CEFRIEL

Deliverable R4.1.5

MAIS adaptive and reconfigurable modem

Giovanni Paltenghi

Roma – 24 Novembre 2005

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Table of Contents

MAIS Adaptive modem architecture

The Supervisor algorithm

Implemented simulator

Simulation results

Future work

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MAIS Adaptive modem architecture

Approach: introduction in 802.16-2004 (WiMAX) baseband standard of a new architectural element (supervisor, SPV) performing optimization procedures

• minimization of the transmitted power for a negotiated level of Quality of Service (appropriately translated at PHY layer)

SPV input• from MAC: Target BER and Target

Bit-Rate (QoS parameters at PHY layer)

• from PHY: channel state information (the set of the channel power gains |Hi|2)

• access to internal look-up tables containing maximum bit-rates and error correction code gains for every RateID in AWGN channel

SPV output• to PHY: RateID, the number and

the position of the ON SCs • to MAC: actual bit-rate and BER

achieved

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WiMAX

channel model

RateID

Modulation

Coding rate

0 BPSK 1/21 QPSK 1/22 QPSK 3/43 16-QAM 1/24 16-QAM 3/45 64-QAM 2/36 64-QAM 3/4

OFDM 256 subcarriers bit-rate: up to 74 Mbit/s in

20 MHz channel

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The Supervisor algorithm (1)

1

0.8

0.6

0.4

0.2

Gai

n

0

1

0.8

0.6

0.4

0.2

Gai

n

0

1

0.8

0.6

0.4

0.2

Gai

n

0

Example: current channel response

The aim of the MAIS SPV algorithm is to find the “optimum” choice of 1) the RateID (constellation size and channel code rate)

2) the number and the position of the active (ON) subcarriers and 3) the transmission power

needed to fit the Target Bit Rate and the Target BER requirements with the “minimum” power constraint, given the current channel

condition

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The SPV selects all the RateIDs that have an achievable bit rate greater than the Target Bit Rate required by the MAC

If there is not a single RateID that meets the requested Target Bit-Rate, the SPV informs the MAC that it is not able to

fulfill the requirements. The MAC layer will then decide the next steps.

SPV does not consider the RateIDs for which the maximum achievable bit-

rate is less than the target one

11

The Supervisor algorithm (2)

Each RateID guarantees, for a given channel bandwidth, a maximum net bit rate that is achievable with all the subcarriers ON

A look-up table containing maximum achievable net bit rates for all possible channel bandwidths in which the transceiver will work is pre-

stored in the SPV

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For each useful RateID the SPV

Calculates the minimum number of subcarriers that must be switched ON to satisfy the Target Bit Rate22

The Supervisor algorithm (3)

192

BitRateMax

RateBit Target N

jj

Identifies the position of the Nj subcarriers which will be switched ON (the best possible choice is to select the Nj subcarriers that are less attenuated by the channel)

33

Reads form a pre-stored look-up table the SNR value that needs to be respected at the receiver by each subcarrier (treated as an AWGN channel) in order to achieve the required Target BER.From the SNR value the SPV estimates the subcarriers amplitude at the receiver, Aj, necessary to achieve the Target BER.

44

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For each useful RateID the SPV (cont’d)

Pre-equalize the Nj active subcarriers of the OFDM symbol on the basis of the channel response, in order to obtain the requested amplitude value of Aj at the receiver (i.e. at the output of the channel)

Calculates the required total transmission power for the current OFDM symbol

55

66

The Supervisor algorithm (4)

ON is SC if H

1A

OFF is SC if 0A

i

jji

256

1i

2ji

j AP

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Among all the useful RateID, the SPV chooses the one that requires the lower transmitted power Pj

77

SPV prepares the OFDM symbol for transmission:- switches off the subcarriers that are not required

- loads the active subcarriers with the selected RateID- multiplies all the active subcarriers by the proper amplitude Ai

j

88

The Supervisor algorithm (5)

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Simulator: general features

IEEE 802.16-2004

OFDM baseband

transceiver

developed and

tested with Matlab

6.5

Full standard

compliant

Flexible and

reconfigurable

-number of simulation bits

-downlink/uplink mode

-guard interval

-Eb/N0 range

-channel model

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Simulation: scenario

To compare performances of standard and SPV transceivers, we measured the mean power transmitted by the two modems in different scenarios

For our simulation we tested the modems in seven different cases (indicated by the letters A, B, C, D, E, F, G), each one of them is related to a particular RateID (A->0, B->1, C->2, D->3, E->4, F->5, G->6), in the sense that the maximum bit rate achievable in one case is the bit rate permitted by the corresponding RateID

We supposed that modems can work in one of the seven cases at a time • Target Bit Rate randomly distributed between 0 and the

maximum bit-rate achievable in the considered case • Target BER randomly distributed among values 10-4, 10-5 and 10-6.

The operations of the standard transceiver imply that OFDM symbols are always transmitted with a power that must guarantee the communication at the maximum bit-rate achievable by the chosen RateID and the minimum BER value requested by the MAC

Instead, the SPV adapts dynamically each OFDM symbol to the requests of the MAC (bit rate and BER) and to the measured channel response

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Simulation Results: scenario C

the standard transceiver requires a transmitted

power of 32 dBm in order to achieve the required

Target Bit Rates (it has to operate with RateID 2,

the minimum RateID that guarantees the

achievement of all the required bit rates) and

BER

When the required Target Bit Rate is low and/or the channel response is good (attenuation is low) and/or the required Target BER is high, SPV transmitted power can be significantly lower

SPV requires 11.34 dB less transmitted power than the standard 802.16-2004

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Simulation Results

CaseMean Powerwithout SPV

[dBm]

Mean Powerwith SPV

[dBm]

Difference[dB]

A 28 11.61 16.39

B 30 16.77 13.23

C 32 20.66 11.34

D 34 24.01 9.99

E 36 27.97 8.03

F 38 30.46 7.54

G 40 34.17 5.83