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WiMax MBS Power Management, Channel Receiving and

Switching Delay Analysis

Thawatt Gopal (Huawei Technologies USA)10180 Telesis Court, Suite 365, San Diego, CA 92121, USA

[email protected]

Abstract In Wimax MBS (Multicast-Broadcast System), the

power savings and channel switching delay are two critical

performance metrics that affects the viewing time capacity and

quality of experience of the end-user. The amount of air-interface

resources allocated for MBS, power savings targeted, channel

receiving/switching delay target, multimedia stream data rate are

all inter-related. This paper analyzes the interactions of these

parameters for Wimax MBS and quantifies the impact of key

parameters on the desired power savings ratio and channel

switching delay.

Keywords MBS, Wimax, MBS Power Management, MBS

Channel Switching

I. INTRODUCTION

There are several key performance metrics for MBS

(Multicast-Broadcast System) type of services. This includes

the areas of power management, channel switching delays,

RF coverage in a SFN (Single Frequency Network),

application layer throughput and packet error rate. This paper

covers the areas of power management and channel switching

delay.

For power management, we make use of models originallydeveloped for the DVB-H system and apply them to a Wimax

MBS system. There are certain similarities between DVB-H

and Wimax MBS namely both uses OFDMA based air-

interfaces and the concept of time-slicing introduced in DVB-

H systems generally applies to Wimax MBS also. Time-

slicing was introduced in DVB-H to prolong the terminal

battery life whereby if the multimedia stream rate was Rc,

then the air-interface burst bitrate is MRc where M is a

function of the supported air-interface capacity and targeted

power savings percentage. This allows the mobile station to

save power while not receiving MBS data that it has not

subscribed or wants to receive and allows the multiplexing of

several multimedia streams that can be decoded

independently in time. Several differences to note between

Wimax MBS and DVB-H are that Wimax supports

simultaneous unicast and MBS traffic in the downlink frame.

It is also possible to multiplex several multimedia streams in

the same MBS Zone in a Wimax frame. The MBS Zone is the

portion of the Wimax frame that can be allocated for MBS

data. The amount of bandwidth allocated determines how

many MBS streams can be supported for a given power

savings target.

The switching delay is an important metric that is

frequently compared for MBS type of services related to

watching broadcast video. The longer the time between data

bursts for the same MBS stream, the better the power savings

benefit whilst the channel switching performance degrades.

II. POWER MANAGEMENTIn terms of power management, a model incorporating the

ON, OFF and Synchronization time (Tsync) provides target

values of the power savings percentage. The Synchronization

time is the amount of time required for the mobile device to

wake-up from sleep and get ready to start demodulation and

decoding of the MBS data stream. Contributions to [1] from

mobile manufacturers suggest that Tsync should be assumed in

the range of 200 to 250ms with power savings target of 90%

but here we investigate a range of power savings target.

For a range of existing handheld device battery capacity,

we provide several points of references. The HP iPAQ 210,

has a relatively large battery capacity of 2200 mAh rating

with features targeted at mobile computing and according totests done in [3] for viewing streaming video it has a battery

life of 3 hours and 28 minutes. Another example is the Nokia

E61 (targeted at the mobile computing application) has a

battery capacity of 1500 mAh while a simpler device like the

Motorola RAZR v3 has a battery life of 710 mAh. Another

point of reference from Media Flo [4] indicates a target video

viewing time of 3.9 hours for video rates of 360 kbps with a

850 mAh battery capacity assuming some form of power

management/bursting technique is used while the test results

for the HP iPAQ does not consider such technique.

In DVB-H, an OFDMA frame is used entirely for

broadcast services while in Wimax MBS the MBS burst

(specified by number of OFDMA symbols) occupies a

portion of the OFDMA frame called the MBS Zone. For a

given power savings percentage target, given average stream

bitrate, we want to find out what is the required burst bitrate.

The following diagram and equations shows the power saving

model used in this paper for Wimax MBS which is adapted

from the model used for DVB-H in [1] and [5].

978-1-4244-2517-4/09/$20.00 2009 IEEE

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t

OFDMA Frame i OFDMA Frame K + i

MBS Burst Stream j MBS Burst stream j

Downlink

subframe

Uplink

subframe

Burst Size

Tb Toff Tsync Tb

t

Burst duration

Ton

MBS Zone

Fig.1: Wimax MBS Power Saving Model

)1(100+

=

offon

offs

TT

TP

)2(syncbon TTT +=

)3(offon

bc

TT

BR

+=

)4()1( hR

BT

b

bb

=

)5(100

)1(

11100

)1(

+

=

+

=b

sync

b

c

c

b

sync

b

b

c

b

B

T

hR

R

R

B

T

hR

B

R

B

Ps

Here Ps is the power savings in percentage, Ton is the time the

receiver is turned on, Toff is the time the receiver is off, Tsync

is the synchronization time required to transition from the Off

to On state, Rc is the average stream bit rate of a MBS stream,

Rb is the MBS Zone burst bitrate, h is the 802.16e protocol

overheads for an MBS stream and Bb is the burst bit size.

Note that Rc for a multimedia stream represents the sum of

the composite audio and video streams plus the outer layer

FEC redundancy information. Rb is also important for MBS

radio resource allocation since it determines how many MBS

streams can be supported. For example, in the case of N CBR

(constant bitrate) encoded video streams with individual

stream rates of RC,i the relationship with Rb is shown below:

)6(

1

, )1( hRR bN

i

ic ==

The stream data rate Rc required to provide sufficientvideo quality is dependent on the characteristics of the

source. For example, for sports channel on a QCIF or QVGA

display, the average bitrate for H.264 encoded video should

be a minimum of 256kbps, without other lower-layer protocoloverheads and FEC overhead. For news and weather channelthe video can be encoded at an average bitrate of 128 kbps,

applications like audio streaming can be delivered using

codecs like AAC with very good quality at 128 kbps and

applications like stock information broadcast may only

require 32 to 64 kbps.

From equation 5, the ratio of Rc/Rb is a critical component

affecting the power savings hence Rb needs to be allocated

sufficiently for a given Rc to achieve the targeted power

savings percentage. Rb is dependent on the spectral efficiency(varies according to modulation/coding scheme, cell-size in

SFN) and on the number of OFDMA symbols allocated for

the MBS Zone. The other parameter affecting the power

savings is the ratio (RcTsync/Bb) which incorporates the impactof Tsync. Bb is dependant on the number of OFDMA symbols

allocated and the MCS used.

To compute Rb and Bb, we rely on the following

information. The Modulation Coding scheme and spectralefficiency assumptions are listed in Table 1 while the system

parameters are listed in Table 2. Note that for 1/4 guardinterval, there would only be 40 OFDMA symbols available

for data instead of 44 symbols for 1/8 guard interval.

Table 1: MCS and Spectral EfficiencyMCS Number of Information bits in 1 PUSC Slot

QPSK (CTC) 1/2 48

16 QAM (CTC) 1/2 96

Table 2: SYSTEM PARAMETERS

Parameter Value

Permutation Type PUSC

BW 10 MHz

FFT Size 1024

Cyclic Prefix 1/8

OFDMA Frame Length 5ms

Symbol Length 102.56

Data Carriers 720

Pilot Carriers 120

The following equation captures the relationship between

the spectral efficiency, number of OFDMA symbols and

Wimax TDD frame duration to derive Rb.

)7(frame

MBSPUSCbitb

T

NNNR

=

Nbit is the number of information bits per PUSC slot, NPUSC isthe number of PUSC slots per OFDMA symbol, NMBS is the

number OFDMA symbols allocated for the MBS Zone perframe and Tframe is the duration of a Wimax TDD frame. We

assume the subcarrier permutation used corresponds to PUSC

all subcarriers and NMBS must be assigned in increments of

two OFDMA symbols since one PUSC slot spans twoOFDMA symbols per 802.16e standard.

As an example, assuming PUSC all subcarriers with 10

MHz bandwidth, there are 30 PUSC slots in 2 OFDMA

symbols for data. For 44 OFDMA symbols in one 5ms frame,MCS corresponding to QPSK (CTC) 1/2, the data rate

corresponds to (48 x 15 x 44 / 0.005) 6.336 Mbps or 31680

bits (3960 bytes) per 44-symbol OFDMA frame. Based on

the number of OFDMA symbols allocated for the MBS Zone,we can compute the burst size (Bb) and the MBS Zone data

rate (Rb) that is used in the power savings computations.

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40

50

60

70

80

90

100

64 128 192 256 320 384 448 512

M B S S trea m B itrate, R c (kbp s)

Powe

avgseete,s

)

Bb 921 kbit, Tb = 0.425 s, 16 sym bol Zone, Q PSK , 1/8 G I



Fig.2: Power Savings Percentage versus Stream Data Rate (16

symbols/frame MBS Zone, QPSK CTC 1/2, Tsync 200ms, h 0.05)

65

70

75

80

85

90

95

100

64 128 192 256 320 384 448 512

M B S S trea m B itrate, R c (kbp s)

Powe

a

gse

tes%)

Bb 1843 k bit, Tb = 0.425 s , 16 sym bol Zone , 16 Q AM , 1/8 G I

Bb 3686 k bit, Tb = 0.845 s , 16 sym bol Zone , 16 Q AM , 1/8 G I

Bb 921 kbit, Tb = 0.215 s, 16 sym bol Zo ne, 16 Q AM , 1/8 G I

Fig.3: Power Savings Percentage versus Stream Data Rate (16symbols/frame MBS Zone, 16 QAM CTC 1/2, Tsync 200ms, h 0.05)

The figures above provide examples of the required

resources. For stream data rates from 384 to 512 kbps usingQPSK, even larger burst sizes (Bb) are not sufficient to

provide power savings above 80%. For such scenarios, use of

higher order modulations such as 16 QAM will be required

but the ability to support 16 QAM will be limited to smallercell-sizes due to the higher required C/I.

III. CHANNEL RECEIVING DELAY TARGETS

The next step is to describe the relationship between thechannel switching delay and the power saving percentage.

Here, we make use of the model from [5]. For the receiving

delay, the average delay consists of two components namely

the delay until the start of the burst and another delay untilreception of the burst. The following equations describe the

relationship:

[ ]

)8()1(2

1

2

1

2

1

2

1

syncb

b

c

bsyncb

c

b

ononc

bononOffonR

ThR

B

R

BTT

R

B

TTR

BTTTTD

+

+=++=

+

+=++=

Combining the results from equations 5 and 8, we obtain:

)9(1002)1()(

11

)1(

1

2

1

1)(

)1(

11

100)1(

11

+

=

+

+

=

+

=

sync

B

Rc

syncR

Bc

syncR

sync

B

b

sync

bc

T

hRDR

TD

RhR

TD

T

hRRc

BT

hRRPs

The following figure provides an example of the power

savings percentage versus the average receiving delay. Here

we consider a 16 symbol/frame MBS Zone with MCScorresponding to QPSK CTC 1/2. Note that the burst durationis 0.635 seconds corresponding to a burst size of 1.448 Mbit.

40

50

60

70

80

90

100

0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4

A verage R eceiving D elay, D R (sec.)Poweravigserentge,s)

R c 128 kbps, Rb 2.3 M bps, 16 sym bol Q PS K 1/2, 1/8 G I




Fig.4: Wimax MBS Power Savings Percentage versus Receiving Delay

IV. CHANNEL SWITCHING DELAY TARGETS

When the user switches channel from one MBS stream toanother MBS stream, besides the delay to wait until the next

burst on the target MBS stream, there are other additional

delays that needs to be considered in order for the user to be

able to view and hear the audio/video stream.

Fig.5: Mean Opinion Score versus channel change time study results

from [7]

A useful reference to note is a presentation [7] made to

ITU on IPTV Channel Zapping MOS evaluation. The figure

above illustrates the relationship between MOS and ChannelZapping time for IPTV. We also note for comparison,

performance targets from [4] indicate a channel switching

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target of 2 seconds although we view this target should be

less than 2 second per results from [7]. Note that we compare performance for Wimax MBS with IPTV since we assume

RTP/UDP/IP as the protocol to transport the media streams.

The receiving delay that was modeled in the previous

sections accounts for variables like the MBS stream data rate, burst data rate and MBS Zone resource allocation size.

However, there are other delays related to the video decoding

process and audio/video synchronization that needs to be

accounted for during channel switching. This sectionprovides an overview and background of these delays.

The I-frame rate impacts several key performance metrics

namely the effective video compression ratio and the channel

switching delay. For H.264 Level 1.2 video codec beingconsidered for Wimax MBS, only I and P frames are used.

The I-frame size is several times the size of P framesdepending on content. Increasing the I-frame rates will

reduce the compression efficiency while improving thechannel switching delays. Note that the terms I-frame and

IDR (Instantaneous Decoder Refresh) are used

interchangeably in this paper.If we consider an example consisting of two group of

pictures (GOP) namely I1P11P12P13P14P15P16P17P18P19P110 and

I1P11P12P13P14P15I2P21P22P23P24P25, it is clear that the second

GOP will consume a larger bandwidth while having a smaller

switching time since the frequency of the I-frames is higher,hence on average a user switching to this channel will

experience a shorter delay to receive the I-frame which also

serve as a Random Access Point (RAP). The I-frame is

required to decode the subsequent P-frames within a GOP,hence even if the user switches to another MBS channel it

will need to wait for the I-frame first before it can begindecoding and viewing that MBS stream.

From the video encoding part, the video encoder typicallyuses a bit budget to encode a video stream, hence if moreIDR frames are used, then this will reduce the bit budget

allocation to the P frames and subsequently reduce the video

quality at the expense of a smaller channel switching delay.

For Wimax MBS applications that are similar to IPTV i.e.

there is an audio and video stream that are synchronized, the

delay between initial tuning into the new channel and

rendering the content of the new channel consists of thesedelay components [6]:

Client stream buffering (network de-jitter buffer,video input buffer)

Random access point acquisition Audio-video synchronization Mechanisms to minimize impact of packet-loss (FEC) Key acquisition times in case of scrambled content

Next, we highlight the differences, optimizations andassumptions for the above delay components in Wimax MBS

in comparison to wireline IPTV.

A. Client/MS Stream Buffering (DBuf):

The terminal/MS needs to buffer incoming packets first, before forwarding to the video decoder to avoid under-run

(due deviation in the arrival time of packets from the relative

presentation timing), deliberately introduced jitter from the

source, traffic smoothing/shaping and to compensate fornetwork introduced jitter.

For Wimax MBS, this delay is specifically for videostreams and occurs during the time the client/MS fills its

buffer before starting to render the video. For IPTV, thisdelay is in the range of 0.5 to 2 seconds from [6]. Here, we

assume a minimum of 0.5 seconds delay is sufficient for

mobile terminals.

B. Random Access Point Acquisition (DRAP):

The I-frame contains information that is necessary to

decode the complete frame while the P-frames are

differentially encoded and depends on the previously

transmitted I-frames in order to decode. For reference, in3GPP MBMS, the recommended range for the I-frame

periodicity is from 500 ms to 4 seconds while current digitalTV deployments over cable and satellite use a typical valueof 600ms, corresponding to a GOP length of 15 frames at 25fps [6]. We assume for Wimax MBS, the I-frame rate is

1000ms corresponding to a GOP length of 15 frames at 15

fps. In a corresponding wireline IPTV system, this would

lead to an additional average RAP delay of 500ms.

I P P P P P P P P P P

P P P P P I P P P P P

Time

Channel Switch Occurs Here

Channel A

Channel B

Random Access Point Delay

One MBS Burst

Fig.6: Channel switching between streams and Random Access Point

Delay

However, in a Wimax MBS system, this depends on theMBS burst scheduling frequency for a particular stream. The

figure above shows the equivalent IPTV RAP delay and also

depicts an example whereby the I-frame is contained within

one MBS burst transmission. From the example shown inFigure 4, the burst duration equals to 0.635 seconds and the

total burst size spanning multiple OFDMA frames equals to

1448178 bits. If we assume 15 fps video stream, an I-frame

average size of 2.2 kbyte and P-frame average size of 1.2kbyte (assumption from [8]), this GOP average size would be19456 bits. Since the total MBS burst size is a huge multiple

of this number (e.g. 74 times average GOP size), one MBS

burst will contain several GOP hence there will be more thanone I-frame in the MBS burst. Hence, for Wimax MBS we

dont need to account for an additional delay for the RAP

delay in addition to the receiving delay as long as the

following constraint ( 1I

N ) is met where NI is the

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number of I-frames in a MBS burst. Based on the power

savings requirement, this requirement is easily met since weneed to schedule multiple GOP in a MBS burst to meet the

power savings target e.g. Bb contains multiple GOP.

C. Synchronization Between RTP Streams (DRTPSR):

When Wimax MBS is used to send video streams, weassume RTP streaming is used. With RTP streaming, the

audio and video is sent as separate RTP/UDP/IP streams.RTCP sender reports (SR) is required for synchronizing the

audio and video streams and at least one RTCP SR has to beacquired for each stream to allow synchronized playouts of

the streams. Some optimization is possible such as sending

the RTCP SR when the I-frame is sent, which is our

assumption here. Based on the similar constraint described

previously ( 1I

N ) for DRAP, we do not need to account

for additional delays for this component.

D. Packet Loss Handling/FEC (DAL-FEC):

We assume application FEC is used for the Wimax MBS

(e.g. such as Raptor FEC). The FEC is applied to a block of

packets to minimize the overhead and increase theinterleaving depth. Packet loss is compensated by

additionally transmitted FEC redundancy. To use the MBS

data, decoding needs to be delayed until all packets of anapplication-layer FEC block are received. Here we assume an

application layer FEC block of 0.5 Mbit is used. Based on the

example in Figure 4 Bb is 1.44 Mbit which would not cause

additional delay due to this component. Generally, as long as

FECALbBB

(hence FECALb DT ) where BAL-FEC is the

application layer FEC block size, we do not need to accountfor this delay component in the overall switching delay.

E. Encryption Key Acquisition (DENC)::

If the MBS content is encrypted, then the decryption keys

must be acquired. There is an additional delay introduced bythe key management if the key for the initial I-frame does not

arrive during the client buffering and random access point

acquisition procedures. Here, we assume the encryption key

acquisition to occur in parallel and within the MBS burstduration. If DENC is greater than the MBS burst duration, then

we need to account for this component delay.

In summary, the total switching delay consists of many

components and is summarized by equation 10 below:

)10(),,,,max()( BufENCFECALRTPSRRAPBBRS DDDDDTTDD ++=

Based on our previous assumptions and optimizations for thevarious delay components, equation 10 would become:

)11(BufRS DDD +=

We note from the literature such as [6] optimization

techniques that have been explored in the wireline IPTVdomain that can also be adapted for Wimax MBS at theexpense of more complexity and/or additional required

bandwidth. These techniques include:

Using a separate tune-in stream (with higherfrequency of I-frames) in parallel to the main stream

Using a low resolution tune-in stream and let deviceup-sample tune in stream

Predictive tuning (client needs to receive and bufferpredicted channels)

Use of a channel change tune-in server located in theAccess Network.

VII. CONCLUSIONS

In this paper, we identified key parameters impactingpower management and channel switching issues for WimaxMBS services. Quantitatively, we evaluate the impact

between the target power savings percentage, MBS Zone

resource allocation and channel receiving delay. We alsoanalyze and quantify the impact of various delay components

besides the receiving delay that needs to be accounted for

when switching video channels based on characteristics of

video decoding procedures at the client/MS. A summary ofoptimization techniques from the wireline IPTV domain that

can be applied to Wimax MBS were presented also.

Optimization techniques to reduce the channel zapping delay

are necessary in order to provide acceptable end-userexperience for viewing video over Wimax MBS. Our analysis

indicates the channel switching delay will be greater than 2seconds if we target a power savings target of at least 80%

with stream data rates between 256 kbps and 384 kbps withan MBS bearer rate of 2.3 Mbps using QPSK (CTC) 1/2

modulation coding scheme. The other option would be to use

16 QAM but this would be limited to smaller cell-sizes.

REFERENCES

[1] ETSI, DVB-H Implementation Guidelines ETSI standard, TR102.377, V1.2.1, 2005

[2] Air Interface for Fixed and Mobile Broadband Wireless AccessSystems,IEEE Std 802.16e, February 2006[3] HP iPAQ 210 Enterprise Handheld Review, Adama D. Brown,http://www.brighthand.com/default.asp?newsID=13810&review=HP+iPAQ+210+Enterprise+Handheld#Battery

[4] Qualcomm White Paper, FLO Technology Overview,www.qualcomm.com

[5] Rezaie, M., Bouaziz, I, Vadakital, V.K.M, Gabboui, M, OptimalChannel Changing Delay for Mobile TV Over DVB-H, IEEEInternational Conference on Portable Information Devices, Pg. 1-5,May 25 - 29 2007

[6] Fuchs, H., Farber, N, Optimizing channel change time in IPTVApplications, IEEE International Symposium on BroadbandMultimedia and Broadcasting, 31 March 02 April 2008.

[7] Admed Kamal, TNO, ITU-T IPTV Global Technical Workshop, Seoul,Korea, Oct. 13, 2006.

[8] Wang, J., Venkatachalam, M. and Fang, Y., System Architectureand Cross-Layer Optimization of Video Broadcast over Wimax, IEEE

Journal on Selected Areas of Communications, Vol. 25, No.4, May2007.

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