cross-layer design of raptor codes for video multicast over 802.11n mimo channels

Communication Systems & Networks© CSN Group 2015

Cross-Layer Design of Raptor Codes for Video Multicast over 802.11n MIMO Channels

Berna Bulut, Evangelos Mellios, Denys Berkovskyy, Farah Abdul Rahman, Angela Doufexi and Andrew Nix

Communication Systems & Networks. University of Bristol, UK

PIMRC 2015


Introduction• This work considered multicast video transmissions over WLANs.• WLAN unicast services do not scale to hundreds of fans, visitors or

customers in a contained area.• Multicast services are required, but these are fundamentally unreliable.• 802.11 provides reliable communications over unicast links via Automatic

Repeat Request (ARQ).• No standardized solution for reliable multicast. • Multicast packets are delivered as a simple broadcast service without

support for ARQ. Therefore, multicast transmission results in high packet loss.

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Introduction• Video applications cannot tolerate higher packet loss rate.• Application Layer Forward Error Correction (AL-FEC) codes are possible

mechanisms of providing reliable multicast delivery without the need for the return channels.

• This paper considered a cross-layer design based on the latest Raptor Q (RQ) codes for transmitting high data rate video over the MIMO channels in realistic outdoor environments.

• In channels with high spatial correlation Spatial Multiplexing (SM) results in high packet loss.

• To address this issue, we explore a combination of SM multicast transmission with RQ codes.

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Background: Raptor Codes• Raptor codes are a form of fountain codes.• Advantages:

- Rateless codes : can generate an unlimited number of encoded symbols from a fixed source block on-the-fly.

- Code rate can be adjusted dynamically according to channel conditions.

- Processing requirements increase linearly with source block length, k.• Disadvantages:

- Have a reception overhead: the decoder needs slightly more symbols than the original k symbols to recover the file with high probability.

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Cross-Layer Simulations: Scenario • This work is part of the AIYP project.• The Wi-Fi performance was evaluated in

our trial location using a geographic model of Bristol Zoo.

• A test user walking along the route was modelled.

• The APs, operating at 2.4 GHz, were placed 3m above ground level.

• The user terminal and APs use a 2x2 antennas.

5

SN

R (dB

)

10

15

20

25

30

35

40

AP2

AP1


Cross-Layer Simulations: Methodology

6

• A 3D ray-tracer was used to model the channel matrix H between the AP and each user location.• A novel Received Bit mutual Information Rate (RBIR) abstraction technique is used to estimate the

Packet Error Rate (PER) statistics.• The video simulator can model the transmission of any H.264 video sequence over the MAC and PHY

layers of 802.11n.

3D ray tracing for a very large

dataset

Spatially & polarimetrically convolve ray data with measured

antenna patterns

+

Determine received packet trace

Determine PER for all MCS modes

Wideband channel frequency responseBit error rate (BER)

Packet error rate (PER)Bit level simulatorSISO downlink in AWGN

channel

RBIRAbstraction technique based on

channel H matrices

Multicast video + RaptorQ simulator


Cross-Layer Design

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• Channel quality metric:

In a MIMO OFDM system the normalised channel capacity is given as

• A simpler channel quality metric which is the determinant of the channel matrix H (det () is used.

• The MIMO channel is characterised by the received SNR and H matrix determinant.

𝐶= 1𝑁 ∑

𝑛=1

𝑁

log2(det( 𝐼𝑁 𝑅+ 𝑆𝑁𝑅𝑁 𝑇

𝐻 (𝑛)𝐻 (𝑛)𝐻 ))


Cross-Layer Design

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• The aim of the cross-layer design process is that for given mean channel SNR and H matrix determinant, select the optimum transmission scheme (SM or STBC), MCS mode m and Raptor code rate CR (if SM is selected) that provide the minimum total transmission time ,with the constraint that .

subject to If , chose SM with otherwise, STBC

with the optimum m.


Cross-Layer Design: Parameters

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• A constant bit rate video sequence which consists of 1500 UDP packets was transmitted at 4 Mbps.

• RaptorQ source block length k=200 and symbol size T=1400 B.• Raptor code rate, CR, range: 0.• The transmission modes for an 802.11n 20 MHz channel profile (with a

400 ns GI) were used.• The peak rates: 144.4 Mbps for SM and 72.2 Mbps for STBC (with the

use of 64-QAM 5/6).• The application QoS requirement: <1%.• One Raptor symbol was placed into one UDP/IP packet.


10 15 20 25 30 3510

-3

10-2

10-1

100

SNR (dB)

PE

R

no RaptorCR=0.95CR=0.9CR=0.85CR=0.8CR=0.75CR=0.7CR=0.65CR=0.6CR=0.55CR=0.5

Results: UDP PER performance with Raptor

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• UDP PER performance of SM with respect to Raptor CR under different MIMO channel conditions: mean H matrix determinant 0.5 (left high correlation) and mean H matrix determinant 1 (right low correlation).

• Depending on the MCS, H matrix determinant and CR there would be as much as 8 dB improvement in the required SNR.

10 15 20 25 30 3510

-3

10-2

10-1

100

SNR (dB)

PE

R

QPSK 1/216-QAM 1/264-QAM 2/3

5 10 15 20 25 30 3510

-3

10-2

10-1

100

SNR (dB)

PE

R

10 15 20 25 30 3510

-3

10-2

10-1

100

SNR (dB)

PE

R

QPSK 1/216-QAM 1/264-QAM 2/3

5 10 15 20 2510

-3

10-2

10-1

100

SNR (dB)

PE

R

no RaptorCR=0.95CR=0.9CR=0.85CR=0.8CR=0.75CR=0.7CR=0.65CR=0.6CR=0.55CR=0.5


Results: Optimum MCS and CR for SM

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• Total transmission time versus SNR for H matrix determinant of 0.5 and corresponding optimum CRs.• For given mean SNR and H matrix determinant, the MCS mode and optimum CR pair that provide the

lowest total transmission time with the constraint that was determined.

5 10 15 20 25 30 35 400

0.5

1

1.5

2

2.5

SNR (dB)

Tota

l tra

nsm

issi

on ti

me

(s)

BPSK 1/2QPSK 1/2QPSK 3/416-QAM 1/216-QAM 3/464-QAM 2/364-QAM 3/464-QAM 5/6

5 10 15 20 25 30 35 400.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

SNR (dB)

Cod

e ra

te, C

R

BPSK 1/2QPSK 1/2QPSK 3/416-QAM 1/216-QAM 3/464-QAM 2/364-QAM 3/464-QAM 5/6opt code rate


Results: Achievable PHY rate with/without Raptor in SM

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• Raptor codes can provide quasi-error free data transmission at low SNR values even though the spatial correlation is high.

• Using Raptor AL-FEC enables higher MCS modes to be selected for data transmission.

• The benefits of Raptor codes are outstanding especially at low SNRs and high correlation conditions.

0 5 10 15 20 25 30 35 400

20

40

60

80

100

120

140150

SNR (dB)

PH

Y ra

te (M

bps)

with Raptor, H matrix determinant 0.5without Raptor, H matrix determinant 0.5with Raptor, H matrix determinant 1without Raptor, H matrix determinant 1


Results: Comparison of SM and STBC

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• Transmission efficiency in terms of total transmission time for STBC and SM with different Raptor codes.

• CR=1 represents the results without Raptor code.

• STBC provides higher transmission efficiency than SM with CR = 0.5 at each MCS mode. Therefore, in the case of MIMO switching the code rate range must be CR > 0.5.

• For higher MCS modes there is little performance difference (gain) that can be exploited by Raptor codes therefore the available CR range is decreased.

1 2 3 4 5 6 7 80

0.5

1

1.5

2

2.5

Mode

Tota

l tra

nsm

issi

on ti

me

(s)

SM, CR=1SM, CR=0.9SM, CR=0.8SM, CR=0.7SM, CR=0.6SM, CR=0.5STBC, CR=1


• Optimum MIMO/MCS mode that provides most efficient data transmission for given mean SNR and H matrix determinant.

• When the correlation is high, STBC is selected until the SNR values that enable higher MCS modes to be implemented in SM, e.g. the required SNRs are 36 dB and 28 dB for the mean H matrix determinant of 0.1 and 0.5 respectively.

• When correlation is low, e.g. mean H matrix determinant 1, the use of Raptor codes allows lower MCS modes such as QPSK 3/4 and 16-QAM 1/2 in SM scheme to be selected.

0 5 10 15 20 25 30 35 40 450

0.5

1

1.5

SNR (dB)To

tal t

rans

mis

sion

tim

e (s

)

0 5 10 15 20 25 30 35 40 450

0.5

1

1.5

2

2.5

SNR (dB)

Tota

l tra

nsm

issi

on ti

me

(s)

SM,H matrix determinant 0.1SM,H matrix determinant 0.5SM,H matrix determinant 1STBC,H matrix determinant 0.1STBC,H matrix determinant 0.5STBC,H matrix determinant 1

5 10 15 20 25 30 35 400

0.5

1

1.5

2

2.5

SNR (dB)

Tota

l tra

nsm

issi

on ti

me

(s)

BPSK 1/2QPSK 1/2QPSK 3/416-QAM 1/216-QAM 3/464-QAM 2/364-QAM 3/464-QAM 5/6

Results: Comparison of SM and STBC


Results: Case study

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0 100 200 300 400 500 6000

5

10

15

20

25

30

35

40

45

SN

R (d

B)

Time (sec)

0 100 200 300 400 500 6000

5

10

15

H m

atrix

det

erm

inan

t

SNR

H matrix determinant

0 200 400 600 800 1000 12000

50

100

150

time slot

PH

Y ra

te (M

bps)

SM no RaptorSM with RaptorSTBC

0 200 400 600 800 1000 12000

50

100

150

time slot

PH

Y ra

te (M

bps)

STBC and SM STBC and SM with Raptor

• In areas with low SNR and high spatial correlation STBC shows better performance than SM. SM fails to provide service without Raptor codes (PHY rate is zero).

• SM takes advantages of Raptor codes, especially when correlation is low, and provides higher transmission efficiency, i.e. the overall channel occupancy time is less than the system without Raptor codes.


Conclusion• This paper presented a cross-layer design approach which considers Raptor

codes in the SM system as a means to enhance the reliability and transmission efficiency for transmitting high data rate video.

• A cross-layer simulator was used to investigate the optimum system parameters under different channel conditions and evaluate the performance in realistic outdoor environments.

• The algorithm implements mean SNR and H matrix determinant to adapt the system to changing channel conditions.

• We have shown that with the use of Raptor codes SM provides robust video transmission in harsh channel conditions and makes use of higher MCS modes when channel conditions are good therefore increases the overall transmission efficiency.

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cross-layer design of raptor codes for video multicast over 802.11n mimo channels

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