04-06-2012, 02:17 PM
Non-Uniform Doppler Compensation for Zero-Padded OFDM over Fast-Varying Underwater Acoustic Channels
[attachment=24163]
INTRODUCTION
The success of multicarrier modulation in the form of
OFDM in radio channels motivates its use in underwater
acoustic communications; see e.g., [1]–[3]. However, underwater
acoustic (UWA) channels are far more challenging
than their radio counterparts, preventing direct application of
OFDM detection methods developed for radio channels, and
requiring a careful receiver design. Recently, there has been
an increased interest in underwater OFDM communication,
including [4] on a low-complexity adaptive OFDM receiver,
[5] on a pilot-tone based block-by-block receiver, and [6] on
a non-coherent OFDM receiver based on on-off-keying.
In this paper, we adopt zero-padded OFDM [7] for underwater
acoustic communications. The performance of a
conventional ZP-OFDM receiver is severely limited by the
intercarrier interference (ICI) due to fast channel variations
within each OFDM symbol. Furthermore, the UWA channel
is wideband in nature due to the fact that the signal bandwidth
is not negligible with respect to the center frequency.
RECEIVER ALGORITHMS
The received signal is directly sampled and all processing is
performed on discrete-time signal. Fig. 1 depicts the receiver
processing. Many steps in the receiver diagram are selfexplanatory.
We next present several key modules.
A. Doppler scale estimation
Doppler scale coarse estimation is based on the preamble
and postamble of a data packet1. The packet structure is shown
in Fig. 2. This idea has been used in [8] for single carrier
transmissions. Via synchronization with the preamble and
postamble, the receiver estimates the time duration of a packet
as Trx. The time duration of this packet at the transmitter side
is Ttr. By comparing Trx with Ttx, the receiver infers how the
received signal has been compressed or dilated by the channel:
SIGNAL DESIGN FOR UNDERWATER EXPERIMENTS
Our transmitted signal is designed as follows. The bandwidth
of our OFDM signal is B = 12 kHz, and the carrier
frequency is fc = 27 kHz. The transmitted OFDM signal
occupies the frequency band of 21 to 33 kHz. We use zeropadded
OFDM with a guard interval of Tg = 25 ms per OFDM
symbol. We test three different settings for the number of
subcarriers: K = 512, K = 1024, and K = 2048. We use rate
2/3 convolutional coding (obtained by puncturing a rate 1/2
code with polynomial (23,35)) and QPSK modulation. We let
each packet have Nd = 30976 information bits. Hence, each
packet will contain Nb = Nd/Ka OFDM blocks, where Ka is
the number of active carriers. For K = 512, 1024, 2048, each
packet contains Nb = 64, 32, 16 OFDM blocks, respectively.
CONCLUSIONS
In this paper we investigated the application of zeropadded
OFDM in fast-varying underwater acoustic channels.
We proposed a two-step approach to mitigating the frequencydependent
Doppler drifts, namely non-uniform Doppler compensation
via resampling, followed by high-resolution uniform
compensation on the residual Doppler. We used null
subcarriers to facilitate the Doppler compensation, and pilot
subcarriers for channel estimation. Our receiver is based on
block-by-block processing, bypassing the need of channel
dependence across OFDM blocks, and is thus suitable for fastvarying
underwater acoustic channels. We tested our methods
in a shallow water experiment. Excellent performance was
achieved even when the transmitter and the receivers had a
relative speed up to 10 knots, where the Doppler drifts were
several times larger than the OFDM subcarrier spacing.
[attachment=24163]
INTRODUCTION
The success of multicarrier modulation in the form of
OFDM in radio channels motivates its use in underwater
acoustic communications; see e.g., [1]–[3]. However, underwater
acoustic (UWA) channels are far more challenging
than their radio counterparts, preventing direct application of
OFDM detection methods developed for radio channels, and
requiring a careful receiver design. Recently, there has been
an increased interest in underwater OFDM communication,
including [4] on a low-complexity adaptive OFDM receiver,
[5] on a pilot-tone based block-by-block receiver, and [6] on
a non-coherent OFDM receiver based on on-off-keying.
In this paper, we adopt zero-padded OFDM [7] for underwater
acoustic communications. The performance of a
conventional ZP-OFDM receiver is severely limited by the
intercarrier interference (ICI) due to fast channel variations
within each OFDM symbol. Furthermore, the UWA channel
is wideband in nature due to the fact that the signal bandwidth
is not negligible with respect to the center frequency.
RECEIVER ALGORITHMS
The received signal is directly sampled and all processing is
performed on discrete-time signal. Fig. 1 depicts the receiver
processing. Many steps in the receiver diagram are selfexplanatory.
We next present several key modules.
A. Doppler scale estimation
Doppler scale coarse estimation is based on the preamble
and postamble of a data packet1. The packet structure is shown
in Fig. 2. This idea has been used in [8] for single carrier
transmissions. Via synchronization with the preamble and
postamble, the receiver estimates the time duration of a packet
as Trx. The time duration of this packet at the transmitter side
is Ttr. By comparing Trx with Ttx, the receiver infers how the
received signal has been compressed or dilated by the channel:
SIGNAL DESIGN FOR UNDERWATER EXPERIMENTS
Our transmitted signal is designed as follows. The bandwidth
of our OFDM signal is B = 12 kHz, and the carrier
frequency is fc = 27 kHz. The transmitted OFDM signal
occupies the frequency band of 21 to 33 kHz. We use zeropadded
OFDM with a guard interval of Tg = 25 ms per OFDM
symbol. We test three different settings for the number of
subcarriers: K = 512, K = 1024, and K = 2048. We use rate
2/3 convolutional coding (obtained by puncturing a rate 1/2
code with polynomial (23,35)) and QPSK modulation. We let
each packet have Nd = 30976 information bits. Hence, each
packet will contain Nb = Nd/Ka OFDM blocks, where Ka is
the number of active carriers. For K = 512, 1024, 2048, each
packet contains Nb = 64, 32, 16 OFDM blocks, respectively.
CONCLUSIONS
In this paper we investigated the application of zeropadded
OFDM in fast-varying underwater acoustic channels.
We proposed a two-step approach to mitigating the frequencydependent
Doppler drifts, namely non-uniform Doppler compensation
via resampling, followed by high-resolution uniform
compensation on the residual Doppler. We used null
subcarriers to facilitate the Doppler compensation, and pilot
subcarriers for channel estimation. Our receiver is based on
block-by-block processing, bypassing the need of channel
dependence across OFDM blocks, and is thus suitable for fastvarying
underwater acoustic channels. We tested our methods
in a shallow water experiment. Excellent performance was
achieved even when the transmitter and the receivers had a
relative speed up to 10 knots, where the Doppler drifts were
several times larger than the OFDM subcarrier spacing.