Unlike most existing forms of wireless access, including 3G technologies, conventional wireless systems have been designed primarily at the physical layer. To address the unique demands posed by mobile users of high-speed data applications, new air interfaces must be designed and optimized across all the layers of the protocol stack, including the networking layers. A prime example of this kind of optimization is found in flash-OFDM™ technology by Flarion. As its name suggests, the system is based on OFDM, however, flash-OFDM is much more than just a physical-layer solution. It is a system-level technology that exploits the unique physical properties of OFDM, enabling significant higher-layer advantages that contribute to very efficient packet data transmission in a cellular network.

Packet Switched Air Interface

The telephone network, designed basically for voice, is an example of circuit-switched systems. Circuit-switched systems exist only at the physical layer that uses the channel resource to create a bit pipe. They are conceptually simple as the bit pipe is a dedicated resource, and there is no control of the pipe required 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 burst data traffic.

Packet-switched systems, on the other hand, are very efficient for data traffic but require control layers in addition to the physical layer that creates the bit pipe. The MAC layer is required for the many data users to share the bit pipe. The link layer is needed to take the error-prone pipe and create a reliable link for the network layers to pass packet data flows over. The Internet is the best example of a packet-switched network.

Because all conventional cellular wireless systems, including 3G, were fundamentally designed for circuit -witched voice, they were designed and optimized primarily at the physical layer. The choice of CDMA¹ as the physical-layer multiple-access technology was also dictated by voice requirements. Flash-OFDM, on the other hand, is a packet-switched designed for data and 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 consideration, but also on MAC–, link-, and network-layer requirements.

Physical-Layer Advantages

As discussed earlier, most of the physical-layer advantages of OFDM are well understood. Most notably, OFDM creates a robust multiple-access technology to deal with the impairments of the wireless channel, such as multipath fading, delay spread, and Doppler shifts. Advanced OFDM–based data systems typically divide the available spectrum into a number of equally spaced tones. For each OFDM symbol duration, information carrying symbols (based on modulation such as QPSK, QAM, etc.) are loaded on each tone.

Flash-OFDM uses fast hopping across all tones in a pseudorandom predetermined pattern, making it a spread spectrum technology. With fast hopping, a user that is assigned one tone does not transmit every symbol on the same tone, but uses a hopping pattern to jump to a different tone for every symbol. Different base stations use different hopping patterns, and each uses the entire available spectrum (frequency reuse of 1). In a cellular deployment, this leads to all the advantages of CDMA systems, including frequency diversity² and out of cell (intercell) interference averaging—a spectral-efficiency benefit that narrowband systems such as conventional TDMA do not have.

As discussed earlier, different users within 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, increasing the total interference in the system. Flash-OFDM therefore has the physical-layer benefits of both CDMA and TDMA and is at least three times more efficient than CDMA. In other words, at the physical layer, flash-OFDM creates the fattest pipe of all cellular technologies. Even though the 3x advantage at the physical layer is a huge advantage, the most significant advantage of flash-OFDM for data is at the MAC and link layers.

MAC and Link-Layer Advantages

Flash-OFDM exploits the granular nature of resources in OFDM to come up with extremely efficient control layers. In OFDM, when designed appropriately, it is possible to send a very small amount (as little as one bit) of information from the transmitter to the receiver with virtually no overhead. Therefore, a transmitter that is previously not transmitting can start transmitting, transmit as little as one bit of information, and then stop, without causing any resource overhead. This is unlike CDMA or TDMA, in which the granularity is much coarser and to merely initiate a transmission wastes a significant resource. Hence, in TDMA, for example, there is 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, as data traffic typically consists of a large number of bits. However, for transmission of control-layer information, the frame structure is extremely inefficient, as the control information typically consists of one or two bits but requires a whole frame. Not having a granular technology can therefore be very detrimental from a MAC– and link-layer point of view.

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 the hooks to handle QoS. It 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. The network layer's traffic therefore experiences small delays and no significant delay jitter. Hence, interactive applications such as (packet) voice can be supported. Moreover, Internet protocols such as TCP/IP run smoothly and efficiently over a flash-OFDM airlink. TCP/IP performance on 3G networks is very inefficient because the link layer introduces significant delay jitter so that channel errors are misinterpreted by TCP as network congestion and TCP responds by backing off to the lowest rate.

Packet switching leads to efficient statistical multiplexing of data users and helps the wireless operators to support a much higher number of users for a given user experience. This, together with QoS support and a 3x fatter pipe, allows the operators to profitably scale their wireless networks to meet the burgeoning data-traffic demand in an all-you-can-eat pricing environment.

¹ 3G system in Europe (WCDMA) and the United States (CDMA 2000) are based on CDMA technology.
² Frequency diversity provides immunity in a fading environment, where a users' signal spans a wide spectrum and usually does not fade at the same time.