Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

3Hybrid Multiple Access Schemes3.1 IntroductionThe simultaneous transmission of multiple data streams over the same medium can beachieved with different multiplexing schemes. Most communications systems, such asGSM, LTE, DECT, and IEEE 802.11a, use multiplexing based on either time division,frequency division, or a combination of both. Space division multiplexing is applied toincrease the user capacity of the system further. The simplest scheme of space divisionmultiplexing is antenna sectorization at the base station, where often antennas with120 ◦ /90 ◦ beams are used. Multiplexing schemes using code division have gained significantinterest and have become part of wireless standards such as WCDMA/UMTS,HSPA, IS-95, CDMA-2000, and WLAN.Time Division MultiplexingThe separation of different data streams with time division multiplexing is carried out byassigning each stream exclusively a certain period of time, i.e. time slot, for transmission.After each time slot, the next data stream transmits in the subsequent time slot. Thenumber of slots assigned to each user can be supervised by the medium access controller(MAC). A MAC frame determines a group of time slots in which all data streams transmitonce. The duration of the different time slots can vary according to the requirements ofthe different data streams.If the different data streams belong to different users, the access scheme is called timedivision multiple access (TDMA). Time division multiplexing can be used with both timedivision duplex (TDD) and frequency division duplex (FDD). However, it is often used incommunication systems with TDD transmission, where up- and downlinks are separatedby the assignment of different time slots. It is adopted in several wireless LAN and WLLsystems including IEEE 802.11a as well as IEEE 802.16x and HIPERMAN.Frequency Division MultiplexingWith frequency division multiplexing, the different data streams are separated by exclusivelyassigning each stream a certain fraction of the frequency band for transmission.Multi-Carrier and Spread Spectrum Systems Second Edition K. Fazel and S. Kaiser© 2008 John Wiley & Sons, Ltd