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LAS-CDMA - The Wireless Technology for 3G and Beyond

Thu, 05/24/2001 - 4:40am
Dr. William C.Y. Lee and Frederic Leroudier

As the wireless industry is poised to enter its third generation (3G), it is enjoying a growth phase projected to rival the computer revolution in its impact on lifestyles and economies worldwide.

Much of this growth will result from a substantial increase in data traffic. While data traffic today represents only about 5 percent of wireless traffic, the proliferation of wireless Internet services will propel it to exceed voice traffic within a few years. As wireless data usage increases, transmission speed requirements also are expected to increase from the current 14.4 Kbps to up to many Mbps. In addition, future wireless data services will be packet-based rather than circuit-based.

To keep up with demand, additional efficient wireless technologies will be required. Current IMT-2000 standards, expected to be deployed in mid 2001, will only satisfy demand in the initial phases of 3G. Soon, however, new and improved technologies will be required to handle the exponential growth in users and traffic. Since more spectrum space is usually not economically viable, these new technologies should provide an increase in capacity and spectral efficiency — the number of bits per second that can be transmitted in one Hertz of spectrum — by a factor of more than two over the current standards. This is especially important in developing countries where density is often very high and wire-line service inadequate.

While this shift creates significant opportunities for software developers, device manufacturers, service providers and others in the wireless community, it also places new demands on wireless networks, including:

Speed. Carrier capabilities must include high-speed, high-efficiency data and voice transmissions to support new devices and data applications.

Data. While data services presently comprise 5 percent of today's wireless network traffic, the rapid growth of the Internet, PDAs and other applications imply that data traffic will soon exceed voice traffic. 3G systems must handle data services as efficiently as they handle voice.

Capacity. Explosive consumer demand for wireless voice service, combined with the rapid development of wireless Internet applications, will exert tremendous pressure on today's networks. 3G networks will require expanded capacity that is used much more efficiently.

Coverage. The industry must facilitate the growth of mobile-enabled services, such as high-speed Internet connections, interactive multimedia, wireless local loop and global mobility management. Expanded coverage will be key for mobile consumers.

The standards that propelled First and Second Generation (1G and 2G) wireless growth are not sufficiently robust or scalable to accommodate 3G and beyond. Current 3G systems will need improvements before they can truly support high capacity and high-speed networks, while maintaining compatibility with today's features and functions.

LinkAir has built such a technology — LAS-CDMA, or Large Area Synchronized Code Division Multiple Access. LAS-CDMA is the first example of a wireless technology that goes beyond the requirements of IMT-2000. LAS-CDMA is an innovative concept that enhances current 3G CDMA technologies by increasing their spectral efficiency by a factor of at least three. It will allow operators to satisfy the capacity and bandwidth needs of operators in the mature phases of 3G and is well positioned to be the leading candidate for Fourth Generation (4G) wireless systems.

LAS-CDMA:
•Has the highest spectral efficiency of any 2G or 3G technology
•Is optimized for data services
•Is backwards compatible, so it optimizes current infrastructure investments by enhancing current GSM (Global Systems for Mobile) and CDMA based systems
•Is a natural choice for future IP packet data systems

LAS-CDMA can significantly enhance today's 2G or 3G networks, supporting the wireless industry's immediate and long-term needs.

Las-CDMA
Attributes/Comments

•High spectral efficiency/ From .7 to 4.4 b/s/Hz
•Voice and data support/ On same RF carrier
•High voice capacity/ At least three times (3X) increase over latest 3G standards
•High-Speed Packet Data/ Up to 5.5 Mbps on a duplex 1.25 MHz carrier (indoor)
•Evolution from current 3G standards/ LAS-CDMA preserves carriers' network and investments and ensures a smooth evolution for their customers.
•Evolution into future generations/ LAS-CDMA has the most efficient radio interface in terms of capacity in a given frequency allocation. It is therefore a natural favorite for next generation wireless services.
•Convergence/ LAS-CDMA offers the first step towards convergence of wireless standards.

2G Limitations in a 3G Wireless World
Historically, wireless growth has been underestimated, leaving many carriers unprepared for the demands placed on their networks for voice and data services. Today, many systems are running out of capacity, just when customers are demanding higher quality voice services and new higher speed data rates. With the fast deployment of new services and heavy volume pricing packages, even CDMA networks will become saturated in a short time.

The design and capabilities of the leading standards for 2G are severely limited in the 3G wireless industry. For example, TDMA uses time-based intervals to divide signals into packets for transmission. As a result, the TDMA standard is extremely inefficient for high-speed data transmissions.

CDMA offers significant improvements over TDMA, and has come to be the recognized standard for 2G. Shifting from time- to code-based packet division greatly increases CDMA's transmission throughput, and CDMA has more than doubled the capacity available with the GSM standard. However, CDMA is not constructed in a way that will accommodate the needs of 3G. For example, the codes CDMA uses to "spread" the transmitted material, actually generate interference, which:

•Limits the traffic/number of users any given network can accommodate
•Limits the acceptable proximity of neighbor base stations
•Barrages the network and weakens overall performance

In addition, CDMA technology relies on soft handoffs to improve coverage at the cell's edge and to reduce interference. But soft handoffs require more channel cards to handle the increased traffic, as well as more backhaul capacity, resulting in higher network infrastructure costs and maintenance.

Limitations such as these not only cause inefficiency and increased costs, but they fail to position CDMA as the technology to meet the 3G wireless industry's burgeoning needs for increased speed, capacity, coverage and data applications.

LAS-CDMA: The Ideal Standard for 3G and Beyond
LinkAir's patented LAS-CDMA technology provides up to 20 times better capacity than existing 2G standards, and up to three times greater capacity than that of CDMA 2000, the fastest-growing 3G standard.

LAS-CDMA delivers exponential leaps in system performance and capacity over CDMA, yet its CDMA-based design ensures compatibility with existing systems. LAS-CDMA offers:

•a minimum of six times more voice and data users than IS95
•higher data throughput
•larger coverage areas
•improved indoor coverage
•less reliance on complex processes, therefore more cost effective

For wireless handset and base station manufacturers, this means the ability to offer higher capacity equipment that meets growing customer demand. For carriers, LAS-CDMA minimizes additional network build-out and expenditures. This allows carriers to compete at a lower cost while efficiently providing customers new and enhanced services, such as high-speed Internet connections, interactive multimedia services, wireless local loop and global mobility management.

Spectral efficiency
LAS-CDMA delivers the highest spectral efficiency of any 2G or 3G technology available. It delivers at least six times more capacity than current CDMA technology and at minimum 20 times more capacity than GSM. It even outperforms CDMA2000 by more than triple.

Emphasis on data services
LAS-CDMA is optimized for data services, enabling voice services to be handled as data — a projected requirement of the 4G wireless industry. LAS-CDMA is designed to handle data capacity requirements efficiently and to facilitate deployment of these services by operators. LAS-CDMA also can be configured for very high-speed data services (up to 5.53 MBPS in a 1.25 MHz carrier), and the bandwidth can be shared among multiple users at the same time. As a result, carriers can deploy these services without acquiring additional spectrum.

Backwards compatibility
LinkAir designed LAS-CDMA to be backwards compatible with today's existing wireless systems, including IS-95, IS-41 and GSM Mobility Application Part (MAP). This allows wireless operators to protect and optimize their current infrastructure investment and maintain a continuous level of service to their customers. It also allows roaming between LAS-CDMA systems and any other wireless system in the world.

Ability to evolve
LAS-CDMA will be the technology of choice as the wireless industry evolves from circuit-oriented/voice-centric to packet-oriented/data-centric networks. LAS-CDMA will work hand-in-hand with the future all-IP architectures that are being developed to handle this shift.

LAS-CDMA is characterized by the use of an innovative spreading and encoding scheme called LAS coding that reduces system generated interference, thus increasing capacity. LAS coding is a combination of two levels of coding called LA and LS codes.

While FDMA (Frequency Division Multiple Access) or TDMA systems are limited by their own design (e.g. by the number of timeslots) classic CDMA systems never reach the limits of their code resource. For example, in a system designed according to the IS-95 standard and using Walsh codes as the multiple access method, capacity rarely exceeds 20 voice users per sector per RF carrier — despite the fact that there are 64 codes available.

This limitation is explained by the fact that classic CDMA systems are designed to tolerate only a certain amount of interference without degrading the quality of the transmission. By contributing to the noise floor, interference makes demodulation increasingly difficult and eventually impossible as the symbol or bit error probability increases. CDMA systems are called interference limited. Capacity of such a system is determined by how much interference it may tolerate.

System generated interference is the sum of three components:
•Inter-Symbol Interference (ISI) caused by reflected signals of the original signal. ISI includes an uplink and a downlink component
•Multiple Access Interference (MAI) caused by signals from other mobile stations operating in the same cell/sector as the mobile, including the sum of all their reflected signals. MAI includes an uplink and a downlink component.
•Adjacent Cell Interference (ACI) caused by signals from neighboring base station. ACI is the sum of all signals received from all neighboring cells in the cellular network. ACI includes an uplink and a downlink component.

LAS-CDMA departs from this limitation by using powerful interference reduction multiple access codes. The LA and LS codes act on the system-generated interference by either reducing or completely eliminating it.

LAS-CDMA also offers wireless operators the first step towards convergence of wireless technologies. Because all LAS-CDMA modes use the same spreading, coding, modulation and system procedures, economies of scale can be achieved for the design and production of LAS-CDMA equipment.

Prepared for 4G

Discussions on 4G services and systems have just begun, and it will be several months before a clear consensus emerges on 4G requirements. However, judging by the direction in which the telecommunications industry is moving, it is expected that many high-level items will be central to 4G definition:

1) Higher spectral efficiency and transmission speeds:
Speeds of up to 15 Mbps could be considered in a fixed environment. In a mobile vehicular environment those speeds could be required to exceed 1 to 2 Mbps. Recent events such as the UK UMTS spectrum auctions have shown that spectrum is a scarce resource, which means that a solution that can deliver these data speeds in a relatively narrow slice of spectrum will be preferred to wide-band solutions.

2) Convergence towards a truly global standard:
3G has failed to produce a truly global standard for the wireless industry...although the different standards are more interoperable than ever. In 4G, global operators are expected to drive the industry towards a truly global standard that will cover all radio technologies.

3) Movement towards an IP-centric world:
As the global telecommunication industry moves towards a packet-oriented paradigm, the wireless industry will surely follow this general trend. This will impact all areas of the wireless industry, from all-IP wireless networks to a generalization of IP applications to the wireless user. For example, it is expected that a form of Voice over IP (VoIP) adapted to the wireless air interface will replace traditional voice service for the next generation.

4) More predominance of software versus hardware:
Many articles have been written on the concept and benefit of Software Defined Radio (SDR). As those technologies develop and become more widespread, introducing new wireless technologies will become easier since changes can be made more easily and faster than if implemented in hardware.

LAS-CDMA has the highest spectral efficiency of all 3G or evolved-3G technical proposals. LAS-CDMA also supports transition from the circuit-oriented services or applications to a general packet oriented since it can support both on a same RF carrier. In addition, the existence of the TD-LAS mode make LAS-CDMA very attractive for Internet traffic patterns.

Improvements can be applied to LAS-CDMA to further improve spectral efficiency:
•Packet-based voice schemes, such as wireless voice over IP: LAS-CDMA will gain from such schemes by taking advantage of voice activity and variable rate voice coders.
•Adaptive array antennas and smart antennas: Improved ability to gain sectors, and the ability to better separate mobile devices with different propagation conditions will further optimize capacity and performance in an LAS-CDMA system.

The sum of all these improvements eventually will converge into a single harmonized standard, bringing all modes of LAS-CDMA into IP-LAS for 4G wireless systems.

Current wireless technology cannot address the needs of tomorrow. Present standards already are falling short of operators' demands for the capacity to provide more robust services, such as high-speed wireless Internet and greater coverage areas. As a result, operators must find a way to upgrade their spectral capacity without having to invest significant dollars to replace or upgrade their existing infrastructure to handle the demands of 3G.

LAS-CDMA is the first example of a wireless technology that goes beyond the ITU's requirements of IMT-2000. LAS-CDMA simply enhances current 3G CDMA technologies by increasing their spectral efficiency. This innovative approach enables operators to leverage their existing infrastructure — not replace or spend significant dollars to upgrade it — to provide the advanced services necessary to stay competitive in the 3G wireless markets. And, because the LAS-CDMA technology offers quantum leap increases in capacity, quality and data speed, operators are well positioned to lead the way into 4G, the next evolution of wireless.

Dr. William C.Y. Lee is the Chairman of LinkAir Communications, Inc.

Frederic Leroudier is the Vice President, Standards and Business Development for LinkAir Communications, Inc.

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