Spread Spectrum Access Methods for Wireless Communications

LA UKkNCE E. MILSTEI>V is aprofessor with the Department ofElecmncal and Computer Engineering at the L’niwsily of California at San D q o . ver the past several years, code division multiple access (CDMA) has been shown to be a viable alternative to both frequency division multiple access (FDMA) and time division multiple acccss (TDMA), and the use of spread spectrum techniques (upon which CDMA is based) in wireless communications applications has become a very active area of research and devclopment [ 1,2]. While there does not appear to be a single multiple accessing techniquethatissuperiortoothersinallsituations,there are characteristics of spread spectrum waveforms that give CDMA certain distinct advantages. The two basic problems which the cellular mobile radio system designer is faced with are multipath fading of the radio link and interference from other users in the cellular reuse environment. Spread spectrum signals are effective in mitigating multipath because their wide bandwidth introduces frequency diversity. They are also useful in mitigating interference, again because of theirwide bandwidth. The result of these effects is a higher capacitypotential compared to that of non-spread accessmethods. Consider the use of direct sequence (DS) spread spectrum. As is well-known, DS waveforms can be used to either reject multipath returns that fall outsidc of the correlation interval of the spreading waveform, or enhance ovcrall performance by diversity combining multipath returns in a RAKE receiver [3]. The above will hold any time the spread bandwidth exceeds the coherence bandwidth of the channel, that is, when the channel appears frequency-selective to the spread spectrumsignal. Alternately, in frequency hopped (FH) spread spectrum, frequency diversity is obtained through coding the data and interleaving it over multiple-hops. Another considcration in using CDMA in cellular systems is the so-called reuse factor. For non-spread multiple accessing techniques (i.e., FDMA and TDMA), frequencies used in a given cell arc typically not used in immediately adjacent cells. This is done so that a sufficient spatial isolation will exist to ensure cells using the same frequency will not cause excessivc interference (ie., co-channel interference) with one another. For example, in the analog AMPS system, a frequency reuse of one-in-seven is employed. However, with spread spectrum signaling, the possibility of a frequency reuse of one-in-one exists. Further, in a CDMA system, performance is typically limited by average (rather than worst-case) interference. For these reasons, in amulticellsystem,CDMAis anticipated to havealargercapacitythaneitherFDMAorTDMA. Note, however, for an isolated cell, because of the nonorthogonalityofthe CDMAwavefom, thecapacity of the cell is less than what would be the case with an orthogonal technique such as TDMAor FDMA. CDMA also provides a natural way to exploit the bursty nature of a source for added capacity. In the case of a two-way telephone conversation, the voice activity of each participant is about 50 percent of the time. If transmission is discontinued during nonactivityperiods,inprincipleonecandouble the numher of simultaneous conversations in the system. The above attributes promise the potential of higher system capacity through spread spectrum techniques. Also, the one cell reuse pattern alleviates the problem of frequency planning required with the narrow band systems (although other planning issues may become necessary, like careful power planning and pilot iming for DS systems, orcarefulchoiceofhoppingpattemsfor~systems). As a result, CDMA has become a serious competitor in the cellular arena. The ability of spread spectrum signals to combat interference has also been applied in a different arena, that of the so-called industrial, scientific, and medical (ISM) bands. The ISM bands are frequency bands which, by Part 18 of the U.S. FCC regulations, were originally designated for operation of equipment which “generate and use locally R F energy for industrial, scientific, and medical” applications, “excluding applications in the field of telecommunications.” In view of the local nature of these RF radiations, it was later suggested touse thesebandsfortelecommnnications, also of alocalnature, such ason-sitecommunications. Spread spectrum techniques, both DSand FHbased, were established, and a byproduct of the use of spread spectrum is the ability to allow unlicensed operation, per Part 15 of the FCC regulations.