WiMAX Forum Certified
Wassim El Khoury, Network Expansion Manager, Pesco
Lebanon
"We have been an Aperto customer for some time and know that PacketMAX is the ideal solution for our offerings... Like many service providers, we are concerned with the cost of deploying and managing WiMAX... with Aperto's WaveCenter EMS product, PacketMAX lets us serve a huge number of customers with exceptional QoS and reliability."

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Antenna Diversity

Obstructed Line-of-Sight (OLOS) and Non Line-of-Sight (NLOS) locations present a special challenge for Fixed Broadband Wireless Access (FBWA) installations, especially if the signal propagates through foliage. Figure 1 shows the received signal in a LOS situation (as observed on a spectrum analyzer), showing less than 1 dB variation in the signal power. Figure 2 shows the received signal after it has propagated though two lines of trees with heavy foliage (over a one minute interval), and it can be seen that the received signal strength varies by as much as 20 dB. The type of fading that takes place in Broadband Wireless Access (BWA) systems occurs due to scattering through leaves, and takes place over relatively large time intervals, for example a fade may easily last for hundreds of milliseconds. Deep fades of this sort are extremely detrimental to the operation of BWA systems, unless preventive counter-measures are taken.

In order to enable OLOS/NLOS operation, the wireless link should be able to continue operating in the presence of deep fades. The Aperto® system incorporates receive diversity techniques to combat deep fades. This involves using antennae with dual polarization at the subscriber site, which have the ability to switch polarizations from burst to burst. The signal received with the vertical polarization is highly un-correlated from that received with the horizontal polarization, as shown in Figure 2. When the signal received by the vertical polarized antenna is in deep fade, then the receiver detects this condition, and switches over to the horizontal polarization, and thus is able to continue operation.

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Figure 1: Received signal power at LOS subscriber site


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Figure 2: Received signal power at OLOS/NLOS subscriber site

Diversity switching as implemented in the Aperto system, results in the behavior shown in Figure 3. The Red curve is the SNR level before diversity switching, while the Blue curve is the SNR level after diversity switching. Due to the diversity switching, the system is able to maintain a steady level of SNR, even though the two individual signals are varying widely.

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Figure 3: Received signal power at OLOS/NLOS subscriber site