Next-Generation Mobile Wireless Internet Technology
Bedminster, New Jersey
Next-generation wireless data systems are expected to offer broadband Internet connectivity. The North American and European standards for these are based on code division multiple access (CDMA) technology, which was originally developed for circuit-switched voice systems and is not as suitable for packet-data transmission. In this paper, I describe flash-OFDM, a new air-interface technology designed specifically for packet-data transmission. Flash-OFDM is a mobile, wide-area-network cellular technology that works well with all existing Internet protocols and delivers a seamless extension to the Internet over the air. Flash-OFDM base stations are access routers that plug directly into the edge routers of an all-IP infrastructure. Mobility is managed using the mobile IP protocol.
Unlike third-generation and other conventional wireless technologies that are first designed primarily at the physical layer, flash-OFDM is a data-oriented technology designed and optimized across all layers of the protocol stack, including the networking layers. As its name suggests, the underlying multiple-access technology for flash-OFDM is orthogonal-frequency division multiple access (OFDMA or OFDM). However, flash-OFDM is much more than OFDM; it is an innovative, system-level technology that exploits the unique properties of OFDM to enable efficient packet-data transmission in a cellular network.
The potential for a mobile wireless data technology that offers cost effective, seamless connectivity to the Internet is virtually unlimited. One of the big drawbacks of conventional cellular wireless data technologies, such as third-generation (3G) technologies, is that they evolved from circuit-switched voice technology, which is inherently inefficient and in many ways unsuitable for
handling data traffic. The problem is compounded because the Internet evolved in the wired world, which offers much more reliable (error-free) connections than the inherently unreliable and error-prone wireless channel. As a consequence, the protocols of the Internet misinterpret channel errors for network congestion resulting in very poor link efficiency.
Flash-OFDM, an air-interface technology designed specifically for packet-data, addresses most of the problems of conventional wireless data systems. Flash-OFDM offers reliable packet-data connectivity with very small delays, thereby supporting interactive data applications, such as voice, video, games, and more. Flash-OFDM also enables complete control over quality of service (QOS) on both uplink and downlink. Thus, with flash-OFDM, operators can offer simplified pricing plans, such as flat-rate, “all-you-can-eat” billing, and can maximize revenue by offering tiered services at tiered prices. Enterprise users, for example, could pay more than consumers for higher quality service.
Because flash-OFDM is data-oriented technology, base stations are access routers designed to connect directly into the edge routers of a managed, all-IP network with all standard components and no special wireless-specific boxes. In contrast, 3G data systems require many expensive, wireless-specific, non-IP boxes in a circuit-oriented access network to manage mobility.
PACKET-SWITCHED AIR INTERFACE
The telephone network, which is designed basically for voice transmission, is an example of a circuit-switched system. Circuit-switched systems exist only at the physical layer and use the channel resource to create a bit pipe, a dedicated resource that requires no control once it is created (some control may be required in setting up or bringing down the pipe). Circuit-switched systems, however, are very inefficient for transmitting burst-data traffic.
Packet-switched systems are very efficient for transmitting data traffic but require control layers in addition to the physical layer that creates the bit pipe. A media-access control (MAC) layer is required for data users to share the bit pipe. A link layer is also necessary to create a reliable link from the error-prone pipe to the network layers so overflows of packet data can be transmitted. The Internet is a good example of a packet-switched network.
Because all conventional cellular wireless systems, including 3G systems, were fundamentally designed as circuit-switched, voice-transmission systems, they were designed and optimized primarily at the physical layer. The choice of code-division multiple access (CDMA)1 as multiple-access technology at the physical layer was also dictated by voice-transmission requirements. Flash-OFDM,
however, a packet-switch system designed for data transmission, is optimized across the physical, MAC, link, and network layers. The choice of OFDM as the multiple-access technology is based not just on physical layer requirements but also on MAC, link, and network layer requirements.
ADVANTAGES OF FLASH-OFDM
OFDM is a robust, multiple-access technology that can address problems with the wireless channel, such as multipath fading, delay spread, and Doppler shifts. OFDM, also called multitone modulation, divides the available spectrum into a number of equally spaced tones. For each OFDM symbol duration, information-carrying symbols (e.g., QPSK or QAM)2 are loaded on each tone. Flash-OFDM is a spread-spectrum technology that fast-hops across all tones in a pseudorandom, predetermined pattern. With fast-hopping, a user assigned one tone does not transmit every symbol on the same tone but uses a hopping pattern to jump to different tones for every symbol duration. Different base stations use different hopping patterns and can use the entire available spectrum (frequency reuse of 1). Thus, in a cellular deployment, flash-OFDM has all the advantages of CDMA systems, including frequency diversity3 and out-of-cell (intercell) interference averaging—a spectral efficiency benefit that narrow-band systems like conventional TDMA (time division multiple access) do not have.
In addition, because the various tones in OFDM are orthogonal, different users in the same cell use different resources (tones) and hence do not interfere with each other. This is similar to TDMA, where different users in a cell transmit at different time slots and do not interfere with one another. In contrast, CDMA users in a cell do interfere with each other, thereby increasing the total interference in the system. At the physical layer, therefore, flash-OFDM has the advantages of both CDMA and TDMA and is at least three times as efficient as CDMA. In other words, at the physical layer, flash-OFDM creates the fattest pipe of all cellular technologies.
MAC AND LINK LAYERS
In addition to the huge advantage of flash-OFDM at the physical layer, the most significant advantages of flash-OFDM for data transmission are at the MAC and link layers. Flash-OFDM exploits the granular nature of resources in OFDM
to create extremely efficient control layers. With appropriately designed OFDM, it is possible to send a very short amount (as little as one bit) of information from the transmitter to the receiver with virtually no overhead. In other words, a transmitter that has not been transmitting can begin transmitting, transmit as little as one bit of information, and stop without causing any resource overhead. With CDMA or TDMA, the granularity is much coarser and initiating a transmission wastes a significant resource. TDMA, for example, has a frame structure, and whenever a transmission is initiated a minimum of one frame (a few hundred bits) of information is transmitted. The frame structure does not cause any significant inefficiency in user data transmission because data traffic typically consists of a large number of bits. However, for transmitting control-layer information, the frame structure is extremely inefficient because control information typically consists of only one or two bits but requires a whole frame. A nongranular technology can, therefore, be a disadvantage at the MAC and link layers.
Flash-OFDM takes advantage of the granularity of OFDM in its control-layer design enabling the MAC layer to perform efficient packet switching over the air and, at the same time, providing all of the hooks to handle QOS. Flash-OFDM also supports a link layer that uses local (as opposed to end-to-end) feedback to create a very reliable link from an unreliable wireless channel with very low delays. Therefore, there are few delays at the network layers and no significant delay jitter. Hence, the system can support interactive applications, such as (packet) voice transmissions. Moreover, Internet protocols like TCP/IP (transport control protocol) run smoothly and efficiently over a flash-OFDM air link. By contrast, TCP/IP performance on 3G networks is very inefficient because the link layer introduces significant delay jitter that TCP interprets as network congestion and, therefore, responds by backing off to the lowest rate.
Packet switching leads to efficient statistical multiplexing of data users and enables wireless operators to support many more users for a given user experience. Combined with QOS support and a pipe three times as fat as with CDMA or TDMA, flash-OFDM enables operators to maximize revenue per Hertz of spectrum.
Flash-OFDM is an air-interface technology designed to meet the needs of packet-data transmission. It provides seamless mobile connectivity to the Internet and can handle all current applications, including interactive data applications and peer-to-peer applications. Flash-OFDM would also impose the fewest constraints on projected future applications.