IEEE Circuits and Systems Magazine - Q3 2019 - 29

A. System Design
Two software defined radios, on which the complete
physical layer is implemented, are connected to both
ends of the described communication channel [48]. The
effective bandwidth of the FSF from 20 to 120 kHz can
be used for an inductive data link. As depicted in Fig. 11
the communication channel has a frequency selective
behavior. Therefore, orthogonal frequency division multiplexing (OFDM) with its robustness against fading is
used for the data link.
The OFDM modulator is realized similar to the IEEE
802.11a standards [49]. It uses 64 subcarriers whereas
subcarrier 0-6 and 59-63 are unused as guard band at the
channel boundaries. The pilots are partitioned onto subcarrier 11, 24, 38 and 51. The subcarriers are modulated
with binary phase shift keying (BPSK) up to 1024 - quadrature amplitude modulation (QAM). A guard interval with a
cyclic prefix of 16 samples is used. With a sample rate of
96 kHz this results in an OFDM symbol time Ts of 0.833 ms.
Referencing to previous section the variation of the group
delay in the passband is below 0.4% of Ts which secures
linear transmission behavior. For channel coding a scrambler, convolution coder and an interleaver are used to increase the transmission robustness against interferers.
For synchronization five OFDM symbols are appended
at the start of every OFDM frame. Its structure is depicted
in Fig. 12. The preamble consists of the short training sequence (STS) for coarse and the long training sequence
(LTS) for fine frequency and timing offset correction. Furthermore, the LTS is used for the channel estimation [50].
The header symbol specifies the payload modulation and
coding scheme (MCS). Furthermore, it contains the number of payload symbols N in one burst. The OFDM burst
here is set to a fixed payload length of 15 symbols which
is a balanced value between overhead and payload.
Fig. 13 shows the power spectral density at the input
of the receiver with the OFDM data signal on the left.
The peaks to the right are the attenuated IPT signal.
B. System Performance
To evaluate the performance of the data link the key
criteria bit error rate (BER) and error vector magnitude
(EVM) are measured at the receiver. Therefore, 10 9 bits
are transferred and analyzed. Table I shows these values
for all MCS without IPT (woIPT) and with IPT (wIPT).
The modulation order M is increased from BPSK (1 bit/
symbol) to 1024-QAM (10 bit/symbol) and the code rate
R c of the convolutional encoder from 1/2 to 5/6.
The transmission rate with channel coding C wCR
ascends from 28.8 kbit/s up to 576 kbit/s with increasing M, but due to the code rate it is always smaller
THIRD QUARTER 2019

than without channel coding C woCR . The EVM of the received subcarrier symbols shows an equal trend, as it
stays constant over all modulation orders. This proves
a functional receiver synchronization. Due to lower modulation order the BER is there influenced less, which is
depicted on the right side of Table I. It can be seen that
for all transmission modes up to a modulation of 64-QAM
no bit errors are detected (BER 1 10 -9 ). Furthermore,
the results for 1024-QAM, where the BER woCR exceeds a
certain limit and too many bit errors appear, show that
channel coding should be turned off.
For 256-QAM the first bit errors can be detected but
corrected by channel coding. However, due to the redundancy of the channel coder, the transmission rate
is only marginally increased. Therefore, 256-QAM with
channel coding and a BER smaller than 10 -6 is identified as the optimum transmission mode achieving transmission rates of 384 kbit/s. In addition, no packet losses
could be measured for the full MCS, which proves that
the presented frame design is sufficient for robust, deterministic and real-time communication.
VI. Discussion
A. Mutual Interference
One of the goals developing this communication system is, that the data transmission must not disturb the
power transmission. This is essential for fail-safe operation. To prove this, Fig. 14 shows the measurement of the

OFDM Burst
STS

LTS

Preamble
Header
(Four OFDM (One OFDM
Symbols)
Symbol)

Payload Data
(N OFDM Symbols)

Figure 12. Structure of used OFDM frame.

Power
Spectral Density (dBm)

V. Communication System

−10
−30
−50
−70
−90
−110
104

105
Frequency (Hz)

106

Figure 13. Power spectrum of shared channel at receiver.

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IEEE Circuits and Systems Magazine - Q3 2019

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