lte introduction part1

LTE Introduction

Jay Chang

Sep. 3 2015

�Wireless Technology Evolution

� LTE Technologies

�Physical Layer

� LTE Test Items

• Overview

• EPC

• E-UTRAN

• UE

Agenda

Part 1

Part 2

• OFDM

• MIMO

• Link Adaptation (AMC)

• HARQ

• Channel Scheduling

• Inter-Cell Interference Coordination (ICIC)

• Frequency Band

• Structure – frame, slots, resource blocks & elements

• Physical signals and channels

• Tx Characteristics

• Rx Characteristics

OverviewLong Term Evolution (LTE) Definition:

� 4G ITU的定義, 靜態DL傳輸速率 = 1Gbps, 高速移動DL = 100Mbps.

� IMT-Advanced的4G標準• LTE FDD: 20MHz, DL = 150 Mbps, UL = 40 Mbps.

• LTE TDD (TD-LTE): 20MHz, DL = 100 Mbps, UL = 50 Mbps.

Technology Evolution Path

� 要加入3GPP主要成員包括3類1.Organizational Partners(OP)具有制訂標準權限(投影片上的國家).

2.Market Representation Partners(MRP)沒有制訂標準權限但可提供 3GPP 市場諮詢資訊的組織(GSMA, UMTS, 4g America, IPV6......).

3.個體會員(就是各單位大老).鬼島加油喇!!

中國聯通

中國移動

中國電信

LTE網路實體網路實體網路實體網路實體

� LTE系統由三個部分組成1. 核心網(EPC, Evolved Packet Core).

2. 接入網(eNB).

3. 用戶設備(UE).

� 核心網EPC分三部分1. MME(Mobility Management Entity,信號處理).

2. S-GW(Serving Gateway,用戶數據處理).

3. P-GW(PDN Gateway,用戶數據包和網路處理).

� 接入網(也叫E-UTRAN)由eNB構成.

� 網路接口• S1接口: eNB與EPC.

• X2接口: eNB與eNB.

• Uu接口: eNB與UE.

LTE Structure



�Physical Layer

� LTE Test Items

• Overview

• EPC

• E-UTRAN

• UE

Agenda

• OFDM

• MIMO


• HARQ



• Frequency Band





Technology Evolution (I)網路架構網路架構網路架構網路架構Long Term Evolution (LTE) or Long Term “Revolution” ?

EPC

E-UTRAN = Evolved Universal Terrestrial Radio Access Network

EPC = Evolved Packet Core = 核心網路RNC = Radio Network Controller =無線網絡控制器E-UTRAN Node B = Evolved Node B = e-NodeB = eNB

SAE = System Architecture Evolution

MME = Mobility Management Entity

S-GW = Serving Gateway, P-GW = PDN Gateway.

WCDMALTE

EPS = LTE systeE-UTRA

Em

PC

N

=> LTE核心網路

=> LTE無線網絡

Revolution What ?

EPC

E-UTRAN

1.為了減少業務的延遲

Revolution What ?

2.核心網路IP化

3.核心網路與無線網絡接口IP化

Technology Evolution (II)

1. Air interface物理層

2. Air interface網路層

3.無線網路接口

4.核心網路 Evolution

Evolution

Evolution

Revolution

CS Domain: CS業務(電路交換) => 語音業務=>打電話=>資源利用率低

PS Domain: PS業務(分組交換) => IP=>上網=>業務訊息用數據包乘載=>傳輸通道共享=>利用率高

EPCCS Domain (Circuit Switched Domain): CS業務(電路交換)=>獨佔資源=>語音業務=>打電話=>資源利用率低.

PS Domain (Package Switched Domain):

PS業務(分組交換)=>IP=>上網=>業務訊息用數據包乘載=>傳輸通道共享=>利用率高.

MME = Mobility Management Entity = 班長SGW = Serving GateWay =業務流接口PGW = PDN GateWay = PDN(Internet)接口HSS = Home Subscribers Server = 儲存用戶信息PCRF = Policy and Charging Rules Function = QoS頻寬管理

LTE不想要!!

革除CS Domain

不過CS業務仍存在LTE中

IP網路網路網路網路PS Domain

CS Domain

無線無線無線無線網路網路網路網路

WCDMA LTE

MME SGW PGW

EPC的宏大目標的宏大目標的宏大目標的宏大目標��承先啟後承先啟後承先啟後承先啟後



�Physical Layer

� LTE Test Items

• Overview

• EPC

• E-UTRAN

• UE

Agenda

• OFDM

• MIMO


• HARQ



• Frequency Band





E-UTRANE-UTRAN通俗講通俗講通俗講通俗講 = LTE無線網路無線網路無線網路無線網路��eNB

� LTE BTS透過X2接口互相連接,透過S1接口與核心網互相連接.

非常有名的圖 in 3GPP TS 36.300

同WCDMA

LTE Air InterfaceLTE air interface分層分層分層分層(Uu層層層層)結構結構結構結構

Macro Cell

BTS, Antenna分離容量大, 輸出功率大, 覆蓋範圍大, GSM

體積大, 室內機房

LTE Base Station

Micro Cell

BTS, Antenna一起容量小, 輸出功率小, 方便佈署, 覆蓋Macro的盲區Pico Cell為LTE-A異質網路的主要成分, WLAN AP

Radio Remote Unit

(RRU)

LTE used

Remote what ?

Macro cell BB and RF part各自獨立, 100 ~ 1000 m

BBU放室內, RRU放天線附近, BBU RRU通過光纖連接(Ir接口)



�Physical Layer

� LTE Test Items

• Overview

• EPC

• E-UTRAN

• UE

Agenda

• OFDM

• MIMO


• HARQ



• Frequency Band





LTE UE

Categories 1 2 3 4 5

Max DL/Mbps 10 50 100 150 300

Max UL/Mbps 5 25 50 50 75

Max DL Mod. Scheme 64 QAM

Max UL Mod. Scheme 16 QAM 64 QAM

Max support layers in

spatial multiplexing

1 2 4

TS 36.306

� LTE UE Cat.在R8, R9只定義五種, 與GPRS HSPA十幾種不同.

� LTE UE可在FDD, TDD網路中同時工作.

� Max support layers in spatial multiplexing與UE天線數量一致.

� 目前商用以Cat. 3為主.

LTE frequency bandTS 36. 101 (Rel 12 Jun 2015)

http://niviuk.free.fr/lte_band.php

Technology Evolution (III)

Technology LTE-A LTE

Rev. R10 R8

BW Max 100 MHz, initial 40 MHz Max 20MHz

DL MIMO Max 8*8 MIMO Max 4*4 MIMO

DL TM TM1 ~ TM9 TM1 ~ TM7

UL MIMO Max 4*4 MIMO None

UL TM TM1 ~ TM2 TM1

spectrum

utilization

(頻譜利用率)

30 bit/Hz 15 bit/Hz

Peak data rate DL: 3000 Mbps

UL: 1500 Mbps

DL: 300 Mbps

UL: 75 Mbps

� 對於語音來講, 頻譜利用率定義為: 每社區每 MHz

支援的多少對用戶同時打電話；

� 而對於資料業務來講, 定義為: 每社區每MHz支持的最大傳輸速率.

Technology Evolution (IV)

LTE WLAN

技術 1. 頻譜靈活2. OFDM

3. MIMO

1. IEEE 802.11n

2. OFDM

3. MIMO

頻率 1. below 2.5 GHz

2. 低頻室外覆蓋率佳1. 2.4 or 5.8 GHz

2. 高頻室內覆蓋率佳

BTS發射功率 Max ~ 40 W (室內施展不開) WLAN AP ~ 100 mW

速度 1 Gbps (LTE-A) 1 Gbps (802.16m)

實施 Licensed Unlicensed

LTE v.s. WLAN (獨孤九劍 v.s. 葵花寶典) ?

版本版本版本版本 IEEE 802.11a/g IEEE 802.11n

生成算法複數 IFFT 複數 IFFT

階數 64 64

基波頻率 312.5 kHz 312.5 kHz

BW 20 MHz 20 MHz

Symbol時長 3.2 us 3.2 us

採樣點時長 50 ns 50 ns

子載波數量 52 56

GI 0.8 us 0.4/0.8 us

OFDM Symbol rate 250 ksps 277.8/250 ksps

OFDM - WLAN

OFDM - LTE

BW 10 MHz 15 MHz 20 MHz

IFFT階數 1024 1536 2048

基波頻率 15 kHz 15 kHz 15 kHz

Symbol時長 66.7 us 66.7 us 66.7 us

採樣點間格 65.1 ns 43.4 ns 32.5 ns

採樣頻率 15.36 MHz 23.04 MHz 30.72 MHz

子載波數量 600 900 1200

GI 4.76 us 4.76 us 4.76 us

OFDM Symbol rate 14 ksps 14 ksps 14 ksps

Major LTE Parameters

Parameter Downlink Uplink

Access scheme OFDMA SC-FDMA (DFTS-OFDM)

Subcarrier spacing 15 kHz

Bandwidth 1.4, 3, 5, 10, 15, or 20 MHz

Modulation QPSK, 16-QAM, 64-QAM

Cyclic prefix length 4.7 μs (short) or 16.7 μs (long)

� OFDMA = orthogonal frequency division multiple access; DFTS = discrete Fourier transform spread

� DFTS-OFDM (also called SC-FDMA = single-carrier frequency division multiple access) is a

transmission scheme that combines the desired properties for uplink :

1. Small variations in the instantaneous Tx signal power (single carrier’s property).

2. Possibility for low-complexity high-quality equalization in the frequency domain.

3. Possibility for FDMA with flexible bandwidth assignment.

� Spectral efficiency is increased up to 4x compared with UTRA, and improvements in architecture

and signaling reduce round-trip latency.

� MIMO antenna technology should enable 10x as many users per cell as 3GPP’s original WCDMA

radio access technology.

� To suit many frequency band allocation arrangements, both paired (FDD) and unpaired (TDD) band

operation is supported. LTE can coexist with earlier 3GPP radio technologies.



�Physical Layer

� LTE Test Items

• Overview

• EPC

• E-UTRAN

• UE

Agenda

• OFDM

• MIMO


• HARQ



• Frequency Band





OFDM is a digital multi-carrier modulation scheme

� Large number of closely-spaced orthogonal sub-carriers (e.g. 300/5 MHz BW).

� Subcarriers modulated with a conventional modulation format (e.g. QPSK, 16/64QAM)

� Low symbol rate similar to conventional single-carrier modulation schemes in the same bandwidth.

LTE symbol rate = 66.7µs, ∆f = 1/symbol rate = 15 kHz for each subcarrier.

In freq. domain 1 RE = 1 subcarrier, so 1 RB = 12 subcarriers = 180 kHz. In time domain 1 RB = 0.5 ms.

Orthogonal Frequency Division Multiplexing

OFDM

把高速的資料分成多個平行的低速資料, 把每個低速的資料分到N個子載波上, 在每個子載波上進行 FSK.

這些在N子載波上同時傳輸的資料符號, 構成一個OFDM符號(=SUM(subcarriers)).

Spectrum of single modulated OFDM subcarrierThe FFT of a rectangular pulse is a sinc or sin(x)/x with zeros at multiples of FP = 1/TP.

LTE symbol rate = 66.7µs, ∆f = 1/ symbol rate =

15 kHz for each subcarrier.

In freq. domain 1 RB = 12 subcarriers = 180 kHz.

In time domain 1 RB = 0.5 ms.

FFT

� OFDM與傳統的多載波調製(MCM)相比, OFDM調製的各子載波間可相互重疊, 並且能夠保持各個子載波之間的正交性.

� 選擇OFDM的一個主要原因在於該系統能夠很好地對抗頻率選擇性衰落或窄帶干擾.

Spectrum of multiple OFDM subcarriers

OFDM Operates as a Number of Orthogonal (Non-Interfering) Narrowband Systems

� Closely spaced carriers overlap.

� Nulls in each carrier’s spectrum land at the center of all other carriers for zero Inter-Carrier

Interference (ICI).

� Carrier spacing creates orthogonality.

� Phase noise, timing and frequency offsets decrease orthogonality.

Fig. Spectrum of multiple OFDM subcarriers of constant amplitude

OFDM v.s. FDM1. Zero guard interval(GI)

• OFDM子載波正交,子載波間不需保護帶, 利用率高.

• FDM因filter特性,需保護帶.

2. BW靈活• 增加減少子載波容易.

• FDM(ex: GSM)每增加一個載波, 需增加一個PA和filter.

3. 減少ISI• OFDM symbol(fundamental mode + each harmonic在基波周期內波形的疊加)減少ISI.

• 信號時延�前一個symbol尾與後一個symbol頭重疊�ISI.

• OFDM symbol時長長�重疊比例少�ISI減小.

4. 對抗freq. select fading• 棄車保帥�不去使用那些衰減大的子載波.

5. MIMO結合• OFDM多個子載波�傳播特性線性化�好實施MIMO.

1. 解決信號multipath delay spread• multipath delay spread帶來(1)ISI, (2)multipath delay與直達信號的干擾(其他頻率子載波異頻干擾�ICI).

• GSM用Equalizer將multipath delay抵消.

• PHS因BTS功率低覆蓋範圍小將multipath delay忽略.

• cdma2000 WCDMA使用Rake接收機.

• OFDM在前後symbol間插入GI解決ISI, GI長, 抗干擾強, 但時間開銷大.(1)解決惹!!

• OFDM讓multipath delay與直達信號正交�給multipath delay多補一塊Tc完整化(Tc時長 = GI時長)�Cyclic Prefix(CP).

2. 處理high PAPR• For single carrier, PA是照Pavg設計的, 讓PA提供更大的DR, 但耗電流功耗都加大.

• 削峰(蕭峰XD) , 但波形失真, 額外干擾.

• 預處理:先選擇子載波疊加後PAPR小的.

3. 對抗頻偏• Doppler shift. chip加強同步設計與tracking能力.

• ex: For B2(1900 MHz) 120 km/hr = 33.3 m/s Max Doppler shift UE = 233 Hz, BTS = 466 Hz.

正交1.子載波頻率是基波整數倍2.積分週期是基波週期3.積分週期幅度一定

(2)也解決惹!!

OFDM PAPR ?

2

Crest factorpeak

rms

xC

x

PAPR C

= =

=

( )/2 2

0

/2

0

1 1sin 0.707

/ 2 2

1 2sin 0.636

/ 2

rms peak peak peak

avg peak peak peak

V V d V V

V V d V V

π

π

θ θπ

θ θπ π

= = =

= = =

∫

∫

For sin wave:

OFDM general link level chains

Rx � Channel estimation � test signal get all freq. response � use Equalizer � lower BER.

( )

∑

∑

∑

−

=

−

=

−

=

∆

=

=

==

1

0

/2

1

0

/2

1

0

2

'N

k

Nknj

k

Nc

k

Nknj

k

Nc

k

fnTkj

ksn

ea

ea

eanTxx s

π

π

π

<≤

<≤=

NkN

Nkaa

c

ck

k0

0'

IDFT

IFFT

OFDM Modulation

OFDM Demodulation

� 各個子載波之間要求完全正交, 各個子載波收發完全同步.

� 發射機和接收機要精確同頻, 同步.

� 多徑效應會引起符號間干擾以及載波間干擾, 積分區間內信號不具有整數週期.

OFDM – Mod. and Demod.

OFDM Fundamentals – Multicarrier Modulation

1. IDFT�代替LO, 產生正交子載波.

2. IDFT�代替PA, 改變正交子載波的幅度.

3. IDFT�代替Combiner, 疊加正交子載波

IDFT(爬樓梯)�IFFT(坐電梯)

快收斂的意思!!

OFDM Fundamentals – Frequency Domain Equalizer

Frequency domain equalizer outperforms with much less complexity !

Rx � Channel estimation � test signal get all freq. response � use Equalizer � lower BER.

OFDM advantages:

� Multiple subcarriers allows.

– Scalable channel bandwidth.

– Frequency selective scheduling within the channel.

� Wide channels are possible which support higher

data rates.

� Resistance to multipath due to very long symbols.

OFDM Advantage and DisadvantageOFDM disadvantages:

� Sensitive to frequency errors and phase noise due to close

subcarrier spacing.

� Sensitive to Doppler shift which creates interference

between subcarriers.

� Pure OFDM creates high PAPR which is why SC-FDMA is

used on UL.

� Guard Interval (GI) necessary (ISI&ICI), reduce data rate.

Table. Comparison of CDMA and OFDM

LTE uses OFDMA (Orthogonal Frequency Division Multiple Access)

� more advanced form of OFDM where subcarriers are allocated to different users over time.

(Freq.)

(Freq.)

OFDM v.s. OFDMA

� 允許多個用戶在不同的時間(time slot), 來使用相同的頻率.

DL OFDMA

� OFDMA provides flexible scheduling in time-frequency domain.

� In case of multi-carrier transmission, OFDMA has larger PAPR than traditional single carrier

transmission. Fortunately this is less concerned with downlink.

� Does OFDMA suits for uplink transmission ?

Uplink being sensitive to PAPR due to UE implementation requirements.

With wider bandwidth in operation, OFDMA in uplink will have lower power per pilot symbol

which in turn leads to deterioration of demodulation performance.

SC-FDMA-FDE general link level chains

�LTE系統中上行鏈路採用SC-FDMA技術,以降低PAPR,提高效率,通過DFT-S-OFDM技術來實現.

�DFT-S-OFDM可以認為是SC-FDMA的頻域產生方式,是OFDM在IFFT調製前進行了基於Fourier Transform的預編碼.

�DFT-S-OFDM與OFDM的區別在於: OFDM是將1個符號資訊調製到1個正交的子載波上，而DFTS-OFDM是將M個輸入符號的頻譜資訊調製到多個正交的子載波上去.

Multiple Access Technology in the Uplink: SC-FDMASC-FDMA is a hybrid transmission scheme:

� low peak to average (PAPR) of single carrier schemes.

� frequency allocation flexibility and multipath protection of OFDMA.

� DFT “pre-coding” is performed on modulated data symbols to transform them into frequency domain.

� IFFT and cyclic prefix (CP) insertion as in OFDM.

� Each subcarrier carries a portion of superposed DFT spread data symbols, therefore SC-FDMA is also

referred to as DFT-spread-OFDM (DFT-s-OFDM).

DFT Sub-carrier Mapping

CP insertion

Size-NTX Size-NFFT

Coded symbol rate= R

NTX symbols

IFFT

Frequency domain Time domainTime domain

Fig. Transmitter structure for SC-FDMA

Low

PAPR

Spreading

High

PAPR

Low

PAPR

Signal at each subcarrier is linear combination of all NTx symbols

� 以長度為M的資料符號塊為單位完成DFTS-OFDM的調製過程.

� 首先通過DFT,獲取與這個長度為M的離散序列相對應的長度為M的頻域序列.

� DFT的輸出信號送入N點的IDFT中去,其中N > M. IDFT的長度比DFT的長度長, IDFT

多出的那一部分輸入為用0補齊.

� 在IDFT之後,為避免符號干擾同樣為這一組資料添加CP.

OFDM

SC-FDMA

SC-FDMA使用DFT變換代替OFDM的S/P變換，使得其可以獲得降低PAPR的作用

UL SC-FDMA (DFTS-OFDM)

UL SC-FDMA

基於基於基於基於DFTS-OFDM的集中式的集中式的集中式的集中式、、、、分散式分散式分散式分散式FDMA

基於基於基於基於DFTS-OFDM的的的的FDMA

� 利用DFTS-OFDM的特點可以方便的實現SC-FDMA multiple access.

� 通過改變不同用戶的DFT的輸出到IDFT輸入端的對應關係, 輸入資料符號的頻譜可以被搬移

至不同的位置,從而實現多用戶multiple access.

Localized and Distributed SC-FDMA

Comparing OFDMA and SC-FDMA

QPSK example using M = 4 subcarriersThe following graphs show how a sequence of eight QPSK symbols is represented in frequency and time.


In freq. domain 1 RE = 1 subcarrier, so 1 RB = 12 subcarriers = 180 kHz. In time domain 1 RB = 0.5 ms.

OFDMA modulation

QPSK example using M=4 subcarriers

SC-FDMA signal generation

QPSK example using M = 4 subcarriers


PAR and constellation analysis at different BW

Transmission scheme OFDMA SC-FDMA

Analysis bandwidth 15 kHzSignal BW

(M x 15 kHz)15 kHz

Signal BW(M x 15 kHz)

Peak to average power ratio (PAR)

Same as datasymbol

High PAR (Gaussian)< data symbol (not

meaningful)Same as data symbol

Observable IQ constellation

Same as data symbol at 66.7 µs rate

Not meaningful (Gaussian)

< data symbol (not meaningful)

. Same as data symbol at M X 66.7 µ s rate


In freq. domain 1 RE = 1 subcarrier

so 1 RB = 12 subcarriers = 180 kHz.

In time domain 1 RB = 0.5 ms.


Multipath protection with short data symbols

15 kHzFrequency

fc

V

CP

OFDMA

Data symbols occupy 15 kHz for

one OFDMA symbol period

SC-FDMA

Data symbols occupy M*15 kHz for

1/M SC-FDMA symbol periods

fc

The subcarriers of each SC-FDMA symbol are not the same across frequency as shown in

earlier graphs but have their own fixed amplitude & phase for the SC-FDMA symbol duration.

The sum of M time-invariant subcarriers represents the M time-varying data symbols.

60 kHz Frequency

V

CP

It is the constant nature of the subcarriers throughout the SC-FDMA symbol

that means when the CP is inserted, multipath protection is achieved despite

the modulating data symbols being much shorter.

Similarities� Block-wise data processing and use of Cyclic Prefix.

� Divides transmission bandwidth into smaller sub-carriers.

� Channel inversion/equalization is done in frequency domain.

� SC-FDMA is regarded as DFT-Precoded or DFT-Spread OFDMA.

Difference� Signal structure: In OFDMA each sub-carrier only carries information related to only one data symbol while in

SC-FDMA, each sub-carrier contains information of all data symbols. 一對一, 多對多.

� Equalization: Equalization for OFDMA is done on per-subcarrier basis while for SC-FDMA, equalization is

done over the group of sub-carriers used by transmitter.

� PAPR: SC-FDMA presents much lower PAPR than OFDMA does.

� Sensitivity to freq. offset: yes for OFDMA but tolerable to SC-FDMA.

OFDMA v.s. SC-FDMA

Time domain: � OFDMA: symbol is a sum of all data symbols by IFFT.

� SC-FDMA: symbol is repeated sequence of data “chips”.

Frequency domain:� OFDMA: modulates each subcarrier with one data

symbol.

� SC-FDMA: “distributes” all data symbols on each

subcarrier.

OFDMA SC-FDMA



�Physical Layer

� LTE Test Items

• Overview

• EPC

• E-UTRAN

• UE

Agenda

• OFDM

• MIMO


• HARQ



• Frequency Band





Multiple Input Multiple Output(MIMO) (I)

MIMO (II)MIMO = Multiple Input Multiple Output Antennas

WHY use Multiple Antennas ?

There are three main types of multiple antenna techniques.

1. Path diversity: one radiated path may be subject to fading loss and another may not.

2. Beamsteering (Beamforming): controlling the phase relationships of the electrical

signals radiated at the antennas to physically steer transmitted energy.

3. MIMO: employs spatial separation (the path differences introduced by separating

the antennas) through the use of spatial multiplexing.

優點1. 信號穩定性提高

2. 信號強度提高

3. 頻譜利用率提高

c.f.

Beamforming is about shaping the beam, to some required angular range.

Beamsteering is about pointing the beam, in some desired direction.

A. Free-space path loss.

B. Reflection.

C. Diffraction.

D. Scattering.

E. Shadow fading.

F. Doppler effect.

Before Diversity

C = Max(A, B) C = A + B




MIMO - Diversity

Diversity技術分為: Rx Diversity, Tx Diversity

Diversity實施方式: space/time/frequency/polarization/path/angle diversity

Diversity信號合併EGC (Equal Gain Combining)

SD (Selection Diversity)

MRC (Max Ratio Combining)對抗信號衰落效果最好

MRC = signal from each antenna is rotated and weighted according to the phase and

amplitude of the channel, such that the signals from all antennas are combined to yield

the maximal ratio between signal and noise terms.

Diversity – some thoughts (I)( )

( )

( )

2

/ 2

2

log 1

log 1

log 1

SISO

Tx Rx

MIMO

C B SNR

C B M SNR

C M B SNR

= +

= + ×

= × +

Diversity – some thoughts (II)( )

( )

( )

2

/ 2

2

log 1

log 1

log 1

SISO

Tx Rx

MIMO

C B SNR

C B M SNR

C M B SNR

= +

= + ×

= × +

Diversity – some thoughts (III)performance of SISO

� No special encoding, and therefore easy to implement.

� Different multipath, Rx can see different fading.

� Rx can use two way to improve SNR.

1. Switched Diversity.

2. Max-Ratio Combining.

� Maximum Ratio Combining depends on different fading

of the two received signals. In other words decorrelated

fading channels.

Rx Diversity (I)

C = Max(A, B) C = A + B

Rx Diversity (II)performance of SIMO

Tx Diversity (I)

Tx diversity �WCDMA

Open-loop:不用建call, 沒有終端feedback

Closed-loop

TSTD

STTD

TSTD (Time Switched Transmit Diversity): SCH同步信道內容在兩根Antenna間輪發.

STTD (Space Time Transmit Diversity): 其他信道採用, Alamouti空時編碼, 兩路正交data stream分別由兩根Antenna傳送.

Tx Diversity (II) –

Space Time Coding

Fading on the air interface

� The same signal is transmitted at different antennas.

Aim: increase of SNR � increase of throughput.

� Alamouti Coding = diversity gain approaches

Rx diversity gain with MRC (Maximal-Ratio Combining) �

benefit for mobile communications.

MRC = signal from each antenna is

rotated and weighted according to the

phase and amplitude of the channel, such

that the signals from all antennas are

combined to yield the maximal ratio

between signal and noise terms.

performance of MISO

相同數據內容透過編碼由不同天線發射至UE

1S 2S

*

1S*

2S−

STBCSFBC

LTE系統中在2 antenna port發送情況下的傳輸分集技術為SFBC

Tx Diversity (III) – LTE

Tx diversity �WCDMA

Open-loop:不用建call, 沒有終端feedback

Closed-loop

TSTD

STTD

� STTD在LTE裡的到了繼承, LTE叫SFTD (Space Frequency Transmit Diversity).

� SFTD利用兩個正交子載波f1, f2來傳送Alamouti coding後的data stream, UE在單根Antenna收到f1, f2疊加訊號,然後解聯立.

� SFTD = SFBC (Space Frequency Block Coding).


2. 信號強度提高Beamforming


MIMO - Beamforming

提升發射功率.

減少距離.

提高Gain.

功耗蓋基地台 No

Dipole antenna G = 2.15 dBi � 2根 +3 dB = 5.15 dBi � antenna array

控制垂直下傾角�同組phase shifter

控制水平波辦�異組phase shifter

� Beamforming技術要求:使用小間距的天線陣列,且天線單元數目要足夠多.

� Beamforming技術的實現方式:是將一個單一的資料流通過加權形成一個指向用戶方向的波束,從而使得更多的功率可以集中在用戶的方向上.

antenna array

Spatial Multiplexing (I)

2

2

log det

bandwidth,

( ( )),

.

C B

B SNR

ρ

ρσ

= + ×

= = =

T

ss

I HH

R

PS.

nTx = # of Tx antennas

nRx = # of Rx antennas.

Consider nTConsider nR

Spatial Multiplexing (II)

� Channel capacity grows linearly with antennas.

� Assumptions

Perfect channel knowledge.

Spatially uncorrelated fading.

� Reality

Imperfect channel knowledge.

Correlation ≠ 0 and rather unknown.

Max Capacity ~ min(nTx, nRx)

PS.

nTx = # of Tx antennas

nRx = # of Rx antennas.

( )

( )

( )

2

/ 2

2

log 1

log 1

log 1

SISO

Tx Rx

MIMO

C B SNR

C B M SNR

C M B SNR

= +

= + ×

= × +




MIMO – Space Division Multiplexing� 單碼字傳輸: 一個資料流程進行通道編碼和調制之後再複用到多根天線上.

� 多碼字傳輸: 複用到多根天線上的資料流程可以獨立進行通道編碼和調制.

� LTE支援最大的碼字數目為2.為了降低回饋的量.

single codewordmultiple codeword

Space Division Multiplexing頻譜利用率提高�單位帶寬能傳更多bit rate

�throughput提升

MIMO (III)Single input single output

Single input multiple output

Multiple input single output

Multiple input multiple output

SIMO = receive diversity.

� This radio channel access mode is suited for low SNR

conditions in which a theoretical gain of 3 dB is

possible when two receivers are used.

� There is no change in the data rate since only one data

stream is transmitted, but coverage at the cell edge is

improved due to the lowering of the usable SNR.

MISO = transmit diversity.

� MISO increases the robustness of the signal to fading and can increase performance in low

SNR conditions.

� MISO does not increase the data rates, but it supports the same data rates using less power.

� MISO can be enhanced with closed loop feedback from the receiver to indicate to the

transmitter the optimum balance of phase and power used for each transmit antenna.

� SIMO + MISO ≠ MIMO.

� If N data streams are transmitted from < N antennas, the data cannot be fully descrambled by any number of

Rx since overlapping streams without the addition of spatial diversity creates interference.

� So N data streams at least N Tx, N Rx will be able to fully reconstruct the original data streams provided the

path correlation and noise in the radio channel are low enough.

� Transmissions from each antenna must be uniquely identifiable.

� The spatial diversity of the radio channel means that MIMO has the potential to increase the data rate.

MIMO (IV)

2

2 2

2 1 2 2

log (1 ),

log (1 ( / ) ) log (1 ( / ) )

where / signal to noise ratio, a singular value of the channel matrix, .

C B SNR

C B N N

N H

σ ρ σ ρ

σ ρ

= +

= + + +

= =

For spatial

multiplexing system

� Streams in a spatially multiplexed link:

ρ = 1, ideal but impractical case of no cross-coupling(double channel capacity).

ρ = 2, total in-phase coupling.

ρ = 0, capacity has dropped back to that of a SISO channel.

� Channel capacity in 2x2 MIMO case ≤ twice SISO case and has substantial improvement in SNR at Rx if the

values of ρi << 1.

� The matrix coefficients are known by Tx, outgoing signals can be modified (precoded) to equalize the

performance between the streams.

� Precoding requires real-time feedback from Rx to Tx, so this is also known as closed-loop spatial multiplexing.

� For effective precoding, the relative signal phase between Tx must be stable over the time interval of the

feedback process.

1 Tx, 1 Rx case

Fig. Orthogonal structure of downlink reference symbols for dual antenna.

LTE Terminology for Multiple AntennasCodeword (想成數據流想成數據流想成數據流想成數據流 ==== 機場行李運輸帶機場行李運輸帶機場行李運輸帶機場行李運輸帶)

Layer

Precoding

� 一個 codeword就是在一個 TTI上發送的包含 encoding和 rate matching之後的獨立傳輸塊 Transport Block (TB).

� TB:實體層需要傳輸的原始資料塊(想成行李箱).

� LTE規定:對於每個 UE一個 TTI最多可以發送兩個 TB.

� TTI: BS 給 UE 安排資源的單位時間. LTE TTI = 1 ms.

� Layer = Stream. 1~4, 層數越多資料容量越大但覆蓋區域越小� 資料被分為不同 layer 進行傳輸, # of layers ≤ # of transmit antennas.

� 根據precoding matrix將transmission layer映射到antenna port.

� precoding matrix 維度為 R × P, R為 rank, 也就是# of transmission layers, P為# of antenna ports.

� IEEE 802.11a/g: AP兩天線, UE單天線, 不過AP也只使用其中接收好的一根做TRx.

� IEEE 802.11n: 支援4x4 MIMO, 不過AP一般配3根(立體極化天線), UE2根.

� WLAN同TDD, 收發同頻, 802.11n引入校正功能.

� 802.11n支援TRx diversity, so Rx用MRC, Tx用STBC.

� LTE BTS TDD: 1/2/4/8根天線, FDD: 1/2根天線.

� LTE UE Cat. 3: 2根天線.

WLAN MIMO v.s. LTE MIMO

LTE Downlink Transmission Modes (TM)3GPP R8 Def

LTE BS發射方式靈活:

� LTE BS會通過調度為用戶的業務選擇合適的發射方式.

� 調度是看業務信號的品質(SINR).

� Ex:離BS近�TM3

� SINR一般�TM8

� 離BS遠�TM2.

� LTE透過PDCCH發送調度.

� 實際上各調度法都是由廠商自行開發不公開!!

TM2: Tx diversity (雪中送炭)

� 避免訊號深衰落, 確保BTS覆蓋範圍.

� LTE基本發射方式.

TM3: Open-loop spatial multiplexing

(錦上添花)

� 信號質量好才能體現.

� 無UE feedback, 支持高速移動.

TM7: TDD Beamforming

� 8x8 MIMO.

� 把main beam打散.

TM8: 2 layers TDD Beamforming

� 8x8 MIMO.

� Beamforming + spatial

multiplexing.

� 在信號好的地方 = TM3.

� 在小區邊界 = TM7.

MIMO transmission modes

7 transmission

modes are defined

Transmission mode 1

SISO

Transmission mode 2

TX diversityTransmission mode 3

Open-loop spatial

multiplexing

Transmission mode 4

Closed-loop spatial

multiplexing

Transmission mode 5

(Multi-User) MU-MIMO

Transmission mode 6

Closed-loop

spatial multiplexing,

using 1 layer

Transmission mode 7

SISO, antenna port 5

= beamforming in TDD

GSM, WCDMA的單發射

Alamouti coding SFBC

Open loop (OL), no need UE feedback

支援4x4 MIMO, 目前商用2x2 MIMO

Close loop (CL), need UE feedback

Feedback: 層數(Rank表示), 傳播特性(Code Book)

SDMAFDD Beamforming = Rank = 1的TM4

8x8 MIMO

TDD Beamforming, 繼承TD-SCDMA

8x8 MIMO

FDD常用: TM2, TM3

TDD常用: TM2, TM3, TM7, TM8

3GPP R8

Overview of physical channel processing TS 36.211



�Physical Layer

� LTE Test Items

• Overview

• EPC

• E-UTRAN

• UE

Agenda

• OFDM

• MIMO


• HARQ



• Frequency Band





Link AdaptationLink adaptation � adaptive modulation and coding (AMC)� Link adaptation技術可以通過兩種方法實現:功率控制和速率控制.

� 一般Link adaptation都指速率控制, LTE中為AMC(Adaptive Modulation and Coding).

� AMC技術可以使得eNB能夠根據UE feedback的通道狀況及時地調整不同的調製方式(QPSK 16QAM

64QAM)和編碼速率從而使得資料傳輸能及時地跟上通道的變化狀況.

� 對於長時延的分組資料, AMC可以在提高系統容量的同時不增加對鄰區的干擾.

功率控制� 通過動態調整功率, 使Rx SNR恆定, 保證linkage的傳輸品質.

� 當信號差時增加Tx power, 信號強時減少Tx power,

保證恆定的傳輸速率.

功率控制可以很好的避免社區內用戶間的干擾

速率控制� 保證發送功率恒定的情況下, 通過調整無線線路傳輸的調製方式與編碼速率,確保linkage的傳輸品質.

� 當通道條件較差時選擇較小的調製方式與編碼速率, 當通道條件較好是選擇較大的調製方式,從而最大化了傳輸速率.

速率控制可以充分利用所有的功率

Link Adaptation in LTE UL DLChannel Quality Indicator

� LTE UL AMC: 基於基站測量的上行通道品質,直接確定具體的調製與編碼方式.

� LTE DL AMC:基於UE feedback的CQI, 從預定的CQI表格中選擇調製與編碼方式(如右圖).

� TPC概念



�Physical Layer

� LTE Test Items

• Overview

• EPC

• E-UTRAN

• UE

Agenda

• OFDM

• MIMO


• HARQ



• Frequency Band





HARQ� Hybrid Automatic Repeat reQuest (HARQ)是一種前向糾錯FEC (Forward Error Correction)和重傳ARQ

(Automatic Repeat reQuest)相結合的技術. ALL because of noise !!

� HARQ與AMC配合使用,為LTE的HARQ進程提供精細的彈性速率調整.

� LTE中的HARQ技術採用增量冗餘(Incremental Redundancy, IR) HARQ, 即通過第一次傳輸發送的資訊bit

和一部分冗餘bit.

� 而通過重傳發送額外的冗餘bit,如果第一次傳輸沒有成功解碼,則可以通過重傳更多冗餘bit降低通道編碼率,從而實現更高的解碼成功率.

� 如果加上重傳的冗餘bit仍然無法正常解碼,則進行再次重傳.

� 隨著重傳次數的增加,冗餘bit不斷積累,通道編碼率不斷降低,從而可以獲得更好的解碼效果.

� HARQ針對每個傳輸塊(TB)進行重傳.

Chase Combining

提高SNR !!

� Info. Bits + Cyclic Redundancy Check, CRC後透過Turbo Encoder編碼產生數據封包�Coded Bits.

� Rx利用Maximum-ratio組合Coded Bits進入Decoder.

� 每次重傳都與第一次傳的資料相同�不會增加Coding

Rate�但每次重傳時都增加SNR.

IR (Incremental Redundancy)� Tx傳送前會將 Coded Bits透過 Circular Buffer用打孔

(Puncturing)的方式分成四種冗餘版本(Redundancy Version,

RV)第一次傳送r.v.=0若需重傳�依次r.v.=2, r.v.=3, r.v.=1, 若傳送四次合併後仍無法正確解碼,才會全部捨棄再從頭重傳.

� LTE中的HARQ結合Soft Combining都是以IR為主.

� FEC及Soft Combining提供的低BER, 可以大幅減少傳統ARQ

所必須重傳的次數.

在MAC layer運作

HARQ搭配Soft Combining在PHY layer

HARQ

� HARQ程序: Tx送出data, 並收到Rx送回的ACK/NACK後�判斷出是否傳送無誤或須再送新資料/重傳.

� HARQ程序依特性可分同步/非同步(Synchronous/Asynchronous)以及適應性/非適應性(Adaptive/Non-

adaptive)

HARQ程序依特性可分兩類� 同步/非同步HARQ

同步HARQ特性是首次傳輸和重傳的時間間隔為固定.

非同步HARQ特性是在首次傳輸後, 重傳的時間無法預先知道.

� 適應性/非適應性HARQ

適應性HARQ特性是重傳的頻率資源對應位置(Frequency Resource Location), 甚至傳輸的格式會有所變動.

非適應性HARQ特性是重傳其頻率資源對應位置及格式皆與初始傳輸時相同.

HARQ

� 單純HARQ機制中, 接收到的錯誤資料包都是直接被丟掉的.

� HARQ與Soft Combining結合: 將接收到的錯誤資料包保存在記憶體中與重傳的資料包合併在一起進行解碼,提高傳輸效率.



�Physical Layer

� LTE Test Items

• Overview

• EPC

• E-UTRAN

• UE

Agenda

• OFDM

• MIMO


• HARQ



• Frequency Band





� LTE系統支援基於頻域的通道調度.

� 相對於單載波CDMA系統, LTE系統的一個典型特徵是可以在頻域進行通道調度和速率控制(AMC).

Channel Scheduling� 基本思想: 對於某一塊資源, 選擇通道傳輸條件最好的使用者進行調度, 從而最大化系統輸送量. MRC的概念.

� LTE(BW=10/15/20MHz)�frequency selective

fading, 在更遠的子載波上衰減特性不同�假如我們知道各用戶子載波上的衰減, 就可為不同用戶選擇好的子載波進行傳輸�提高頻譜效率.



�Physical Layer

� LTE Test Items

• Overview

• EPC

• E-UTRAN

• UE

Agenda

• OFDM

• MIMO


• HARQ



• Frequency Band





ICIC

� ICIC = Inter-Cell Interference Coordination

小區干擾原因� LTE,系統中個小區採用相同的頻率進行TRx.

� 與CDMA系統不同, LTE系統不能通過合併不同小區的信號來降低鄰近小區信號的影響所以小區干擾嚴重,尤其在邊緣處.

小區間干擾消除技術方法包括小區間干擾消除技術方法包括小區間干擾消除技術方法包括小區間干擾消除技術方法包括：：：：

1. 加擾加擾加擾加擾

2. 跳頻傳輸跳頻傳輸跳頻傳輸跳頻傳輸

3. Tx Beamforming以及以及以及以及IRC

4. 小區間干擾協調小區間干擾協調小區間干擾協調小區間干擾協調

5. 功率功率功率功率控制控制控制控制

� LTE系統充分使用序列的隨機化避免小區間干擾.

� 一般情況下, 加擾在通道編碼之後, 資料調製之前進行即比特級的加擾

• PDSCH, PUCCH format 2/2a/2b, PUSCH: 擾碼序列與UE id、社區id以及時隙起始位置有關.

• PMCH: 擾碼序列與MBSFN id和時隙起始位置有關.

• PBCH, PCFICH, PDCCH: 擾碼序列與社區id和時隙起始位置有關.

� PHICH物理通道的加擾是在調製之後, 進行序列擴展時進行加擾.

• 擾碼序列與社區id和時隙起始位置有關.

Turbo Coding Interleaver

Scrambling A

User A

小區間干擾消除小區間干擾消除小區間干擾消除小區間干擾消除��加擾加擾加擾加擾

ICI Cancellation (I)

ICI Cancellation (II)

� 目前LTE UL DL都可以支持跳頻傳輸, 通過進行跳頻傳輸可以隨機化小區間的干擾.

• 除了PBCH之外, 其他下行物理控制通道的資源映射均於社區id有關.

• PDSCH、PUSCH以及PUCCH採用sub-frame內跳頻傳輸.

• PUSCH可以採用sub-frame間的跳頻傳輸.

小區間干擾消除小區間干擾消除小區間干擾消除小區間干擾消除��跳頻傳輸跳頻傳輸跳頻傳輸跳頻傳輸

ICI Cancellation (III)

� 提高Wanted UE的信號強度.

� 降低信號對其他使用者的干擾.

� 如果Beamforming時已經知道Interfering UE的方位, 可以主動降低對該方向輻射能量.

小區間干擾消除小區間干擾消除小區間干擾消除小區間干擾消除��Tx Beamforming

� 當接收端也存在多根天線時,接收端也可以利用多根天線降低使用者間干擾.

� 其主要的原理是通過對接收信號進行加權, 抑制強干擾, 稱為IRC(Interference

Rejection Combining).

DL

UL

小區間干擾消除小區間干擾消除小區間干擾消除小區間干擾消除��IRC

ICI Cancellation (IV)

ICI Cancellation (V) 小區間干擾消除小區間干擾消除小區間干擾消除小區間干擾消除��小區間干擾協調小區間干擾協調小區間干擾協調小區間干擾協調

� 基本思想: 以小區間協調的方式對資源的使用進行限制, 包括限制哪些時頻資源可用, 或者在一定的時頻資源上限制其發射功率.

Scheduling

HARQ

3 4

5 6

AMC

ICI Cancellation

• OFDM

• MIMO


• HARQ





�Physical Layer

� LTE Test Items

• Overview

• EPC

• E-UTRAN

• UE

Agenda

• OFDM

• MIMO


• HARQ



• Frequency Band





LTE FDD Frequency bands

P.S.

ARFCN = Absolute radio-frequency channel number

UARFCN = UMTS Absolute radio-frequency channel number

EARFCN = EUTRA Absolute radio-frequency channel number

A good website: http://niviuk.free.fr/lte_band.php

LTE TDD Frequency bands

FDD & TDDFDD

� Pair spectrum.

� GSM, cdma2000, WCDMA.

� Duplexer.

TDD� Un-pair spectrum.

� PHS, TD-SCDMA

� Switch.

Table. LTE (FDD) downlink and uplink peak data rates.

Table. Peak data rates for UE categories.

� In order to scale the development of equipment, UE categories have been defined to limit

certain parameters.

� The most significant parameter is the supported data rates:

Theoretical LTE Data Rate Calculation

� Question: Assume 20 MHz bandwidth (100 RB) and normal CP calculate data rate = ?

� Throughput � symbols per second �bits per second.

� 1 RB = 1 time domain(1 slot = 0.5 ms = 7 OFDM symbols) x 1 freq. domain(12 subcarriers)

= 7 x 12 x 2 = 168 symbols per ms

� 64 QAM = 26 QAM = 6 bits per symbol.

� 16800 symbols per ms = 16,800,000 symbols per sec = 16.8 Msps.

� Throughput = data rate = 16.8 x 6 = 100.8 Mbps for single chain.

� LTE 4x4 MIMO (4T4R) 100.8 x 4 = 403.2 Mbps for DL.

� But there is 25% overhead use for controlling and signaling so 403.2 x 0.75 = 302.4 Mbps ~ 300 Mbps.

� For UL we have only one transmit chain at UE end so after 25% 100.8 x 0.75 = 75.6 Mbps ~ 75 Mbps.

� There is why we get the # of throughput 300 Mbps for DL and 75 Mbps for UL shown everywhere!!

Use 3GPP Spec. 36.213 for Throughput Calculation� Coding rate described the efficiency of the particular modulation scheme.

� Example: 16 QAM with 0.5 coding rate means its can only carry 2 information bits.

� The combination of the modulation and coding rate is called Modulation Coding Scheme (MCS).

� Example: 100 RBs MCS Index = 28, the TBS = 75376, assume 4x4 MIMO so the peak data rate

= 75376 x 4 = 301.5 Mbps.Table 7.1.7.2.1-1: Transport block size table (dimension 27××××110)

DL/UL Throughput calculation for LTE FDD

� BW = 20 MHz

� Multiplexing scheme = FDD

� UE category = Cat 3

� Modulation supported =

per Cat 3 TBS index 26 for DL (75376 for 100 RBs) and 21 for UL (51024 for 100 RBs)

� Throughput = # of Chains x TB size.

DL throughput = 2 x 75376 = 150.752 Mbps.

UL throughput = 1 x 51024 = 51.024 Mbps.

Good website: http://niviuk.free.fr/ue_category.php

DL/UL Throughput calculation for LTE TDD

Table. LTE TDD frame configuration.

Table. Special subframe configuration.

� BW = 20 MHz

� Multiplexing scheme = TDD

� UE category = Cat 3

� Modulation supported = per Cat 3 TBS index 26 for DL (75376 for 100 RBs)

and 21 for UL (51024 for 100 RBs)

� TDD frame configuration 2 (D-6, S-2 and U-2)

� Special subframe configuration 7 (DwPTS-10, GP-2 and UpPTS-2)

� DL Throughput = # of Chains x TB size x (DL Subframe + DwPTS in SSF)

� UL Throughput = # of Chains x TB size x (UL Subframe + UpPTS in SSF)

� DL Throughput = 2 x 75376 x (0.6 + 2(10/14)) = 112 Mbps.

� UL Throughput = 1 x 51024 x (0.2 + 0.2(2/14)) = 12 Mbps.



�Physical Layer

� LTE Test Items

• Overview

• EPC

• E-UTRAN

• UE

Agenda

• OFDM

• MIMO


• HARQ



• Frequency Band





Frame StructureFDD Frame Structure

TDD Frame Structure

1/ (15000 2048) 32.6 nssT = × =� Type 1 is defined for FDD mode.

� Each radio frame is 10 ms long and

consists of 10 subframes. Each

subframe contains two slots.

� In FDD, both uplink and downlink

have the same frame structure but use

different spectra.

� Type 2 is defined for TDD mode.

� Each radio frame is 10 ms long and

consists of two half frames. Each half

frame contains five subframes.

� Subframe #1 and sometimes subframe

#6 consist of three special fields:

1. downlink pilot timeslot (DwPTS),

2. guard period (GP),

3. uplink pilot timeslot (UpPTS).

Frame Structure type 1 (FDD) FDD: Uplink and downlink are transmitted separately

#0 #2 #3 #18#1 ………. #19

One subframe = 1ms

One slot = 0.5 ms

One radio frame = 10 ms

Subframe 0 Subframe 1 Subframe 9

Frame Structure type 2 (TDD)

DwPTS, T(variable)

One radio frame, Tf = 307200 x Ts = 10 msOne half-frame, 153600 x Ts = 5 ms

#0 #2 #3 #4 #5

One subframe, 30720 x Ts = 1 ms

Guard period, T(variable)

UpPTS, Ｔ(variable)

•5ms switch-point periodicity: Subframe 0, 5 and DwPTS for downlink,

Subframe 2, 5 and UpPTS for Uplink

•10ms switch-point periodicity: Subframe 0, 5,7-9 and DwPTS for downlink,

Subframe 2 and UpPTS for Uplink

One slot, Tslot =15360 x Ts = 0.5 ms

#7 #8 #9

For 5ms switch-point periodicity

For 10ms switch-point periodicity

Frame Structure

� Max FFT size generate OFDM symbols = 2048

Subcarrier frequency spacing = 15 kHz

Sampling rate = 15 kHz*2048 = 30.72 MHz.

� Ts (sampling period)

= 1/sampling rate = 32.6 ns.

� Sampling rate = 8*3.84 MHz = 30.72 MHz.

Frame Structure - Type 1 (FDD)

� For 20 MHz BW:

There are 15360 samples in one time slot

(add all numbers in the red circle)

Ts (sampling period) = 0.5 ms/15360 = 32.6 ns.

OFDM symbols (= 7 OFDM symbols @ Normal CP)

The Cyclic Prefix is created by prepending each symbol with a copy of the end of the symbol

160 2048 144 2048 144 2048 144 2048 144 2048 144 2048 144 2048 (x Ts)

1 frame= 10 sub-frames= 10 ms

1 sub-frame= 2 slots= 1 ms

1 slot= 15360 Ts= 0.5 ms

0 1 2 3 4 5 6etc.

CP CP CP CP CPCPCP

P-SCH - Primary Synchronization Channel

S-SCH - Secondary Synchronization Channel

PBCH - Physical Broadcast Channel

PDCCH -Physical Downlink Control Channel

PDSCH - Physical Downlink Shared Channel

Reference Signal – (Pilot)

DL

symbN

#0 #1 #8#2 #3 #4 #5 #6 #7 #9 #10 #11 #12 #19#13 #14 #15 #16 #17 #18

Downlink Frame Structure - FDD

10 2 3 4 5 6 10 2 3 4 5 6

Table. Sample rates and FFT sizes for each LTE BW configuration.

Sample rate = 15 kHz*2048 = 30.72 MHz

64QAM16QAM QPSK

Downlink mapping - FDD

P-SCH - Primary Synchronization Channel

S-SCH - Secondary Synchronization Channel

PBCH - Physical Broadcast Channel

PDCCH -Physical Downlink Control Channel

PDSCH - Physical Downlink Shared Channel

Reference Signal – (Pilot)

Uplink Frame Structure & PUSCH Mapping

- FDD

10 2 3 4 5 6

#0 #1 #8#2 #3 #4 #5 #6 #7 #9 #10 #11 #12 #19#13 #14 #15 #16 #17 #18

1 frame

10 2 3 4 5 6

1 sub-framePUSCH - Physical Uplink Shared Channel

Demodulation Reference Signal for PUSCH

• • • • •

OFDM symbols (= 7 SC-FDMA symbols @ Normal CP)

The Cyclic Prefix is created by prepending each symbol with a copy of the end of the symbol

160 2048 144 2048 144 2048 144 2048 144 2048 144 2048 144 2048 (x Ts)

1 slot= 15360 Ts= 0.5 ms

0 1 2 3 4 5 6etc.

CP CP CP CP CPCPCP

UL

symbN

PUSCH

Zadoff-ChuPUSCH ≥ 3RB

QPSKPUSCH < 3RB

or PUCCH

Demodulation Reference Signal (for PUSCH)

PUCCH

Demodulation Reference Signal

for PUCCH format 1a/1b

64QAM QPSK BPSK(1a) QPSK(1b)16QAM

Uplink Mapping - FDD

Frame Structure - Type 2 (TDD)

� Special subframes consist of the 3 fields

1. Downlink Pilot Timeslot (DwPTS),

2. Guard Period (GP), and

3. Uplink Pilot Timeslot (UpPTS).

� Seven uplink-downlink configurations

with either 5 ms and 10 ms downlink-to-

uplink periodicity are support.Table. Uplink-downlink configurations

“D” denotes a subframe reserved for downlink transmission,

“U” denotes a subframe reserved for uplink transmission, and

“S” denotes the special subframe.

Downlink

P-SCH

S-SCH

PBCH

PDCCH

PDSCH

Reference Signal

DL/UL subframe

Uplink

Reference Signal(Demodulation)

PUSCH

UpPTS

Physical Layer Definitions

Frame Structure - TDD (5ms switch periodicity)

10 2 3 4 5 6 10 2 3 4 5 6 10 2 3 4 5 6 10 2 3 4 5 6 10 2 3 4 5 6 10 2 3 4 5 6

DwPTS(3-12 symbols)

UpPTS(1-2 symbols)

NsymbDL OFDM symbols (=7 OFDM symbols @ Normal CP)

Cyclic Prefix

160 2048 144 2048 144 2048 144 2048 144 2048 144 2048 144 2048 (x Ts)

1slot = 15360

0 1 2 3 4 5 6

Ts = 1 / (15000x2048)=32.552nsec1 slot

1 subframe

GP(1-10 symbols)

Downlink

P-SCH

S-SCH

PBCH

PDCCH

PDSCH

Reference Signal

Physical Layer Definitions

Frame Structure - TDD (10ms switch periodicity)

10 2 3 4 5 6 10 2 3 4 5 6 10 2 3 4 5 6 10 2 3 4 5 6 10 2 3 4 5 6 10 2 3 4 5 6

DwPTS

NsymbDL OFDM symbols (=7 OFDM symbols @ Normal CP)

Cyclic Prefix

160 2048 144 2048 144 2048 144 2048 144 2048 144 2048 144 2048 (x Ts)

1slot = 15360

0 1 2 3 4 5 6

Ts = 1 / (15000x2048)=32.552nsec1 slot

DL/UL subframe

Uplink

Reference Signal(Demodulation)

PUSCH

UpPTS

LTE User Equipment Categories

There are five UE categories, the main differences are data rates and MIMO capabilities.

Parameters Cat 1 Cat 2 Cat 3 Cat 4 Cat 5

Peak data rate (Mbps) – downlink 10 50 100 150 300

Peak data rate (Mbps) – uplink 5 25 50 50 75

RF bandwidth (MHz) 20 20 20 20 20

Modulation – downlink QPSK

16-QAM

64-QAM

QPSK

16-QAM

64-QAM

QPSK

16-QAM

64-QAM

QPSK

16-QAM

64-QAM

QPSK

16-QAM

64-QAM

Modulation – uplink QPSK

16-QAM

QPSK

16-QAM

QPSK

16-QAM

QPSK

16-QAM

QPSK

16-QAM

64-QAM

Rx diversity � � � � �

2x2 MIMO � � � � �

4x4 MIMO � � � � �

Slot Structure (I)OFDM Symbol and Cyclic Prefix

� Key advantage in OFDM systems is the ability to protect against multipath delay spread.

� The long OFDM symbols allow the introduction of a guard period between each symbol to

eliminate inter-symbol interference (ISI) due to multipath delay spread.

� If the guard period is longer than the delay spread in the radio channel, and if each OFDM

symbol is cyclically extended into the guard period (by copying the end of the symbol to the

start to create the cyclic prefix), then the ISI can be completely eliminated.

CP is created by prepending each symbol

with a copy of the end of the symbol.

Fig. OFDM symbol structure for normal cyclic prefix case (downlink).

Table. SC-FDMA CP length (uplink). Table. OFDM CP length (downlink).

5.2 s for first symbol

4.7 s for other symbols.

µ

µ

512 32.6 ns 16.7 s.µ× =

Resource Element and Resource Block

Slot Structure (II)

� A resource element is the smallest unit in the physical layer and occupies one OFDM or

SC-FDMA symbol in the time domain and one subcarrier in the frequency domain.

� A resource block (RB) is the smallest unit that can be scheduled for transmission. An RB

physically occupies 0.5 ms (= 1 slot) in the time domain and 180 kHz in the frequency domain.

Fig. Resource grid for uplink (a) and downlink (b).

Table. RB parameters for the uplink.

Table. RB parameters for the downlink.

• 7.5 kHz subcarrier spacing, which is used for multimedia

broadcast over single frequency network (MBSFN).

• Symbols are twice as long, which allows the use of a

longer CP to combat the higher delay spread in larger

MBSFN cells.

Configurable Channel Bandwidth

� In CDMA systems, the transmission bandwidth is fixed and determined by the

inverse of the chip rate.

� In OFDM systems, the subcarrier spacing is determined by the inverse of the FFT

integration time. So number of subcarriers and transmission bandwidth can be

determined independently. More flexibility.

Table. Transmission bandwidth configuration.

1 RB includes 12 subcarriers

LTE symbol rate = 66.7µs, ∆f = 1/ symbol rate = 15 kHz for each subcarrier.

In freq. domain 1 RE = 1 subcarrier, so 1 RB = 12 subcarriers = 180 kHz.

To Be Continued …

lte introduction part1

Engineering