adaptive modulation and coding method

8/2/2019 Adaptive Modulation and Coding Method

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(12) United States PatentYang et al.

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(10) Patent No.:(45) Date of Patent: US 7,889,703 B2F e b . 1 5 , 2 0 1 1

(54) ADAPTIVE MODULATION AND CODINGMETHOD

(75) Inventors: Sung-Kang Yang, Taoyuan (TW);Ta-Sung Lee, Hsinchu (TW)

(73) Assignee: Mediatek Inc., Hsin-Chu (TW)Subject to any disclaimer, the term of thispatent is extended or adjusted under 35U.S.C. 154(b) by 699 days.

(21) Appl. No.: 11/273,144

( *) Notice:

(22) Filed: Nov. 14, 2005(65) Prior Publication Data

US 2007/0110002 Al May 17, 2007(51) Int. Cl.

H04B 71216 (2006.01)(52) U.S. Cl. 370/335; 370/204; 370/205;

370/212; 370/230; 370/232; 370/395.21(58) Field of Classification Search . 370/204,

370/205,212See application file for complete search history.

(56) References CitedU.S. PATENT DOCUMENTS

6,012,161 A * 112000 Ariyavisitakul et al. 714/7956,631,127 Bl * 10/2003 Ahmed et al 370/349

2003/0118031 Al * 6/2003 Classon et al. 370/395.54

210

2007/0076810 Al * 4/2007 Herrera et al. 375/261FOREIGN PATENT DOCUMENTS

TW 200302642 8/2003OTHER PUBLICATIONS

Liu et al., "Cross-layer modeling of adaptive wireless links for QoSsupport inmultimedia networks," Oct. 2004, Proceedings of the FirstInternational Conference on Quality of Service in HeterogeneousWiredlWireless Networks, pp. 68-75, IEEE Computer Society.*Liu et al., "Cross-layer modeling of adaptive modulation and codingwith truncated ARQ over wireless links," Sep. 2004, IEEE transac-tions on wireless communications, vol . 3, No.5, pp. 1746-1755,IEEE Computer Society. *English abstract of TW200302642, pub. Aug. 1,2003.* cited by examinerPrimary Examiner-Daniel J RymanAssistant Examiner-lae Y Lee(74) Attorney, Agent, or Firm-Thomas,Horstemeyer & Risley

Kayden,

(57) ABSTRACTAn adaptive modulation and coding (AMC) method for datatransmission by various modulation orders and coding rates isprovided. A signal-to-noise ratio (SNR) of a transmissionchannel is estimated. Quality of service (QOS) parameters areprovided, defining maximum allowable delay and packeterror rate (PER). A retransmission limit is determined fromthe maximum allowable delay. An optimum packet length isdetermined based on the QOS parameters and the SNR. Amodulation order and a coding rate most suitable for theestimated SNR and the defined PER are selected based on theoptimum packet length and the retransmission limit.

7 Claims,S Drawing Sheets

r-MAC layer

,212 _c_202 ,204QOS packetparameter scheduler length/retransmissionlimit

SNR,220

(208 (206PHY layer modulator thresholdtable


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provide SNR and QOS parameters

(504 (506"fonnaMAClayer packetbased on theSNRandQOSparameters

establish athreshold tableaccording tothe given PERlimit

"form a physical layer packet by amodulation order and coding rateselected based on the threshold tableandSNR

FIG. 5


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US 7,889,703 B21

ADAPTIVE MODULATION AND CODINGMETHOD

BACKGROUNDThe invention relates to adaptive modulation and coding

(AMC), and in particular, to a cross layer AMC method com-bining media access control (MAC) and physical (PHY) lay-ers to enhance system performance.AMC provides the flexibility to match the modulation-

coding scheme to the average channel conditions for eachuser. With AMC, the power of the transmitted signal is heldconstant over a frame interval, and the modulation and codingrate is changed to match the current received signal quality orchannel conditions. Different order modulations allow tran- 15si tion of more bits per symbol, thus, higher throughputs andbetter spectral efficiencies are provided. When using a modu-lation technique such as QAM64, however, better signal-to-noise ratios (SNRs) are needed to overcome any interferenceand maintain a certain bit error ratio (BER). The use of adap- 20tive modulation allows a wireless system to choose the high-est order modulation depending on the channel conditions, asrange increases, to step down to lower modulations. Con-versely, a closer target can utilize higher order modulationssuch as QAM for increased throughput. Additionally, adap- 25tive modulation allows the system to overcome fading andother interference.FIG. 1 shows the environment for adaptive modulation and

coding (AMC) scheme. In a system withAMC, users close tothe transmitter 100 are typically assigned higher order modu _ 30lation with higher coding rates (e.g. QAM64 with R=3/4turbo codes), but the modulation-order and/or coding rate willdecrease as the distance from the transmitter increases, suchas QAMI6, QPSK and BPSK.The implementation of AMC offers several challenges.

AMC is sensit ive to measurement error and delay. In order toselect the appropriate modulation, the scheduler must beaware of the channel quality. Errors in the channel estimatewill cause the scheduler to select the wrong data rate ortransmit at too high a power, wasting system capacity, or tooIowa power, raising the block error rate. Delay in report ingchannel measurements also reduces the reliability of thechannel quality estimate due to the constantly varying mobilechannel. Changes in the interference also add to the measure-ment errors.

SUMMARYAnexemplary embodiment of an adaptive modulation and

coding (AMC) method for data transmission by variousmodulation orders and coding rates is provided. A signal-to-noise ratio (SNR) of a transmission channel is estimated.Quality of service (QOS) parameters are provided, definingmaximum allowable delay and packet error rate (PER). Aretransmission limit is determined from the maximum allow-able delay. Anoptimum packet length is determined based onthe QOS parameters and the SNR. A modulation order and acoding rate most suitable for the estimated SNR and thedefined PER are selected based on the optimum packet lengthand the retransmission limit.A MAC layer packet can be formed with the optimum

packet length, and a physical layer frame comprising theMAC layer packet can be formed based onthe selected modu-lation order and the coding rate.The coding rate is utilized in a forward error correction

algori thm conforming to the Reed-Solomon or LDPC stan-

2dard. The modulation order is one of the BPSK, QPSK,QAM16 and QAM64 modulation standards.Optimum relationships between SNR and PER for each

modulation order may be predetermined. A threshold table5 can therefore be established by substituting the defined PERinto the optimum relationships, in which a plural ity of sec-tions are generated each defining a SNR range correspondingto amodulation order and a coding rate. The modulation orderand coding rate selection is performed by searching the

10 threshold table.DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way ofexample and not intended to limit the invention solely to theembodiments described herein, will best be understood inconjunction with the accompanying drawings, in which:FIG. 1 shows the enviroument for adaptive modulation and

coding (AMC) scheme;FIG. 2 shows an embodiment of the cross layer AMC

procedure;FIG. 3 shows a transit ion diagram of SNR versus PER;FIG. 4 is a threshold table corresponding to a given PER;

andFIG. 5 is a flowchart of the cross layer AMC method.

DESCRIPTIONA detailed description of the invention is provided in the

following.FIG. 2 shows an embodiment of the cross layer AMC

procedure. Typically, a plurality of MAC layer packets areformed in the MAC layer 210 and packed in a physical layer220 for transmission. Inthe MAC layer 210, a QOS parameter

35 212 defines various essential conditions to form the MAClayer packet. For example, requirements for maximum allow-able delay, throughput, bit error rate (BER), and packet errorrate (PER) are defined in the QOS parameter 212. A signal-to-noise ratio (SNR) of a transmission channel is estimated

40 and input to a scheduler 202, and an optimal packet length andretransmission limit 204 are calculated in the MAC layer.A retransmission limit can be determined by equating the

worst-case delay to the maximum allowable delay, where theworst-case delay is (Rj-l ) (TDATA+TAcK+2TsIFs), and the

45 maximum allowable delay D can be given by:(1)

Where R is the retransmission limit, and TDATA' TACK andTsIFS are transmission time for data, acknowledge (ACK)50 packets and short inter frame space (SIFS) intervals accord-

ing to the IEEE 802.11 standard. For an N channel decoder,the maximum buffer delay is NxD. Assume a frame size of2000 bytes with R equal to 5, the maximum allowable delayDis 6.876 milliseconds when utilizing QAM64 modulation.

55 The value is within acceptable ranges for non-interactivevideo streaming applicat ions, which is between 1 to 10 mil-liseconds.In the scheduler 202, the packet length and retransmission

60 limit are calculated from the QOS parameters and the SNR,and are then sent to the physical layer 220 to form the physicallayer frame. The packet length is determined by calculatingmaximum acceptable throughput nnder bonnded PERrequirements. Assume a packet with L bytes payload istrans-

65 mitted, the probability of successful transmission isexpressed as:

Por(1-P err ,da,a)(l -P err ,ack) (2)


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3US 7,889,703 B2

4where POK is the successful packet transmission probabil-

ity, Perr data is data packet error probability, and Perr ack isACK e{yor probability. The total successful transmissionwithin transmission limit R, Psucc' comprises POK+P oK (I -P OK )+ PO K(I-P OK )2 + ... P OK (I-P OK t-l.The probability that a packet is successfully transmitted atthe n-th transmission, among the total successful transmis-

sion, PnlSUCO can be expressed as:[PO~I-POK)"-l]lPsucc

Therefore the average time taken for the PnlSUCC is:

RTAV,SUCC = P nls uc c.L ( iT ba d,i + Tgood,i)

i=O

where Tbad'; is the time taken for a failed cycle, and Tgood';is the time taken for a successful cycle. Also the average timetaken for a fai led transmission can be derived as:

RTAv.FAlL= .L (Thad.;)

i=O

Thus, the average transmission cycle for the L bytes packetwithin transmission limit R is given by

1 - PscccTAV = ---(TAV suee + TAvFAIL!Psuee' .

The effective throughput can then be calculated by:

where La is the total data length required to transmit apacket, comprising the L bytes payload, cyclic redundancycheck (CRC) codes, and forward error correction (FEC)codes. The KIN is coding rate. The constant 8 transfers thethroughput from byte units to bit units. By substituting theworst case delay (R+ I) (TDATA+TAcK+2TsIFs) as the TAn theoptimum packet length associated with the coding rate can beestimated.In the physical layer, a modulator 208 selects a modulation 50

order according to the information provided from the packetlength and retransmission limit 204. Specifically, an optimalmodulation order and coding rate most suitable for the esti-mated SNR and the defined PER are selected, based on thepacket length and the retransmission limit determined by the 55scheduler 202. The coding rate is utilized in an error correc-tion algorithm conforming Reed-Solomon or LDPC stan-dard, and the modulation order is one of the BPSK, QPSK,QAMI6 and QAM64 modulation standards.In the physical layer, a threshold table 206 is established for 60

modulation order selection. The threshold table 206 providesoptimum relationships between SNR and PER for eachmodulation order. When receiving the defined PER from thepacket length and retransmission limit 204, the thresholdtable 206 establishes a threshold table by substituting the 65defined PER into the optimum relationships, in which a plu-rality of sections are generated each defining a SNR range

(3)

(4)

(5)

(6)

(7)

corresponding to a modulation order and a coding rate. Themodulator 208 thus looks up the threshold table to determinewhich modulation order and coding rate to use.FIG.3 shows a transition diagram ofSNR versus PER. The

optimum relationships between SNR and PER for eachmodulation order, can be expressed in the form:

PERn(y)~1 ifO


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6selecting a modulation order from a plurality of predeter-mined modulation orders and a coding rate from a plu-rality of predetermined code rates based on the optimumpacket length and the retransmission limit, wherein theselected modulation order and the selected coding rateare most suitable for the estimated SNR and the definedPER.

2. The AMC method as claimed in claim 1, further com-prising:forming a MAC layer packet with the optimum packet 10length;

forming a physical layer frame comprising the MAC layerpacket based on the selected modulation order and thecoding rate.

3. The AMC method as claimed in claim 1, wherein the 15coding rate is utilized in a forward error correction algorithmconforming to the Reed-Solomon or Low Density ParityCheck (LDPC) standard.4. The AMC method as claimed in claim 1, wherein the

modulation order is one of the BPSK, QPSK, QAM16 andQAM64 modulation standards.5. The AMC method as claimed in claim 1, further com-prising:deriving optimum relationships between SNR and PER for 25each of the plurality of predetermined modulationorders;

establishing a threshold table by substituting the definedPER into the optimum relationships, in which a plurality

of sections are generated each defining a SNR rangecorresponding to a modulation order and a coding rate,wherein

the step of selection is performed by searching the thresh-old table.

6. The AMC method as claimed in claim 5, wherein theoptimum packet length is determined by:deriving transmission successful probability within theretransmission limit from the PER;

calculating an average transmission time consumed tocomplete a successful transmission based on the trans-mission successful probability within the retransmissionlimit;

calculating an effective throughput from total payloadstransmitted during the average transmission time; and

determining the optimum packet length based on the effec-tive throughput.

7. The AMC method as claimed in claim 5, wherein theoptimum relationships between the SNR and PER are derived

20 by an approximation function:

P E Rn (y )~ an ex p( -g nY ) if Y> ~Y pnwhere n is the mode index, a value associated with themodulation order and coding rate;

Yis the estimated SNR; andan' gn and Yp n are mode dependent constants.

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adaptive modulation and coding method

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