Multicast and Unicast Real-Time Video Streaming Over Wireless LANs

Abhik Majumdar, Daniel Grobe Sachs, Igor V. Kozintsev, Kannan Ramchandran, and Minerva M. YeungIEEE 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 802.11b

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 conversionpacket

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[di]min is the minimum expected distortion for the ith cl

ient. E[di(R)] is the expected distortion for the used coding s

cheme.

minmax iii

dERdER

Minimax regret

Comparison of penalty in distortion

Distortion penalty