This paper will show that wireless local loop systems provide more capacity than mobile cellular systems. Code Division Multiple Access technology has proven to have higher capacity than other multiple access schemes.
Industrial countries require quality and service features from their mobile networks, while developing countries require low-cost high capacity systems-even at the expense of voice quality. Therefore, the system design aspects are different depending on the customer’s needs.
Some history
WLL systems could have been implemented in the 1970s in U.S. rural areas, but were not. AT&T Corp. developed fixed-to-fixed microwave links, but the cost justifications at the time for WLL were:
In the short term, using the radio link would be the solution because the cost of laying a long wire cable for just two remote points is high.
Using the wire link would be the long-term solution. Although the cost of laying a long wire cable was high in the beginning, adding more users along the wire link later would have driven the cost-per-user down over the years.
AT&T could have used its Advanced Mobile Phone System in wireless local loop systems in the 1970s for two advantages:
It could have been easier for AT&T to acquire the spectrum from the Federal Communications Commission for WLL.
AT&T could have bundled services. For example, AT&T could have served wireless local loop and cellular together using the same spectrum and operating the same wireless communications network.
Since AT&T did not take advantage of this opportunity in the 1970s, wireless operators were busy developing cellular systems in the 1980s. Now, in the 1990s, we are starting to look at WLL domestic and international markets.
Since a WLL system is a fixed-to-fixed link system without the wireline connected, as shown in Figure 1, the advantages are:
WLL can eliminate many problems and be cost effective in many areas and many countries.
WLL can shorten the time to deploy a telephone network.
WLL can be easily moved around to accommodate new changes.
WLL is a quick start for start-up telephone systems.
Deploying WLL
To measure the spectral efficiency of a wireless communication system, we have to take two parameters; the channel bandwidth, and the C/I (the received signal (carrier) power to interference ratio).
Narrowing the channel bandwidth or lowering the required carrier-to-interference ratio (C/I), but keeping an accepted voice quality, will increase spectrum efficiency in the network. Since the required C/I in a WLL is always lower than that in the mobile wireless systems, the spectrum efficiency for a WLL increases.
The general key drivers of deploying a WLL system are: capacity, coverage and quality. However, the three key drivers are related among themselves. With an allocated spectral band as a given, both the coverage and the capacity are inversely proportional to the quality and they are related to C/I.
The three key drivers (capacity, coverage and quality) can be expressed as a function of C/I. When the received C/I increases, the quality improves. When the received C/I decreases, the capacity or coverage increases.
Advantages
Due to the nature of the wireless communication medium, there are advantages to implementing a WLL system. These advantages are created by the following attributes and shown in Figure 1:
Coverage increase: The coverage of a WLL is based on a fixed-to-fixed propagation. The path loss of the fixed-to-fixed propagation in a WLL is based on 20 dB/decade. However, the path loss of mobile propagation (fixed-to-mobile) is based on 40 dB/decade, which shows a high excessive loss. Therefore, the same wireless communication system can cover more area for WLL services than for the mobile radio services.
Capacity increase: A WLL is under no multipath condition because the antennas are usually placed high, on top of buildings. The required carrier-to-interference ratio, (C/I), under no fading fixed-to-fixed condition for a 30 kHz FM channel is C/I = 10 dB. Nevertheless, the required C/I under mobile radio condition for a 30 kHz FM channel is C/I = 18 dB. This means that in a WLL, the received signal in a non-fading condition can tolerate more interference. The frequency reuse distances for WLL is much shorter than for mobile radio. The capacity for WLL, then, is much higher than for mobile radio by operating the same communication system.
Interference decrease: In a WLL, the frequency reuse distance can be further reduced because the WLL fixed-to-fixed link is using directional antennas on both ends so that the interference area becomes small. Reducing the frequency reuse distance more means that the capacity is further increasing.
In a WLL, no hand-offs occur because it is a fixed-to-fixed link. Furthermore, the air link from each building to the cell site can be customarily installed to reduce the interference. This link remains unchanged after installation, and the design of the WLL system is much simpler.
The WLL system will be developed differently and will serve in different markets, as described below.
Industrial
The requirement of a WLL system for those subscribers in industrial countries is good voice quality. This requirement is needed to compete with the service from wireline telephones.
The requirements of WLL systems for the system operators in industrial countries are high capacity and large coverage. How can we meet these two requirements and still lower the cost of deploying the WLL system and use the spectrum efficiently? This is a big challenge.
Since the three keys drivers, voice quality, coverage and capacity are always against each other among themselves, we may have to determine an accepted voice quality level first, and choose a wireless communication system that can provide the high capacity and large coverage. The bottom line is to calculate the deployment cost per line and justify the potential of the business in that area.
CDMA can be used for high capacity. The reason is, the frequency reuse factor K is approaching one. All of the cell sites are using the same radio channel. In deploying a CDMA-based wireless local loop system, the power control or adjustment is used to reduce near-far interference. This power control will be set at the beginning of the service, according to all the fixed-to-fixed links, and left untouched until the environment changes. No frequency planning is needed. The CDMA system uses up its processing gain to increase capacity. Of course, due to the nature of a WLL environment, the high gain directional antenna can be used to reduce the interference. One 1.25 MHz radio channel can provide 20-45 traffic channels for mobile radio and 70-80 traffic channels for WLL.
CDMA can be used as a bundled service system. CDMA has been chosen to be a standard system for cellular, personal communications services and for mobile satellite service (via Globalstar) already. Therefore, it is natural trend to have WLL using CDMA because CDMA would then become a bundled service system; providing cellular and PCS services in urban and densely populated areas, and providing WLL and mobile satellite services in rural areas. All the networks can use CDMA technology. All can share the same integrated network so that the investment cost will be much lower. These bundled services can also even-up the whole network traffic density; cellular and PCS operate in urban areas, and WLL and mobile satellite services operate heavily, outside urban areas. The same CDMA phone can be used in all the bundle-services.
(Figure 2 goes here)
Developing countries
The radio capacity of WLL systems is a key issue in developing countries. Voice quality is not an issue in WLL systems because a customer may not even have a phone. The possibility of a customer even adds great value to the network. Because of this, the spectrum efficiency can be reached at any
high level as long as the voice quality can be sacrificed. This means the physical system capacity is not a problem.
The system capacity vs. voice quality is illustrated in Figure 2. In CDMA, when the system required E/I (energy per bit/interference per Hz) is reduced, i.e., the voice quality decreases, the radio capacity in terms of the number traffic channels/cell increases. (The calculation is based on a CDMA radio channel of 1.23 MHz.)
Cost is a major issue in WLL systems in developing countries. Most people have to justify a telephone by the cost they can afford. Therefore, the business plan has to drive the cost down.
We are creating a new term called customer payment-to-investment ratio, c/i in WLL. The notation, c/i, is distinguished from the regular term for the carrier-to-interference ratio (C/I), which is used for the system design parameter. c/i is a business planning parameter, where:
c: customer payment in dollars in X months.
i: investment (including capital, operation and maintenance) in dollars per line over X months.
In a WLL system in a developing country, c is very low. Therefore, you need to have an i value that is much lower than c in order to make profit. This is a big challenge.
Building WLLs
The cost of each customer provided equipment (CPE) needs to be lower. Cost considerations at the equipment side for a WLL system include:
Using an existing cellular system-the same phone can be used both in WLL and in cellular networks. This will lower the cost by sharing the same wireless communications network.
Developing a low-cost WLL system with low-cost CPE and having the capability of upgrading the system in the future. In developing countries, the markets for WLL are huge, therefore, it may be justified to choose a low-cost system solely for WLL.
In a WLL system, the standard is not an issue. The WLL links are fixed, not moving, therefore, the CPE does not require many standard, low-cost WLL systems on the market, such as the Metricom system, General Wireless system and Adicom system, etc. Most of them are using CDMA technology.
In order to lower the cost on equipment per line, we may have to have more users to share one line. The following examples can reduce the cost:
Use multiplexing on one radio. We can use Frequency Division multiplexing or Time Division multiplexing on one radio to increase more talking channels, but of course, the voice quality may be reduced.
Use party-line feature for those subscribers who are willing to share one subscribed line.
Conclusion
The WLL always provides more capacity than mobile cellular systems. The requirements of WLL systems for industrial countries are the quality and service features. It is wise to choose a bundled service system to share the same wireless communication network in order to lower the operational cost and to only offer to the subscriber, in the meantime, a single handset.
The WLL system for the future, in developing countries, is to make a low-cost system, even if the voice quality has to be sacrificed. Of course, to adapt an existing cellular system, which can have both services: mobile cellular and WLL, to share the same spectrum, is possible. Many suggestions to lower the cost by developing low-cost WLL systems, have also been addressed. WLL is a future business for both industrial countries and developing countries.
Dr. William C. Y. Lee is vice president and chief scientist at AirTouch Communications Inc., Walnut Creek, Calif.
