4G Mobile Communications (WiMAX and LTE)

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GSM, CDMA, UMTS…3GPP

802.16 WiMAX

802.11 Wi-Fi

802.15.3Bluetooth60 GHzUWB

802.22

Local Metro

Regional

Personal

Wide

TVWS

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IEEE 802.22, is a standard for wireless regional area network (WRAN) using white spaces (vacant TV channels) in the television (TV) frequency spectrum (in the VHF and UHF bands). 

Operates in the range of frequencies between 54 MHz and 862 MHz.

Operates in lower population density areas.

The development of the IEEE 802.22 WRAN standard is aimed at using cognitive radio (CR) techniques to allow sharing of geographically unused spectrum allocated to the television broadcast service.

Cognitive Radio The IEEE 802.22 standard

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Overview of IEEE 802.22 Standard

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In March 2008, the International Telecommunications Union-Radio communications (ITU-R) specified a set of requirements for 4G standards, named the International Mobile Telecommunications Advanced (IMT-Advanced) specification, setting peak speed requirements for 4G service at 300 Mbit/s for high mobility communication (such as from trains and cars) and 1Gbit/s for low mobility communication (such as pedestrians and stationary users).

Since the first-release versions of Mobile WiMAX (first used in South Korea in 2007) and LTE (in Oslo, Norway and Stockholm, Sweden since 2009 )support much less than 1 Gbit/s peak bit rate, they are not fully IMT-Advanced compliant, but are often branded 4G by service providers.

On December 6, 2010, ITU-R recognized that these two technologies, as well as other beyond-3G technologies that do not fulfill the IMT-Advanced requirements, could nevertheless be considered "4G", provided they represent forerunners to IMT-Advanced compliant versions .

History

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During the spring 2011 above two system provide their advanced version as: Mobile WiMAX Release 2 (also known as WirelessMAN-Advanced or IEEE 802.16m') and LTE Advanced (LTE-A, Based on UMTS 3G technology) and promising speeds in the order of 1 Gbit/s in 2013.

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3G 4GData Rates of 100 Kbps to 2 Mbps

Goal is 'to provide multimedia multirate mobile communications anytime and anywhere'.

Connection between the cellular world and the wired Internet firmly established.

Mobile devices used mainly for Human-to-Human and Human-to-Machine communication

Data Rates up to 100 Mbps

Expansion on the 3G goal to provide a wider range of new and improved multimedia services.

Integration of broadcast, cellular, cordless, Wireless LAN, short-range and fixed wire systems to appear as a single seamless network.

Not only the 3G modes of communication but also characterized by a great deal of Machine-to-Machine traffic

Comparison of 3G and 4G

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Some Key Challenges

• Coverage – Transmit power limitations and higher frequencies limit the

achievable cell size• Capacity

– Current air interfaces have limited peak data rate, capacity, and packet data capability

• Spectrum– Lower carrier frequencies (< 5 GHz) are best for wide-area

coverage and mobility

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WiMAX• 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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In 1998 IEEE802.16 protocol was developed to provide high speed service of WMAN (Wireless Metropolitan Area Network). Next two new version of above protocol were found as: IEEE 802.16d (in 2004) was developed to support high speed wireless data service of fixed user and its later version IEEE 802.16e (in 2005) supports both fixed and mobile users.

With the advent of OFDMA based IEEE 802.16e, research is now going on to implement VoIP service with adaptive modulation and channel coding (MCS) 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.

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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Three common types of BW allocation algorithms are: Dedicated Resource Allocation (Unsolicited Grand Service known as UGS Algorithm) where fixed amount of BW is allocated to each user hence possibility of waste of BW when a user needs to data send data at low rate; Polling-Based Resource Allocation (Real-Time Polling Service called rtPS Algorithm) where BS allocates the BW dynamically therefore incurs some protocol overhead and delay; Hybrid Resource Allocation Algorithm is the combination of above two.

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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WiBro, Korean version of WiMAX has been deployed in Korea.

WiFi and WiMAX are the B3G (Beyond 3G) systems. WiMAX may be an interim system of a 4G system.

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Some important features of WiMAX are given below: OFDM in physical layer: The access technique used in physical layer of WiMAX is OFDM; where the high speed serial data is converted to low rate parallel streams and each stream is modulated by separate carrier each one is known as subcarrier. Subcarriers are mutually orthogonal and deals with low data rate hence can protect multipath fading.  Very high peak data rates: The data rate of WMAX is 70Mbps under the channel of bandwidth of 20 MHz. The rate can be further increased using space division multiplexing i.e. incorporation of multiple antennas.

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Adaptive Modulation and Coding: The IEEE 802.16e standard changes modulation and channel coding scheme based on received SNR. For example a SS close to the BS can use a high modulation scheme (more bits per symbol) i.e. the system can get more capacity but when the SS is at the cell boarder the system permits lower modulation scheme (increased signal space on orthogonal basis function coordinate system) to avoid huge symbol error rate. Therefore the system can overcome the time selective fading (the channel condition is better at some instant than other). Error Correction Techniques: WiMAX incorporates two types of strong error correction techniques: FEC (Forward Error Correction) for multimedia traffic and ARQ (Automatic Repeat Request) for data traffic to improve throughput. 

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Support for TDD and FDD: Like mobile cellular communication it supports both FDD (Frequency division duplexing) and TDD (Time division duplexing), as well as a half-duplex FDD. Above features provide the flexibility of using same or different carriers for up and down link.

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IEEE 802.16 general architecture

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WiMAX applications and missions

BWA (Broadband Wireless Access)

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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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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?

UMB (Ultra Mobile Broadband) was the brand name for a project within 3GPP2 to improve the CDMA 2000 mobile phone standard for next generation applications and requirements. No carrier had announced plans to adopt UMB, and most CDMA carriers in Australia, USA, Canada, China, Japan and South Korea have already announced plans to adopt either WiMAX or LTE as their 4G technology.

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The progress tree for communication technology

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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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The IEEE 802.16 data link layer layer is composed of three sub-layers Service Specific Convergence Sub-layer (CS), MAC Common Part Sub-layer (CPS) and the Security Sub-layer. Each sub-layer has a specific function to perform.

The 802.16 Protocol Stack

The 802.16 protocol stack

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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

Layers of WiMAX

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802.16 PHYThe IEEE 802.16e 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 (downlink) DL and (uplink)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.

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An SS (subscriber Station) close to the BS could use a high modulation scheme, thereby giving the system more capacity. In contrast, a weak signal from a more remote subscriber might only permit the use of a lower modulation scheme to maintain the connection quality and link stability.

This feature enables the system to overcome time-selective fading.

The coding rate also change according received SNR of fading channel.

Adaptive Modulation and Coding

ModulationQPSK, 16-QAM, 64-QAM

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Modulation Coding Schemes (MCSs)

PDU → Packet Data UnitSDU → Service Data Unitlm → the number of PDU allocated for a TDMA slot

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where the parameter lm is the size of the VoIP PDU, which is modulated with the mth MCS level after encoding and xm is the number of PDU at mth MCS level.

Uplink scheduling is feasible if the allocated uplink resources are less than the total of available resources (number of PDU/slot) Nslot,u.Hence, we have

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For example, we consider Nslot,u = 50 and M = 4. Then, the MCS-level distributions of packets are denoted as,

X =(x1, x2, x3, x4).

If the MCS-level distributions of six packets in the uplink queue are (0, 0, 0, 6) or (0, 0, 1, 5) and the MCS level of the seventh packet in the queue is not four, the BS schedules six packets according to the uplink feasibility condition.

= 0 + 0 + 0 + 6*6 = 36 <50

X = (0, 0, 0, 6)

X = (0, 0, 1, 5)

= 0 + 0 + 1*12 + 5*6 = 42 < 50

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However, if the MCS level of the seventh packet is four, the BS can schedule more packets than six because the MCS-level distribution of seven packets becomes (0, 0, 0, 7) or (0, 0, 1, 6), which satisfies feasibility condition.

= 0 + 0 + 0 + 6*7= 42 <50

X = (0, 0, 0, 7)

X = (0, 0, 1, 6)

= 0 + 0 + 1*12 + 6*6= 48 <50

Data link layer

Upper layers

Service specific convergence layer

MAC sub-layer

Security sub-layer

Transmission convergence sub-layer

QPSK 16-QAM 64-QAM

Fig.5 The 802.16 protocol stack

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

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

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 Common Part Sublayer

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Similarly , multiple MSDUs from the same higher-layer service 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 MSDUGMH Other

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).

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

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.

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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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GMH Other SH CRCFSH MSDU Fragment

Fig.7 MAC PDU frame carrying a single fragmented MSDU

FSH → Fragmentation Sub-header

PSH → Packing Sub-header

GMH Other SH CRCPSH Variable size MSDU or

Fragment PSH Variable size MSDU orFragment

…

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

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

Long Term Evolution (LTE)

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Long-term evolution (LTE) standard is one of the newly developed fourth generation standards for mobile communications. In the standard, either frequency division duplexing (FDD) or time division duplexing (TDD) schemes can be used to achieve two-way communications.

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Evolution of LTE

1G 2G 3G 4G2.5G

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Comparison of LTE Speed

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Features of LTEThe LTE- Advanced (Long Term Evolution- Advanced) is 4G wireless service

proposed by 3GPP (Third generation Partnership Project). In 2009 4G LTE started its commercial service in Scandinavia. Three important features of LTE are: femtocell deployment , OFDMA-based physical layer access and MIMO.

The FBS (Femto BS) is named as Home evolved Node-B (HeNB) in LTE-A placed in public places to provide higher data rate and improve resource usage to a number of users. Femtocells are different in the sense that they are installed by customers in an ad hoc fashion without any RF planning. Objective of eNodeB Femtocells lies in off-loading of traffic.

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Two-tier macro-femto network architecture

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LTE provides OFDMA-based physical layer access where:

• OFDMA minimizes separation between carriers

• Carriers are selected so that they are orthogonal over symbol interval

• Carrier orthogonality leads to frequency domain spacing ∆f =1/T, where T is the symbol time

• In LTE carrier spacing is 15KHz and useful part of the symbol is 66.7 microsec

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MIMO communication takes place between eNB and UE. LTE standard requires support for:

eNodeB can have maximum 4 antennasUE can have maximum 2 antennas

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Architecture of LTE

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The architecture of LTE consists of two major parts: the E-UTRAN (Evolved Universal Terrestrial Radio Access Network) and the EPC (Evolved Packet Core). The first part provides air interface between MS or UE to BS and the second part is interconnected switching network called backbone or core network.

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User Equipment (UE)As the name suggests, a UE is the actual device that the LTE

customers use to connect to the LTE network and establish their connectivity. The UE may take several forms; it can be a mobile phone, a tablet, or a data card used by a computer/notebook.

Similar to all other 3GPP systems, the UE consists of two main entities: a SIM-card or what is also known as User Service Identity Module (USIM), and the actual equipment known as Terminal Equipment (TE).

SIM-card carries the necessary information provided by the operator for user identification and authentication procedures. The terminal equipment on the other hand provides the users with the necessary hardware (e.g., processing, storage, operating system) to run their applications and utilize the LTE system services.

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Evolved UTRAN (E-UTRAN)The E-UTRAN in LTE consists of directly interconnected eNodeBs which are

connected to each other through the X2 interface and to the core network through the S1 interface.

This eliminates one of the biggest drawbacks of the former 3GPP systems (UMTS): the need to connect and control the NodeBs through the Radio Network Controller (RNC), which make the system vulnerable to RNC failures.

The LTE E-UTRAN architecture can be seen in Figure below.

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Evolved Packet Core (EPC)The EPC (also known as the LTE core network) consists of three main entities: Mobility Management Entity (MME), Serving Gateway (S-GW) and the Packet Data Network Gateway (PDN-GW). In addition, there are some other logical entities like the Home Subscriber Server (HSS) and Policy and Charging Rules Function (PCRF).

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The EPC consists of six nodes:

Home Subscriber Server (HSS) is like the combination of HLR and AUC of UMTS or GSM 

The Packet Data Network (PDN) Gateway (P-GW) provides connectivity between UE and external packer switching network. It works like a gateway SGSN (Serving GPRS Support Node) of UMTS.

The serving gateway (S-GW) works as a router whose function is to forward data between the BS and the PDN gateway(P-GW). It also works as the mobility anchor of eNB handovers and do the similar job between LTE and other 3GPP technologies. 

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The MME (for Mobility Management Entity) deals with the signaling (between UE and CR) related to mobility of users, paging of UE in idle-mode and security for E-UTRAN access.

The Policy Control and Charging Rules Function (PCRF): This module works like: Packet filtering and billing on flow basis. 

ePDG (Evolved Packet Data Gateway) provides secured data transmission between UE to un-trusted non-3GPP access through EPC.

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S-GW and MME HSS

P-GW

E-UTRAN

Macro

Femto

Radio tower

Radio tower

Radio tower Trusted non 3GPP access

ePDG

Untrusted non 3GPP access

Fig.1 Architecture of LTE

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InterfacesS1 (eNode B to SGSN)S1 (eNode B to MME)X2 between two eNode Bs (required for handover)Uu (UE to eNode B)

eNB

MME / S-GW MME / S-GW

eNB

eNB

S1 S1

X2 E-UTRAN

Uu

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LTE Protocol Stack

PDCP → Packet Data Convergence ProtocolRLC → Radio Link ControlMAC → Medium Access ControlGTP-U→ GPRS Tunneling Protocol

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PDCP layer or layer 2 Packet Data Convergence Protocol is responsible for data ciphering and IP header compression to reduce the IP header overhead. The service provided by PDCP to transfer IP packets is called a radio bearer. A radio bearer is defined as an IP stream corresponding to one service for one UE.

RLC layer or layer 2 Radio Link Control performs the data concatenation and then generates the segmentation of packets from IP-Packets of random sizes which comprise a Transport Block (TB) of size adapted to the radio transfer. The RLC layer handles a retransmission scheme of lost data through a first level of Automatic Repeat reQuests (ARQ).

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There are two types of LTE frame structure:Type 1:   used for the LTE FDD mode systems.Type 2:   used for the LTE TDD systems.

Type 1 LTE Frame StructureThe duration of one LTE radio frame is 10 ms. One frame is divided into 10

subframes of 1 ms each, and each subframe is divided into two slots of 0.5 ms each.

LTE Frame Structure

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Each slot contains either six or seven OFDM symbols, depending on the Cyclic Prefix (CP) length. The useful symbol time is 1/15 kHz= 66.6 mircosec. Since normal CP is about 4.69 microsec long, seven OFDM symbols can be placed in the 0.5-ms slot as each symbol occupies (66.6 + 4.69) = 71.29 microseconds.

When extended CP (=16.67 microsec) is used the total OFDM symbol time is (66.6 + 16.67) = 83.27 microseconds. Six OFDM symbols can then be placed in the 0.5-ms slot.

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Detailed time domain structure

TCP: 160Ts (5.1us) for first symbol;v144Ts (4.7us) for other six symbolsTCP-e: 512 Ts (16.7 us) for all symbols

Need for two different CP:1. To accommodate environments with large

channel dispersion2. To accommodate MBSFN (Multi-Cast

Broadcast Single Frequency Network) transmission

In case of MBSFN it may be beneficial to have mixture of sub-frames with normal CP and extended CP. Extended CP is used for MBSFN sub-frames

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In LTE, radio resources are allocated in units of Physical Resource Blocks (PRBs). Each PRB contains 12 subcarriers and one slot.

72LTE physical resource block

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• LTE – radio resource = “time-frequency chunk”• Resource Block (RB) = 12 subcarriers in one TS (12*15KHz x 0.5ms)• Time domain

1 frame = 10 sub-frames 1 subframe = 2 slots 1 slot = 7 (or 6) OFDM symbols

• Frequency domain 1 OFDM carrier = 15KHz

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Time Unit ValueFrame 10 msHalf-frame 5 msSubframe 1 msSlot 0.5 msSymbol (0.5 ms) / 7 for normal CP

(0.5 ms) / 6 for extended CPBasic timing unit: Ts 1/(15000 * 2048) sec » 32.6 ns

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OFDMA time-frequency scheduling• Minimum allocateable resource in LTE is

Resource Block pair• Resource block pair is 12 subcarriers wide in

frequency domain and lasts for two time slots (1ms)

• Depending on the length of cyclic prefix RB pair may have 14 or 12 OFDM symbols

• PHY channels consist of certain number of allocated RB pairs

• Overhead channels are typically in a predetermined location in time frequency domain

• Allocation of the radio block is done by scheduler at eNode B

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Type 2 LTE Frame StructureThe frame structure for the type 2 frames used on LTE TDD is somewhat different. The 10 ms frame comprises two half frames, each 5 ms long. The LTE half-frames are further split into five subframes, each 1ms long.

The subframes may be divided into standard subframes of special subframes. The special subframes consist of three fields;DwPTS - Downlink Pilot Time SlotGP - Guard PeriodUpPTS - Uplink Pilot Time Stot.

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LTE OFDMParameter Value

Bandwidth (MHz) 1.4 3 5 10 15 20

Frame /subframe duration 10/1 ms

Subcarrier spacing 15KHz

Useful symbol part 66.7us

FFT size 128 256 512 1024 1536 2048

Resource blocks 6 15 25 50 75 100

Number of used subcarriers 72 180 300 600 900 1200

Cyclic prefix length Normal: 5.1us for first symbol in a slot and 4.7us for other symbols , Extended: 16.7us

OFDM symbols /slot 7 (normal CP), 6 (extended CP)

Error coding 1/3 convolutional (signaling); 1/3 turbo (data)

Basic timing unit: Ts = 1/(2048 x 15000) ~ 23.552 ns

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Downlink reference signals• For coherent demodulation – terminal needs channel estimate for each subcarrier• Reference signals – used for channel estimation• There are three type of reference signals

1. Cell specific DL reference signals 2. UE specific DL reference signals3. MBSFN reference signals

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Cell specific reference signals

• DL transmission may use up to four antennas• Each antenna port has its own pattern of reference signals• Reference signals are transmitted at higher power in multi-

antenna case• Reference signals introduce overhead

– 4.8% for 1 antenna port– 9.5% for 2 antenna ports– 14.3 % for 4 antenna ports

• Reference symbols vary from position to position and from cell to cell – cell specific 2 dimensional sequence

• Period of the sequence is one frame

Four port TX

Two port TX

One port TX

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UE Specific Reference Signal RS• UE specific RS – used for beam forming• Provided in addition to cell specific RS• Sent over resource block allocated for DL-SCH (applicable only for

data transmission)

Note: additional reference signals increase overhead. One of the most beneficial use of beam forming is at the cell edge – improves SNR

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Relays and RepeatersRelaying is used in order to deploy cells in areas where no (or very expensive) wired backhaul exists. It is often used to improve the coverage and throughput, for example, in urban, and rural scenarios. Figure below shows a typical scenario where a Relay Node (RN) is connected to an eNodeB wirelessly. The eNodeB that the RN is connected to is called in that case, a Donor eNodeB (DeNB).