The promise of local multipoint distribution services and other broadband wireless access networks has yet to be realized because cost-effective and architecturally sound examples have been slow to develop over the last five years. However, next-generation approaches that far exceed traditional legacy-based broadband wireless access (BWA) equipment have earned the attention of carriers waiting for the proper systems that will make it economically feasible to compete with truly differentiated service offerings.
Ranging from frequencies of 2 to 43.5 GHz, BWA services in the United States include local multipoint distribution at 28 and 31 GHz, multipoint multichannel distribution at 2.5 GHz and digital electronic messaging at 24 GHz and 38 GHz. In addition, several countries have allocated spectrum at 26, 28 and other gigahertz frequencies. To ensure successful market entry, wireless competitive local exchange carriers (CLECs) with spectrum licenses must understand the numerous financial and operational problems while facing crucial system purchase and deployment decisions. These new carriers require an efficient, flexible BWA system and backhaul network that optimizes their capital investment while maximizing revenue and customer satisfaction.
Point-to-point (PTP) radio is a proven technology that integrates well with backhaul PTP, but these systems were designed to supply large, constant-need capacities to single locations. Where BWA customers are geographically dispersed or numerous or both, PTP is not appropriate because of the need for many antennas and the inability to spread costs.
On the other hand, point-to-multipoint (PMP) uses an improved architecture and cost model that can better serve customers with data rate needs from 2 Mbits/second to 20 Mbits/s. These customers need greater data capacity than is available from xDSL and cable modem technologies and cannot be reached with the limited fiber deployed today. Further, PMP is the only technology that allows wireless CLECs to become facilities-based service providers and to achieve spatial, statistical multiplexing, which is the ability to oversubscribe services over a group of geographically dispersed buildings.
The need for higher-bandwidth connections and for mixing traffic types-voice, data and video-on a single infrastructure provides new challenges to system designers and network architects. Therefore, a fresh approach is required to solve the new challenges of the BWA market. Next-generation PMP systems, in particular those that incorporate proven time-division duplexing (TDD) technologies, are enabling wireless CLECs to compete effectively. Without the constraints of using existing technologies or legacy products, a solution built from the ground up can be realized that benefits both carriers and small- and medium-sized business users.
To develop the optimum PMP system, true innovation is required in three technology areas: the physical layer, the Media Access Control (MAC) layer and the network layer. The physical layer consists of digital modem technology, RF circuitry and millimeter-wave front ends and antennas. The MAC layer is the real-time software layer driven by class-of-service issues to allow the simultaneous access of bandwidth resources by thousands of subscribers. Finally, the network layer is the layer between wireless and wireline components. These components include customer premise equipment inputs/outputs, which require standard ports and signaling interfaces, as well as a basestation.
Starting at the lowest level, a duplexing scheme of some type is required to enable the two-way simultaneous exchange of information between two devices. Several solutions exist. The two best suited for PMP communications work by separating the communications signals in frequency or in time.
Frequency division duplexing (FDD) has been used in recent system designs. FDD requires a "guard band," that is, a large, otherwise unused frequency block for separating upstream and downstream bands for proper operation. This results in inefficient spectrum use.
In contrast to this legacy method, TDD is a commercially proven technique that separates upstream and downstream traffic in time. TDD has been successfully deployed in Personal Handyphone Service networks throughout Japan and Digital European Cordless Telecommunications networks in Europe. As a low-cost and low-power wireless platform for both base-stations and customer premises equipment, TDD systems perform well in the rigorous mobile telephone environment. These systems can be optimized for fixed wireless access to deliver multiple types of communications.
Adaptive TDD represents a next-generation technique. It improves TDD by accommodating instantaneous changes in end-user traffic asymmetry by making real-time adjustments to upstream and downstream capacity. At any given time, each radio sends and receives a variable number of time slots based on real-time traffic demands, allowing for infinitely variable asymmetry and the optimum bandwidth allocation in each direction for end users.
An adaptive TDD BWA system provides numerous benefits. As a software-enabled technology it is more efficient, requiring only one RF carrier and no duplexer, which can be an expensive radio component. An adaptive TDD system also simplifies the radio by reusing filters, frequency sources, mixers and synthesizers, while further eliminating isolation complexity for each antenna. In addition, since guard bands are eliminated, there is no spectrum waste. Consequently, there is a substantial increase in usable spectrum, especially in the case of block allocations. An adaptive TDD radio can be used in many of the frequencies allocated worldwide, from 10 to 43.5 GHz, with only a simple change to the RF front end. As a result, this portable platform can be manufactured in volume.
Adaptive modulation technology allows the system to automatically select the correct modulation scheme for each customer, based upon the distance to that customer and current environmental conditions. Initially, wireless CLECs will want to maximize system range in order to minimize the number of basestations deployed. That will require the use of a simple, robust modulation scheme. But using only this modulation scheme means that there is wasted equipment when communicating with users who need a more efficient, but more sensitive, modulation scheme. Later, as customer traffic grows, the need for greater capacity will further drive the need for these more complex modulation schemes. The best solution is to accomplish this trade-off automatically in real-time on a single channel in a single system from the beginning.
With adaptive modulation, both capacity and distance can be optimized simultaneously while meeting customer commitments. Essentially, the number of basestations can be decreased by using quadrature phase shift keying, and the amount of equipment needed to serve customers can be minimized by using higher-order modulation schemes, such as 16 and 64 quadrature amplitude modulation for users who are closer to each basestation.
To achieve simultaneous broadband connectivity to numerous users in a PMP configuration, a BWA system must have very fast burst modems. More recently, modems have been designed either to perform at continuous high speeds or to provide data burst capabilities, but not both.
The proper technology combines these two functions to allow many high-bandwidth users to share the same frequency and basestation resources. This results in higher statistical gains and lower capital expenditures. Broadband burst modems enable an efficient implementation of bandwidth on demand in the MAC layer as well as the additional benefits of differentiated-services offerings and greater infrastructure leveraging.
Given a fast, efficient physical layer, with all of the inherent benefits of minimum spectrum and capital equipment use, a sophisticated MAC protocol is needed to allocate the proper bandwidth to each user in real-time. Using time division multiple access (TDMA), the MAC performs this function based on the services each user has purchased and the quality of service (QoS) required for their particular applications.
A more advanced MAC layer is one based on adaptive TDMA technology. This next-generation approach represents a more flexible technique that incorporates instantaneous bandwidth on demand and QoS while supporting all the various network protocols in use today.
With instantaneous bandwidth on demand, the system allocates only the necessary bandwidth required by an end user at any given time, thus allowing wireless CLECs to meet their service-level commitments while freeing unused bandwidth for other users. Instantaneous bandwidth on demand also enables wireless CLECs to oversubscribe their spectrum and their equipment.
A MAC layer must also handle the bundling of various service offerings such as voice, data and video with their variable traffic patterns on the same infrastructure. Adaptive TDMA supports various QoS parameters on a customer-by-customer basis. This ensures that customers can buy exactly the data rate and service priority they require while allowing the service provider to meet specific service-level agreements with each customer and, even better, to oversubscribe the air link for increased revenues.
Most important, adaptive TDMA works equally well with the various network protocols in use today. Agnostic to ATM, IP, TDM and frame relay protocols, adaptive TDMA systems are designed to carry traffic to and from these various networks seamlessly. This permits carriers to equally support these protocols, multiple deployment environments and the evolution of networks over time.
As network edge devices, next-generation BWA systems must address network protocol and back-haul concerns. Today, this means that seamless connections to Sonet rings are required and that TDM, IP and ATM protocols must be supported equally by the system, because different carriers will require different support either city by city in their deployments or over time. At the very least, each carrier will have a somewhat different focus in the market in providing services with the appropriate protocols.
With the growing demand for converged voice, video and data on a single infrastructure and at broadband speeds, recent advances in BWA systems now offer a 21st-century solution for what is, in fact, a 21st-century problem.
By addressing every technology layer with recent advances-adaptive TDD, adaptive TDMA, adaptive modulation and broadband burst modem technology-wireless CLECs can take advantage of the inherent flexibility and high functionality that next-generation systems offer.
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