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support for time division multiple access (TDMA) and frequency division multiple access
(FDMA) transmission as well as advanced antenna techniques.
The end-to-end services are delivered over an IP architecture allowing WiMAX to ride the
declining cost curves of IP processing and facilitate easy convergence with other networks [26].
The base stations, usually equipped with multiple antennas employed in various downlink
multi-antenna schemes such as beamforming, spatial multiplexing or spatial diversity coding
[23, 26], are connected to the ISP by means of either T1/E1 interfaces, Ethernet or via line-of-
sight millimetre-wave connections [27, 28].
Figure 2-2: WiMAX deployment scenario
As demonstrated in Figure 2-2, each base station is serving a single cell, and is able to transmit
at the 2.3GHz, 2.5GHz, 3.5GHz and/or 5.7GHz bands depending on regulatory issues [22, 29].
For Europe, the 3.5GHz licensed band is typically allocated to service providers [22, 30, 31].
To increase the spectral efficiency across a cell, a frequency reuse technique is employed so that
each cell is being sectorised and WiMAX base stations accessing the individual sectors with
Chapter 2 Next Generation Access Networks
22
directional antennas [23]. An illustration of that is shown in Figure 2-2 where a frequency reuse
with a factor of 1:3:3 is assumed [23]. However, this approach could impose strong inter-cell
interference and as a result interference management should be applied with multi-site
processing [32]. Coordination, via direct communication, between the cooperating base stations
needs to be established, requiring therefore additional capacity on the backhaul link.
In addition, the WiMAX signals are broadcasted from a base station to all subscribers in a cell
or a sector within a radius of typically 3-5 miles [30]. Therefore, due to the inherited point-to-
multi-point (P2MP) distribution, TDMA is typically applied in upstream [33] to serve as the
required contention control mechanism, similar to TDM-PONs. To reduce the traffic
congestion, coverage and throughput due to TDMA, more densely spaced base stations with
improved spectral efficiency and QoS could be deployed resulting, however, in increased
deployment cost.
Current WiMAX standards include the existing IEEE802.16d [24] and IEEE802.16e [25],
providing fixed and mobile broadband wireless operation respectively. The main characteristics
of these standards are summarised in Table 2-1. Compared to initial WiMAX standard
definitions, IEEE802.16-2001 [34], at the physical layer these standards are operating at 2-11
GHz transmission bands, licensed and license-exempt, driven by the need for non-line-of-sight
(NLOS) operation [24, 25]. For efficient multipath propagation mitigation the modulation
technique utilised is primarily based on orthogonal frequency division multiplexing (OFDM)
where data is carried over closely spaced orthogonal subcarriers, generated by Fast Fourier
Transform (FFT), with long symbol duration [23]. Each individual subcarrier is modulated with
QPSK, 16-QAM or 64-QAM digital modulation formats [23]. In addition a wide range of guard
times, attached to an OFDM symbol, are defined in the WiMAX standards to allow for the
necessary trade-off between spectral efficiency and delay spread robustness [24, 25]. The
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