Fast-Hopped OFDM Technology Offers A New Opportunity For Mobile Broadband - Technology Information

Computer Technology Review, May, 2001 by Rajiv Laroia

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 wide-band 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.