LAS-CDMA-A New Generation Wireless Technology
William C.Y. Lee, Ph.C.
William C.Y. Lee, Ph.D., graduated from the electrical engineering of Ohio State University. He serves as Chairman of the Board of Directors for LinkAir Communications, Inc., and has been selected as an IEEE Fellow. He has served as a member of numerous Councils, including the California State Council on Science and Technology, the US Council on Competitiveness and the FCC Technical Advisory Council. Regarded as a world-class scholar in wireless communications, Dr. Lee is renowned for his leading contributions in making analog and CDMA technologies commercially viable. He has published more than 200 articles and several books on CDMA theory and technology. His latest book-"Lee's Essentials of Wireless Communications"-was published in 2000. Dr. Lee is the inventor of Microcell, a leading technology. He holds more than 25 US patents, with 11 more pending. Dr. Lee was one of a team of pioneers in developing advanced wireless technology-AMPS-for Bell Labs for 15 years. He then joined the ITT Defense Communications Division, where he headed the advanced mobile communications system. During his subsequent tenure with Airtouch Vodafone-one of the largest wireless carriers in the United States-Dr. Lee assisted in CDMA research and the initial trial of the technology. The pioneer in personal communications network (PCN) technology, Dr. Lee led PacTel's PCS experimental trial, and under his leadership, the first CDMA phone call was completed in Los Angeles in 1995. He has earned many prestigious industry awards, including the IEEE VTS Avant Garde Award, the CTIA Award, the CDMA Industry Achievement Award, the SATEC Award, a Bell Lab Award and the IEEE Third Millennium Medal Award.
Wireless telecommunications has grown from a niche application ten years ago to what many analysts believe will be the most significant technological evolution in the next decade. The wireless industry is enjoying an unparalleled growth phase projected to rival the Internet revolution in its impact on lifestyles and global economy.
Much of this growth will result from three fundamental factors:
* A substantial increase and shift in data traffic: While data traffic represents only about five percent of wireless traffic, the proliferation of wireless Internet services will propel it to exceed voice traffic within a few years. As wires data usage increases, data transmission requirements also are expected to evolve from the current low data rate circuit switched connections (e.g.14.4 Kbps) to very high data rates (up to more than 5 Mbps) packet switched services.
* A fundamental shift from legacy circuit based core and access networks to "All IP" packet networks: This move goes between the different types of networks. In order to reduce network upgrade costs and produce greater returns on investment, wireless operators are looking for technology that can move them into a new generation of all IP and packet based wireless networks.
* A convergence of Wireless telecommunication and the Internet: The convenience and ubiquity of "always on" portable wireless devices will replace the old desktop/wireline modem paradigm of the last century.
In order for these elements to exist, there must be an economical way to realize those applications. Indeed, to keep up with demand and to provide cost-effective ways of deploying new high-bandwidth applications, wireless operators will need to find ways to enhance their networks' capacity. However, given the ever-increasing costs of acquiring spectrum, (as seen in the recent auctions in the United Kingdom ($32.44B), Germany ($46.2B) and Italy ($11.6B), operators are the ratio of serviceable traffic capacity over available spectrum), but also be able to efficiently serve the few wireless packet data applications. The existing 3G technologies, as currently defined by the ITU, while appropriate for early 3G deployments will soon need to be upgraded to meet the needs of the changing market place.
In fact, neither the 1G and 2G technologies that propelled early wireless growth, nor the current 3G standards are sufficiently robust or scalable to accommodate 3G requirements and beyond. New and improved technologies will be required to handle the exponential growth in users and data traffic and should provide an increase in both capacity and spectral efficiency.
Therefore, the wireless industry must identify a new technology that will drive future growth, while accommodating and enhancing systems designed for existing standards. The new generation of wireless needs to happen today. That is bringing 4G wireless technologies in today to improve 3G technologies and lead the evolution into an all IP future.
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 technologies by increasing their spectral efficiency by a factor of at least three. By incorporating LAS-CDMA technology into their planning,, operators' current systems will satisfy their capacity and bandwidth needs. LAS-CDMA is also positioned to be the leading candidate for 4G wireless systems through its innovative merging of LAS-CDMA and TDD (time division duplex) technologies for large area systems.
LAS-CDMA:
* Has the highest spectral efficiency of any wireless technology.
* Supports both data and voice transmission over the same carrier
* is the only technology today that can utilize FDD networks to deploy TDD air interfaces that will enable the asymmetric communications ideal for wireless Internet traffic without sacrificing spectral efficiency.
LAS-CDMA Attributes
Comments
High spectral efficiency
Changes from the interference limited environment to the noise limited environment
Voice and data
On same RF carrier
High voice capacity
Significant 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 wireless standards
LAS-CDMA preserves carriers' network and investments and ensures a smooth evolution for their customers.
Next generation technology for today
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 a leap forward in the convergence of wireless communications and the internet.
2G Technology Limitations in a 3G Wireless World
Historically, wireless growth has been consistently underestimated, leaving many carriers unprepared for the demands being placed on their networks for voice and data services. Today, many systems-CDMA, GSM, and TDMA (Time Division Multiple Access) included-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 3G 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 industries. For example, current TDMA standards have severe capacity limitations due to their relatively low spectral efficiency.
CDMA is a significant improvement over TDMA, and has come to be the recognized standard for 3G. Shifting from time to code-based multiple-access division and allowing same frequency reuse from cell to cell greatly increases the transmission throughput of a CDMA system. For instance, CDMA has more than doubled the voice capacity available with the GSM or TDMA (IS-136) standard. However, current CDMA does have its limits in particular when it comes to further increasing spectral efficiency. This is because the use of the current spreading and scrambling codes used in CDMA systems to multiplex users in a multi-cell network. The use of these codes results in system generated interference such as:
* Inter-Symbol Interference (ISI) and Multiple Access Interference (MAI) (also known as same cell or intra-cell interference):
Limits the traffic/number of users any given network can accommodate in a single cell.
* Adjacent Cell Interference (ACI) (also know as inter-cell or other-cell interference): Limits the acceptable proximity of neighbor base stations and generates capacity limiting interference.
Another consequence of these code properties is the inability of WCDMA and cdma2000 to efficiently accommodate voice and data traffic over the same carriers, leading to the deployment of inefficient overlay networks (e.g. 1x EV/DO, which is a RF carrier overlay solution to carry packet data on top of a cdma2000 network). The unstoppable movement towards global telecommunications networks being able to carry voice, multimedia, e-mail and Internet browsing traffic on the same network will make these solutions obsolete.
These two critically limiting factors (limited spectrum efficiency and non-integrated solutions) mean that new technological evolutions are urgently required for the wireless industry. The penalty for not doing so will undoubtedly be a slow-down in the growth of the wireless industry, as other telecommunication segments will continue to enjoy high growth.
LAS-CDMA: The Ideal Standard for 3G Evolution
LinkAir's patented LAS-CDMA is the next generation wireless technology that utilizes smart spreading codes to eliminate interference thus dramatically in creasing the capacity of the wireless network and enabling voice and data transmission over the same carrier.
LAS-CDMA delivers exponential leaps in system performance and capacity over current CDMA, yet its design ensures maximum reuse of existing systems. LAS-CDMA offers:
* greater capacity than current CDMA and GSM technologies
* higher data throughput
* larger coverage areas
* improved indoor coverage
* simpler designs, making it 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 costs associated with new network build-out and capacity upgrade 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.
LAS-CDMA technology overview
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 dimension (e.g. by the number of timeslots in TDMA), CDMA systems typically never reach the limits of their code resource with which they are designed. 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. For this reason, CDMA is said to be "interference limited". Capacity of a CDMA system is determined by how much interference it might tolerate.
In current CDMA systems this limitation is severe due to the high self-and cross-interference (or auto and cross-correlation) properties of the codes used in current CDMA systems. By design current CDMA systems can only tolerate a certain portion of severe 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.
System generated interference is the sum of three components:
· ISI caused by reflected signals of the original signal. ISI includes an uplink and a downlink component
· * MAI caused by signals from other mobile stations operating in the same call/sector as the mobile, including the sum of all their reflected signals. MAI includes an uplink and a downlink component.
* 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 and uplink and a downlink component.
LAS-CDMA is a smart spreading code, which greatly improves this limitation by using powerful zero interference multiple access codes. The LA and LS codes act on the system-generated interference by either reducing or completely eliminating it.
Spectral efficiency
LAS-CDMA delivers the highest spectral efficiency of any 2G or 3G technology available, Current CDMA technology is an interference-limited system. By using the LAS smart codes, the MAI and ISI are all reduced to zero and ACI reduced to minimum. The LAS-CDMA prototype system tested in China proved that the LAS smart codes perform in both indoor and outdoor environments and in moving vehicles.
Emphasis on data services
LAS-CDMA is optimized for data services, thanks to the following two key properties:
* The low cross-correlation properties of the LA and LS codes make them ideal for supporting voice and data traffic on the same RF frequency and network. Since the codes used to carry voice traffic will have minimal interference with the codes used for data, it becomes very easy to engineer an integrated voice and data network using the same RF resources.
* LAS-CDMA high spectral efficiency allows any combination of high-speed data and high capacity. This means that an operator will be able to offer new high-speed packet data services without sacrificing voice capacity as compared to current systems. This makes LAS-CDMA the most economical solution for a carrier wanting to deploy high-speed wireless Internet access integrated with their voice networks.
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 also offers wireless operators the first step towards convergence of wireless technologies. Since 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.
Frequency Division Duplexing (FDD)
FDD is a component of all current 1G, 2G and 3G wireless technologies. FDD is the air interface utilized by most current wireless systems. FDD utilizes two channels for transmitting and receiving data. LAS-CDMA technology can enhance FDD systems to further reduce the various forms of interference. For example, in 1x EV/DO systems, the high-speed data rate can only be carried out by a dedicated radio carrier. With LAS-CDMA technology, the high-speed data rate traffic channel can be mixed with the low rate voice channel on the same radio carrier, resulting in improved spectral efficiency for carriers. Also, LAS-CDMA technology can be used for any other FDD systems for reducing or eliminating the interference.
Breakthrough 4G technology
The emergence of a totally different telecommunication landscape, with wireless and Internet at its core has already triggered the movement toward next generation wireless technology, a.k.a 4G. The 4G system will not be classified based on timetables but rather on technology revolution. It is expected that a number of high-level requirements will be central to the defining of 4G:
Higher spectral efficiency and transmission speeds
Speeds of up to 15Mbps could be considered in a fixed environment. In a mobile vehicular environment those speeds could be required to exceed 1 to 2Mbps. Recent events such as the UK UMTS spectrum auctions have shown that spectrum is scarce resource, 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.
Convergence towards a truly global standard:
Today's 3G technologies have failed to demonstrate the way to converge all 3G technologies into a single global cohesive mode. 4G technology shall enable the wireless industry to move into a common IP centric air interface.
Movement towards an IP-centric world:
As the global telecommunication industry moves towards a packet-oriented paradigm to replace the traditional circuit-oriented networks and services inherited from a world where voice represented the quasi-totality of the traffic and revenue, the wireless industry will surely follow this general trend. This will have impact across 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.
The Advantages of TDD Systems
Beyond these 4G "must-have's," LinkAir goes a step further to position carriers in front of the competition by incorporating and fully utilizing TDD technology in the LAS-CDMA system. LAS-CDMA is a breakthrough technology for TDD in a large area cellular system. Using the LS coding technology, a signal transmitted from a neighboring base station will not interfere with the reception of a weak mobile signal at the home base station even while both are transmitting and receiving at the same frequency. Until now, no other technologies can perform this way. Because of this, the advantages of using TDD with LAS-CDMA technology are as follows:
* Any single spectrum band of 1.25 MHz or 1.6 MHz can be considered for the TDD application.
* The system structure of TDD is relatively simple.
* The simple configuration of the handsets will drive down the production costs.
* Since no duplexer is needed in TDD, the size of the handset can be substantially reduced.
* Power consumption is lower.
* TDD can efficiently handle asymmetrical traffic with a single spectrum band.
* TDD has greater spectrum efficiency than FDD.
TDD is the preferred technology for IP based mobile communications due to its simplicity in radio access and its high value in spectrum efficiency. LAS-CDMA supports transition from the circuit-oriented services or applications to a general packet oriented since it can support on the same RF carrier. Until now, the use of TDD has been severely limited due to imperfect power control, which results in a severe degradation of the system's performance. However in a TD-LAS system, all signals are kept within an interference free window. As a result, TD-LAS achieves unprecedented efficiencies, including:
* No complex power control needed: The need for power control is greatly reduced because the smart code eliminates MAI.
* No joint detection scheme is needed: As the smart code can eliminate the MAI interference the need for joint detection
schemes is eliminated.
* No new antennas (e.g. smart antennas ) are needed to enhance spectrum efficiency with LAS-CDMA: The smart coding
eliminates the need for smart antennas, which are primarily used to detect and eliminate interference from a specific direction.
* No soft handoff is needed because TDD utilizes hard handoffs. Hard handoffs require fewer system resources than soft handoffs and free up spectrum within the systems.
* The TD-LAS system is much less complicated than the existing FDD systems.
* TD-LAS can change the transmission environment from interference limited to noise limited.
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.
Summary
In conclusion, 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 in FDD systems simply offers solutions for developing high speed, high capacity, IP friendly networks. 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 in TDD systems 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. |
