Adaptive Video Encoding and Dynamic Channel Access for Real-time Streaming over SDRs

In this paper we study and implement real-time adaptation schemes for video encoding and channel selection that work in tandem to facilitate HD video streaming for secondary users in a dynamic spectrum access network. Out-of-band feedbacks on instantaneous pathloss of the signal between the transmitter and the receiver, the received signal strength indicator (RSSI) at the receiver and the quality of the reconstructed video are used to continuously determine the most apt encoding parameters. At the same time, the radio transmitter continuously adjusts the channel parameters (i.e., center frequency and channel bandwidth) based on the transmission activities of the primary users who have prioritized rights on these channels. We consider the physical limitations of the encoder along with the channel statistics to determine when to change the encoder parameters and when to switch to a new channel. We propose a multi-level threshold based mechanism to find the optimal number of encoding bit rates. We also propose a threshold based algorithm to find the best available channel between the transmitter-receiver pair. We validate our theoretical propositions on an indoor testbed using software defined radios (SDRs) and the GNU Radio suite. Live video was captured, encoded using open source H.264 software libraries, streamed using GStreamer and transmitted over the 915 MHz ISM bands with omni-directional antennas. For the SDRs, we chose the universal software radio peripheral (USRP) B210s from Ettus Research and use them as the transmitter and the receiver. A third B210 was used to sense the energy levels on all the channels to detect the presence of primary transmissions. GNU Radio was used to build the signal processing pipeline, both for the transmitter and the receiver. We use PSNR and SSIM to measure the video quality and report experimental results that show that: (i) the video encoder and the USRP transmitter-receiver pair are able to adapt to the changing RF conditions, (ii) the adaptation schemes yield better video quality than non-adaptive schemes, and (iii) the USRPs can switch the channels fast enough allowing uninterrupted HD video streaming even when primary users preempt the secondary user's transmission.