Σάββατο, 6 Ιουλίου 2013

WiMax spectrum, bandwidth and modulation formats

The spectrum used in WiMax is around the regions of 2.5, 3.5 , 5.25 and 5.8 GHz. It partially intersects with the spectrum used in Wi-Fi (Wireless Fidelity) home networks, which arises interference issues [6]. The big range of the spectrum slices used and the absence of one global central frequency has  discouraged vendors, providers and consumers from widely deploying WiMax globally.
Figure 2: initial and future spectral allocation of WiMax

The spectrum initially used was 2500-2690 MHz, 2700-2900 MHz, 3400-3600 MHz and 5725-5850 MHz. The future spectrum to be used is 2305-2320 MHz, 2345-2360 MHz, 5150-5350 MHz and 5470-5725 MHz [16]. The spectral use is presented in Figure 2. The lowest frequency slice (2305-2320) is technically the most attractive of all, as it combines the minimum attenuation of all (the lower the frequency, the smaller the signal attenuation during propagation in the air) and the non-interference with Wi-Fi spectrum. There has been a different commercial allocation of the offered frequencies. As we can see in Figure 3, the most popular spectral regions are at 3.5 (which has to be licensed) and 5 GHz (which is unlicensed). Unlicensed spectrum is in principle cheaper and therefore more attractive for both commercial providers and subscribers.
Figure 3: WiMax commercial frequency allocation around the world
WiMax forum is an industry-led, not-for-profit organization that certifies and promotes the compatibility and interoperability of broadband wireless products based upon IEEE 802.16 standard [14]. The WiMax commercial products that pass conformance and interoperability tests achieve the “WiMax certified” designation and can display this mark on their products. Some vendors claim their equipment to be “WiMax ready”, “WiMax compliant” or “pre-WiMax” , if they are not officially WiMax certified [11]. WiMax consists of nine Working Groups (WG) : Application WG, Global Roaming WG, Regulatory WG, Certification WG, Marketing WG, Service Provider WG, Network WG and Technical WG and the Technical Steering Committee (or Technical Plenary).
There are four different channel bandwidths in WiMax: 1.25 MHz, 3.5 MHz, 5 MHz and 10 MHz, where 128, 256, 512 and 1024 OFDM (Orthogonal Frequency-Division Multiplexing) respectively are used. Depending on the channel bandwidth, the modulation and code rate differ among BPSK (Binary Phase Shift Keying), QPSK (Quad Phase Shift Keying), 16 and 64 QAM (Quadrature Amplitude Modulation), thus resulting to a bitrate ranging from 946 kbps to 22.500 kbps per channel at the downstream side. The upstream bandwidth is around the one quarter to one third of the downstream, starting from 326 kbps and reaching up to 6.720 kbps. The code rate can be heavily protected at a rate of ½ , but it can go up to 5/6 . The smaller bandwidths ad the heavy code rates are combined for users at a long distance with large attenuation and/or severe interference problems, while the lighter code rates with higher bandwidths are used in a short range from the Base Station [13].
OFDM is a spectrally very efficient modulation. In 802.16-2004 the spectral efficiency is about 3.7 bits/Hz, while in other modern wireless technologies it does not exceed ~0.5 bits/Hertz .This creates a huge competitive advantage for WiMax. As we have seen, WiMax can be deployed in both licensed and unlicensed parts of the spectrum. It supports a mix of IPv4 and IPv6 protocols in both providers’ and subscriber’s side and also different Radio Access Network topologies. It has defined two MAC system profiles: the basic ATM and the basic IP. Even though it does operate better as a fixed point wireless technology, it can also handle mobility issues [13].
Figure 4: IP-Based WiMax Network Architecture
A WiMax network consists of three different parts:
·         the Mobile Station which is the terminal of the subscriber (end user) and can be an electronic device (smartphone, tablet, laptop, desktop PC etc) with a WiMax network card. If the device is mobile, the user can be nomadic.
·         the Access Service Network, which combines one or more Base Stations and one or more gateways that form the radio access network at the edge. For the classic phone network (POTS – Plain Old Telephone Service) , the equivalent is the twisted copper pairs of the last mile. This exact equivalency has motivated the idea of using WiMax as a substitute for the copper last mile.

·         The Connectivity Service Network, which provides IP connectivity and all the IP core network functions. For classic telecommunications networks, this consists of both the backhaul (closer to the access part) and the backbone (closer to the core) network. This is a potential alternative too, especially for parts of the backhaul/backbone network that need a fast setup and which today are mainly served by microwave links until an optical fiber is installed [13].

Δεν υπάρχουν σχόλια: