Fast-hopped OFDM technology offers a new opportunity for mobile broadband
Source: Computer Technology Review
Publication date: 2001-05-01
Arrival time: 2001-06-24
Figure 1
Current cellular wireless technologies are prone to errors, which makes them unreliable for data transmission.
Cellular wireless systems operate under challenging conditions. The wireless channel is unpredictable because of factors such as multipath fading, shadowing, Doppler spread, and time dispersion or delay spread. These factors are all related to variability introduced by the mobility of the user, who may be close or distant from a cell. Also, the spectrum is a scarce resource for wireless systems, and thus is reused within cellular systems. This means that the same frequencies are allocated to each cell or to a cluster of cells, and are shared. As a result, there is potential for interference within a cell and between cells as each channel is used by a number of users.
These challenges have not impeded the widespread adoption of cellular services because voice is not as sensitive to the vagaries of the RF (radio frequency) environment. Data, however, generally consists of high-bandwidth bursts that need to be transported reliably-more consistent with the quality afforded by the wired connections around which the Internet was designed. Wireless systems originally architected for circuit-voice, then adapted for data, e.g., 3G, cannot cost effectively provide the whole Internet experience to mobile users. Because the fundamental structure of the Internet is built for data, architecture designed in the same spirit is needed for wireless data.
OFDM
Orthogonal Frequency Division Modulation (OFDM) is a new technology that promises to overcome signaling transmission barriers and step up transmission speeds. It is very robust to the unpredictable changes of the RF environment, such as multipath delay- - spread. It is especially well suited for mobility applications in cellular networks because different users within a cell do not interfere with each other.
OFDM, also known as multitone modulation, is a modulation and multiple access technique that divides a digital signal across 1,000 or more signal carriers simultaneously. OFDM divides the spectrum into a number of equally spaced tones, and carries a portion of a user's information on a tone. The OFDM tones are orthogonal, which means that the individual tones do not interfere with one another, where the peak of each tone corresponds to a zero level, or null, in every other tone.
Figure 1 illustrates the basic principle of flash-OFDM. For simplicity, consider two adjacent cells and in each cell there are two mobiles. The x-axis indicates the indices of symbol periods in time and the y-axis indicates the indices of tones that are used in individual symbol periods in frequency. In this figure, mobile #1 in cell 1 uses tone 1 in symbol 1, tone 3 in symbol 2, and so forth, while mobile #2 in cell 1 uses tone 4 in symbol 1, tone 6 in symbol 2, and so forth. Similarly one can see how tones are used by mobiles A and B in cell 2.
OFDM offers better spectral efficiency and interference immunity. First consider the interference within a cell. Clearly, in any given symbol period, different mobiles in a cell use different tones. From the orthogonality discussed previously, there is no interference between mobiles within a cell. Recall that CDMA is not orthogonal. This property gives flash-OFDM a capacity of almost three times that of CDMA, and provides additional advantages of supporting bursty data traffic. Now consider the interference between cells. After carefully examining the tone hopping sequences, we can see that mobile #1 interferes with mobiles A and B once every 7 symbols in this figure, thus inter-cell interference being averaged. Hence, the spectrum reuse in flash-OFDM is not limited by the worst-case but by the average inter-cell interference. This property enables universal spectrum reuse which represents a significant advantage over TDMA systems.
OFDM accommodates for wireless communications by adding a small amount of overhead, called a cyclic prefix, to each symbol. By choosing appropriate parameters for the cyclic prefix each tone can be made orthogonal, even in the presence of multipath signals. Because of its orthogonal nature, OFDM can avoid interference.
flash-OFDM
flash-OFDM refers to fast-- hopped OFDM, and is a wideband spread- spectrum technology. The flash-OFDM system is a mobile wireless data system that was designed and optimized from the ground up to enable cost effective broadband mobile data communications. This design approach avoids the compromises inherent in other mobile data systems because it is based on a packet-switched data centric architecture instead of a circuit-switched voice centric architecture.
The flash-OFDM format is tolerant of both multipath and high- speed Doppler. There is no interference between users in the same cell and interference from users from adjacent cells is averaged effectively by the use of hopping patterns. The capability to work around interfering signals is a distinct advantage over CDMA technology.
The flash-OFDM system contains a single broadband receiver, greatly simplifying implementation (and reducing cost) as compared to TDMA and CDMA systems. Because the system's multitone modulation mitigates multipath without the use of an equalizer, a dramatic reduction in system complexity, power consumption, and cost is realized, especially in the wireless user terminals.
The flash-OFDM system tames latencies-which determine its effectiveness in an interactive environment-to 20 milliseconds, while cranking up data rates scalable from 384Kbps to 1Mbps downstream, and up to 1 Mbps upstream. (The upstream limitation has more to do with the battery power required to deliver more bits per symbol as the data rates go up.) flash-- OFDM can be likened to putting time- division multiple access on top of OFDM, yielding all the benefits of both technologies in terms of robustness under mobility and channel- delay spreads, interference averaging from other cells, and orthogonality inside the cell.
flash-OFDM Integrated System Design Approach
From a PHY layer standpoint, OFDM delivers a larger pipe than CDMA- based systems. OFDM has to be coupled with a MAC (media access control) layer that tells the system how to manage the data. Many fixed access or short-range LAN systems have chosen OFDM as their physical layer, but typically combine it with pre-existing non- wireless MAC protocols (such as DOCSIS and CSMA). This hybrid approach is not suitable for mobile data applications, where spectrum is scarce and high-speed mobility is required. The flash-OFDM system was designed to be directly compatible with the Internet for interactive data applications.
Figure 2
flash-OFDM's physical and media-access control layers were jointly designed for data to address many of the problems with the wireless channel, in terms of reliability, low latency and high bandwidth efficiency, and to create an extremely efficient and high performance system. Another major focus of the design is low cost: to enable service providers to realize an orderof-magnitude reduction in the cost to deliver a megabyte of data to the end user, relative to other wireless data systems (such as the planned 3G wireless systems). The primary reason that low cost can be achieved is attributable to an architecture based on data, which takes advantage of the statistical multiplexing advantages of packet switching and enables a flat decentralized network design, leveraging the benefits of open IP standards. The unique physical layer combined with the throughput advantages realized by statistical multiplexing via a packetbased air interface enables the flash-OFDM system to dramatically reduce the cost per megabyte of data delivered.
The design of the technology incorporates low overhead contention free access, which yields high throughput while delivering low latency and highly reliable communications with support for QoS. Important features of the flash-- OFDM system that help reduce overall system complexity and cost include autonomous basestations, which are not required to be synchronized to one another, and a network architecture that enables each basestation to be connected directly to IP networks (Fig 2).
The MAC itself is key to the system's potential success over the very difficult air interface. When it comes to error correction, end- to-end retransmission is very expensive. The most unreliable part of the link is the air link, so flash-OFDM has link protocols built into the MAC that offer a very reliable link layer to the IP traffic, along with proprietary error-- control mechanisms that are not end to end.
The link layer thus has feedback built in that does not cause end- to-end retransmission-only across the wireless link. This is another benefit of the PHY/MAC linkage, which in this case lowers the retransmissions and, therefore, latencies.
flash-OFDM Physical Layer
OFDM is used for multiple access. That is, mobiles are assigned different channel resources (frequency tones) to share the spectrum. Moreover, tone assignments hop from time to time rapidly to achieve frequency diversity and interference averaging. Frequency diversity helps mitigate the effects of the frequency-selective multipath fading that is characteristic of wireless channels. The interference is averaged as a result of the geographic distribution of users and the fact that they are power-controlled. In thi\s way, the amount of interference seen by a particular tone will change from hop to hop, resulting in an averaging process. These two properties enable spectrum reuse factor to be one. In flash-OFDM's PHY layer, a signal hops from tone to tone at the rate of roughly 10,000 times per second. Every user ends up signaling across the band on all tones, effectively turning OFDM into a spread-spectrum technology. The flash- OFDM system has been designed to minimize the cyclic prefix overhead while maintaining the multipath immunity of the system for the vast majority of measured multipath scenarios.
The property of orthogonality is a key design consideration that provides the flash-OFDM system capacity and data capability advantages. In addition, channel equalization to combat intersymbol interference is essentially eliminated (it becomes a trivial one-tap equalizer). The cyclic prefix ensures that the tones within a cell are orthogonal; therefore, the flash-OFDM system has no interference between users on the same cell. This results in the spectral efficiency of flash-- OFDM being approximately three times that of CDMA and 3G systems, which suffer a loss of orthogonality between codes in the presence of multipath.
Further, 3G systems are not orthogonal systems. This means that a significant portion of the interference in a cell actually is generated within the cell itself, by the users. This effectively limits the capacity of the cell. A flash-OFDM system does not suffer from this problem, resulting in a capacity advantage of almost three times that of equivalent deployment scenarios.
The use of the proprietary flash-OFDM technology gives the new cellular proposal a robustness not yet seen in this field. That's important in an application plagued by the vagaries of signals bouncing off many surfaces-a phenomenon known as multipath interference-or off moving objects. Where conventional single-- carrier transmission schemes send only one signal at a time using one radio frequency, OFDM sends multiple highspeed signals concurrently on different frequencies. This results in very efficient use of bandwidth and robust communications in the presence of noise and interference.
In a limited amount of radio spectrum, voice optimized cellular technologies (e.g., TDMA, CDMA) are not capable of cost-effectively achieving an acceptable level of reliability for data without trading off data rates, response time (latency), and cost. Existing and proposed third-generation cellular systems were conceived and designed before the explosion of the Internet; hence, they are encumbered with a legacy, voice-optimized architecture that will not scale up to cost effectively meet mass market demand for mobile broadband services.
Third generation (3G) mobile networks, although meant to carry voice and data traffic simultaneously, retain a circuit-switched, hierarchical architecture. Consequently, there is tension between the design objectives and the current environment of the wired Internet and mobile voice networks. The resulting design compromises of 3G networks, which are optimized for voice, impair their ability to deliver high-speed, low-latency data cost effectively. The resulting high cost-per-megabyte of data delivery over 3G networks will prevent the emergence of mass-market wireless Internet access. An alternative approach, focusing directly on high speed, low cost, and low latency wireless data delivery (such as flash-OFDM) is required.
Rajiv Laroia is the founder and CTO of Marion Technologies (Bedminster, NJ).
Copyright West World Publications, Inc. May 2001
Publication date: 2001-05-01
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