1 WiMax Wireless and Mobile Communications Systems.

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WiMax

Wireless and Mobile Communications Systems

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WiMAXWiMAX• Wi-MAX : The Worldwide Interoperability for Microwave Access, is a

technology aimed at providing wireless data over long distances It is

based on the IEEE 802.16 standard.

Fig.1

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The main objective of wireless communication is to provide high data rate with guaranteed QoS. The Mobile WiMAX at high throughput is implemented based on IEEE Std 802.16-2009.

With the advent of OFDMA based IEEE 802.16e, research is now going on to implement VoIP service with adaptive modulation and channel coding scheme. To enhance the throughput of the wireless system the modulation and coding scheme of the transmitter is changed according to the fading condition of the channel. Such dynamic modulation and coding schemes are called MCSs (Modulation Coding Schemes).

Therefore the service becomes a variable bit rate service where the bit rate depends on the fading condition of the wireless channel.

WiMAX

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The new wireless platform, WiMAX holds promise of high-speed Internet delivered to handheld devices.

It delivers higher speeds than Wi-Fi over a much longer distance.

The technology does not require a line of sight (LOS), and it is more efficient in transporting bandwidth-intensive applications, particularly time-sensitive services such as real-time video. WiMAX also can be used as a complementary system to Wi-Fi. Both of the two major 3G systems, CDMA2000 and UMTS, compete with WiMAX.

WiMAX

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We consider a single cell in a WiMAX network with a base station and multiple subscriber stations (Fig.2). Each subscriber station serves multiple connections. Admission control is used at each subscriber station to limit the number of ongoing connections through that subscriber station. At each subscriber station, traffic from all users for uplink connections are aggregated into a single queue.

OFDMA Based WiMAX Network

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The size of this queue is finite (i.e., X packets) in which some packets will be dropped if the queue is full upon their arrivals. The OFDMA transmitter at the subscriber station receives packets and transmits them to the base station. The base station may allocate different number of subchannels to different subscriber stations. For example, a subscriber station with higher priority could be allocated more number of subchannels.

Fig.2 System model

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(Beyond 3G)• WiFi and WiMAX are the B3G systems• Using OFDM technology• Intel provided WiMAX chip imbedded in PC in

2007 and in handset in 2008• WiBro, Korean version of WiMAX has been

deployed in Korea• WiMAX may be an interim system of a 4G

system

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IEEE 802 Activities Wired

802.3: Ethernet 802.17: Packet Ring (new)

Wireless 802.11: Wireless LAN

• Local Area Network

802.15: Wireless PAN– Personal Area Network (e.g. BluetoothTM)

802.16: WirelessMANTM

– Metropolitan Area Networks

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Wireless Technology Evolution to 3.9G

CDMA(IS-95A)

GSM

CDMA(IS-95B)

cdma2000

1xEV-DORev 0/A/B

UMB802.20

2G

2.5G

3G

3.5G

3.9G

GPRS

E-GPRSEDGE

HSDPAFDD/TDD

TDMAIS-136

WCDMAFDD/TDD

TD-SCDMA

LCR-TDD

HSUPAFDD/TDD

HSPA+LTE

E-UTRA

IEEE802.16

Fixed WiMAX802.16d

Mobile WiMAX802.16e

WiBRO

IEEE802.11

802.11g

802.11a

802.11g

802.11n

CDMA GSM/UMTS IEEE Cellular IEEE LAN

1. What is 4G?

Fig.3

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4G - The Future Technology

• Requirement based on ITU• 1 Gbps data speed when stationary

• 100 Mbps data speed when moving

• Search for 4G technology• Meeting the requirement with minimum bandwidth

will be the winner

• User’s Expectation• Multimedia application

• Operating with long hours

• Become a personal mobile office if possible

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In a hierarchical telecommunications network the backhaul portion of the network comprises the intermediate links between the core network or backbone, of the

network and the small sub-networks at the "edge" of the entire hierarchical network.

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• Wi-Fi, which stands for “wireless fidelity,” is a radio technology that networks computers so they connect to each other and to the Internet without wires. Users can share documents and projects, as well as an Internet connection, among various computer stations and easily connect to a broadband Internet connection while traveling. By using a Wi-Fi network, individuals can network desktop computers, laptops, and PDAs and share networked peripherals such as servers and printers.

Wi-Fi

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A Wi-Fi network operates just like a wired network, but without the restrictions imposed by wires. It uses radio technologies called IEEE 802.11a, 802.11b, or 802.11g to provide secure, reliable, fast wireless connectivity. A Wi-Fi network can be used to connect computers to each other, to the Internet, and to wired networks (which use IEEE 802.3 or Ethernet). Wi-Fi networks operate in the unlicensed 2.4-and 5-GHz radio bands with an 11-Mbps (802.11b) or 54-Mbps (802.11a) data rate, or with products that contain both bands (dual band).

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Layers of WiMax

Mobile WiMAX systems are based on the IEEE 802.16e-2005 specifications which define a physical (PHY) layer and a medium access control (MAC) layer.

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802.16 PHYMobileWiMAX is generally considered to be the IEEE 802.16e-2005 adopting OFDMA PHY. The IEEE 802.16e-2005 supports both time division duplexing (TDD) and frequency division duplexing (FDD) modes. However, the initial release of Mobile WiMAX profiles only considers the TDD mode of operation for the following reasons:

Time Division DuplexingTime division duplexing (TDD) refers to the interleaving of transmission and reception of data on the same frequency. A common frequency is shared between the upstream and downstream, the direction in transmission being switched in time. Frequency Division DuplexingFrequency division duplexing (FDD) refers to the simultaneous transmission and reception of data over separate frequencies, allowing for bidirectional full-

duplex communications.

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A single frequency channel in DL and UL can provide more flexibility for spectrum allocation.It enables dynamic allocation of downlink (DL) and uplink (UL) radio resources to effectively support asymmetric DL/UL traffic that is common in Internet applications.It supports link adaptation, multi-input-multi-output (MIMO) techniques, and closed loop advanced antenna technique such as beam-forming.

ModulationQPSK, 16-QAM, 64-QAM

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802.16 PHY• Burst Uplink

• Downlink

– Mode A: continuous transmission stream supporting concatenation of Reed Solomon coding, interleaving, and convolutional coding for use in an FDD only system

– Mode B: burst format supporting FDD with adaptive modulation as well as TDD and half-duplex FDD

• Modulation

– QPSK, 16-QAM, 64-QAM

Fig.4

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From the reference model as illustrated in Figure 5, there are three sub-layers in the data link layer. The MAC layer is composed of i) a security sublayer, ii) a MAC common part sublayer, and iii) a convergence sublayer. It provides only connection oriented service.

Data link layer

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Upper layers

Service specific convergence layer

MAC sub-layer

Security sub-layer

Transmission convergence sub-layer

QPSK 16-QAM 64-QAM

Data link layer

Physical layer

Fig.5 The 802.16 protocol stack

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The MAC layer of WiMAX is divided into thee distinct parts.

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The CS, which is the interface between the MAC layer and layer 3 of the network, receives data packets from the higher layer. These higher layer packets are known as service data unit (SDU). The CS is responsible for performing all operations that are dependent on the nature of higher-layer protocol, such a header compression and address mapping. The CS can be viewed as an adaptation layer that masks the higher-layer protocol.

Packet header suppression (PHS): At the transmitter it involves removing the repetitive part of the header of each SDU. For example, if the SDUs delivered to the CS are IP packets, the source and destination addresses contained in the header of each IP packet do not change from one packet to the next and thus can be removed before being transmitted over the air. Similarly at the receiver: the repetitive part of the header can be reinserted into the SDU before being delivered to the higher layer.

Service Specific Convergence Sub-layer (CS):

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CS is also responsible for the mapping the higher layer address, such as IP address, of the SDUs into the identity of the PHY and MAC connections to be used for its transmission. The WiMAX MAC layer is connection oriented and identifies a logical connation between the BS and the MS by a unidirectional connection identifier (CID). The CID for UP and DL connections are different.

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IEEE Standard 802.16 defines two general service-specific convergence sublayers for mapping services to and from 802.16 MAC connections.

The ATM convergence sublayer is defined for ATM services, and the packet convergence sublayer is defined for mapping packet services such as IPv4, IPv6, Ethernet, and virtual local area network (VLAN).

The primary task of the sublayer is to classify service data units (SDUs) to the proper MAC connection, preserve or enable QoS, and enable bandwidth allocation. The mapping takes various forms depending on the type of service.

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Segmentation and Reassembly Sublayer (continuedSegmentation and Reassembly Sublayer (continued))

Segmentation and Reassembly SublayerSegmentation and Reassembly Sublayer

Convergence Sublayer

Application Layer

Convergence Sub-layerConvergence Sub-layer

MessageMessage

CS CS HeaderHeader

CSCSTrailerTrailer

PadPad

SARSARHdrHdr

SARSARTrlrTrlr

SARSARHdrHdr

SARSARTrlrTrlr

SARSARHdrHdr

SARSARTrlrTrlr

SARSARHdrHdr

SARSARTrlrTrlr

SARSARHdrHdr

SARSARTrlrTrlr

ATM LayerATM Layer

ATMATMHdrHdr

ATMATMHdrHdr

ATMATMHdrHdr

ATMATMHdrHdr

ATMATMHdrHdr

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The MAC layer takes packets from the upper layer-these packets are called MAC service data units (MSDUs)- and organize them into MAC protocol data units (MPDUs) for transmission over the air.

The IEEE 802.16-2004 and IEEE 802.16e-2005 MAC design includes a convergence sub-layer that can interface with variety of higher-layer protocols, such as ATM, TDM voice, IP etc.

The WiMAX MAC uses a variable length MPDU and offer a lot of flexibility to allow for their efficient transmission. For example multiple MPDUs of same or different lengths may be arranged into a single burst when they are destined to the same receiver.

MAC layer

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Similarly , multiple MSDUs from the same higher-layer aservice may be concatenated into a single MPDU to save MAC header overhead.

Large MSDUs may be fragmented into smaller MPDUs and send across multiple frames. When an SDU is fragmented, the position of each fragment within the SDU is tagged by a sequence number. The sequence number enables the MAC layer at the receiver to assemble the SDU from its fragments in the correct order.

WiMAX has two types of PDUs, each with a very different header structure.

1. The generic MAC PDU is used for carrying data and MAC-layer signaling messages.

2. The bandwidth request PDU is used by the MS to indicate to the BS that more BW is required in UL, due to pending data transmission. A bandwidth request

PDU consists only of a bandwidth-request header, with no payload or CRC.

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Packed fixed size MSDU

GMHOther

SHPacked fixed size MSDU

CRC…….

Fig.6 MAC PDU frame carrying several-fixed length MSDUs packed together

GMH → Generic MAC Header (used for carrying data and MAC-layer signaling messages)

SH → Sub-header

Each MAC frame is prefixed is prefixed with GMH (generic MAC header). The GMH contains the following fields,

(a) Connection Identifier (CID), tells which connection this frame belongs to. Connection identifier on which the payload is to be sent. Its length is 16 bits.

(b) The length of frame

(c) Header CRC field is a checksum over the header only using the generator polynomial x8+x2+x+1. The length of this field is 8bits.

(d) Field (in SH) to indicate whether the payload is encrypted or not. If the payload is encrypted then the encryption key is also given.

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GMHOther

SHCRCFSH MSDU Fragment

Fig.7 MAC PDU frame carrying a single fragmented MSDU

FSH → Fragmentation Sub-header

PSH → Packing Sub-header

GMHOther

SHCRCPSH

Variable size MSDU orFragment

PSHVariable size MSDU or

Fragment…

Fig.8 MAC PDU frame carrying several variable length MSDUs packed together

Besides MSDUs the payload may contains bandwidth requests or retransmission requests. The type of payload is identified by the sub-header immediately precedes it. For example FSH or PSH of above figure.

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CRC(optional)MAC PDU payload (optional)

Generic MACHeader

(6 bytes)

msb lsb

LENmsb(3)

HT

CID msb (8)LEN lsb (8)

Generic MAC Header Format(Header Type (HT) = 0)

EC

Type (6 bits)rsv

CI

EKS(2)

rsv

HCS (8)CID lsb (8)

BW Req. Header Format(Header Type (HT) =1)

BW Req.msb (8)

HT

CID msb (8)BWS Req. lsb (8)

EC

Type (6 bits)

HCS (8)CID lsb (8)

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Field Length descriptionHT 1 Header type (set 0 for such header)

EC 1 Encryption control (0 = payload not encrypted; 1 = payload encrypted

Type 6 type

ESF 1 (1= ES present; 0 = ES not present)

CI 1 CRC indicator (1=CRC included; 0=CRC not included)

EKS 2 Encryption key sequence (index of the traffic encryption key and the initialization vector used to encrypt the payload)

Rsv 1 Reserved

LEN 11 Length of MAC PDU in bytes, including the header)

CID 16 Connection identifier on which the payload is to be sent

HCS 8 Header check sequence; generation polynomial x8+x2+x+1

Generic MAC Header Fields

LENmsb(3)

HT

CID msb (8)LEN lsb (8)

EC

Type (6 bits)rsv

CI

EKS(2)

rsv

HCS (8)CID lsb (8)

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Field Length DescriptionHT 1 Header type (set 1 for such header)

EC 1 Encryption control (set 0 for such header)

Type 3 type

BR 19 BW request ( the number of bytes of UL BW requested by the SS for the given CID)

LEN 11 Length of MAC PDU in bytes, including the header

CID 16 Connection identifier

HCS 8 Header check sequence

Bandwidth Request MAC Header Fields

BW Req.msb (11)

HT

CID msb (8)BWS Req. lsb (8)

EC

Type (3 bits)

HCS (8)CID lsb (8)

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Privacy (or Security) Sub-layer: supporting authentication, secure key exchange, and encryption.