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:
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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.
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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.
| 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) |
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[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). | ||||
Open table as spreadsheet 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:
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WirelessMAN-OFDM, known as OFDM and using OFDM transmission;
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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;
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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.
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