WiMAX Technology for Broadband Wireless Access 2007 phần 3 pptx

It is planned that WiMAX system profi les with a 5 MHz channel bandwidth Table 4.4 Fixed WiMAX certifi cation profi les, all using the OFDM PHY and the PMP modes Frequency band GHz Dup

widely announced WiMAX frequency band We here mention that third-generation (3G) cellular systems operating in the 2.5 GHz band as an extension band for these systems have been reported.

• License-exempt bands: 5 GHz The 2004 WiMAX unlicensed frequency fi xed profi le used the upper U-NII frequency band, i.e the 5.8 GHz frequency band (see Table 4.1) In the future, various bands between 5 GHz and 6 GHz can be used for unlicensed WiMAX, depending on the country involved

Table 4.3 shows (globally) the present expected WiMAX frequencies around the world Other frequencies are sought These frequencies should not be higher than the 5.8 GHz already cho-sen because, for relatively high frequencies (3.5 GHz is itself not a very small value), NLOS operation becomes diffi cult, which is an evident problem for mobility The Regulatory Work-ing Group (RWG), introduced in Chapter 2, is trying to defi ne both new frequencies (reports talk about 450 MHz and 700 MHz) and also the conditions for an easy universal roaming with (possible) different frequencies in different countries Regulator requirements mainly allow both Time Division Duplexing (TDD) and Frequency Division Duplexing (FDD) The attributed frequency spectrum size is a function of the country Some elements about the WiMAX situation in some countries are given below

4.3.1 France

In France, as elsewhere, the authorities wish to have (at least fi xed) broadband access in the highest possible percentage of the territory WiMAX has been seen as a means to provide this broadband access Altitude Operator (owned by Iliad) has a WiMAX license in the 3.5 GHz band Altitude obtained it in 2003 when the regulating authority, Autorité de Régulation des Télécommunication (ART), accepted that Altitude takes a WLL license owned (and not used)

by another operator Since then, ART has changed its name to become ARCEP (Autorité de Régulation des Communications Electroniques et des Postes, http://www.arcep.fr)

Table 4.2 Transmit spectral mask parameters [1] A, B,

C and D are in MHz Channelisation

Table 4.3 Expected WiMAX frequencies (based on RWG documents)

Region or country Reported WiMAX frequency bands

Central and South America 2.5, 3.5 and 5.8 GHz

South-East Asia 2.3, 2.5, 3.3, 3.5 and 5.8 GHz

Middle East and Africa 3.5 and 5.8 GHz

In August 2005, ARCEP started the process of attribution of two other WiMAX licenses (2⫻15 MHz each):

• BLR 1: 3465–3480 and 3565–3580 MHz;

• BLR 2: 3432.5–3447.5 and 3532.5–3547.5 MHz

This process ended in July 2006 by the allocation of these two licences to two operators in each of the 22 French metropolitan regions However, Altitude is the only French operator with a national WiMAX license The choice was made based on three equally important criteria:

• contribution to the territorial development of broadband access;

• aptitude to ameliorate a high data rate concurrence;

• allowances paid by the operator

The operators should have a minimum number (in total) of 3500 WiMAX sites by June 2008 They will be paying 125 million euros in 2006

4.3.2 Korea

In Korea, the frequencies attributed to WiBro are in the 2.3–2.4 GHz band In 2002, 100 MHz bands were decided for WiBro in Korea and WiBro licenses were attributed in January 2005 The three operators are Korea Telecom (KT), SK Telecom (SKT) and Hanaro Telecom Pilot networks are already in place (April 2006) Relatively broad coverage public commercial offers should start before the end of 2006

4.3.3 USA

In the USA, a large number of 2.5 GHz band licenses (the BRS, or Broadband Radio Service, and the EBS, or Educational Broadband Service) and 2.3 GHz band licenses (WCS, or Wire-less Communications Service) are owned by many operators Sprint and Nextel have joined forces, providing them with by far the greatest number of population served by their license

In the USA, until now the 2.5 GHz band had often been attributed for the MMDS However, EBS licenses have been given to educational entities so that they can be used for educational purposes and the Federal Communications Commission (FCC) has allowed EBS license holders to lease spectra to commercial entities under certain conditions

4.3.4 UK

Currently, two operators have BWA licenses in the UK: PCCW (UK Broadband) and Pipex Their licenses are in the 3.4 GHz (PCCW) and 3.5 GHz (Pipex) bands A number of smaller operators use or plan to use a license-exempt WiMAX frequency band for limited operations

4.3.5 China

China is a country with big dimensions and a still developing telecommunications network For the moment (October 2006), no license for commercial service of WiMAX has been allocated However, WiMAX trials are taking place in many regions and are regularly

reported Leading Chinese telecommunications equipment suppliers, Huawei and ZTE, are reported to be active in the WiMAX fi eld (members of the WiMAX Forum, contributing to experiments, preparing WiMAX products, etc.).

4.3.6 Brazil

Brazil is another country with high expectations for WiMAX Auction of 3.5 GHz and 10 GHz BWA spectra were launched in July 2006 Expectations about the possible use of the 2.5 GHz band for WiMAX have been reported

4.4 WiMAX System Profi les

A WiMAX system certifi cation profi le is a set of features of the 802.16 standard, selected by the WiMAX Forum, that is required or mandatory for these specifi c profi les This list sets, for each of the certifi cation profi les of a system profi les release, the features to be used in typical implementation cases System certifi cation profi les are defi ned by the TWG in the WiMAX Forum The 802.16 standard indicates that a system (certifi cation) profi le consists of fi ve components: MAC profi le, PHY profi le, RF profi le, duplexing selection (TDD or FDD) and power class The frequency bands and channel bandwidths are chosen such that they cover as much as possible of the worldwide spectra allocations expected for WiMAX

Equipments can then be certifi ed by the WiMAX Forum according to a specifi c system certifi cation profi le Two types of system profi les are defi ned: fi xed and mobile These profi les are introduced in the following sections

4.4.1 Fixed WiMAX System Profi les

Table 4.4 shows the fi xed WiMAX profi les [11] These system profi les are based on the OFDM PHYsical Layer IEEE 802.16-2004 (in fact, this PHY Layer did not change very much with 802.16e) All of the profi les use the PMP mode This was the fi rst set of choices decided in June 2004 (at the same time as approval of IEEE 802.16-2004) Each certifi ca-tion profi le has an identifi er for use in documents such as PICS proforma statements Fur-ther system profi les should be defi ned refl ecting regulatory (band opportunities) and market development Among others, new fi xed certifi cation profi les should be approved before the end of 2006 It is planned that WiMAX system profi les with a 5 MHz channel bandwidth

Table 4.4 Fixed WiMAX certifi cation profi les, all using

the OFDM PHY and the PMP modes Frequency

band (GHz)

Duplexing mode

Channel bandwidth (MHz)

Profi le name

and 2.5 GHz frequency band schemes will be added Fixed certifi cation profi les, based on 802.16e, are also planned.

4.4.2 Mobile WiMAX System Profi les

Along with the work on the 802.16e amendment, the mobile WiMAX system profi les were defi ned These certifi cation profi les, known as Release-1 Mobile WiMAX system profi les and shown in Table 4.5, were approved in February 2006 They are based on the OFDMA PHYsical Layer (IEEE 802.16e) and all include only the PMP topology These profi les are defi ned by the Mobile Task Group (MTG), a subgroup of the TWG in the WiMAX Forum Release 1 certifi cation will probably be separated in different Certifi cation Waves, starting with Wave 1 having only part of all Release 1 features

In the OFDMA PHYsical Layer as amended in 802.16e, the number of OFDMA riers (equivalent to the FFT size, see the next chapter) is scalable OFDMA of WiMAX is called scalable OFDMA The TDD mode is the only one that has been chosen for this fi rst set, one of the reasons being that it is more resource-use effi cient FDD profi les may be defi ned in the future The frame length is equal to 5 ms Other technical aspects of the selected profi les will be introduced in the following chapters

subcar-Table 4.5 Release 1 Mobile WiMAX certifi cation profi les, all using the

OFDMA PHY and the PMP modes

Frequency

band (GHz)

Duplexing mode

Channel bandwidth and FFT size (number

of OFDMA subcarriers) 2.3–2.4 TDD 5 MHz, 512; 8.75 MHz, 1024; 10 MHz, 1024

2.305–2.320 TDD 3.5 MHz, 512; 5 MHz, 512; 10 MHz, 1024

2.496–2.690 TDD 5 MHz, 512; 10 MHz, 1024

3.3–3.4 TDD 5 MHz, 512; 7 MHz, 1024; 10 MHz, 1024

3.4–3.8 TDD 5 MHz, 512; 7 MHz, 1024; 10 MHz, 1024

Part Two

WiMAX Physical Layer

WiMAX: Technology for Broadband Wireless Access Loutfi Nuaymi

© 2007 John Wiley & Sons, Ltd ISBN: 0-470-02808-4

Many digital modulations can be used in a telecommunication system The variants are obtained by adjusting the physical characteristics of a sinusoidal carrier, either the frequency, phase or amplitude, or a combination of some of these Four modulations are supported by the IEEE 802.16 standard: BPSK, QPSK, 16-QAM and 64-QAM In this section the modulations used in the OFDM and OFDMA PHYsical layers are introduced with a short explanation for each of these modulations.

5.1.1 Binary Phase Shift Keying (BPSK)

The BPSK is a binary digital modulation; i.e one modulation symbol is one bit This gives high immunity against noise and interference and a very robust modulation A digital phase modulation, which is the case for BPSK modulation, uses phase variation to encode bits: each modulation symbol is equivalent to one phase The phase of the BPSK modulated signal is π

or
π according to the value of the data bit An often used illustration for digital modulation

is the constellation Figure 5.2 shows the BPSK constellation; the values that the signal phase can take are 0 or π

5.1.2 Quadrature Phase Shift Keying (QPSK)

When a higher spectral effi ciency modulation is needed, i.e more b/s/Hz, greater lation symbols can be used For example, QPSK considers two-bit modulation symbols

modu-WiMAX: Technology for Broadband Wireless Access Loutfi Nuaymi

© 2007 John Wiley & Sons, Ltd ISBN: 0-470-02808-4

Table 5.1 shows the possible phase values as a function of the modulation symbol Many variants of QPSK can be used but QPSK always has a four-point constellation (see Figure 5.3) The decision at the receiver, e.g between symbol ‘00’ and symbol ‘01’, is less easy than a decision between ‘0’ and ‘1’ The QPSK modulation is therefore less noise- resistant than BPSK as it has a smaller immunity against interference A well-known

Digital Modulator Digital Signal

Figure 5.2 The BPSK constellation

Table 5.1 Possible phase values for QPSK modulation

Even bits Odd bits Modulation symbol {k

digital communication principle must be kept in mind: ‘A greater data symbol modulation

is more spectrum effi cient but also less robust.’

5.1.3 Quadrature Amplitude Modulation (QAM): 16-QAM and 64-QAM

The QAM changes the amplitudes of two sinusoidal carriers depending on the digital quence that must be transmitted; the two carriers being out of phase of π/2, this amplitude modulation is called quadrature It should be mentioned that according to digital communica-tion theory, QAM-4 and QPSK are the same modulation (considering complex data symbols) Both 16-QAM (4 bits/modulation symbol) and 64-QAM (6 bits/modulation symbol) modula-tions are included in the IEEE 802.16 standard The 64-QAM is the most effi cient modulation

se-of 802.16 (see Figure 5.4) Indeed, 6 bits are transmitted with each modulation symbol.The 64-QAM modulation is optional in some cases:

• license-exempt bands, when the OFDM PHYsical Layer is used

• for OFDMA PHY, yet the Mobile WiMAX profi les indicates that 64-QAM is mandatory

in the downlink

5.1.4 Link Adaptation

Having more than one modulation has a great advantage: link adaptation can be used (this cess is also used in almost all other recent communication systems such as GSM/EDGE, UMTS, WiFi, etc.) The principle is rather simple: when the radio link is good, use a high-level modula-tion; when the radio link is bad, use a low-level, but also robust, modulation Figure 5.5 shows this principle, illustrating the fact that the radio channel is better when an SS is close to the BS Another dimension is added to this fi gure when the coding rate is also changed (see below)

pro-5.2 OFDM Transmission

In 1966, Bell Labs proposed the Orthogonal Frequency Division Multiplexing (OFDM) patent Later, in 1985, Cimini suggested its use in mobile communications In 1997, ETSI included OFDM in the DVB-T system In 1999, the WiFi WLAN variant IEEE 802.11g

considered OFDM for its PHYsical Layer The purpose of this chapter is not to provide a complete reference for the OFDM theory and the associated mathematical proofs Rather, the aim is to introduce the basic results needed for a minimum understanding of WiMAX.OFDM is a very powerful transmission technique It is based on the principle of trans-mitting simultaneously many narrow-band orthogonal frequencies, often also called OFDM

subcarriers or subcarriers The number of subcarriers is often noted N These frequencies

are orthogonal to each other which (in theory) eliminates the interference between channels Each frequency channel is modulated with a possibly different digital modulation (usually the same in the fi rst simple versions) The frequency bandwidth associated with each of these channels is then much smaller than if the total bandwidth was occupied by a single modula-

tion This is known as the Single Carrier (SC) (see Figure 5.6) A data symbol time is N times

longer, with OFDM providing a much better multipath resistance

Having a smaller frequency bandwidth for each channel is equivalent to greater time periods and then better resistance to multipath propagation (with regard to the SC) Better resistance to multipath and the fact that the carriers are orthogonal allows a high spectral effi ciency OFDM is often presented as the best performing transmission technique used for wireless systems

5.2.1 Basic Principle: Use the IFFT Operator

The FFT is the Fast Fourier Transform operator This is a matrix computation that allows the discrete Fourier transform to be computed (while respecting certain conditions) The

QPSK 1/2 5.33 Mb/S

16-QAM 1/2 10.67 Mb/s

64-QAM 2/3 21.33 Mb/s

BS1

Figure 5.5 Illustration of link adaptation A good radio channel corresponds to a high-effi ciency

Mod-ulation and Coding Scheme (MCS)

FFT works for any number of points The operation is simpler when applied for a number

op-erator and realises the reverse operation OFDM theory (see, for example, Reference [12])

shows that the IFFT of magnitude N, applied on N symbols, realises an OFDM signal, where each symbol is transmitted on one of the N orthogonal frequencies The symbols are the

data symbols of the type BPSK, QPSK, QAM-16 and QAM-64 introduced in the previous section Figure 5.7 shows an illustration of the simplifi ed principle of the generation of an OFDM signal In fact, generation of this signal includes more details that are not shown here for the sake of simplicity

Figure 5.6 Time and frequency representation of the SC and OFDM In OFDM, N data symbols are

transmitted simultaneously on N orthogonal subcarriers

IFFT

Each (modulation) symbol is modulated with a possibly different modulation

Figure 5.7 Generation of an OFDM signal (simplifi ed)

If the duration of one transmitted modulation data symbol is Td, then Td 1/∆f, where ∆f

is the frequency bandwidth of the orthogonal frequencies As the modulation symbols are transmitted simultaneously,

Td duration of one OFDM symbol

 duration of one transmitted modulation data symbol

This duration, ∆f, the frequency distance between the maximums of two adjacent OFDM

subcarriers, can be seen in Figure 5.8 This fi gure shows how the neighbouring OFDM carriers have values equal to zero at a given OFDM subcarrier maximum, which is why they are considered to be orthogonal In fact, duration of the real OFDM symbol is a little greater due to the addition of the Cyclic Prefi x (CP)

sub-5.2.2 Time Domain OFDM Considerations

After application of the IFFT, the OFDM theory requires that a Cyclic Prefi x (CP) must

be added at the beginning of the OFDM symbol (see Figure 5.9) Without getting into

Figure 5.8 Presentation of the OFDM subcarrier frequency

Figure 5.9 Cyclic Prefi x insertion in an OFDM symbol

mathematical details of OFDM, it can be said that the CP allows the receiver to absorb much more effi ciently the delay spread due to the multipath and to maintain frequency orthogonal-

ity The CP that occupies a duration called the Guard Time (GT), often denoted TG, is a

tem-poral redundancy that must be taken into account in data rate computations The ratio TG/Td

is very often denoted G in WiMAX/802.16 documents The choice of G is made according

to the following considerations: if the multipath effect is important (a bad radio channel), a

high value of G is needed, which increases the redundancy and then decreases the useful data rate; if the multipath effect is lighter (a good radio channel), a relatively smaller value of G can be used For OFDM and OFDMA PHY layers, 802.16 defi ned the following values for G:

1/4, 1/8, 1/16 and 1/32 For the mobile (OFDMA) WiMAX profi les presently defi ned, only the value 1/8 is mandatory The standard indicates that, for OFDM and OFDMA PHY layers,

an SS searches, on initialization, for all possible values of the CP until it fi nds the CP being used by the BS The SS then uses the same CP on the uplink Once a specifi c CP duration has been selected by the BS for operation on the downlink, it cannot be changed Changing the

CP would force all the SSs to resynchronize to the BS [1]

5.2.3 Frequency Domain OFDM Considerations

All the subcarriers of an OFDM symbol do not carry useful data There are four subcarrier types (see Figure 5.10):

• Data subcarriers: useful data transmission

• Pilot subcarriers: mainly for channel estimation and synchronisation For OFDM PHY, there are eight pilot subcarriers

• Null subcarriers: no transmission These are frequency guard bands

• Another null subcarrier is the DC (Direct Current) subcarrier In OFDM and OFDMA PHY layers, the DC subcarrier is the subcarrier whose frequency is equal to the RF centre frequency of the transmitting station It corresponds to frequency zero (Direct Current) if the FFT signal is not modulated In order to simplify Digital-to-Analogue and Analogue-to-Digital Converter operations, the DC subcarrier is null

In addition, subcarriers used for PAPR reduction (see below), if present, are not used for data transmission

Left guard

Nused

Data subcarriers Pilot subcarriers

Figure 5.10 WiMAX OFDM subcarriers types (Based on Reference [10].)

5.2.4 OFDM Symbol Parameters and Some Simple Computations

The main WiMAX OFDM symbol parameters are the following:

• The total number of subcarriers or, equivalently, the IFFT magnitude For OFDM PHY,

NFFT 256, the number of lower-frequency guard subcarriers is 28 and the number of er-frequency guard subcarriers is 27 Considering also the DC subcarrier, there remains

high-Nused, the number of used subcarriers, excluding the null subcarriers Hence, Nused 200 for OFDM PHY, of which 192 are used for useful data transmission, after deducing the pilot subcarriers

• BW, the nominal channel bandwidth

The sampling frequency, denoted fs, is related to the occupied channel bandwidth by the lowing (simplifi ed) formula:

fol-fs n BW.

This is a simplifi ed formula because, according to the standard, fs is truncated to an 8 kHz

multiple According to the 802.16 standard, the numerical value of n depends of the channel

bandwidths Possible values are 8/7, 86/75, 144/125, 316/275 and 57/50 for OFDM PHY and 8/7 and 28/25 for OFDMA PHY

5.2.4.1 Duration of an OFDM Symbol

Based on the above-defi ned parameters, the time duration of an OFDM symbol can be computed:

OFDM symbol duration  useful symbol time  guard time (CP time)

 1/(one subcarrier spacing)  G  useful symbol time

 (1/∆f) (1 G)

 [1/( fs / NFFT)] (1G)

 [1/( n BW / NFFT)] (1G).

The OFDM symbol duration is a basic parameter for data rate computations (see below)

5.2.4.2 Data Rate Values

In OFDM PHY, one OFDM symbol represents 192 subcarriers, each transmitting a tion data symbol (see above) One can then compute the number of data transmitted for the duration of an OFDM symbol (which value is already known) Knowing the coding rate, the number of uncoded bits can be computed Table 5.2 shows the data rates for different Modula-

modula-tion and Coding Schemes (MCSs) and G values The occupied bandwidth considered is 7 MHz

and the sampling factor is 8/7 (the value corresponding to 7 MHz according to the standard).Consider the following case in Table 5.2: 16-QAM, coding rate  3/4 and G  1/16 It can

be verifi ed that the data rate is equal to:

Data rate  number of uncoded bits per OFDM symbol/OFDM symbol duration

 192  4  (3/4)/{[256/(7 MHz  8/7)] (1  1/16)}

 16.94 Mb/s

It should be noted here that these data rate values do not take into account some overheads such as preambles (of the order of one or two OFDM symbols per frame) and signalling mes-sages present in every frame (see Chapter 9 and others in this book) Hence these data rates, known as raw data rates, are optimistic values.

5.2.5 Physical Slot (PS)

The Physical Slot (PS) is a basic unit of time in the 802.16 standard The PS corresponds to four (modulation) symbols used on the transmission channel For OFDM and OFDMA PHY Layers, a PS (duration) is defi ned as [1]

PS  4/fs.Therefore the PS duration is related to the system symbol rate

This unit of time defi ned in the standard allows integers to be used while referring to an amount of time, e.g the defi nition of transition gaps (RTG and TTG) between uplink and downlink frames in the TDD mode

5.2.6 Peak-to-Average Power Ratio (PAPR)

A disadvantage of an OFDM transmission is that it can have a high Peak-to-Average Power Ratio (PAPR), relative to a single carrier transmission The PAPR is the peak value of trans-mitted subcarriers to the average transmitted signal A high PAPR represents a hard con-straint for some devices (such as amplifi ers) Several solutions are proposed for OFDM PAPR reduction, often including the use of some subcarriers for that purpose These subcarriers are then no longer used for data transmission The 802.16 MAC provides the means to reduce the PAPR PAPR reduction sequences are proposed in Reference [2]

5.3 OFDMA and Its Variant SOFDMA

5.3.1 Using the OFDM Principle for Multiple Access

The OFDM transmission mode was originally designed for a single signal transmission Thus, in order to have multiple user transmissions, a multiple access scheme such as TDMA

or FDMA has to be associated with OFDM In fact, an OFDM signal can be made from many user signals, giving the OFDMA (Orthogonal Frequency Division Multiple Access) multiple access

Table 5.2 OFDM PHY data rates in Mb/s (From IEEE Std 802.16-2004 [1] Copyright

IEEE 2004, IEEE All rights reserved.)

G ratio BPSK

1/2

QPSK 1/2

QPSK 3/4

16-QAM 1/2

16-QAM 3/4

64-QAM 2/3

64-QAM 3/4