ELP 4003 LTE Air Interface

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LTE Air Interface

ESB 4003 R2D

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This book is a training document and contains simplifications. Therefore, it must not be considered as a specification of the system.

The contents of this document are subject to revision without notice due to ongoing progress in methodology, design and manufacturing.

ENKI Adam Girycki assumes no legal responsibility for any error or damage resulting from the usage of this document.

Copyright c⃝ April 28, 2014 by ENKI Adam Girycki.

This document was produced in Poland by ENKI Adam Girycki. It is used for training purpose only and may not be copied or reproduced in any manner without the express written consent of ENKI.

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5 LTE downlink physical channels 71 5.1 Cell search . . . 71 5.2 P-SS. . . 72 5.3 S-SS . . . 73 5.4 RS . . . 75 5.5 PBCH . . . 77 5.5.1 MIB. . . 78 5.5.2 SIB . . . 79 5.6 PCFICH . . . 79 5.7 PDCCH . . . 80 5.7.1 PDCCH usage . . . 80 5.7.2 PDCCH mapping. . . 81 5.7.3 PDCCH format. . . 81 5.7.4 PDCCH processing . . . 81 5.7.5 PDCCH blind decoding . . . 83 5.8 PDSCH . . . 84 5.8.1 CRC attachment . . . 88

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CONTENTS

5.8.3 Channel coding. . . 88

5.8.4 Rate matching . . . 89

5.8.5 Code block concatenation . . . 92

5.8.6 Scrambling . . . 92

5.8.7 Modulation mapper . . . 92

5.8.8 Layer mapper. . . 92

5.8.9 Precoding . . . 95

5.8.10 Resource element mapping . . . 98

5.9 PHICH . . . 99

5.10 PMCH. . . 99

5.11 Downlink physical channels modulation summary . . . 100

6 LTE uplink physical channels 101 6.1 PUSCH . . . 102

6.2 Uplink reference signals. . . 104

6.2.1 RS . . . 104

6.2.2 SRS. . . 105

6.3 PUCCH . . . 105

6.3.1 PUCCH format 1A/1B. . . 107

6.3.2 PUCCH format 1. . . 108

6.3.3 PUCCH format 2. . . 108

6.4 PRACH . . . 109

7 Physical layer procedures 113 7.1 Timing advance . . . 113

7.1.1 Uplink-downlink frame timing . . . 113

7.1.2 Timing advance range . . . 113

7.1.3 Random access . . . 113

7.1.4 Other cases. . . 115

7.1.5 Maintenance of uplink time alignment . . . 116

7.2 Random Access (RA) . . . 116

7.3 Resource allocation . . . 117

7.3.1 Resource allocation type 0 . . . 119

7.4 MIMO . . . 120 7.4.1 Spatial multiplexing . . . 121 7.4.2 Transmit diversity . . . 123 7.4.3 Transmission modes . . . 123 7.4.4 MIMO antennas . . . 124 7.5 UE reporting . . . 126 7.5.1 CQI definition . . . 127

7.5.2 Aperiodic CQI/PMI/RI reporting using PUSCH . . . 127

7.5.3 Periodic CQI/PMI/RI reporting using PUCCH . . . 129

7.6 Modulation order and transport block size determination . . . 130

7.6.1 Modulation determination . . . 131

7.6.2 Transport block size determination . . . 131

7.7 UL power control . . . 134

7.7.1 PUSCH power control . . . 134

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8 LTE mobility 141

8.1 Idle mode mobility . . . 141

8.1.1 PLMN selection . . . 142

8.1.2 Cell selection . . . 144

8.1.3 Cell reselection . . . 146

8.2 Connected mode mobility. . . 148

8.2.1 X2 handover . . . 149

8.2.2 Event triggered reporting . . . 151

8.2.3 A3 event . . . 154

A System information 157

List of Figures 165

List of Tables 167

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1 OFDMA principles

1.1 Two way communication

In order to provide two way communication, so called duplex, two directions of transmission must exist and they must be separated from each other to avoid col-lisions. Transmission from the User Equipment (UE) to the Base Station (BS) is referred to as Uplink (UL), while the transmission from the BS to the UE is referred to as Downlink (DL), see Figure 1.1.

Figure 1.1: Two way communication.

The UL and DL transmissions can be separated in frequency or time domain, as presented in Figure 1.2.

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1.1.1 FDD

The FDD system uses diﬀerent frequency bands for UL and DL, separated by the duplex distance, see Figure 1.2. In case of FDD, the UL is usually placed on the lower frequency band because the transmission of lower frequency radio wave re-quires less energy comparing to the higher frequency band, on which the DL is placed. In FDD solution the transmission and reception may take place contin-uously or discontincontin-uously. An example of the FDD system is Global System for Mobile communication (GSM).

1.1.2 TDD

The TDD system uses the same frequency band for both UL and DL, which is time shared as presented in Figure 1.2. TDD requires only one frequency to realise two way communications, which may be an advantage when the availability of radio resources is a limiting factor. On the other hand, to avoid any collisions, TDD system requires a time structure (synchronisation) to separate the UL and DL transmission, which is always discontinuous. An examples of the TDD system is cordless telephony system.

1.2 Access network evolution overview

Apart from duplex transmission separation, a harmonised access of multiple UEs to the shared radio resources must exist, see Figure 1.3. In uplink direction a number UEs transmit to the base station. Thus the multiple access technology is required, which allows the base station to separate transmissions from diﬀerent UEs. In downlink direction a single base station has to keep a connection with multiple users. For that reason a multiple access method is applied, which allows multiplexing of signals at the base station and demultiplexing the signal at the receiving side.

Figure 1.3: Multiple access.

Each generation of cellular telecommunications system provided diﬀerent, more ef-fective, radio access technology, which are discuss in the next sections. The briefly summary of cellular technologies evolution is presented in Figure 1.4.

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1.2 Access network evolution overview

Figure 1.4: Cellular technologies evolution.

1.2.1 1G FDMA

1st Generation (1G), the first generation of wireless telecommunications technology, was introduced in the 1980s and it was an analogue system. The oﬀered service was the voice.

Examples of 1G system:

• Nordic Mobile Telephony (NMT) introduced in 1981 and developed in Nordic

countries, Switzerland, Netherlands, Eastern Europe and Russia

• Advanced Mobile Phone Systems (AMPS) introduced in 1983 and developed

in North America and Australia

In 1G system, Frequency Division Multiple Access (FDMA) method of radio re-sources usage was applied. The available radio rere-sources were divided in frequency domain. For each connection a separate bandwidth was allocated and the user trans-mission on the allocated channel was continuous, which is illustrated in Figure 1.5. The allocated one way channel had bandwidth of 25 kHz in NMT and 30 kHz in case AMPS, resulting in a total of 50 kHz in NMT and 60 kHz in AMPS for each duplex channel.

1.2.2 2G TDMA

2G, the second generation of wireless telecommunications technology, was introduced in 1990s. It was a digital system. The oﬀered services were voice, Short Message Service (SMS), Circuit Switched (CS) data transfer with the rate of 9.6 kbit/s. Examples of 2G systems:

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Figure 1.5: Frequency Division Multiple Access (FDMA).

• GSM introduced in 1991 and used across more than 212 countries and

territo-ries.

• Digital Advanced Mobile Phone Systems (D-AMPS) introduced 1991 and used

in North America.

• CDMAOne introduced in 1995 and used in the Americas and parts of Asia.

GSM, which is the dominant 2G system, employs the Time Division Multiple Access (TDMA) method of radio resources usage combined with FDMA. Available radio resources are first divided into Radio Frequency (RF) channels of 200 kHz bandwidth (FDMA concept) and next each RF channel is divided in time domain into timeslots (TDMA concept). A certain number of timeslots create so called TDMA frame. The number of timeslots in the TDMA frame is system specific. In GSM system eight timeslots make up the TDMA frame. A user has a cyclic access to the common radio resources during the allocated timeslot. Thus the transmission is discontinuous. The Figure 1.6 presents the TDMA system with 4 timeslots in the TDMA frame.

Figure 1.6: Time Division Multiple Access (TDMA).

General Packet Radio Service (GPRS), which is an add-on to the CS GSM also called 2.5G, oﬀers Packet Switched (PS) data transfer with the rate of approximately 50 kbps.

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Enhanced GPRS (EGPRS), also called 2.75G, oﬀers higher date rate of PS data transfer with the maximum rate of approximately 500 kbps, thanks to higher order modulation.

1.2.3 3G WCDMA

3G, the third generation of wireless telecommunication technology, was introduced in 2000s.

Examples of 3G system:

• CDMA2000 introduced in 2000 in South Korea and used in Asia, America and

Africa.

• Universal Mobile Telecommunications System (UMTS) introduced in 2001 in

Japan and used in Europe, Asia and Africa.

3G systems employ Code Division Multiple Access (CDMA) method of radio re-sources usage. CDMA allows for simultaneous transmission of multiple users in the same frequency band, which is presented in Figure 1.7. Separation of diﬀerent connections is achieved by means of diﬀerent codes. The codes must be orthogonal (independent of each other).

Figure 1.7: Code Division Multiple Access (CDMA).

In CDMA2000 the initial frequency band width was 1.25 MHz, which was next tripled to 3x1.25 MHz.

In UMTS, the Wideband Code Division Multiple Access (WCDMA) method is ap-plied, which utilizes wide frequency band of 5 MHz. Wide frequency channel allows for lowering the power density, thus signal may be even weaker than thermal noise level.

High Speed Packet Access (HSPA) provides downlink throughput of approximately 14 Mbps, while Evolved HSPA (also called HSPA+) provides throughput of 84 Mbps.

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1.2.4 4G OFDMA

4G, the fourth generation of mobile telecommunications technology, must support 1 Gbit/s downlink bit rate. Currently there is no system, that is able to support mobile communications with the required bit rate. However there are two technologies, which are on a way to achieve this goal in the nearest future:

• LTE oﬀers approximately 100 Mbit/s bit rate. The world’s first publicly

avail-able LTE-service was opened in the two Scandinavian capitals Stockholm and Oslo on the 14 December 2009.

• Worldwide Interoperability for Microwave Access (WiMAX) oﬀers

approxi-mately 40 Mbit/s bit rate. WiMAX access was used to assist with communi-cations in Aceh, Indonesia, after the tsunami in December 2004.

Both LTE and WiMAX employ Orthogonal Frequency Division Multiple Access (OFDMA) method of radio resources usage. Theoretical foundation of OFDMA had been already laid in 1960’, but due to high costs and lack of appropriate tech-nologies for a long time it remained purely theoretical. This situation has changed with advent of cheap, small and fast microchips capable of processing the FFT and Inverse Fast Fourier Transform (IFFT) algorithms. Nowadays, OFDM is widely used in wireless networking (Wireless Local Area Network (WLAN)), digital televi-sion (Digital Video Broadcasting – Terrestrial (DVB-T)), audio broadcasting (Digi-tal Audio Broadcasting (DAB)) and broadband wireless communications (WiMAX, LTE).

OFDMA is a special type of the FDMA. OFDMA allows for transfer messages simultaneously, using multiple narrow ranges of frequencies, called subcarriers, see Figure 1.8.

Figure 1.8: Orthogonal Frequency Division Multiple Access (OFDMA). To avoid Inter Carrier Interference (ICI), in ordinary FDMA system, all such subcar-riers are separated in frequency domain with guard bands, therefore some spectrum is wasted. OFDM provides much better spectrum eﬃciency, as it does not need gaps between subcarrier bands. Moreover, the subcarrier bands are overlapping, which allows to additionally save some spectrum. ICI is mitigated here by taking advan-tage of the fact that under the following conditions the subcarriers are orthogonal with one another:

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• The careful choice of subcarrier spacing. The subcarrier spacing ∆f should be

exactly equal to the reciprocity of the OFDMA symbol duration Tsymbol, see

Figure 1.9, which provides that the subcarriers are mathematically orthogonal and thus independent.

• Keeping the synchronisation in the frequency domain, providing there are no

frequency shifts, e.g. due to Doppler eﬀects.

Figure 1.9: OFDM subcarriers.

Multiplexing and demultiplexing of OFDMA symbols into subcarriers can be per-formed using Inverse Inverse Discrete Fourier Transform (IDFT) and DFT. These mathematical procedures, that transform signal from frequency to time domain and opposite, can be implemented with IFFT and FFT algorithms.

The presented above FDMA, TDMA and CDMA multiple access methods are single carrier modulation. OFDMA is a multi carrier modulation. In other words, it means that a large number of closely spaced orthogonal subcarriers are used to carry data. Each subcarrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying) at a low symbol rate, maintaining total data rates similar to conventional single carrier modulation schemes in the same bandwidth.

Advantages of OFDMA:

• OFDMA eﬀectively diminishes also the problem of multipath selective

fad-ing. Due to multipath radio waves propagation in typical urban environment, signal at the receiver can be constructively or destructively interfered by the same signal delayed over different path. This effect can dramatically change depending on frequency used as a signal carrier – some of the frequencies will suffer from deep fading, while neighbouring ones may not be affected at all. As

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OFDMA uses very small subcarrier widths, the fading within every subcarrier can be considered as relatively flat.

• Another problem mitigated by OFDMA is Inter Symbol Interference (ISI).

One of the causes of this effect is signal reflection from distant object (typi-cally mountain). The delayed signal, which propagates over much longer path, interferes with the direct signal because it carries another (older) symbol than the direct signal and therefore the receiver is unable to detect the correct sym-bol. The ISI effect is diminished when the symbol duration is longer, thus only very far objects will lead to ISI. But the signal reflected from very far object is usually week enough and does not lead to interference. In OFDMA, the symbol duration can be lengthened, because a few symbols can be transmitted simultaneously on different subcarriers. As already mentioned, longer symbol makes the radio path less vulnerable to ISI. Additionally, to avoid overlapping, the adjacent symbols are always separated in time by short guard period. In the guard period, from technical reasons, it is not effective to stop transmission at all, thus, so called, cyclic prefix is inserted here, which is simply a copy of the signal tail end.

• OFDMA can achieve a higher Multiple Input Multiple Output (MIMO)

spec-tral eﬃciency due to providing flatter frequency channels than a CDMA rake receiver can.

• No cell size breathing as more users connect.

Recognised disadvantages of OFDMA:

• Higher sensitivity to frequency oﬀsets and phase noise.

• Asynchronous data communication services such as web access are

charac-terised by short communication bursts at high data rate. Few users in a base station cell are transferring data simultaneously at low constant data rate.

• The complex OFDMA electronics, including the FFT algorithm and forward

error correction, is constantly active independent of the data rate, which is ineﬃcient from power consumption point of view, while OFDMA combined with data packet scheduling may allow that the FFT algorithm hibernates during certain time intervals.

• The OFDMA diversity gain, and resistance to frequency-selective fading, may

partly be lost if very few sub-carriers are assigned to each user, and if the same carrier is used in every OFDMA symbol. Adaptive sub-carrier assignment based on fast feedback information about the channel, or sub-carrier frequency hopping, is therefore desirable.

• Dealing with co-channel interference from nearby cells is more complex in

OFDMA than in CDMA. It would require dynamic channel allocation with advanced coordination among adjacent base stations.

• The fast channel feedback information and adaptive sub-carrier assignment is

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1.3 Complex numbers

Complex numbers are used in OFDMA signal processing. A complex number is a number comprising a real (Re) and imaginary (Im) part.

1.3.1 Rectangular notation

The complex number can be written in the form of rectangular notation (also called Cartesian notation) a + ib, where a and b are real numbers, and i is the standard imaginary unit with the property i2 =−1. Figure 1.10 shows geometric representa-tion of a complex number z = a+ib in the complex plane. The complex plane can be thought of as a Cartesian plane, with the real part of a complex number represented by a displacement along the x-axis, and the imaginary part by a displacement along the y-axis.

Figure 1.10: Geometric representation of a complex number in the rectangular notation in a complex Cartesian plane.

Each complex number z has a conjugate z∗, which has the same real part but opposite imaginary part, see 1.11:

z = a + ib (1.1)

z∗= a− ib (1.2)

1.3.2 Polar notation

Figure 1.12 presents another notation, so called polar notation, of a complex number. In the polar plane the complex number is represented by its modulus (absolute value)

r and argument (angle) φ.

1.3.3 Relation between rectangular and polar notation

Relation between rectangular and polar notation of a complex number is the follow-ing:

a = r cos φ (1.3)

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Figure 1.11: Conjugate z∗ of a complex number z.

Figure 1.12: Geometric representation of a complex number in the polar notation.

Thus, the complex number z = a + ib may be expressed as follows:

z = a + ib = r cos φ + ir sin φ = r(cos φ + i sin φ) (1.5)

1.3.4 Euler’s formula

Leonhard Euler, Swiss mathematician and physicist, discovered a mathematical re-lationship between the trigonometric functions (sin and cos) and the complex expo-nential function (see also Figure 1.13):

cos φ + i sin φ = eiφ (1.6)

Euler’s formula was called by Richard Feynman ”one of the most remarkable, almost astounding, formulas in all of mathematics”.

1.3.5 Exponential notation

Using the Euler’s formula the complex number z may be written as follows, which is called the exponential notation of a complex number:

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1.4 Fourier analysis

Figure 1.13: Euler’s formula.

In the exponential notation certain calculations, particularly multiplication and di-vision of complex numbers, are easier than in rectangular notation. On the other hand, addition and subtraction are easier with the use of rectangular notation. The exponential notation of a complex number is in widespread use in engineering and science.

Using the Euler’s formula the conjugate z∗ may be written as:

z∗= e−iφ (1.8)

1.4 Fourier analysis

1.4.1 Fourier Transform (FT)

Fourier Transform (FT) is an operation that transforms time domain function into frequency domain function. Therefore FT is often called the frequency domain representation of the original time domain function, see Figure 1.14.

1.4.2 Discrete Fourier Transform (DFT) and Fast Fourier

Transform (FFT)

Discrete Fourier Transform (DFT) is a specific kind of FT. The input to the DFT is a finite sequence of real or complex numbers making the DFT ideal for processing information stored in computers. In particular, the DFT is widely employed in signal processing and related fields to analyse the frequencies contained in a sampled signal, to solve partial diﬀerential equations, and to perform other operations such as convolutions or multiplying large integers.

DFT transforms the sequence of N complex numbers a0, a1, ..., aN−1(usually in time domain) into a sequence of A0, A1, ..., AN−1 complex numbers (usually in frequency

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domain) according to the following formula:

Ak = N_∑−1

n=0

anwkn k = 0, ..., N− 1 (1.9)

w = e−2πNi (1.10)

The inverse transform to the DFT, which transforms the sequence of complex num-bers Akback to the sequence of complex values an, is called Inverse Discrete Fourier Transform (IDFT) and is given by the following formula:

an= 1 N N_∑−1 k=0 Akw−kn n = 0, ..., N− 1 (1.11)

In practice, the DFT can be computed eﬃciently using a Fast Fourier Transform (FFT) algorithm and IDFT using Inverse Fast Fourier Transform (IFFT) algo-rithm.

DFT example

We are going to apply the DFT to the following sequence of N = 8 numbers in the time domain:

a = [2, 1, 0, 1, 2, 1, 0, 1] (1.12)

We will show that the DFT of the above sequence is the following sequence of numbers in the frequency domain:

A = [8, 0, 4, 0, 0, 0, 4, 0] (1.13)

Figure 1.15 shows the graphical presentation of the example, where the sequence of real numbers an is transformed into the sequence of complex numbers Ak. The complex numbers Ak are expressed by their modulus r and argument φ (see section 1.3). The modulus r represents the amplitude of the cosinusoidal signal of a given frequency f and the argument φ corresponds to the phase shift of the cosinusoidal signal. Because, in this example, the phase shift of the cosinusoidal signals is zero (which means that the imaginary parts of complex numbers Ak are equal zero) therefore Ak are actually real numbers. For the sequence of 8 numbers, the DFT formula may be expressed by the following matrix form:

              A0 A1 A2 A3 A4 A5 A6 A7               =               1 1 1 1 1 1 1 1 1 w w2 w3 w4 w5 w6 w7 1 w2 w4 w6 w8 w10 w12 w14 1 w3 w6 w9 w12 w15 w18 w21 1 w4 _w8 _w12 _w16 _w20 _w24 _w28 1 w5 w10 w15 w20 w25 w30 w35 1 w6 w12 w18 w24 w30 w36 w42 1 w7 w14 w21 w28 w35 w42 w49               ·               a0 a1 a2 a3 a4 a5 a6 a7               (1.14)

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Figure 1.15: Example of the Discrete Fourier Transform (DFT). w = e−2π8i = e− π 4i = cos (_π 4 ) − i sin(π 4 ) = √1 2 − i √ 2 (1.15)

When raising the coeﬃcient w to any integral power, one of eight values is obtained, which are illustrated in Figure 1.16. Let us denote these eight complex values by arrows according to Figure 1.16. Now, the matrix form of DFT can be noted in the following way:               A0 A1 A2 A3 A4 A5 A6 A7               =               → → → → → → → → → ↘ ↓ ↙ ← ↖ ↑ ↗ → ↓ ← ↑ → ↓ ← ↑ → ↙ ↑ ↘ ← ↗ ↓ ↖ → ← → ← → ← → ← → ↖ ↓ ↗ ← ↘ ↑ ↙ → ↑ ← ↓ → ↑ ← ↓ → ↗ ↑ ↖ ← ↙ ↓ ↘               ·               2 1 0 1 2 1 0 1               (1.16)

We may calculate Ak numbers from the above matrix notation. As an example

A0, A1 and A2 are calculated below:

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Re Im

Figure 1.16: The coeﬃcient wn in the DFT for N = 8.

A1 = 1· 2 + ( 1 √ 2− i √ 2 ) · 1(−i) · 0 + ( −√1 2 − i √ 2 ) · 1+ + (−1) · 2 + ( −√1 2 + i √ 2 ) · 1 + i · 0 + ( 1 √ 2+ i √ 2 ) · 1 = 0 (1.18) A2 = 1· 2 − i · 1 − 1 · 0 + i · 1 + 1 · 2 − i · 1 − 1 · 0 + i · 1 = 4 (1.19)

You may calculate the remaining Ak values to confirm that the DFT transforms the sequence a = [2, 1, 0, 1, 2, 1, 0, 1] into the sequence A = [8, 0, 4, 0, 0, 0, 4, 0]. It is important to observe that the duration of our signal sample in the time domain was 8 s, while the shift between transformed signals in frequency domain is equal

1 8 s =

1 8Hz.

Inverse Discrete Fourier Transform (IDFT) example

We are going show that the IDFT transforms the sequence of N = 8 numbers in the frequency domain:

A = [8, 0, 4, 0, 0, 0, 4, 0] (1.20)

back into the following sequence of numbers in the time domain:

a = [2, 1, 0, 1, 2, 1, 0, 1] (1.21)

Values an may be calculated from formula 1.11, as presented below, and values w−n are shown in Figure 1.17:

a0= 1 N N_∑−1 k=0 Ak= 1 8(8 + 0 + 4 + 0 + 0 + 0 + 4 + 0) = 2 (1.22)

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a1 = 1 N N∑−1 k=0 Akw−k = 1 8 ( 8w−0·1+ 4w−2·1+ 4w−6·1)= = 1 8(8 + 4i− 4i) = 1 (1.23) a2 = 1 N N_∑−1 k=0 Akw−2k= 1 8 ( 8w−0·2+ 4w−2·2+ 4w−6·2)= = 1 8(8− 4 − 4) = 0 (1.24)

Figure 1.17: The coeﬃcient w−n in the IDFT for N = 8. When comparing with Figure 1.16 notice that w−n is a conjugate of wn.

Figure 1.18 shows the graphical presentation of the IDFT example.

1.5 OFDM concept

The OFDM concept, which uses DFT, is shown in Figure 1.19. In the picture, the information to be transmitted is represented by different Ak values. The process of converting bits into Ak values is called modulation. Each of the Ak values is sent on another subcarrier. In the picture there are N = 10 subcarriers. Ak values, which are sent on different subcarriers, are represented by different heights of the bars. With the use of IDFT the Akvalues are transformed to signal in time domain, which is physically transmitted during symbol time Tsymbol. The time domain signal

is denoted by an values and represented by circles. Because there are 10 bars in the frequency domain before DFT, therefore there are also 10 circles of the time domain signal after IDFT. As already mentioned, the 10 time domain samples are to be transmitted during Tsymbol, therefore the time between samples is equal

Ts=

Tsymbol

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1.5 OFDM concept

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So far we had to do with digital operations (modulation and IDFT are digital opera-tions). Next, the 10 time domain circles anare used to generate an analogue signal, which is physically transmitted from an antenna.

Figure 1.19: OFDM concept.

The receiver performs an opposite operation. It samples the time domain signal every Ts and collects 10 time domain samples an, which are next transformed, with use of the DFT, to frequency domain values Ak. The frequency domain values Ak carry information about bits which were transmitted. The bits are retrieved by demodulation of values Ak.

After time Tsymbol the next symbol may be transmitted. Figure 1.19 illustrates

transmission of 3 symbols. Please observe that there could be a break between consecutive symbols transmission. This break is used to transmit cyclic prefix.

1.5.1 OFDM transmitter

In OFDM the carrier signal is a sum of orthogonal subcarriers. In each subcarrier processing Quadrature Amplitude Modulation (QAM) or Phase Shift Keying (PSK) can be used. A simplified scheme of an OFDM transmitter has been shown in Figure 1.20.

s[i] is input bit stream. First, bits are separated into N parallel streams. Streams

are assigned for Quadrature Amplitude Modulation (QAM) or Phase Shift Keying (PSK) modulation. Depending on the modulation, subcarriers may have diﬀerent transmission bit rate.

Next, IFFT is computed for the sequence of complex data symbols A0, ..., AN−1, which results in a sequence of complex time symbols a0, ..., aN−1 of the signal. For each symbol, after imaginary and real part separation, both parts are converted to analogue in Digital-to-Analogue converter (D/A). Next, analogue signals are quadrature modulated (multiplied by cosine and sine functions) and summed up giving the output modulated signal s(t).

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1.6 Modulation

Figure 1.20: OFDM transmitter.

1.5.2 OFDM receiver

Figure 1.21 presents the simplified OFDM receiver model. Receiver is detecting the signal rx(t). Besides the wanted signal also signal with 2f frequency is created. Therefore low pass filter is used to filter it out. Next, the signal is sampled and converted to digital by the Analogue-to-Digital converter (A/D). The series of com-plex time symbols is then corrected for frequency drifts and global phase oﬀsets (not shown in the diagram). In the next step FFT is carried out and frequency symbol detection takes place, which results in N parallel bit streams, joined finally into one initial bit stream s(i).

Figure 1.21: OFDM receiver.

1.6 Modulation

In telecommunications, modulation is the process of conveying a message signal, for example digital information bit stream, inside another signal that can be physically

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transmitted. Modulation of a sine waveform is used to transform a baseband message signal to a passband signal, for example a RF signal. Electrical signals can only be transferred over a limited passband frequency spectrum, with specific (non-zero) lower and upper cut-oﬀ frequencies. Modulating a sine wave carrier makes it possible to keep the frequency content of the transferred signal as close as possible to the centre frequency (typically the carrier frequency) of the passband.

For the purpose of LTE it is a good idea to think about the modulation as a tech-nique, which changes a digital signal of bits into another digital signal of complex numbers. The complex numbers represent amplitude and phase shift of OFDM subcarriers.

The modulation techniques used in LTE are based on phase and amplitude modu-lation of the carrier frequency, see also Figure 1.22:

• Binary Phase Shift Keying (BPSK) allows for transmission of one information

bit during one modulation symbol.

• Quadrature Phase Shift Keying (QPSK) allows for transmission of two

infor-mation bits during one modulation symbol.

• 16 Quadrature Amplitude Modulation (16QAM) allows for transmission of 4

information bits during one modulation symbol.

• 64 Quadrature Amplitude Modulation (64QAM) is the fastest modulation used

in LTE and allows for transmission of 6 information bits during one modulation symbol.

Only QPSK, 16QAM and 64QAM are used in LTE for user data bit. QPSK is only used for some control information bits, which require robust modulation.

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1.6 Modulation

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2 EPS architecture

2.1 LTE requirements

Operators around the world have been rapidly deploying 3rd Generation (3G) net-work technologies, including UMTS, HSPA, and CDMA2000 1xEV-DO, to support increasing subscriber demand for mobile broadband services. LTE is a step toward the 4th Generation (4G). LTE requirements are specified by TS 25.913:

• Capability-related requirements. ◦ Peak data rate.

Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) should support significantly increased instantaneous peak data rates. The sup-ported peak data rate should scale according to size of the spectrum allocation.

Note that the peak data rates may depend on the numbers of transmit and receive antennas at the UE. The targets for DL and UL peak data rates are specified in terms of a reference UE configuration comprising:

1. DL capability – 2 receive antennas at UE. 2. UL capability – 1 transmit antenna at UE.

For this baseline configuration, the system should support an instanta-neous downlink peak data rate of 100 Mbps within a 20 MHz downlink spectrum allocation (5 bps/Hz) and an instantaneous uplink peak data rate of 50 Mbps (2.5 bps/Hz) within a 20 MHz uplink spectrum alloca-tion. The peak data rates should then scale linearly with the size of the spectrum allocation.

In case of spectrum shared between downlink and uplink transmission, E-UTRAN does not need to support the above instantaneous peak data rates simultaneously.

◦ Control Plane (CP) latency.

Transition time (excluding downlink paging delay and Non-Access Stra-tum (NAS) signalling delay) of less than 100 ms from a camped-state (Idle Mode) to an active state, in such a way that the User Plane (UP) is established.

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E-UTRAN UP latency reduced to less than 5 ms in unload condition for small Internet Protocol (IP) packets.

◦ CP capacity.

The system should be able to support a large number of users per cell with quasi instantaneous access to radio resources in the active state. It is expected that at least 200 users per cell should be supported in the active state for spectrum allocations up to 5 MHz, and at least 400 users for higher spectrum allocation.

A much higher number of users is expected to be supported in the camped state.

• System performance requirements. ◦ DL user throughput.

Target for user throughput per MHz at the 5% point of the C.D.F., 2 to 3 times Release 6 HSDPA.

Target for averaged user throughput per MHz, 3 to 4 times Release 6 HSDPA. Both targets should be achieved assuming Release 6 refer-ence performance is based on a single Tx antenna at the Node B with enhanced performance type 1 receiver in UE whilst the E-UTRA may use a maximum of 2 Tx antennas at the Node B and 2 Rx antennas at the UE.

The supported user throughput should scale with the spectrum band-width.

◦ UL user throughput.

Target for user throughput per MHz at the 5% point of the C.D.F., 2 to 3 times Release 6 Enhanced Uplink (deployed with a single Tx antenna at the UE and 2 Rx antennas at the Node B).

Target for averaged user throughput per MHz, 2 to 3 times Release 6 Enhanced Uplink (deployed with a single Tx antenna at the UE and 2 Rx antennas at the Node B).

Both should be achievable by the E-UTRAN using a maximum of a single Tx antenna at the UE and 2 Rx antennas at the Node B. Greater user throughput should be achievable using multiple Tx an-tennas at the UE.

The user throughput should scale with the spectrum bandwidth pro-vided that the maximum transmit power is also scaled.

◦ Spectrum eﬃciency.

Downlink.

In a loaded network, target for spectrum eﬃciency (bits/sec/Hz/site), 3 to 4 times Release 6 HSDPA. This should be achieved assuming Release 6 reference performance is based on a single Tx antenna at the Node B with enhanced performance type 1 receiver in UE whilst

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2.2 EPS architectural principles

the E-UTRA may use a maximum of 2 Tx antennas at the Node B and 2 Rx antennas at the UE.

Uplink

In a loaded network, target for spectrum eﬃciency (bits/sec/Hz/site), 2 to 3 times Release 6 Enhanced Uplink (deployed with a single Tx antenna at the UE and 2 Rx antennas at the Node B). This should be achievable by the E- UTRA using a maximum of a single Tx antenna at the UE and 2Rx antennas at the Node B.

◦ Mobility.

The E-UTRAN shall support mobility across the cellular network and should be optimised for low mobile speed from 0 to 15 km/h. Higher mobile speed between 15 and 120 km/h should be supported with high performance. Mobility across the cellular network shall be maintained at speeds from 120 km/h to 350 km/h (or even up to 500 km/h depending on the frequency band).

◦ Coverage.

E-UTRAN should support the maximum cell range of 100 km.

2.2 EPS architectural principles

The LTE radio network is called E-UTRAN. System Architecture Evolution (SAE) is the core network architecture of the LTE wireless communication standard. SAE is the evolution of the GPRS Core Network. The main component of the SAE architecture is the Evolved Packet Core (EPC).

The Long Term Evolution/System Architecture Evolution (LTE/SAE) system, which consists of E-UTRAN and EPC, is called Evolved Packet System (EPS), see Figure 2.1. LTE/SAE is specified from 3GPP Technical Specification (3GPP TS) Release 8.

2.2.1 Evolved Packet Core (EPC)

The EPC provides access to external data networks (e.g., Internet, corporate net-works) and operator services (e.g., Multimedia Messaging Services (MMS)1, Multi-media Broadcast and Multicast Services (MBMS)2). It also performs functions re-lated to security (authentication, key agreement), subscriber information, charging and inter-access mobility (GSM EDGE Radio Access Network (GERAN)/Univer-sal Terrestrial Radio Access Network (UTRAN)/E-UTRAN/Interworking Wireless

1

Multimedia Messaging Services (MMS) is a standard way to send messages that include mul-timedia content to and from mobile phones. It extends the core SMS capability which only allows exchange of text messages up to 160 characters in length. The most popular use of MMS is to send photographs from camera-equipped handsets, although it is also popular as a method of delivering news and entertainment content including videos, pictures, text pages and ringtones.

2

Multimedia Broadcast and Multicast Services (MBMS) is a broadcasting service, which may be oﬀered via existing GSM and UMTS cellular networks. The main application is mobile TV. The infrastructure oﬀers an option to use an uplink channel for interaction between the service and the user.

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Figure 2.1: EPS architecture.

Local Area Network (IWLAN)/Code Division Multiple Access 2000 (CDMA2000) etc.). The EPC also tracks the mobility of inactive terminals (i.e., terminals in power saving state).

The number of user plane nodes3 in the core network has been reduced from two in Release 6 (Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN)) to only one in EPS called Packet Data Network/Serving Gateway (P/S-GW). The P/S-GW can be divided into a S-GW and P-GW but often resides in the same physical node referred to as P/S-GW or System Architecture Evolution Gateway (SAE-GW). In typical implementations the P/S-GW is realised by software upgrade of GGSN.

The control plane node is called MME and it may be realised by software upgrade of SGSN.

2.2.2 Evolved UTRAN (E-UTRAN)

E-UTRAN performs all radio related functions for active terminals (i.e. terminals sending data). The number of user plane nodes in E-UTRAN has been reduced to one only and the node is called Evolved Node B (eNB) The interface between the 3_{User plane is a communication strata responsible for user data transmission, in contrast to}

control plane, which is responsible for signalling transmission. The strata concept is explained in the next section.

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2.3 Strata

EPC and the E-UTRAN is called S1 and the interface between the eNBs is called X2.

2.3 Strata

To keep the questions of mobility and connection management independent of the air interface technology, the concept of communication strata has been employed in UMTS and it is also used in LTE/SAE. The stack of protocols has been divided into:

• NAS – containing Core Network (CN) protocols between the CN and UE,

which do not terminate in the E-UTRAN, but in the CN itself. E-UTRAN is completely transparent for these protocols, and hence they can be independent of the radio technology used.

• Access Stratum (AS) – containing radio access protocols between the UE and

the E-UTRAN. These protocols are diﬀerent in GSM, UMTS and LTE, since the radio access technology is diﬀerent here (OFDMA instead of TDMA or WCDMA).

2.3.1 Non-Access Stratum (NAS)

The concept of Non-Access Stratum (NAS) is almost the same as in UMTS, however it is implemented in much diﬀerent way.

The UMTS uses the same mobility and connection management protocols as the earlier generation networks (GSM, GPRS) and they are the following:

• Connection Management (CM) and Mobility Management (MM) for the CS

part of the network,

• Session Management (SM) and GPRS Mobility Management (GMM) for the

PS part.

The fact that LTE/SAE is totally packet oriented eliminates the protocols connected with the CS network part and modifies the NAS operation in PS part (i.e. the entire network).

Consequently, the NAS in EPS:

• Introduces the new EPS Mobility Management (EMM) layer, • Inherits the SM layer after UMTS.

From the changes presented above, one can deduct that lower layer EMM had to be redefined for EPS to meet the requirements of the new concept of UE mobility for the PS transmission only. The SM remains the same due to the fact of common way of handling the session management in LTE/SAE, UMTS and GSM (GPRS) systems.

The examples of functions performed by NAS:

• Mobility management for idle UEs, • UE authentication,

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• EPS bearer management,

• Configuration and control of security, • Paging initiation for idle UEs.

The NAS messages are transported by the Radio Resource Control (RRC) layer – the signalling layer of the AS. There are two ways to transport the NAS messages by RRC, either by concatenating the NAS messages with other Radio Resource Control (RRC) messages, or by including the NAS messages in dedicated RRC messages without concatenation.

The NAS messages are protected using the ciphering and integrity protection services provided by the Packet Data Convergence Protocol (PDCP) layer. However, NAS is also protected by its own security functions terminated in the UE and MME, respectively.

On the network side, the NAS layers are in 3rd Generation Partnership Project (3GPP) agreed to be terminated by the MME.

The NAS state model is based on a two-dimensional model which consists of EMM states describing the mobility management states that result from the mobility man-agement procedures e.g. attach and Tracking Area Update (TAU) procedures, and of EPS Connection Management (ECM) states describing the signalling connectivity between the UE and the EPC.

The ECM and EMM states are independent of each other and when the UE is in EMM-CONNECTED state this does not imply that the user plane (radio and S1 bearers) is established.

2.3.2 Access Stratum (AS)

The services, access signalling, mobility and subscriber management specific to CN are completely outside the AS, and are transferred transparently through the Ra-dio Access Network (RAN). AS protocols are specific to the RAN being used by the mobile system. This RAN may be implemented as the GSM Base Station Sys-tem (BSS), GERAN, UTRAN or E-UTRAN. AS provides radio access bearers for both connection-oriented, packet-switched services and connectionless (store-and-forward) services. In LTE/SAE there is no CS network part thus the AS diﬀers significantly from the one in older technologies.

The AS provides the connectivity between the nodes in the E-UTRAN. There are three interfaces that are involved in the AS concept:

• Radio interface – connectivity between the UE and the E-UTRAN node – the

eNB.

• S1 – connectivity between eNB and the core network nodes: ◦ S1-MME – eNB and MME, responsible for control plane. ◦ S1-U – eNB and S-GW, responsible for user plane. • X2 – connectivity between eNBs in E-UTRAN.

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2.4 EPS Bearer and QoS

The EPS defines bearers for services and strictly binds them with QoS level provided. This strict mapping leads to definition of certain QoS level for certain applications using the bearers in the network. Consequently, the bearers will always obtain appropriate QoS classes, according to the requirements of the service provided by the application the UE utilises.

2.4.1 EPS Bearer

Similarly to UMTS, EPS implements a bearer concept for supporting end-user data services. The EPS bearer (similar to a Packet Data Protocol (PDP) context of previous 3GPP releases) is defined between the UE and the P-GW node in the EPC (which provides the end-users IP point of presence towards external networks), see Figure 2.2.

Figure 2.2: EPS bearer concept.

End-to-end services (e.g. IP services) are multiplexed on diﬀerent EPS Bearers. There is a many-to-one relation between end-to-end services and EPS Bearers. An UL Traﬃc Flow Template (TFT) in the UE binds an Service Data Flow (SDF) to an EPS Bearer in the uplink direction. Multiple SDFs can be multiplexed onto the same EPS Bearer by including multiple uplink packet filters in the UL TFT. A DL TFT in the P-GW binds an SDF to an EPS Bearer in the downlink direction. Multiple SDFs can be multiplexed onto the same EPS Bearer by including multiple downlink packet filters in the DL TFT.

The EPS Bearer is further sub-divided into a E-UTRAN Radio Access Bearer (E-RAB) and S5/S8 Bearer. An E-RAB transports the packets of an EPS Bearer between the UE and the EPC. When an E-RAB exists, there is a one-to-one map-ping between this E-RAB and an EPS Bearer. An S5/S8 Bearer transports the packets of an EPS Bearer between a S-GW and a P-GW.

A Radio Bearer transports the packets of an EPS Bearer between a UE and an eNB. When a Radio Bearer exists, there is a one-to-one mapping between this

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Radio Bearer and the EPS Bearer/E-RAB. An S1 Bearer transports the packets of an E-RAB between an eNB and a S-GW.

A UE stores a mapping between an uplink packet filter and a Radio Bearer to create the binding between an SDF and a Data Radio Bearer in the uplink. A P-GW stores a mapping between a downlink packet filter and an S5/S8a Bearer to create the binding between an SDF and an S5/S8a Bearer in the downlink.

An eNB stores a one-to-one mapping between a Radio Bearer and an S1 Bearer to create the binding between a Radio Bearer and an S1 Bearer in both the uplink and downlink.

A S-GW stores a one-to-one mapping between an S1 Bearer and an S5/S8a Bearer to create the binding between an S1 Bearer and an S5/S8a Bearer in both the uplink and downlink.

2.4.2 Quality of Service (QoS)

QoS concept

QoS has been defined by the International Telecommunication Union (ITU) as:

the collective eﬀect of service performance, which determines the degree of satisfaction of a user of a service.

Thus, QoS is connected with the way the user perceives the service. The user is not interested in how a service is provided but only whether or not he or she is satisfied with that service. So, from a user’s perspective the QoS level is a very sub-jective thing and if the network does not provide the desired level of satisfaction, the user may simply stop using the service and possibly change to some other operator oﬀering a similar service with the desired QoS level.

QoS classes

In UMTS four diﬀerent QoS classes (referred also to as traﬃc classes) have been defined. These QoS classes are:

• Conversational class, • Streaming class, • Interactive class, and • Background class.

The main distinction between these QoS classes follows from how delay-sensitive the traffic is: Conversational class is meant for traffic, which is very delay-sensitive, while Background class is the most delay-insensitive traffic class.

QoS Class Identifier (QCI)

In case of LTE, 3GPP in Release 8 introduces another concept: QoS Class Identifier (QCI). QCI is a scalar that is used as a reference to node specific parameters that

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2.5 Integration with 2G and 3G

control packet forwarding treatment. They should be pre-configured by the operator owning the node.

QCI values indicate the QoS characteristics for edge-to-edge packet forwarding be-tween UE and Policy and Charging Enforcement Function (PCEF). Each QCI is associated with the following standardized performance characteristics:

• Resource Type (Guaranteed Bit Rate (GBR) or Non-GBR), • Priority,

• Packet Delay Budget, • Packet Error Loss Rate.

To control the edge-to-edge packet forwarding QCI is signaled to diﬀerent network nodes while the above standardized characteristics are not. It is up to the operator to map QCI values to the corresponding performance characteristics. The charac-teristics of QCI from 1 to 9 are standardized though and should be considered as guidelines when pre-configuring the node specific parameters. The goal of this op-eration is to ensure that applications mapped to a particular QCI receive the same minimum level of QoS regardless access network they use (e.g. when UE is roaming or if the network operator uses equipment from diﬀerent vendors).

Table 2.1 presents standardized QCI values mapped to the corresponding perfor-mance characteristics, as specified in 3GPP TS 23.203.

Mapping between QCI and QoS classes

In order to provide backward compatibility, the mapping between QCI and QoS classes parameters was specified in Time Slot (TS) 23.401. It is presented in Table 2.2.

2.5 Integration with 2G and 3G

When an E-UTRAN system is deployed in a network already supporting GERAN and/or UTRAN it is possible to use a common core network for all accesses. In practice this means that the P-GW will provide GGSN functionality towards the existing GPRS CN. Therefore an E-UTRAN/UTRAN/GERAN capable terminal will not need to change the GGSN (i.e., the IP point of presence towards external networks) when it changes Radio Access Technology (RAT)) and switches between GERAN, UTRAN or E-UTRAN. Figure 2.3 shows how the EPS inter-works with existing 2nd Generation (2G)/3G networks. The figure presents the UTRAN when utilizing the GPRS one tunnel approach standardized in 3GPP Release 7. This feature makes it possible to bypass the SGSN in the user plane.

Figure 2.4 shows a standardization view on how GERAN, UTRAN and E-UTRAN are integrated into the SAE. It should however be noted that the SGSN and MME shares a lot of common functionality. It is also required that the CN protocols, SM and MM, used in 2G/3G are compatible with the respective protocols used in EPS meaning that the SGSN and MME share a common evolution in the 3GPP standard. In a typical implementation/deployment view, it is likely that the 2G/3G SGSN and the MME are merged into one node, as illustrated in Figure 2.4. This

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QCI Resource type Priority Packet delay budget Packet error loss rate Example service 1 GBR 2 100 ms 10−2 Conversational voice

2 4 150 ms 10−3 Conversational video (live

streaming) 3 3 50 ms 10−3 Real-time gaming 4 5 300 ms 10−6 Non-conversational video (buﬀered streaming) 5 non-GBR 1 100 ms 10−6 IMS signalling 6 6 300 ms 10−6 Video buﬀered streaming,TCP based services (e.g. www, e-mail, chat, ftp, p2p file sharing, progressive video, etc.)

7 7 100 ms 10−3 Voice, video live streaming,

interactive gaming

8 8 300 ms 10−6 ”Premium bearer” for video

buﬀered streaming, TCP based services (e.g. www, e-mail, chat, ftp, p2p file sharing, progressive video, etc) for premium subscribers

9 9 300 ms 10−6 ”Default bearer” for video,

TCP based services (etc. for non-privilaged subscribers Table 2.1: QoS Class Identifier (QCI) defined for LTE/SAE.

QCI Traﬃc class

Traﬃc Handling Priority Signalling indication Source statistics descriptor

1 Conversational N/A N/A Speech

2 Conversational N/A N/A Unknown

3 Conversational N/A N/A Unknown

4 Streaming N/A N/A Unknown

5 Interactive 1 Yes N/A

6 Interactive 1 No N/A

9 Background N/A N/A N/A

Table 2.2: Mapping between standardized QCIs and pre-Relese-8 QoS parameter values.

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2.6 Interfaces overview

Figure 2.3: E-UTRAN, UTRAN and GERAN architecture. GPRS one tunnel approach.

will make it possible to support intra SGSN/MME and inter P/S-GW/GGSN node mobility between the diﬀerent accesses.

2.6 Interfaces overview

This section contains a brief overview of the LTE/SAE interfaces.

Gi

Gi is the interface to external packet data networks (e.g. Internet) and contains the end-user’s IP Point of Presence (PoP). All user-plane and control-plane functions that use the Gi interface are handled above the end-user’s IP layer, whereas all terminal mobility within 3GPP is handled below the Gi interface.

S1

S1 is the interface between eNB and MME and between eNB and S-GW. In the user plane this interface will be based on GTP User data tunnelling (GTP-U) (similar to Iu and Gn interface in UMTS). In the control plane the interface is more similar

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Figure 2.4: Typical implementation of LTE/SAE. Combined SGSN/MME one tunnel approach.

to RAN Application Part (RANAP), with some simplifications and changes due to the diﬀerent functional split and mobility within EPS.

It has been agreed to split the S1 interface into a CP (control plane) and S1-UP (user plane) part. The signalling transport on S1-CP will be based on Stream Control Transmission Protocol (SCTP). The signalling protocol for S1 is called S1 Application Protocol (S1AP). S1AP protocol has the following functions:

• EPS Bearer management function.

This overall functionality is responsible for setting up, modifying and releasing EPS bearers, which are triggered by the MME The release of EPS bearers may be triggered by the eNB as well.

• Initial context transfer function.

This functionality is used to establish an S1 UE context in the eNB, to setup the default IP connectivity, to setup one or more SAE bearer(s) if requested by the MME, and to transfer NAS signalling related information to the eNB if needed.

• Mobility functions for UEs in LTE ACTIVE in order to enable:

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S1 interface (with EPC involvement),

◦ a change of RAN nodes between diﬀerent RAT (inter-3GPP-RAT

han-dovers) via the S1 interface (with EPC involvement).

• Paging.

This functionality provides the EPC the capability to page the UE.

• S1 interface management functions:

◦ Reset functionality to ensure a well defined initialisation on the S1

inter-face.

◦ Error Indication functionality to allow a proper error reporting/handling

in cases where no failure messages are defined.

◦ Overload function to indicate the load situation in the control plane of

the S1 interface.

• NAS signaling transport function between the UE and the MME is used

to:

◦ transfer NAS signalling related information and to establish the S1 UE

context in the eNB,

◦ transfer NAS signalling related information when the S1 UE context in

the eNB is already established.

• S1 UE context release function.

This functionality is responsible to manage the release of UE specific context in the eNB and the MME.

S1 is a many-to-many interface.

X2

X2 is the interface between eNBs. The interface is mainly used to support active mode UE mobility (Packet Forwarding). This interface may also be used for multi-cell Radio Resource Management (RRM) functions. The X2-CP interface consists of a signalling protocol called X2 Application Protocol (X2AP) on top of SCTP. The X2-UP interface is based on GTP-U. The X2-UP interface is used to support loss-less mobility (packet forwarding).

The X2-AP protocol provides the following functions:

• Mobility Management (MM).

This function allows the eNB to move the responsibility of a certain UE to another eNB. Forwarding of user plane data is a part of the mobility manage-ment.

• Load management.

This function allows eNBs to indicate overload and traﬃc load to each other.

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This function allows reporting of general error situations, for which function specific error messages have not been defined.

The X2 interface is a many-to-many interface.

S3

S3 is a control interface between the MME and 2G/3G SGSNs. The interface is based on Gn/GTP Control plane (GTP-C) (SGSN-SGSN), possibly with some new functionality to support signalling free idle mode mobility between E-UTRAN and UTRAN/GERAN. S3 will not support packet forwarding; instead this will be sup-ported on the S4 interface.

The S3 interface is similar to the S10 interface between MMEs which will be used for intra-LTE mobility between two MME pool areas.

S4

S4 is the interface between the P-GW and 2G/3G SGSNs. The interface is based on Gn/GPRS Tunnelling Protocol (GTP) (SGSN-GGSN). The user plane interface is based on GTP-U (same as S1-UP and Iu-UP) and the control plane is based on GTP-C (similar to S11).

The S4 interface is backwards compatible with the Gn interface.

S6

S6a enables transfer of subscription and authentication data for authenticating/au-thorizing user access to the evolved system (Authentication, authorisation and ac-counting (AAA) interface) between MME and Home Subscriber Server (HSS). S6d is between the SGSN and the HSS. S6 is based on Diameter.

S5/S8

S5/S8 is the interface between the S-GW and P-GW. In principle S5 and S8 is the same interface, the diﬀerence being that S8 is used when roaming between diﬀerent operators while S5 is network internal. The S5/S8 interface will exist in two variants one based on Gn/GTP (SGSN-GGSN) and the other will use the Internet Engineer-ing Task Force (IETF) specified Proxy Mobile IP (PMIP) for mobility control with additional mechanism to handle QoS.

The usage of PMIP or GTP on S5/S8 will not be visible over the S1 interface or in the terminal. In the non roaming case the S-GW and P-GW functions can be performed in one physical node.

It has been agreed in 3GPP that the usage of PMIP or GTP on S5 and S8 should not impact RAN behaviour or impact the terminals.

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In the roaming case S8 is providing user and control plane between the S-GW in the Visited PLMN (VPLMN) and the P-GW in the Home PLMN (HPLMN). S8 is the inter Public Land Mobile Network (PLMN) variant of S5.

S5/S8 is a many-to-many interface.

S9

S9 provides transfer of QoS policy and charging control information between the Home Policy and Charging Rules Function (PCRF) and the Visited PCRF in order to support local breakout function.

S10

S10 is a control interface between the MMEs which will be very similar to the S3 interface between the SGSN and MME. The interface is based on Gn/GTP-C (SGSN-SGSN) with additional functionality.

S11

S11 is the interface between the MME and S-GW. The interface is based on Gn/GTP-C (interface between SGSN and GGSN) with some additional functions for paging coordination, mobility compared to the legacy Gn/GTP-C (SGSN-GGSN) interface.

S12

S12 is the interface between UTRAN and S-GW for user plane tunnelling when direct tunnel is established. It is based on the Iu-u/Gn-u reference point using the GTP-U protocol as defined between SGSN and UTRAN or respectively between SGSN and GGSN. Usage of S12 is an operator configuration option.

S13

S13 enables UE identity check procedure between MME and Equipment Identify Register (EIR).

SGi

SGi is the interface between the P-GW and the packet data network. Packet data network may be an operator external public or private packet data network or an intra operator packet data network, e.g. for provision of IP Multimedia Subsystem (IMS) services. This interface corresponds to Gi for 3GPP accesses.

Rx

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Gx

Gx provides transfer of QoS policy and charging rules from PCRF to PCEF in the P-GW.

2.7 Evolved Packet Core (EPC) functions

EPC is the core network of the SAE system and is built up with P/S-GW nodes, together with MME nodes.

2.7.1 Mobility Management Entity node

The EPS architecture defines MME node, which contains core network control func-tionality. Although the functionality is not entirely the same, the MME conceptually constitutes a control plane SGSN node. The CP terminal protocols terminate at the MME, which also manages the mobility contexts of the UEs. The same MME re-mains in control of a UE as long as the UE moves within an MME pool area. The MME handles the mobility and session management functions listed below:

• UE attach/detach handling.

This allows UE to register and de-register to the network.

• Security.

The MME implements functions for Authentication and Authorization to ver-ify users’ identities, grant access to the network and track users’ activities, respectively. In addition, the MME performs ciphering and integrity protec-tion of NAS message signalling.

• EPS Bearer handling.

The MME manages the setting up, modification and tearing down of EPS Bearers. It is assumed that a UE in E-UTRAN will always have one default EPS Bearer established at the time of attachment to the network.

• MM for idle mode UEs.

The MME manages mobility of idle mode UEs. Idle mode UEs are tracked with the granularity of Tracking Areas (TAs).

2.7.2 P-GW and S-GW nodes

The EPS architecture defines the Packet Data Network/Serving Gateway (P/S-GW) node. The P/S-GW is the anchor point for the user plane for a terminal moving between eNBs. The S-GW is only changed when the UE move to a new S-GW pool area while the P-GW is normally kept as long as the UE is attached to the network.

The P/S-GW functionality is very similar to the existing GGSN node. The main additions are adding support for packet buﬀering during E-UTRAN paging and additional support for Non-3GPP interworking (e.g. CDMA2000, WLAN). The

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2.7 Evolved Packet Core (EPC) functions

P-GW provides an interface to the outside world (e.g. the Internet). The P/S-GW can mainly be seen as a user plane node, however it also performs some QoS related signalling (it terminates the interface for policy control).

The P/S-GW is involved in the following control plane functions:

• EPS Bearer Handling.

The P/S-GW triggers the setup of EPS Bearers upon request from the policy control functions.

• Mobility Anchor – IP PoP.

The P-GW acts as a mobility anchor point which hides UE mobility from the fixed network. When a UE attaches to the network it is assigned an IP address from a P-GW, which then also assumes the role of mobility anchor to the UE. While the control of a UE may be transferred to another MME or S-GW as a consequence of a Handover (HO), the UE’s IP PoP will remain at the P-GW. Thus, the mobility of UEss is transparent to the fixed network.

Further, the P/S-GW handles the following user plane functions:

• QoS Policy Control and Enforcement.

To simplify bearer requests from an application point of view, increase op-erator’s control over its network resources and limit the potential for abuse by users, EPS QoS is network controlled. The policy control and enforcement functions associate users’ traffic flows with appropriate QoS classes and execute rate policing to prohibit users or flows from exceeding the QoS limits speci-fied in users’ subscription agreements. DL traffic is policed in the P/S-GW whereas UL traffic is policed in the eNB.

• Charging.

The charging function is responsible for charging the user for its traﬃc accord-ing to the rate that applies for a particular service, subscription etc.

• Lawful Intercept.

This function enables communications to be electronically intercepted, or eaves-dropped, by law enforcement agencies, should it be authorized by judicial or regulatory mandates.

2.7.3 MME and S-GW pooling concept

It is possible to pool a number of MME and S-GW nodes together in order to elimi-nate the risk that one node failure will cause parts of the network to be out of service. This is possible since there is a many-to-many relation interface between eNBs and EPC nodes where each eNB is associated with a set of MMEs and S-GWs called an MME and S-GW pool. The resulting network is non-hierarchical. Independent pooling of MME and S-GW are supported, it is however not possible to change a S-GW without involving the MME.

An operator may pool MMEs and S-GWs into one or several pools depending on organisation, regulatory requirements, transport providers etc. This is illustrated in Figure 2.5. The flexibility of the pooling concept makes it possible to enable partial

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sharing of networks; i.e., to use only a part of the operator’s network as a shared network.

Figure 2.5: Inter-pool mobility.

The individual pooled MMEs and S-GW do not have to be located on the same physical site, but can be distributed in the network. All pools of a particular op-erator are assumed to be interconnected by means of an interface similar to the S3/S4/S10/S11 interface.

When a UE attaches to the network, it is assigned to one of the MMEs that belong to the MME pool associated with the eNB through which the UE is attaching, the MME then selects an S-GW in the S-GW pool. No change of MME or S-GW is required while the UE moves around among eNBs belonging to the same MME or S-GW pool. If the UE moves out of the pools coverage it is reassigned to an MME or S-GW in the pool associated with the new eNB.

The P-GW, which performs charging, policy enforcement and UE’s IP PoP is not changed when the S-GW is relocated. The main purpose of the S-GW is to act as a local mobility anchor and to buﬀer packets during E-UTRAN paging. In some equipment vendors views (for example Ericsson) S-GWs are rare and in most cases the S-GW and P-GW functions are performed by the same physical node. MME relocation may be more motivated since there may be limits on how many eNBs the MME is connected to.

ELP 4003 LTE Air Interface

LTE Air Interface

Contents

1 OFDMA principles

1.1

Two way communication

1.1.1

FDD

1.1.2

TDD

1.2

Access network evolution overview

1.2.1

1G FDMA

1.2.2

2G TDMA

1.2.3

3G WCDMA

1.2.4

4G OFDMA

1.3

Complex numbers

1.3.1

Rectangular notation

1.3.2

Polar notation

1.3.3

Relation between rectangular and polar notation

1.3.4

Euler’s formula

1.3.5

Exponential notation

1.4

Fourier analysis

1.4.1

Fourier Transform (FT)

1.4.2

Discrete Fourier Transform (DFT) and Fast Fourier

Transform (FFT)

1.5

OFDM concept

1.5.1

OFDM transmitter

1.5.2

OFDM receiver

1.6

Modulation

2 EPS architecture

2.1

LTE requirements

2.2

EPS architectural principles

2.2.1

Evolved Packet Core (EPC)

2.2.2

Evolved UTRAN (E-UTRAN)

2.3

Strata

2.3.1

Non-Access Stratum (NAS)

2.3.2

Access Stratum (AS)

2.4

EPS Bearer and QoS

2.4.1

EPS Bearer

2.4.2

Quality of Service (QoS)

2.5

Integration with 2G and 3G

2.6

Interfaces overview

2.7

Evolved Packet Core (EPC) functions

2.7.1

Mobility Management Entity node

2.7.2

P-GW and S-GW nodes

2.7.3

MME and S-GW pooling concept