Broadband Access: Dynamic, flexible links key for user

New-generation systems must meet service providers' demands to simultaneously address subscribers under line-of-sight, obstructed-line-of-sight and non-line-of-sight conditions relative to the serving basestation. And new systems must address very diverse service-grade requirements, from multitier data to voice and video.

Recent breakthroughs in dynamic per-subscriber link optimization are enabling fixed broadband wireless access (FBWA) to meet multiservice mass-market requirements for capacity, coverage, quality-of-service (QoS) and cost-effectiveness. This is accomplished by moving away from static, preset subscriber parameters and going toward a service-intelligent network architecture for personalized multiservice delivery. Emerging architectures are based on a highly flexible and adaptable range of parameters to optimize performance dynamically to accommodate the user.

Wireless links operate in a relatively hostile environment compared with their wired counterparts. Broadband wireless channels are subject to high error rates, vary enormously over time and vary by subscriber location. Each of those conditions is determined by the occurrence of multipath, interference and fading in differing combinations in each subscriber link, and each must be accounted for in the system design.

A complete system-level approach to network performance is required to achieve a multitiered optimization of network capacity, coverage, QoS and cost. After all, for mass-market service delivery, network operators must be able to address subscribers in multiple environments, ranging from suburban to dense urban, using multicell networks with multiple sectors that efficiently reuse spectrum.

Dynamic per-subscriber link optimization automatically optimizes each subscriber link for simultaneous management of multipath, interference, signal strength and fading. That link optimization takes service intelligence to a new level for differentiated services and personalized service delivery. For the network operator, it addresses the performance gaps of previous generation wireless systems in maximizing capacity, coverage and QoS to meet critical mass-market business model objectives.

Dynamic per-subscriber link optimization utilizes multiple physical-layer (i.e., modem, radio and antenna-related) and media-access-control (MAC) layer adaptive parameters. Adaptive PHY parameters include adaptive modulation, adaptive forward error correction (FEC), output power control and antenna diversity. Adaptive MAC-layer parameters include adaptive frame size and adaptive automatic repeat request (ARQ), which is a retransmission protocol at the link layer of a wireless system that enables recovery from packet errors.

Multipath management consists of minimizing the effects of intersymbol interference, fading and intrasymbol distortion. Signal-strength management is required to counter the mean path loss and variations caused by fading and obstructions, as well as to minimize interference. Interference mitigation and management are required to manage both external and intranetwork interference caused by aggressive frequency reuse in cellular networks.

Multipath can be managed by adapting such parameters as equalization, modulation, FEC, ARQ retransmissions and antenna diversity. Interference can be mitigated and managed by adapting modulation, output power, frame size, FEC, ARQ retransmissions, channel frequency and antenna diversity.

Adaptive modulation is defined as the ability of a basestation to choose the right level of modulation for each transmission to or from a subscriber unit. Using that feature, the basestation can simultaneously support multiple sets of subscribers, each of which may require a different modulation. Since different modulation types have different signal-to-noise thresholds, adaptive modulation can be put to use in several different situations.

The most common use is in the context of increasing the cell coverage, where hard-to-reach subscriber locations with excessive link attenuation can be served using lower order modulation. In addition, adaptive modulation can serve as a useful tool against co-channel interference.

Adaptive modulation can adapt in both directions, so if the co-channel interference goes down and the link quality improves, the modulation level should increase. Considerations related to adaptive modulation are independent of whether the system employs single- or multicarrier modulation.

When transmissions occur between the basestation and a subscriber unit (or vice versa), the resulting power from that signal leaks into neighboring cells and sectors and is directly responsible for all occurrences of co-channel interference. Transmit-power control plays an integral role in interference reduction techniques. To reduce the amount of signal going into neighboring cells, the most straightforward policy is to transmit with lower power. But that conflicts with the requirement for reliable signal reception, that is, that the signal-to-interference-plus-noise ratio (SINR), as defined by SINR = received signal power/(noise power + interference power), needs to be greater than a well-defined threshold.

Harnessing power

Hence the optimal policy is to transmit with just enough power to enable the SINR at the receiver to be maintained above the threshold. That optimal transmit power is then a function of the link attenuation between the transmitter and the receiver, as well as the interference power at the receiver. The path loss in a fixed wireless system changes relatively slowly, if at all. But the interference power is a more dynamic quantity and varies in a stochastic manner, depending on the current transmission environment.

The basestation or subscriber antennas can be configured to transmit or receive, using either horizontal or vertical polarization. Polarization can be used as a tool to reduce co-channel interference because of a property of polarized transmissions known as cross polarization discrimination, or XPD. Because of XPD, when a horizontally polarized antenna receives a co-channel signal sent from a vertically polarized antenna (and vice versa), the effective signal strength is reduced by several decibels. The amount of XPD is reduced if the signal undergoes extensive scattering.

But it is possible for each receiver-transmitter pair in the system to dynamically choose the polarity that leads to higher SINR. Such a scheme works as follows: The transmit and receive polarities at the basestations and subscribers are not fixed but are chosen dynamically as a function of the combination that leads to the highest SINR. That requires that the basestation be able to change its transmit and receive polarity from burst to burst, depending on the subscriber from which the burst is coming or vice versa.

Automatic repeat request is a retransmission protocol implemented at the link layer of a wireless system that enables recovery from packet errors. The ARQ protocol maintains a high-quality link for end-user applications and is significantly more efficient than TCP retransmissions. When a packet is lost to short-term interference or fading, the receiver sends a signal back to the transmitter. In response, the transmitter attempts to retransmit the errored packet at a later time.

Optimization challenges ARQ is an effective way to combat interference but is most effective in recovering low to moderate error rates. When the interference is strong enough to cause high error rates, ARQ must be combined with other techniques, such as adaptive modulation, to combat interference effectively. Depending on the application, ARQ can be selectively turned on or off. For example, for real-time flows such as voice or video that can tolerate some packet loss, ARQ is automatically turned off.

Numerous technical challenges have to be addressed to implement dynamic per-subscriber link optimization. Since the technology entails a broad range of interrelated variables across both the PHY and MAC layers of the system, an overall systems approach must be taken. That approach includes the ability to:

• Control multiple link parameters dynamically and on a per-burst basis. The PHY and radio layers should be agile enough to change their configurations quickly from burst to burst, without degrading the signal.

• Optimally vary the link parameters. Different parameter sets take effect over different time scales and interact in a complex manner. Optimal control involves the solution of a multiparameter, multidimensional optimization problem that considers overall system optimization, along multiple performance axes.

• Have sufficient processing power in the basestation so that it can simultaneously optimize the link parameters for multiple customer-premises equipment sites.

• Interact with the scheduler function so that customer QoS guarantees are maintained, even though the link on which the data is being sent continuously changes its effective bit rate because of link adaptation.

Dynamic per-subscriber link optimization maximizes subscriber channel capacity, network capacity and service providers' spectrum utilization. It provides advanced interference mitigation and management, allowing high-frequency reuse in multicell networks. It extends system coverage in line-of-sight, obstructed line-of-sight and non-line-of-sight conditions. In addition, it plays a key role in maintaining the high quality of service requirements of network operators. The optimal use of resources and automated, service intelligent capabilities minimize infrastructure cost as well as operating costs. This service view thus enables the creation of personalized services to enhance the business model.

Subscribers have very diverse service-grade requirements, driven by applications ranging from multitier data to voice and video. One difference between broadband wireless links and their wired counterparts is that channel variations render traditional, wireline-based QoS schemes meaningless. Adaptive MAC-PHY layers are critical to deliver QoS over broadband wireless access. The design of the fixed BWA system must accommodate channels that are constantly changing, in order to allocate bandwidth predictably for each flow.

Managing flows

New-generation systems that leverage a multilayer architecture will be able to intelligently manage personalized service delivery on a per-flow basis. Service class and bandwidth will be granular down to the host or application level, with complete per-flow statistics available to the service provider and subscriber. Subscribers can have multiple flows, to match the service-grade requirements of their data, voice and video applications.

Carriers have had limited success deploying first-generation FBWA equipment. The static, one-size-fits-all nature of the early wireless technologies severely constrains their ability to expand capacity, coverage, QoS and cost-effectiveness. In essence, they cannot extend their current wireless technical or business model beyond the tens of thousands they now serve, to reach millions of new users whose environments entail more demanding conditions.

The newest generation of solutions uses dynamic per-subscriber link optimization to cost-effectively break these constraints and maximize real-world BWA network performance. By fully exploiting the potential of intelligent, advanced PHY and MAC parameters, network operators can boost both system and network capacity by orders of magnitude, boost coverage to millions of sites demanding service, and enable the delivery of QoS to fulfill multiservice requirements, all at lower overall capital and operational expenses.