5.3 OFDMA and Its Variant SOFDMA

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.

In OFDMA, the OFDMA subcarriers are divided into subsets of subcarriers. each subset representing a subchannel (see Figure 5.11). In the downlink, a subchannel may be intended for different receivers or groups of receivers; in the uplink, a transmitter may be assigned one or more subchannels. The subcarriers forming one subchannel may be adjacent or not. The standard ^[1] indicates that the OFDM symbol is divided into logical subchannels to support scalability, multiple access and advanced antenna array processing capabilities. The multiple access has a new dimension with OFDMA. A downlink or an uplink user will have a time and a subchannel allocation for each of its communications (see Figure 5.12). Different subchannel distributions and logical renumberings are defined in the 802.16 standard, as will be seen in the rest of this chapter. First, the SOFDMA concept is introduced.

Figure 5.11: Illustration of the OFDMA principle. (Based on Reference ^[1].)

Figure 5.12: Illustration of OFDMA multiple access

5.3.2 Scalable OFDMA (SOFDMA)

OFDMA multiple access is not the only specificity of OFDMA PHY. Another major difference is the fact that its OFDM transmission is scalable. Although this word does not appear in the standard, OFDMA PHY is said to have Scalable OFDMA (SOFDMA). The scalability is the change of the FFT size and then the number of subcarriers. The supported FFT sizes are 2048, 1024, 512 and 128. FFT size 256 (of the OFDM layer) is not included in the OFDMA layer. Only 1024 and 512 are mandatory for mobile WiMAX profiles.

The change in the number of subcarriers, for a fixed subcarrier spacing, provides for an adaptive occupied frequency bandwidth and, equivalently, an adaptive data rate, as shown in the following example. See the example shown in Table 5.3. In this example, the sampling factor is equal to 28/25, chosen according to the channel bandwidth. SOFDMA provides an additional resource allocation flexibility that can be used in the framework of radio resource management policy taking into account the dynamic spectrum demand, among others.

Table 5.3: Example of SOFDMA figures. (Inspired from Reference ^[10].)
Open table as spreadsheet

Parameters	Numerical values
Subcarrier frequency spacing	10.95 kHz
Useful symbol duration (Td= 1/Δf)	91.4 μs
Guard time (TG= Td/8)	11.4 μs
OFDMA symbol duration (Ts= Td + TG)	102.9 μ
Number of OFDMA symbols in the 5 ms frame	48
FFT size (Nfft) or number of subcarriers	512	1024
Channel occupied bandwidth	5MHz	10MHz

5.3.3 OFDMA in the OFDM PHYsical Layer: Subchannelisation

As a matter of fact, the OFDM PHY includes some OFDMA access. Subchannelisation was included in 802.16-2004 for the uplink and also for the downlink in amendment 802.16e. The principle is the following. The 192 useful data OFDM subcarriers of OFDM PHY are distributed in 16 subchannels made of 12 subcarriers each. Each subchannel is made of four groups of three adjacent subchannels each (see below).

A subchannelised transmission is a transmission on only part of the OFDM subcarrier space. The subchannelised transmission can take place on 1, 2, 4, 8 or 16 subchannels. A five-bit indexation shown in Table 5.4 indicates the number of subchannels and the subcarrier indices used for each subchannel index for the uplink. As shown in this table, one or more pilot subcarrier(s) (there are eight in total) are allocated only if two or more subchannels are allocated. The subcarriers other than the ones used for subchannelised transmission are nonactive (for the transmitter). The five-bit subchannel index is used in the uplink allocation message UL-MAP (see Chapter 9 for the UL-MAP).

Table 5.4: The number of subchannels and the subcarrier indices used for each (five bits) subchannel index. (Based on Reference ^[1].)
Open table as spreadsheet

Subchannell index				Pilot frequency index	Subchannel index (continued)	Subcarrier frequency indices
		0b0010	0b00010	−38	0b00001 0b00011	100: −98; −37: −35; 1: 3; 64: 66
					0b00100	−97: −95, −34; −32, 4; 6, 67: 69
				0b000110	13	−94: −92, −31: −29,7: 9, 70: 72
				0b01000	0b00111	−91: −89, −28: −26, 10: 12, 73: 75
		0b01000	0b01010	−88	0b01001	−87: −85, −50: −48, 14: 16,51: 53
					0b1011	−84, −82, −47: −45, 17: 19, 54: 56
			0b01100		0b01101	−81: −79, −44: −42, 20: 22, 57: 59
			0b01110	63	0b01111	−78: −76, −41: −39, 23: 25, 60: 62
0b10000 (no subchannelisation						−75: −73, −12: −10, 26:28,89:91
			0b10010	−13	0b10011	−72: −70, −9: −7, 29: 31. 92: 94
			0b10100		0b10101	−69: −67, −6: −4, 32: 34, 95: 97
			0b10110	38	0b10111	−66: −64, −3: −I. 35: 37, 98: 100
			0b11000		0b11001	−62: −60. −25: −23, 39: 41, 76: 78
			0b11010	−63	0b11011	−59: −57, −22: −20, 42: 44, 79: 81
		0b11100			0b11101	−56: −54, −19: −17, 45: 47, 82: 84
			0b11110	88	0b11111	−53: −51, −16; −14, 48: 50, 85: 87

Subchannelised transmission in the uplink is an option for an SS. It can be used only if the BS signals its capability to decode such transmissions. The BS must not assign to any given SS two or more overlapping subchannelised allocations in the same time.

The standard ^[1] indicates that when subchannelisation is employed, the SS maintains the same transmitted power density unless the maximum power level is reached. Consequently, when the number of active subchannels allocated to an uplink user is reduced, the transmitted power is reduced proportionally, without additional power control messages. When the number of subchannels is increased the total transmitted power is also increased proportionally. The transmitted power level must not exceed the maximum levels dictated by signal integrity considerations and regulatory requirements. The subchannelisation can then represent transmitted power decreases and, equivalently, capacity gains.

5.2 OFDM Transmission

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 transmitting 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 first 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 modulation. 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.

Figure 5.6: Time and frequency representation of the SC and OFDM. In OFDM, N data symbols are transmitted simultaneously on N orthogonal subcarriers

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 efficiency. 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 FFT works for any number of points. The operation is simpler when applied for a number N which is a power of 2 (e.g. N = 256). The IFFT is the Inverse Fast Fourier Transform operator 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 simplified 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 5.7: Generation of an OFDM signal (simplified)

If the duration of one transmitted modulation data symbol is T_d, then T_d = 1 Δf, where Δf is the frequency bandwidth of the orthogonal frequencies. As the modulation symbols are transmitted simultaneously,

This duration, Δf, the frequency distance between the maximums of two adjacent OFDM subcarriers, can be seen in Figure 5.8. This figure shows how the neighbouring OFDM subcarriers 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 Prefix (CP).

Figure 5.8: Presentation of the OFDM subcarrier frequency

5.2.2 Time Domain OFDM Considerations

After application of the IFFT, the OFDM theory requires that a Cyclic Prefix (CP) must be added at the beginning of the OFDM symbol (see Figure 5.9). Without getting into mathematical details of OFDM, it can be said that the CP allows the receiver to absorb much more efficiently the delay spread due to the multipath and to maintain frequency orthogonality. The CP that occupies a duration called the Guard Time (GT), often denoted T_G, is a temporal redundancy that must be taken into account in data rate computations. The ratio T_G/T_d 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 defined the following values for G: 1/4, 1/8, 1/16 and 1/32. For the mobile (OFDMA) WiMAX profiles presently defined, 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 finds the CP being used by the BS. The SS then uses the same CP on the uplink. Once a specific 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].

Figure 5.9: Cyclic Prefix insertion in an OFDM symbol

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.

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

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

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, N_FFT = 256, the number of lower-frequency guard subcarriers is 28 and the number of higher-frequency guard subcarriers is 27. Considering also the DC subcarrier, there remains N_used, the number of used subcarriers, excluding the null subcarriers. Hence, N_used, = 200 for OFDM PHY, of which 192 are used for useful data transmission, after deducing the pilot subcarriers.

• BW, the nominal channel bandwidth

• n, the sampling factor.

The sampling frequency, denoted f_s, is related to the occupied channel bandwidth by the following (simplified) formula:

This is a simplified formula because, according to the standard, f_s is truncated to an 8kHz 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-defined parameters, the time duration of an OFDM symbol can be computed:

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 modulation 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 Modulation 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).

Table 5.2: OFDM PHY data rates in Mb/s. (From IEEE Std 802.16-2004 ^[1]. Copyright IEEE 2004, IEEE. All rights reserved.)
Open table as spreadsheet

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
1/32	2.92	5.82	8.73	11.64	17.45	23.27	26.18
1/16	2.82	5.65	8.47	11.29	16.94	22.59	25.41
1/8	2.67	5.33	8.00	10.67	16.00	21.33	24.00
1/4	2.40	4.80	7.20	9.60	14.40	19.20	21.60

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 messages present in every frame (see Chapter 9 and others in this book). Hence these data rates. known as raw 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 defined as ^[1]

PS = 4/f_s.

Therefore the PS duration is related to the system symbol rate.

This unit of time defined in the standard allows integers to be used while referring to an amount of time, e.g. the definition 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 transmitted sub carriers to the average transmitted signal. A high PAPR represents a hard constraint for some devices (such as amplifiers). 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].

^[12]van Nee, R. and Prasad, R., OFDM for Wireless Multimedia Communications. Artech House, 2000.

^[1]IEEE 802.16-2004, IEEE Standard for Local and Metropolitan Area Networks, Air Interface for Fixed Broadband Wireless Access Systems, October 2004.

Chapter 5: Digital Modulation, OFDM and OFDMA

5.1 Digital Modulations

As for all recent communication systems, WiMAX/802.16 uses digital modulation. The now well-known principle of a digital modulation is to modulate an analogue signal with a digital sequence in order to transport this digital sequence over a given medium: fibre, radio link, etc. (see Figure 5.1). This has great advantages with regard to classical analogue modulation: better resistance to noise, use of high-performance digital communication and coding algorithms, etc.

Figure 5.1: Digital modulation principle

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.

Figure 5.2: The BPSK constellation

5.1.2 Quadrature Phase Shift Keying (QPSK)

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

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 noiseresistant than BPSK as it has a smaller immunity against interference. A well-known digital communication principle must be kept in mind: ‘A greater data symbol modulation is more spectrum efficient but also less robust.’

Figure 5.3: Example of a QPSK constellation

Table 5.1: Possible phase values for QPSK modulation
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Even bits	Odd bits	Modulation symbol	ϕk
0	0	00	π/4
1	0	01	3π/4
1	1	11	5π/4
0	1	10	7π/4

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 sequence 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 communication 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) modulations are included in the IEEE 802.16 standard. The 64-QAM is the most efficient modulation of 802.16 (see Figure 5.4). Indeed, 6 bits are transmitted with each modulation symbol.

Figure 5.4: A 64-QAM constellation

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 profiles 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 process 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 modulation; 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 figure when the coding rate is also changed (see below).

Figure 5.5: Illustration of link adaptation. A good radio channel corresponds to a high-efficiency Modulation and Coding Scheme (MCS)

4.4 WiMAX System Profiles

4.4 WiMAX System Profiles

A WiMAX system certification profile is a set of features of the 802.16 standard, selected by the WiMAX Forum, that is required or mandatory for these specific profiles. This list sets, for each of the certification profiles of a system profiles release, the features to be used in typical implementation cases. System certification profiles are defined by the TWG in the WiMAX Forum. The 802.16 standard indicates that a system (certification) profile consists of five components: MAC profile, PHY profile, RF profile, 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 certified by the WiMAX Forum according to a specific system certification profile. Two types of system profiles are defined: fixed and mobile. These profiles are introduced in the following sections.

4.4.1 Fixed WiMAX System Profiles

Table 4.4 shows the fixed WiMAX profiles ^[1]. These system profiles 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 profiles use the PMP mode. This was the first set of choices decided in June 2004 (at the same time as approval of IEEE 802.16-2004). Each certification profile has an identifier for use in documents such as PICS proforma statements. Further system profiles should be defined reflecting regulatory (band opportunities) and market development. Among others, new fixed certification profiles should be approved before the end of 2006. It is planned that WiMAX system profiles with a 5 MHz channel bandwidth and 2.5 GHz frequency band schemes will be added. Fixed certification profiles, based on 802.16e, are also planned.

Table 4.4: Fixed WiMAX certification profiles, all using the OFDM PHY and the PMP modes
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Frequency band (GHz)	Duplexing mode	Channel bandwidth (MHz)	Profile name
3.5	TDD	7	3.5T1
3.5	TDD	3.5	3.5T2
3.5	FDD	3.5	3.5F1
3.5	FDD	7	3.5F2
3.5	TDD	10	5.8T

4.4.2 Mobile WiMAX System Profiles

Along with the work on the 802.16e amendment, the mobile WiMAX system profiles were defined. These certification profiles, known as Release-1 Mobile WiMAX system profiles 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 profiles are defined by the Mobile Task Group (MTG), a subgroup of the TWG in the WiMAX Forum. Release 1 certification will probably be separated in different Certification Waves, starting with Wave 1 having only part of all Release 1 features.

Table 4.5: Release 1 Mobile WiMAX certification profiles, all using the OFDMA PHY and the PMP modes
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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

In the OFDMA PHYsical Layer as amended in 802.16e, the number of OFDMA subcarriers (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 first set, one of the reasons being that it is more resource-use efficient. FDD profiles may be defined in the future. The frame length is equal to 5 ms. Other technical aspects of the selected profiles will be introduced in the following chapters.

4.3 WiMAX Frequencies, Regulations and Availability

4.3 WiMAX Frequencies, Regulations and Availability

In this section, some of the frequencies that are expected to be used for WiMAX are given. The frequency bands that will be used in one country or another for the moment (October 2006) are:

• Licensed bands: 2.3 GHz, 2.5 GHz (remember that the 2.4 GHz band is a free band used, among others, by WiFi), 3.3 GHz and 3.5 GHz, the latter being the most (geographically) 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 fixed profile 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 chosen because, for relatively high frequencies (3.5 GHz is itself not a very small value), NLOS operation becomes difficult, which is an evident problem for mobility. The Regulatory Working Group (RWG), introduced in Chapter 2, is trying to define 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.

Table 4.3: Expected WiMAX frequencies (based on RWG documents)
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Region or country	Reported WiMAX frequency bands
USA	2.3, 2.5 and 5.8 GHz
Central and South America	2.5, 3.5 and 5.8 GHz
Europe	3.5 and 5.8 GHz; possible: 2.5 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

4.3.1 France

In France, as elsewhere, the authorities wish to have (at least fixed) 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).

In August 2005, ARCEP started the process of attribution of two other WiMAX licenses (2X15 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 Wireless 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 field (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.

4.2 Licensed and Unlicensed Frequencies

4.2 Licensed and Unlicensed Frequencies

3G networks have licensed fixed frequency bands. These bands are 1885–2025 GHz and 2110–2200 GHz. WiFi WLANs (11b) also have a fixed frequency band: 2410–2480 GHz. This latter band is in the ISM (Industrial, Scientific and Medical), a free band also used by systems other than WiFi.

For WiMAX, the IEEE 802.16 standard considers that the system works on frequencies smaller than 11 GHz (for the PHYsical layers considered for WiMAX). Precise operating frequencies are indicated in WiMAX Forum system profiles (see Sections 4.3 and 4.4 below).

Both types of frequency bands are part of WiMAX:

• licensed bands;

• license-exempt bands.

It is often mentioned that unlicensed frequencies of WiMAX will be used for limited coverages, campuses (enterprise or academic), particular initiatives, etc. In other words, operator revenue should come only from licensed frequencies where the service can be more easily guaranteed.

In some countries and regions, attributed licenses are known as agnostic licenses, i.e. no specific technology or other requirements are mandatory. Only frequency filter shapes and maximum transmitted power are indicated. For example, in these bands, an operator can have 3G or UMTS. However, not all attributed frequencies are agnostic. In some countries, a WiMAX licensed band may only be used for WiMAX operation. Additional constraints may also exist. For example, WiMAX cannot be used for mobile operations in some countries.

The standard indicates that the RF (Radio Frequency) centre frequency is the centre of the frequency band in which a BS or an SS may transmit. Uplink and downlink centre frequencies must be multiples of 250kHz. The required precision is of the order of 10⁻⁵ (depending on the PHYsical Layer and whether it is the BS or the SS).

4.2.1 Frequency Channels and Spectral Masks

The spectral mask of the interfering signal of WiMAX working on licensed bands depends on local regulatory requirements (in Europe, ETSI requirements). When changing from one frequency (burst profile) to another, margins must be maintained to prevent saturation of the amplifier and to prevent violation of emission masks.

Concerning license-exempt bands (^[11], Section 8.5), the channel centre frequency is given by

where n_ch = 0,1,…,199 is the channel number. This definition provides a numbering system for all channels, with 5 MHz spacing, from 5 GHz to 6 GHz. The standard indicates that this provides flexibility to define channelisation sets for current and future regulatory domains.

Channelisation has been found to be compatible with the WiFi WLAN 802.11 a variant, for interference mitigation purposes, in the 5 GHz (US definition) Unlicensed National Information Infrastructure (U-NII) frequency band (see Table 4.1).

Table 4.1: License-exempt band channels. Current applicable regulations do not allow this standard to be operated in the CEPT band B. (From IEEE Std 802.16-2004 ^[1]. Copyright IEEE 2004, IEEE. All rights reserved.)
Open table as spreadsheet

Regulatory domain	Band (GHz)	Channel Number
		20 MHz channels	10 MHz channels
USA	U-NII middle 5.25–5.35	56, 60, 64	55, 57, 59, 61, 63, 65, 67
	U-NII upper 5.725–5.825	149, 153, 157, 161, 165	148, 150, 152, 154, 156, 158, 160, 162, 164. 166
Europe	CEPT band B 5.47–5.725	100. 104. 108. 112. 116. 120. 124. 128. 132. 136	99. 101. 103. 105. 107. 109. 111. 113. 115. 117. 119. 121. 123. 125. 127. 129. 131. 133. 135. 137
	CEPT band C 5.725–5.875	148. 152. 156. 160. 164. 168	147. 149. 151. 153. 155. 157. 159. 161. 163. 165. 167. 169

In license-exempt bands, the transmitted spectral density of the transmitted signal must fall within the spectral mask of Figure 4.9. The measurements must be made using the 100 kHz resolution bandwidth. The 0 dBr level is the maximum power allowed by the relevant regulatory body.

Figure 4.9: Transmit spectral mask (see also Table 4.2). (From IEEE Std 802.16-2004 ^[1]. Copyright IEEE 2004, IEEE. All rights reserved.)

Table 4.2: Transmit spectral mask parameters ^[1]. A, B, C and D are in MHz
Open table as spreadsheet

Channelisation (MHz)	A	B	C	D
20	9.5	10.9	19.5	29.5
10	4.75	5.45	9.75	14.75

^[11]

4.1 The Cellular Concept

4.1 The Cellular Concept

The global objective of a wireless network is rather simple: to connect wireless users to a core network and then to the fixed network. Figure 4.1 illustrates the principle of a Public Land Mobile Network (PLMN), as defined for second-generation GSM networks.

Figure 4.1: Illustration of a Public Land Mobile Network (PLMN) offering a cellular service

The first wireless phone systems date back as far as the 1930s. These systems were rather basic and had very small capacity. The real boost for wireless networks came with the cellular concept invented in the Bell Labs in the 1970s. This simple but also extremely powerful concept was the following: each base station covers a cell; choose the cells small enough to reuse the frequencies (see Figure 4.2). Using this concept, it is theoretically possible to cover a geographical area as large as needed!

Figure 4.2: The cellular concept. simple and so powerful!

The cellular concept was applied to many cellular systems (mostly analogue) defined in the 1980s: AMPS (US), R2000 or Radiocom 2000 (France), TACS (UK), NMT (Scandinavian countries), etc. These systems being incompatible, a unique European cellular system was invented, GSM, which is presently used all over the world.

WiMAX applies the same principle: a BS covers the SSs of its cell. In this section, some elements of the cellular concept theory needed for WiMAX dimensioning are provided. First sectorisation is reminded.

4.1.1 Sectorisation

A base station site represents a big cost (both as investment, CAPEX, and functioning, OPEX) for a network operator. Instead of having one site per cell, which is the case for an omnidirectional BS, trisectorisation allows three BSs to be grouped in one site, thus covering three cells (see Figure 4.3). These cells are then called sectors. Three is not the only possibility. Generally, it is possible to have a sectorisation with n sectors. Yet, for practical reasons, trisectorisation is very often used. Sectorisation evidently needs directional antennas. Trisectorisation needs 120° antennas (such that the three BSs cover the 360°).

Figure 4.3: Omnidirectional antennas and trisectorisation

For economical reasons, sectorisation is almost always preferred to omnidirectional antennas for cellular networks unless in rare specific cases, e.g. for very large cells in very low populated geographic areas. WiMAX is not an exception as sectorisation is also recommended.

4.1.2 Cluster Size Considerations

Cellular networks are based on a simple principle. However, practical deployment needs a complicated planification in order to have high performance, i.e. great capacity and high quality. This planification is made with very sophisticated software tools and also the ‘know-how’ of radio engineers.

Frequency reuse makes room for an interference that should be kept reasonably low. As illustrated in Figure 4.4, in the downlink (for example) and in addition to its useful signal (i.e. its corresponding BS signal), an SS receives interference signals from other BSs using the same frequency. Signal-to-Noise Ratio (SNR) calculations or estimations are used for planification. The SNR is also known as the to-Carrier-Interference-and-Noise Ratio (CINR). The term SNR is used more for receiver and planification considerations, while CINR is used more for practical operations. In this book, SNR and CINR represent the same physical parameter. An appropriate cellular planification is such that the SNR remains above a fixed target value (depending on the service, among others) while maximising the capacity.

Figure 4.4: Illustration of a useful signal and interference signal as used for SNR calculations

A parameter of cellular planification is the cluster size. A cluster is defined as the minimal number of cells using once and only once the frequencies of an operator (see Figure 4.5). It can be verified that having a small cluster increases the capacity per cell while big clusters decrease the global interference and then represent high quality. The choice of the cluster size must be done very carefully. In the case of GSM networks, the value of the cluster size was initially 12 or 9. With time and due to different radio techniques (such as frequency hopping, among others), the cluster size of the GSM may be smaller (down to 3, possibly).

Figure 4.5: Example of a (theoretical) regular hexagonal network. Cluster size = 3. In each cluster all the operator frequencies are used once and only once

The regular hexagonal grid (as in Figure 4.5) is a model that can be used for first estimations. For this model, a relation can be established between the minimal SNR value of the network and the cluster size ^[9]:

where n is the cluster size and α a value depending on the radio channel (of the order of 4). This is only an approximated formula used for a nonrealistic channel (too simple) and cell shapes used only for a first estimation of n. Practical deployment uses more sophisticated means but still the minimal SNR (used for cells planification or dimensioning) increases with n.

What about WiMAX frequency reuse? WiMAX is an OFDM system (while GSM is a Single Carrier, SC, system) where smaller cluster sizes can be considered. Cluster size values of 1 or 3 are regularly cited. On the other hand, it seems highly probable that sectorisation will be applied. This gives the two possible reuse schemes of Figure 4.6. Consequently, an operator having 10.5 MHz of bandwidth will have 3.5 MHz of bandwidth per cell (respectively 10.5 MHz) if a cluster of 3 (respectively 1) is considered. Yet, it is not sure that a cluster of 1 will lead to a higher global capacity. With a cluster of 1, reused frequencies are very close and then the SNR will be (globally) lower, which means less b/s Hz if link adaptation is applied (as in WiMAX, see Chapter 11). The choice of cluster size is definitely not an easy question.

Figure 4.6: Possible frequency reuse schemes in WiMAX

Other cellular frequency reuse techniques can be also be used. Reference ^[10] mentions fractional frequency reuse. With an appropriate subchannel configuration, users operate on subchannels, which only occupy a small fraction of the whole channel bandwidth. The subchannel reuse pattern can be configured so that users close to the base station operate with all the subchannels (or frequencies) available, while for the edge users, each cell (or sector) operates on a fraction of all subchannels available. With this configuration, the full-load frequency reuse is maintained for centre users in order to maximise spectral efficiency and fractional frequency reuse is implemented for edge users to assure edge user connection quality and throughput. The subchannel reuse planning can be dynamically optimised across sectors or cells based on network load and interference conditions on a frame-by-frame basis. Figure 4.7 shows an example of operating frequencies for each geographical zone in a fractional frequency reuse scheme. In the OFDMA PHYsical Layer (Mobile WiMAX), flexible subchannel reuse is facilitated by the subchannel segmentation and permutation zones. A segment is a subdivision of the available OFDMA subchannels (one segment may include all subchannels).

Figure 4.7: Example of operating frequencies for each geographical zone in a fractional frequency reuse scheme. The users close to the base station operate with all subchannels available. (Based on Reference ^[10])

The tools for efficient radio resource use and other radio engineering considerations for WiMAX are described in Chapter 12.

4.1.3 Handover

Handover operation (sometimes also known as ‘handoff’) is the fact that a mobile user goes from one cell to another without interruption of the ongoing session (whether a phone call, data session or other). Handover is a mandatory feature of a cellular network (see Figure 4.8). Many variants exist for its implementation. Each of the known wireless systems have some differences. WiMAX handover is described in Chapter 14.

Figure 4.8: Illustration of handover in a cellular network

^[9]Lee, W. C. Y., Mobile Cellular Telecommunications: Analog and Digital Systems, McGraw Hill, 2000.

^[10]WiMAX Forum White Paper, Mobile WiMAX - Part I: a technical overview and performance evaluation, March 2006.

3.7 WiMAX Topologies

3.7 WiMAX Topologies

The IEEE 802.16 standard defines two possible network topologies (see Figure 3.6):

• PMP (Point-to-Multipoint) topology (see Figure 3.5);

  
  Figure 3.5: PMP topology

  
  Figure 3.6: Mesh topology. The BS is no longer the centre of the topology, as in the classical PMP mode

• Mesh topology or Mesh mode (see Figure 3.6).

The main difference between the two modes is the following: in the PMP mode, traffic may take place only between a BS and its SSs, while in the Mesh mode the traffic can be routed through other SSs until the BS and can even take place only between SSs. PMP is a centralised topology where the BS is the centre of the system while in Mesh topology it is not. The elements of a Mesh network are called nodes, e.g. a Mesh SS is a node.

In Mesh topology, each station can create its own communication with any other station in the network and is then not restricted to communicate only with the BS. Thus, a major advantage of the Mesh mode is that the reach of a BS can be much greater, depending on the number of hops, until the most distant SS. On the other hand, using the Mesh mode brings up the now thoroughly studied research topic of ad hoc (no fixed infrastructure) networks routing.

When authorised to a Mesh network, a candidate SS node receives a 16-bit Node ID (IDentifier) upon a request to an SS identified as the Mesh BS. The Node ID is the basis of node identification. The Node ID is transferred in the Mesh subheader of a generic MAC frame in both unicast and broadcast messages (see Chapter 8 for frame formats).

First WiMAX network deployments are planned to follow mainly PMP topology. Mesh topology is not yet part of a WiMAX certification profile (September 2006). It has been reported that some manufacturers are planning to include the Mesh feature in their products, even before Mesh is in a certification profile.

3.6 Network Management Reference Model

3.6 Network Management Reference Model

The 802.16f amendment ^[8] provides enhancements to IEEE 802.16-2004, defining a Management Information Base (MIB) for the MAC and PHY and the associated management procedures. This document describes the use of a Simple Network Management Protocol (SNMP), an Internet Engineering Task Force (IETF) protocol (RFCs 1902, 1903, 3411-5 and 3418), as a network management reference model.

802.16f consists of a Network Management System (NMS), managed nodes and a service flow database (see Figure 3.4). BS and SS managed nodes collect and store the managed objects in the format of WiressMan Interface MIB and wmanDevMib, defined in the 802.16f document, which are made available to NMSs via management protocols, such as the Simple Network Management Protocol (SNMP). The service flow database contains the service flow and the associated QoS information that need to be associated to the BS and the SS when an SS enters into a BS network.

Figure 3.4: Network management reference model as defined in 802.16f. (From IEEE Std 802.16f-2005 ^[8]. Copyright IEEE 2005, IEEE. All rights reserved.)

^[8]IEEE 802.16f, IEEE Standard for Local and Metropolitan Area Networks. Air Interface for Fixed Broadband Wireless Access Systems, Amendment 1: Management Information Base, December 2005.

3.5 PHYsical Layer

3.5 PHYsical Layer

WiMAX is a BWA system. Hence, data are transmitted at high speed on the air interface through (radio) electromagnetic waves using a given frequency (operating frequency).

The PHY Layer establishes the physical connection between both sides, often in the two directions (uplink and downlink). As 802.16 is evidently a digital technology, the PHYsical Layer is responsible for transmission of the bit sequences. It defines the type of signal used, the kind of modulation and demodulation, the transmission power and also other physical characteristics.

The 802.16 standard considers the frequency band 2-66GHz. This band is divided into two parts:

• The first range is between 2 and 11 GHz and is destined for NLOS transmissions. This was previously the 802.16a standard. This is the only range presently included in WiMAX.

• The second range is between 11 and 66 GHz and is destined for LOS transmissions. It is not used for WiMAX.

Five PHYsical interfaces are defined in the 802.16 standard. These physicals interfaces are summarised in Table 3.1. The five physical interfaces are each described in a specific section of the 802.16 standard (and amendments). The MAC options (AAS, ARQ, STC, HARQ, etc.) will be described further in this book (see the Index). Both major duplexing modes, Time Division Duplexing (TDD) and Frequency Division Duplexing (FDD), can be included in 802.16 systems.

Table 3.1: The five PHYsical interfaces defined in the 802.16 standard. (From IEEE Std 802.16e-2005 ^[2]. Copyright IEEE 2006, IEEE. All rights reserved.)
Open table as spreadsheet

Designation	Frequency band	Section in the standard	Duplexing	MAC options
WirelessMAN-SC (known as SC)	10–66 GHz (LOS)	8.1	TDD and FDD
WirelessMAN-SC (known as SCa)	Below 11 GHz (NLOS); licensed	8.2	TDD and FDD	AAS (6.3.7.6), ARQ (6.3.4), STC (8.2.1.4.3), mobility
WirelessMAN-OFDM (known as OFDM)	Below 11 GHz; licensed	8.3	TDD and FDD	AAS (6.3.7.6.), ARQ (6.3.4), STC (8.3.8.), mesh (6.3.6.6) mobility
WirelessMAN-OFDMA (known as OFDMA)	Below 11 GHz: licensed	8.4	TDD and FDD	AAS (6.3.7.6), ARQ (6.3.4), HARQ (6.3.17), STC (8.4.8), mobility
WirelessHUMAN	Below 11 GHz; license exempt	8.5 (in addition to 8.2, 8.3 or 8.4)	TDD only	AAS (6.3.7.6), ARQ (6.3.4), STC (see above), only with 8.3, mesh (6.3.6.6)
[2]IEEE 802.16e, IEEE Standard for Local and Metropolitan Area Networks, Air Interface for Fixed Broadband Wireless Access Systems, Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1, February 2006 (Approved: 7 December 2005).

For frequencies in the 10–66 GHz interval (LOS), the WirelessMAN-SC PHY is specified. For frequencies below 11 GHz (LOS) three PHYsical interfaces are proposed:

• WirelessMAN-OFDM, known as OFDM and using OFDM transmission;

• WirelessMAN-OFDMA, known as OFDMA and using OFDM transmission, and Orthogonal Frequency Division Multiple Access (OFDMA), with the OFDMA PHY Layer, described in Section 8.4 of the 802.16 standard, being completely rewritten between 802.16-2004 and 802.16e;

• WirelessMAN-SCa, known as SCa and using single-carrier modulations.

Some specifications are given for the unlicensed frequency bands used for 802.16-2004 in the framework of the WirelessHUMAN (High-speed Unlicensed Metropolitan Area Network) PHYsical Layer. Unlicensed frequency is included in fixed WiMAX certification. For unlicensed frequency bands, in addition to the features mentioned in Table 3.1, the standard ^[2] requires mechanisms such as Dynamic Frequency Selection (DFS) to facilitate the detection and avoidance of interference and the prevention of harmful interference into other users, including specific spectrum users identified by regulations ^[7] as priority users.

WiMAX considers only OFDM and OFDMA PHYsical layers of 802.16 (see Figure 3.3). The PHYsical Layer is described in Chapter 7, where the OFDM transmission technique is described. Efficiency of the use of the frequency bandwidth is treated in Chapter 12.

Figure 3.3: IEEE 802.16 common MAC Layer can be used with two possible PHYsical layers in WiMAX

3.5.1 Single Carrier (SC) and OFDM

The use of OFDM increases the data capacity and, consequently, the bandwidth efficiency with regard to classical Single Carrier (SC) transmission. This is done by having carriers very close to each other but still avoiding interference because of the orthogonal nature of these carriers. Therefore, OFDM presents a relatively high spectral efficiency. Numbers of the order of magnitude of 3.5–5 b/s Hz for spectral efficiency are often given. This is greater than the values often given for CDMA (Code Division Multiple Access) used for 3G, although this is not a definitive assumption as it depends greatly on the environment and other system parameters.

The OFDM transmission technique and its use in OFDM and OFDMA physical layers of WiMAX are described in Chapter 5.

^[7]Recommendation ITU-R M.1652, Dynamic frequency selection (DFS) in wireless access systems including radio local area networks for the purpose of protecting the radio determination service in the 5 GHz band, 2003.