Multicast and Unicast Real-Time Video Streaming Over Wireless LANs Abhik Majumdar, Daniel Grobe Sachs, Igor V. Kozintsev, Kannan Ramchandran, and Minerva M. Yeung IEEE Transactions on Circuit and Systems for Video Technology

Introduction  Addressing the problem of real-time video streaming over wireless LANs.  Unicast Forward error control (FEC) Automatic Repeat ReQuest (ARQ)  Multicast ARQ Optimization of a maximum regret cost function  Experimental results

Challenges for wireless streaming  Fluctuations in channel quality  High bit-error rates  Heterogeneity among receivers Each user will have different channel conditions, power limitations, processing capabilities, etc.

Packet-erasure probability in b

Source coding  Fixed-rate (single resolution) coding Compressing the data to a target size. High compression ratio Ex: DPCM, JPEG, MPEG1  Progressive (scalable) coding Data is divided into coding units. The decoding of the data within a coding unit can be partial. More data can be decoded implies the better quality. Ex: MPEG4-FGS

Rate distortion characteristics

Dependencies between data units

Communication protocols  Asynchronous Reliable but have unbounded delay Need acknowledgment Such as ARQ  Synchronous Bounded delay No feedback Such as FEC

FEC coding  Can protect data against channel erasures by introducing parity packets.  Cannot guarantee that the receiver receives all the packets without error.  This paper employs Reed-Solomon (RS) codes. Described by two numbers (n, k)  n is the length of the codeword.  k is the number of data symbols in the codeword. The original data can be recovered if at least k of the original n symbols are received.

MDFEC  MDFEC Converts a prioritized multi- resolution bitstream into a nonprioritized multiple description bitrstream.  An (n, i) RS code is applied to it to form the N packets.  The ith resolution layer can be decoded on the reception of at least i packets.

MDFEC conversion packet layer

Hybrid ARQ  Algorithm Split data into “ packet groups ” consisting of k packets each. For each packet group, append n-k RS parity packets. Transmission  Transmitter initially sends only the first k data packets.  Transmitter starts sending parity packets until: An ACK is received The deadline of the transmission is reached  Once at least k packets are received, the receiver sends an ACK.

Coding schemes

Advantages of HARQ  Require less parity packets than FEC.  Require less acknowledgements than ARQ.  When acknowledges are lost, the transmitter simply assumes that more parity is needed.

Block diagram of experimental system

Throughput for FEC, ARQ, and HARQ n=150 k=100

Discussion  The throughput is defined as (time to send k data packets / the average time actually need to send them). The probability of successfully sending a data packet.  HARQ method is better than that of the ARQ system because fewer ACKs are sent.  Both ARQ and HARQ outperform FEC in this setup.  FEC will become optimal as the block size (n) increases.

A multicast case  ARQ-based schemes are less appropriate for the multicast case.  Problem formulation for multicast Arriving at an overall quality criterion for the multi- user case is difficult. This paper focuses on a maximal regret criterion.   R is the rate partition  E[d i ] min is the minimum expected distortion for the ith client.  E[d i (R)] is the expected distortion for the used coding scheme.

Minimax regret

Comparison of penalty in distortion

Distortion penalty