WiMAX Forum Certified
Wassim El Khoury, Network Expansion Manager, Pesco
"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."


Time Division Duplexing (TDD)

Time division duplexing (TDD) is a duplexing scheme whereby upstream and downstream transmissions are separated in time while using a single radio frequency (RF) channel. This differs significantly from frequency division duplexing (FDD) where the downstream and upstream transmissions are separated in frequency, thus requiring two RF channels to operate. One of the most significant advantages of TDD in an IP network is that the upstream to downstream bandwidth allocation can be dynamically adjusted to accommodate the traffic pattern. This implementation, known as Adaptive TDD, leads to dramatically lower latency performance in a heavily loaded system than is possible with FDD. The low latency performance of Adaptive TDD becomes especially critical when time dependent applications such as voice are being served over an IP network.

TDD: Improving immunity to interference

The Aperto PacketMAX System incorporates one of the most sophisticated TDD architectures in the market, one which has several advantages over other TDD, and even FDD systems, in the matter of interference rejection and recovery techniques. Co-channel Interference (CCI) is a source of performance degradation and capacity loss in a broadband wireless access network. CCI is caused when nearby cells and/or sectors use identical frequencies, in order to boost the capacity of the system by frequency re-use. Specifically, in dense radio conditions or while operating in license-exempt environments, the following techniques enable PacketMAX systems to maintain reliable link performance in presence of co-channel interference:

Learn how these features improve immunity to interference in the license-exempt environment:

Bi-directional Power Control and Burst Transmission Mode: Almost all current Broadband Wireless Architectures, whether FDD or TDD, employ power control only in the upstream direction. Furthermore, transmissions in the downstream direction are sent in the continuous mode, using the maximum transmit power. Such a design significantly increases the total amount of radiated power, and hence the resulting CCI. PacketMAX on the other hand, employs power control in both upstream and downstream directions. In addition, both upstream and downstream transmissions are sent in the burst mode. These features lead to the following benefits:

  • Since downstream power is radiated only when there is traffic to be sent, this reduces the amount of CCI
  • Due to downstream power control, customer premise equipment that is close to the base station does not require a large amount of radiated power, thus reducing the total amount of radiated power and the resulting CCI.


Polarization Diversity: The Aperto PacketMAX System employs antennae that can radiate power in both horizontal and vertical polarizations. The significance of this feature in mitigating CCI is due to the fact that a signal which is of a certain polarity causes very little interference to another signal which is at the same frequency but of opposite polarity. As a result, by employing polarization diversity, it is possible to achieve a high degree of frequency re-use while keeping the amount of CCI under control. The PacketMAX system allows both transmit and receive polarity to be adjusted dynamically, as a function of current conditions. It employs an advanced algorithm that adapts the optimal polarity orientation at each customer premise site as a function of CCI and propagation properties of the wireless link.

Adaptive Wireless Packet Size and Schedule Adjustment: In a burst mode transmission system, the amount of interference caused by a burst, and also its susceptibility to being interfered by another burst, is proportional to the size of the data burst. The PacketMAX system employs a novel wireless transport layer in which the size and parameter set of each wireless burst can be adaptively tailored to the link between the base station and the customer premise equipment. Hence, if CCI is a significant issue in the transmission environment (such as license-exempt spectrum), then the system can adaptively reduce the size of the wireless bursts, thus reducing its effect. In addition, the PacketMAX system also employs an advanced scheduling algorithm that is capable of staggering the transmissions from sectors that are subject to CCI, thus reducing its effect.

Wireless Packet Re-transmissions: To further combat CCI, the PacketMAX system uses wireless packet re-transmissions. This is a very effective and powerful tool against CCI, and it enables the system to function effectively and provide good performance even in the presence of CCI. The wireless transport layer fragments an IP packet into smaller Wireless Protocol Data Units (WPDU), each of which can be individually re-transmitted over the wireless link. The number of re-transmissions allowed can be tailored to be a function of the degree of CCI existing in the system. TDMA-based burst transmission, in combination with re-transmissions in case of CCI-induced errors, provides an extremely powerful tool against all kinds of interference conditions and significantly increases the robustness of the system.