range of frequencies suffers from interference, the carriers within that range can be disabled or made to run more slowly. Another feature of OFDM is that the speeds of the upstream and down- stream channels can be varied by allocating either more or fewer carriers for each purpose. Some forms of Rate-Adaptive DSL (RADSL) use this feature in real-time SO that bandwidth is allocated to whichever stream needs it most. Orthogonal Frequency Division Multiple Access (OFDMA) is a proprietary enhancement to OFDM technology, introduced by Runcom Technologies (www. runcom.com) in 2000. Whereas OFDM involves 256 subcarriers, OFDMA defines either 2,048 or 4,096 subcarriers. In current OFDM systems, only a single user can transmit on all the subcarriers at any given time, and TDMA or FDMA is used to support multiple users. OFDMA, on the other hand, allows multiple users to trans- mit simultaneously on the different subcarriers per OFDM symbol. The end result is improved spectral efficiency. Because OFDMA enables more efficient duplexing techniques, including FDD and TDD, the created signal is immune to interference and is therefore capable of high data throughput. In addition, OFDMA improves the power gain per channel, providing better coverage and range availability, larger cell radius, and smaller power amplifiers and antennas, ultimately leading to lower investment in capital equipment. OFDMA, also referred to as Multiuser-OFDM, is being considered as a modula- tion and multiple-access method for upcoming fourth-generation (4G) wireless networks, and it is being embraced by a number of current technologies, including DVB and WiMax. It is also a preferred technology driving the BWA market. Improving Error Detection and Correction Remember that in radio transmission systems, it is important to balance expanding capacity (i.e., increasing bandwidth efficiency) with maintaining quality (i.e., reducing the effects of noise and interference). Now that we've covered the various spectrum reuse techniques designed to achieve great