Divide and Conquer: In the air, bandwidth is a highway that can’t be widened, but it can be built to accommodate maximum traffic
Concerns about efficient bandwidth usage certainly are not limited to broadband wireless operators. But for those who are spending millions of dollars on spectrum, the issue of how to deliver a variety of competitive, revenue-generating services efficiently is demanding a lot of attention. And their competitors are watching closely.
If broadband wireless is a highway, how do you divide the lanes to maximize traffic flow and minimize slowdowns?
Do you dedicate each lane to a specific vehicle, ensuring uninterrupted travel? Or do you open it up and let the traffic flow freely, accommodating more vehicles but possibly slowing things down during rush hour? On the broadband wireless highway, there is room for both.
With the recent auction of local multipoint distribution service (LMDS) spectrum now complete, a slew of new highway-in-the-sky owners are looking closely at how to divide their lanes to provide connectivity to a diversified customer base.
They need to deliver the quality-of-service (QOS) requirements that high-end business users demand while maintaining connections to cost-sensitive residential and small office/home office users. Their highway needs to accommodate both time division multiplexing (TDM) and bursty services from a common network infrastructure. And it needs to use the bandwidth efficiently and cost-effectively. In the air, bandwidth is a highway that can’t be widened, but it can be built to accommodate maximum traffic.
Asynchronous transfer mode is favored consistently over Internet protocol or TDM-based solutions for the infrastructure.
On this platform, effective lane division can be accomplished by employing shared lanes, or radio access channels, using time division multiple access (TDMA), and dedicated lanes using frequency division multiple access (FDMA).
TDMA, combined with quaternary phase shift keying (QPSK) modulation, addresses the bandwidth requirements of lower data rate connections, while FDMA, combined with higher efficiency quadrature amplitude modulation (QAM), addresses higher bandwidth connections.
Multiple services with ATM Before we paint the lines on the highway to address these divisions, it’s important to understand the rationale behind the use of ATM construction.
With ATM, multiple services can be delivered through a common network infrastructure. Exploiting ATM functionality to the edge of the network enables the implementation of QOS, statistical gain and service flexibility. A common air interface is achieved that creates a flexible, robust and relatively future-proof service delivery platform. The common air interface offers benefits within the wireline backbone and throughout the wireless access network including:
* high-capacity design concepts throughout
* statistical gain throughout the system, not just on the ATM backbone
* reduced backhaul requirements
* end-to-end network management, including radio resources
* definable combinations of services-switched, permanent and multiplexed
* a variety of customer access services, including support of legacy services such as T-1
* the ability to administer and maintain a range of QOS performance attributes
The use of high-frequency millimeter wave spectrum (such as that associated with LMDS) allows the bandwidths in the radio access portion of the system to be matched effectively to the available bandwidths within the ATM backbone.
ATM construction allows the wireless highway to carry all the service cargo accommodated on the wireline backbone.
When considering the variety of service connection possibilities, several attributes can be used to group the associated traffic types according to the following categories:
* low-bandwidth, symmetrical fixed rate connections such as T-1/ E-1 (or fractionals)
* low-bandwidth, asymmetrical variable data rate connection, such as Ethernet
* high-bandwidth, fixed data rate connections such as DS-3/E-3 or OC-3c.
Radio access options ATM offers the ability to handle all of the above traffic types from a common base station, provided the radio access layer is fitted with suitable connections. Efficiently handling these categories of traffic in the wireless environment brings us back to our highway analogy and the need to divide the lanes.
Several factors determine which radio access method makes sense for different traffic types (Table 1).
Shared access radio interconnection is best accommodated using TDMA in combination with QPSK modulation. The primary factor here is the ability to accurately demodulate the TDMA upstream signals without significantly affecting efficiency.
QPSK can achieve a maximum of 2b/Hz base efficiency and is less efficient than QAM. Given the combined factors of capacity, power and available technology, it remains the most effective modulation to support a TDMA structure.
A dedicated bandwidth structure such as FDMA requires optimizing spectral efficiency, best achieved through the use of QAM. Conversely, QAM is optimally deployed using FDMA-based channels able to provide dedicated connection channels with full forward error correction running in both the up and down links.
By using an ATM switch in the wireless base station, traffic control through the different over-the-air channels (QPSK or QAM) is relatively straightforward. An integrated approach (Figure 1) further allows for optimized coupling among medium access control, switching and control fabrics. End-to-end network management and path management are facilitated when the architecture employs ATM in the backbone as well as over the air, with adaptation to the native interface carried out within the customer premises equipment.
QAM and QPSK in tandem Dividing some lanes of our highway for shared access (TDMA) and the rest for dedicated use (FDMA) requires accommodating both QPSK and QAM radio signals from the same base station. Because these modulation schemes differ in RF bandwidths and signal-to-noise requirements, their radio ranges-maximum cell radius, for example-vary.
A number of options can accommodate both, including dynamic switching between modulation schemes; deploying closer customers on QAM/FDMA channels, with farther customers using QPSK/TDMA and creating annular coverage rings where the service availabilities are dissimilar; and deploying customers on fixed modulation schemes and engineering link budgets so that both the QPSK and QAM based services are available equally throughout the cell coverage area.
Dynamic switching creates complex bandwidth control and allocation problems and is not well suited for constant bit rate traffic such as in legacy TDM services. Customer premises equipment costs also are increased because of the added radio complexities involved.
Deploying cells with annular service rings is inconvenient and usually unacceptable to the service operators given the additional complexities inherent in managing and matching locations of a given subscriber within the rings to the respective service/modulation offering.
The last option drives the service operator to select the modulation scheme that the customer will use through the service connection being purchased. This lets the operator offer different types and levels of service and price them appropriately.
It also necessitates designing the radio links for both TDMA and FDMA, and their associated modulation schemes, to achieve common RF operating ranges. This is achieved through appropriate effective radiated power and bandwidth design techniques. A typical radio range achieved in a millimeter wave system in the 28 GHz range (such as LMDS) can provide a wide variety of service typ es using either QAM or QPSK connectivity (Figure 2).
Through appropriate design, figure 2 shows how QPSK and 16 and 64 QAM data links let the supported services be deployed out to the same approximate cell radius (shown as the 0 dB margin point).
So when building a multiservice highway in the air, ATM provides a convenient infrastructure for multimedia and multiservice transport.
Switching and traffic control can be exploited to direct traffic onto different channels supported by different modulation techniques. The ability to use these different techniques allows for optimized use of the radio license to provide customer connectivity from a broad service portfolio.
It also enables carriers to deploy several network configurations: shared access, using TDMA/QPSK; low-bandwidth, symmetrical, fixed-rate connections; low-bandwidth, asymmetrical, variable data rate connections; or dedicated access using FDMA/QAM; and high-bandwidth, fixed data rate connections.
Wireless service providers are also giving consideration to the use of QAM for TDMA radio access. But that, for now, is the road less taken.
RELATED ARTICLE: Wireless Highway Holds Its Own
* Residential multimedia
– Internet access
– TV/video distribution
* PCS backhaul
* Future applications – ATM offers significant
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