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31-01-2009, 12:54 AM

Multiple access (MA) schemes are used to simultaneously share available bandwidth in communication systems. In conventional MA applications a finite and contiguous amount of spectrum which is designated for a particular service is allocated to multiple users. Therefore, the use of designated radio spectrum is unconstrained and can be utilized in many ways to achieve maximum efficiency. Time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA) are the three major MA techniques. In recent years growing demand on available radio spectrum, as the fundamental shared resource of wireless communication systems, has forced marry applications to use spectrum which is partially in use by other services or by legacy applications. Ultra wide band (UWB) communication systems , which share their spectrum with marry services, an example of such applications where minimizing the impact of interference or co-existence with weaker narrow band systems is one of the main challenges. Another example is the modernized GPS system whose signal must share the LI and L2 bands with existing commercial C/A Code in the center of the bands . Additionally, a recent guideline from the FCC Spectrum Policy Task Force , which recommends maximum flexibility in spectrum use by both licensed and unlicensed users, has motivated extensive research activities in the area of dynamic resource allocation.

One of the primary technical challenges in flexible radio spectrum allocation is shaping the signal's spectrum to limit mutual interference with existing licensed users. The simplest approach to interference mitigation is to filter the interfering signal to suppress spectral content in certain bands. This will result in removing useful information along with the interference. A more efficient approach is to shape the spectrum at the transmitter. Spectral shaping reduces interference to the existing signals while preserving information signal and potentially enables us to have higher gains in other bands.

The above applications require design of MA schemes for noncontiguous radio spectrum. In these spectrally constrained MA applications, conventional TDMA and CDMA are generally not considered since all the users have to use the whole bandwidth for transmission. The most intuitive and most frequently used approach is based on FDMA or its more efficient version orthogonal frequency division multiple access (OFDMA) where the spectrum shaping is achieved by turning off a set of carriers at the frequencies occupied by existing licensed users or at which strong narrow-band interference is present. Aside from well studied advantages and disadvantages of orthogonal frequency division multiplexing (OFDM), there are several drawbacks in applying OFDM to spectrum shaping. The main issue is spectral leakage that occurs when the existing signals or narrowband interferences are not centered at one of the OFDM carriers. Depending on the power of the existing signal, spectral leakage could potentially impact a considerable number of OFDM carriers, forcing us to turn off a large number of carriers.

In this seminar and presentation, we discuss a novel orthogonal CDMA-based spectral shaping technique with potentially higher spectral efficiency. In our approach, which we refer to as Code-Hopping CDMA (CH-CDMA), users are allowed to hop between available spreading codes in an orthogonal fashion.
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