Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

244 Applicationsto the base station. In addition, for most of the remaining users LOS is obtained throughrooftop positioning of the antenna, which requires very accurate pointing, thereby makingthe installation both time- and skill-consuming. Therefore, a system operating in NLOSconditions enabling self-installation will play an important role, especially in the successof BWA for the residential market.In response to these trends, under the ETSI BRAN project HIPERMAN (HIgh PERformanceMetropolitan Area Networks) (HM) [19] and the IEEE 802.16-project WirelessMan(Wireless Metropolitan Area Networks, WMAN) [23], several specifications have beenpublished. Both standards offer at the first stage a wide range of data services (especiallypacket-based IP with a peak data rate > 50 Mbps at 10 MHz) for residential customers(i.e. single- or multi-dwelling households) and for small to medium sized enterprises byadopting multi-carrier transmission for carrier frequencies (RF) below 11 GHz with linkcoverages up to 20 km in the LOS case and 3–5 km in NLOS conditions. In a later stagethese standards will also offer nomadic and mobility applications [24]. The promotionand the certification of these two standards are taking place in the WiMAX forum [34].The aim of this section is to highlight the most important features of these publishedstandards and to give a deeper understanding of the WiMAX systems.5.3.4.1 Network Topology and Reference ModelAs shown in Figure 5-22, the BWA system will be deployed to connect user networkinterfaces (UNIs) physically positioned in customer premises to a service node interface(SNI) of a broadband core network (e.g. IP), i.e. offering last mile connectivity. The basestation typically manages communications of more than one carrier or sector. For eachbase station sector one antenna or more is positioned to cover the deployment region. Theterminal station antenna can be directional or omni-directional. At the terminal station sidethe network termination (NT) interface connects the terminal station with the local usernetwork (i.e. LAN/WLAN).The BWA network deployments will potentially cover large areas (i.e. cities, ruralareas) [19, 22]. Due to large capacity requirements of the network, large amounts ofspectrum with high transmission ranges (up to 20 km) are needed. For instance, a typicalnetwork may therefore consist of some cells each covering part of the designateddeployment area. Each cell will operate in a point to multi-point (PMP) or mesh manner.Two duplex schemes can be used: (a) frequency division duplex (FDD) and (b) timedivision duplex (TDD). The channel size is between 1.5 and 28 MHz wide in both the FDDand the TDD case. The downlink data stream transmitted to different terminal stations ismultiplexed in the time domain by MC-TDM (multi-carrier-time division multiplexing) byusing OFDM or OFDMA transmission. In the uplink case, MC-TDMA (multi-carrier-timedivision multiple access) with OFDM or OFDMA will be used.In Figure 5-23 the IEEE 802.16x reference model is depicted [23]. The IEEE 802.16standard covers mainly the PHY and the MAC layers. The MAC layer in addition to theclassical functions of the MAC comprises also the specific convergence sub-layer andthe security sub-layer specifications. The goal of the specific convergence sub-layer (CS)specification is to enable the adaptation of the IEEE 802.16 standard specification for thetransport of the higher layer services (e.g. IP, Ethernet, ATM). The PHY layer deals withthe wireless transport medium, Layer 1.