
Multiplier
Input
trits
c[1]
c[0]
Time [µs]
Decoded Header
Chip Summary
Multiplier Coefficients
2 bits
2 bits
3 bits
4 bits
5 bits
Short
payload
0
0.8
4
Node responds with
beacon + header
0.1
−0.1
v+ [V]
Parallel
Correlator
en[3]
Basestation
Requests Data
v− [V]
Multiplier Coefficients
ShiftSynchronization
Interleave
Registers
Threshold
Bits Out [V]
clk{3}
clk{2}
AQ{3}
2
convert
Figure 27.6.4: RX front end (FE) consisting of four time-offset acquisition (AQ)
blocks.
Figure 27.6.3: Supply-rail-coupled (SRC) differential ternary transmitter.
Re-Timing
hold
Φ1{0}
Φ2{0}
clk{0}
sysclk
v+/v-
Technology
Core area
Supply voltage
Clock frequency
Multi-access
: 0.18μm
: 0.83mm2
: 0.9V
: 10MHz
: TDMA/CSMA
RX 10-3 sensitivity : 15mV
TX output swing : 6-to-290mV
[1]
[2]
[3]
Decoded Payload
[4]
[5]
This
Work
wireless/wired* wireless
wired
eTextiles
eTextiles eTextiles
Type
UWB
BCC
BCC
RX FE E/bit
45pJ
250pJ
225pJ
1.1 / * pJ
7.2pJ
2pJ
TX FE E/bit
--
70pJ
90pJ
2.9 / * pJ
0.9pJ
0.7-18pJ
RX & BB Pwr 4.3mW
Range
Data Rate
10m
40Mb/s
-1m
3.7mW 0.18 / 2.7mW 0.42mW
1.8m
8.5Mb/s 10Mb/s
0.11mW
1.2 / 85 cm
3cm
1m
10Mb/s
4Mb/s
10Mb/s
* Wired front-end power not available, only total power is shown
Figure 27.6.5: RX back end (BE) used for synchronization.
Figure 27.6.6: Measured transient waveforms and table of measured results.
DIGEST OF TECHNICAL PAPERS •
497
27
ISSCC 2010 PAPER CONTINUATIONS
TX +
RX
Digital BB
& MAC
TX -
Figure 27.6.7: Die photograph of the eTextiles transceiver.
• 2010 IEEE International Solid-State Circuits Conference
978-1-4244-6034-2/10/$26.00 ©2010 IEEE