Page 251 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

228 Chapter 9. Error-Resilience Video Coding Techniques

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NACK(3)

(a) Reference picture selection with negative acknowledgment messages

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ACK(1) ACK(2) ACK(4)

(b) Reference picture selection with positive acknowledgment messages
Figure 9.10: Reference picture selection based on feedback

In the positive acknowledgment mode, illustrated in Figure 9.10(b), the de-
coder sends an acknowledgment (ACK) message whenever a frame is received
error-free. At the encoder, only acknowledged frames are used as references.
In the example shown, the encoder continues to use frame 1 for prediction
until it receives the acknowledgment for frame 2. The encoder then starts
using the acknowledged frame 2 for prediction until the acknowledgment of
the next error-free reference frame is received. Note that since the erroneous
frame 3 is not acknowledged, it is never used for prediction and its errors do
not propagate to subsequent frames.
Note that during error-free transmission, the NACK mode is more e$cient
than the ACK mode since the most recent reference frame is used for pre-
diction. During erroneous transmission, however, the NACK mode results in
longer periods of error propagation than the ACK mode. Thus, the NACK
mode is more suitable if errors occur only rarely after long periods of error-
free transmission, whereas the ACK mode is preferred for highly error-prone
transmissions.