Atmospheric turbulence causes the receive signal intensity on free space optical (FSO) communication links to vary over time. Scintillation fades can stymie connectivity for milliseconds at a time. To approach the information-theoretic limits of communication in such time-varying channels, it necessary to either code across extremely long blocks of data – thereby inducing unacceptable delays – or to vary the code rate according to the instantaneous channel conditions. We describe the design, laboratory testing, and over-the-air testing of an FSO modem that employs a protocol with adaptive coded modulation (ACM) and hybrid automatic repeat request. For links with fixed throughput, this protocol provides a 10dB reduction in the required received signal-to-noise ratio (SNR); for links with fixed range, this protocol provides the greater than a 3x increase in throughput. Independent U.S. Government tests demonstrate that our protocol effectively adapts the code rate to match the instantaneous channel conditions. The modem is able to provide throughputs in excess of 850 Mbps on links with ranges greater than 15 kilometers.
Atmospheric turbulence causes the receive signal intensity on free space optical (FSO) communication links to vary over time. Scintillation fades can stymie connectivity for milliseconds at a time. To approach the information-theoretic limits of communication in such time-varying channels, it necessary to either code across extremely long blocks of data – thereby inducing unacceptable delays – or to vary the code rate according to the instantaneous channel conditions. We describe the design, hardware implementation, and system performance of an FSO modem that employs low-density parity-check (LDPC) coding in an incremental redundancy (IR) hybrid automatic repeat request (HARQ) protocol. Independent tests performed by the U.S. Government demonstrate that our protocol effectively adapts the LDPC code rate to match the instantaneous channel conditions. For links with fixed throughput, this translates to the longest possible range in the presence of optical scintillation; for links with fixed range, this translates to the highest possible average throughput. By leveraging an LDPC that is amenable to low-complexity, high-throughput implementation in hardware, our modem is able to provide throughputs in excess of 850 Mbps on links with ranges greater than 15 kilometers.
This paper discusses the design and implementation of a burst mode Continuous Phase Modulation (CPM) modem over flat, Rayleigh fading channels in the 220MHz frequency band. Our design focused on bandwidth efficiency while maintaining good synchronization performance and low complexity. The designed preamble is based on the combination of Minimum Shift Keying (MSK) signals and 3RC (Raised Cosine) signals. The use of MSK allows a closed form result for an initial Maximum Likelihood (ML) timing estimate. The 3RC signal, which has a better timing characteristic than MSK, is used to refine the timing and frequency estimates. The payload uses 3RC signals. Pilot symbol assisted modulation (PSAM) is used to assist channel estimation. The designed packet structure meets the Federal Communications Commission (FCC) frequency emission mask for 220MHz frequency band. A CPM modem using the designed packet structure has been implemented on the testbed and the simulation, bench test, and field test results are presented in this paper.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.