3-WiMAX Key Technologies

7/30/2019 3-WiMAX Key Technologies

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WiMAX Key Technologies


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Contents

Advance Antenna Technologies

Hybrid Automatic Repeat Request (HARQ)

Adaptive Modulation and Coding (AMC)

Scheduling Algorithms

Handoff

Power Control

Power Saving Modes

Quality of Service (QoS)


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AAS Adaptive Antenna System Beamforming (Smart Antenna)

MIMO Multiple Input Multiple Output

Space Time Coding (STC), Spatial Multiplexing (SM)

Advantages: Increased range/coverage

Increased capacity

Improved link quality/reliability

Increase spectral efficiency Lower power requirements


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Beamforming

Beamforming is a technique tocontrol the directionality/sensitivity of

an array antennas radiation pattern

based on the interference.

Each users signal is multiplied with

complex weights that adjust the

magnitude and phase of the signal

to and from each antenna. It causes

the output from the array of

antennas to form a transmit/receivebeam in the desired direction and

minimizes the output in other

directions.


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Space Time Coding (STC)

Space Time Coding transmit

multiple, redundant copies of a

data stream to the receiver in the

hope that at least some of them

may survive the physical path

between transmission and

reception in a good enough state to

allow reliable decoding.

WiMAX specifies Alamouti scheme

for space-time coding.

Types:

Trellis codes (STTC)

provide both coding

gain and diversity gain.

Block codes (STBC)

provide only

diversity gain but is

less complex. Used in

WiMAX.


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Spatial Multiplexing (SM)

In SM-MIMO, multiple streams aretransmitted over multiple antennas.

If the receiver also has multiple

antennas, it can separate the different

streams to achieve higher throughput.

The mobile WiMAX standard covers a

suite of MIMO encoding techniques for

up to 4 antennas at each end of the link(4*4 MIMO)

UL Collaborative MIMO

Two separate WiMAX devices,

each having a single transmit

lineup, utilize the same frequency

allocation to communicate with the

dual-antenna WiMAX base

station.

Double the uplink capacityof the

WiMAX system.


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Adaptive Switching for Antenna


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Contents



Adaptive Modulation and Coding (AMC) Scheduling Algorithms

Handoff

Power Control

Power Saving Modes



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HARQ

HARQ (Hybrid ARQ) is a combination of ARQ (Automatic Repeat

Request) and FEC (Forward Error Correction) coding.

An HARQ scheme uses an error control code in conjunction with theretransmission scheme to ensure reliable transmission of datapackets.

Every sent packet includes parity bits for error correction anddetection.

Within the FEC capability, the errors will be corrected.

Beyond the FEC capability, ARQ is used.

HARQ types:

Chase Combining

Incremental Redundancy

Base

Station


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Type I Chase Combining

In chase combining, the receiver keeps the framewith error and combines it with the retransmitted

frame before attempting to decode the information.

The combination can obtain a data frame with higher

SNR. It provides powerful error correction capability

and is very suited for channels with strong

interference.


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Type II Incremental Redundancy

Incremental Redundancy improves the correctdecoding probability by gradually reducing the

encoding rate -- the redundancy of the code words

is increased incrementally.

It can adaptively adjust the encoding rate of the

transmit end according to the instantaneous

decoding condition.

Only applicable to CC & CTC (Convolutional Turbo

Coding)


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Forward Error Correction

The transmit end sends correctable codes and thereceiver uses a code correction decoder to

automatically detect errors and perform error

correction.

Coding schemes:

Convolutional Coding (CC)

Block Turbo Coding (BTC)

Convolutional Turbo Coding (CTC)

Low Parity Density Check (LDPC)

CC is a mandatory scheme, while the others are

optional schemes. However, most manufacturers

implement CTC for its better performance.


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Contents





Handoff

Power Control

Power Saving Modes



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AMC

AMC stands forAdaptiveModulation and Coding

Dynamically varies the size

of the raw channel.

Channel capacity can vary

depending on linkconditions

Key Features:

Increases the effective

coverage range.

Allows the system to adapt

to the actual fadingconditions, and use higher

modulation schemes when

possible.


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AMC

Mobile WiMAX supports AMC in both downlink and

uplink with variable packet size.

Technology DL Modulation DL Code Rate UL Modulation UL Code Rate

MobileWiMAX

64QAM

16QAM

QPSK

CC, CTC:

1/2, 2/3, 3/4, 5/6

Repetition:x2, x4, x6

16QAM

QPSK

64QAM(optional)

CC, CTC:

1/2, 2/3, 5/6

Repetition:x2, x4, x6


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AMC

The system should have timely and accurate understanding of

the channel performance.

OFDM system can use pilot signals or reference code

words with known data to test the SINR of each channel

According to the SINR, determine the most appropriate

modulation mode.


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Contents





Handoff

Power Control

Power Saving Modes



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Resource allocation

Resource allocation uses schedulingalgorithms to determine which users

to schedule, how to allocate sub-

carriers to them, and which power

level is appropriate.

Aim of resource allocation:

Take advantage ofmultiuserdiversity and adaptive

modulation to provide high

throughput while considering

fairness among users in the

system.

Multiuser diversity describes the gains available by selecting

a user or sub-set of users having good conditions

CSI Channel State Information


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Fair throughput scheduling

Allocate same throughput to all subscribers. Different Subscribers can obtain the same throughput.

Fair time scheduling

Allocate same occupation time to all subscribers.

Different Subscribers can get equivalent bandwidth.

Max C/I scheduling All the subscribers to be served according to the expected

C/I value of signals.

Round Robin(RR)

All the subscribers can occupy resources by turns in aspecific sequence.

Proportional Fairness Scheduling (PFS)

In a short term, the algorithm takes channel conditions intoprior consideration.

In a long term,ensures fair throughput for every subscriber.


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Algorithm Comparison

Algorithm Sum Capacity Fairness Complexity

Fair throughput Medium Best High

Fair time Low Better Low

Max C/I Highest Worst Lower

RR Medium Better Low

PFS Higher Better Lower


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Contents





Handoff

Power Control

Power Saving Modes



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Handoff

Mobility management, enables the MS

to retain its connectivity to the networkwhile moving from the coverage area ofone BS to the next thru the handoffprocess.

Handover Schemes Optimized Hard Handover (OHHO)

Fast Base Station Switching (FBSS)

Macro Diversity Handover (MDHO)

Handover Control Mobile initiated

BS initiated

Network initiated

Cell Selection Neighbor advertisements from

Serving BS

Periodic intervals for scanningNeighbor BS


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Handoff Process

Cell Selection

Release of

previous BS

Handoff decision

and initiation

Synchronization to

target BS

Ranging to target BS

MS performs scanning and association with one or

more neighboring BSs to determine their suitability

as a handoff target

MS sends request message to the BS indicating one or

more BSs as handoff targets. BS sends a response

message indicating the target BSs for handoff. MS selects

the target BS by sending an handoff indication message.

MS receives the DL preamble to get time and frequency

synchronization for the DL transmission, and decode

DL/UL-MAP for more information on the ranging channel.

MS uses the ranging channel to synchronize its UL

transmission with the BS and get information about

initial timing advance and power level

Previous BS starts resource retain

timerand holds MAC PDU associated

with MS until the timer expires


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Contents





Handoff

Power Control

Power Saving Modes



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Power Control

Purpose of power control

Remove the influence of slow fading in the wireless

channel

Prevent power increase to decrease the overall interference

level of the system

Reduce the Tx power to the minimum level that meets thecommunication quality requirement.

Principles of power control

Power balance

Ensure approximately equal power of the wanted signals at

the receive end SNR balance

Ensure approximately equal C/I at the receive end

BER/FER balance

Ensure approximately equal BER/FER at the receive end


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Power Control

Power Control Methods:

Open loop MS vary their own transmit power according toreceived signal quality.

Closed loop - power is varied according to the base station

decision from comparison of the received signal quality to a

setpoint.

Mobile BTS

Signal Strength

Measurement

Setpoint

or

Reverse Closed Loop

Power Control

Reverse Open LoopPower Control


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Contents





Handoff

Power Control

Power Saving Modes



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Power Saving Modes

Power management allows an MS to conservebattery resources, a critical feature for handheld

devices.

Power saving modes:

Sleep Mode

Idle Mode (optional)


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Sleep Mode

In sleep mode, an MS with active connections negotiates with

the BS to temporarily disrupt its connection over the air

interface for a predetermined amount of time, called the sleep

window.

Each sleep window is followed by a listen window, during

which the MS restores its connection.

Active Sleeps Listens Sleeps Listens

Periodic sleep intervals

Periodic listen intervals

MS request BS to

enter sleep mode


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Idle Mode

In Idle mode, MS can receive broadcast/multicast service

without registering to the network.

Cell selection may occur but no handoff happens thus saving

power on the MS and saving resources on the BS.

Paging is used to alert MS of incoming message.

MS associates to a broadcast region formed by a paging group.

PagingGroup 1 Paging

Group 2

Paging

Group 3


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Contents





Handoff

Power Control

Power Saving Modes



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With fast air link, symmetric downlink/uplink capacity, fine

resource granularity and a flexible resource allocationmechanism, Mobile WiMAX can meet QoS requirements for awide range of data services and applications.

Unsolicited grant services (UGS) - support fixed-size datapackets at a constant bit rate (CBR).

Real-time polling services (rtPS) - support real-time service

flows, such as MPEG video, that generate variable-sizedata packets on a periodic basis.

Non-real-time polling service (nrtPS) - support delay-tolerant data streams, such as an FTP, that requirevariable-size data grants at a minimum guaranteed rate.

Best-effort (BE) service - support data streams, such as

Web browsing, that do not require a minimum service-levelguarantee.

Extended real-time variable rate (ERT-VR) service -support real-time applications, such as VoIP with silencesuppression, that have variable data rates but requireguaranteed data rate and delay


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Qos Service Types


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