Chapter 2 Multiple access methods 2.1 Basic problem It is a usual situation in radio communications that a number of connections are active in the same frequency domain, in the same time domain, in the same geographical, that is spatial domain. An old example is audio broadcasting, see for instance the FM-broadcast system in Europe: f =87.5 MHz 108 MHz B =20.5 MHz, B u 400 khz, hundreds of FM radio stations. In order to control and minimize mutual disturbances, multiple access methods are utilized. Information theoretical background: The resources available for transmission are total available bandwidth per cell B/r termed cell bandwidth, total available transmission time T total.
2.2 Frequency Division Multiple Access 19 The below mentioned multiple access methods differ in how these resources are shared among the individual users. Available multiple access (MA) methods: Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Space Division Multiple Access (SDMA), Code Division Multiple Access (CDMA), Combinations of these methods, that is hybrid methods (GSM: FDMA+TDMA; IS-95: FDMA+CDMA). 2.2 Frequency Division Multiple Access Principle: The resource cell bandwidth B/r is subdivided into K partial frequency bands of the width B u = B/r K. (2.1) The K users of each cell are assigned different partial frequency bands. In these partial frequency bands all K users are active all the time, see Fig. 1.6, which means that the resource total available transmission time T total is not subdivided among the K users. The use of FDMA was presupposed in the considerations made in Section 1. User separation at receiver: In the spectral (frequency) domain by filtering. Advantages: User separation very robust (no synchronization required, user separation not impaired by adverse propagation properties of the radio channel), easy implementation.
2.2 Frequency Division Multiple Access 20 spektrale Leistungsdichte B=r B u K tes Teilnehmerfrequenzband 0 f 0 B=r 1 2......... K f 2 0 f 0 + B=r 2 Fig. 2.1. FDMA f Disadvantages: User bandwidth B u considerably smaller than the cell bandwidth B/r, which increases the impact of adverse channel properties (frequency selective fading), The only practicable MA method in analog transmission. Remarks: Oldest MA method. Most widely spread MA method. Applied in analog mobile radio systems as for instance C-Net, NMT, AMPS.
2.3 Time Division Multiple Access 21 2.3 Time Division Multiple Access Principle: The resource total available transmission time T total is periodically subdivided into K time slots of length T u each as shown in Fig. 2.2. Bandbreite TR B u Tu 1...... K 1...... K 1...... K 0 t Rahmen (Frame) Fig. 2.2. TDMA One period of time slots is termed a frame. Each of the K users of a cell is assigned an individual time slot within each frame. In its time slot the user disposes of the total cell bandwidth B/r without sharing this resource with other users, that is B u = B/r. (2.2) In the case of TDMA the user bandwidth B u is K times larger than in the case of FDMA. However, this bandwidth is only available in one K th of the total transmission time. Therefore, the effective user bandwidth is B u /K with B u of (2.2) User separation at receiver: In the time domain by time-gating. Advantages: User bandwidth B u is equal to the cell bandwidth B/r, which mitigates the impact of adverse channel properties (frequency selective fading). Data rate can be rather easily changed by assigning more or less time slots to a user. Mobile assisted handover is enabled. This means that in time slots in which a MS is not active the attenuation to BSs other than the MS s BS can be monitored and eventually a handover to another BS, to which the attenuation is lower than to the original BS, can be initiated by the MS.
2.4 Space Division Multiple Access 22 Disadvantages: User separation less robust than in the case of FDMA (synchronization required, user separation impaired by multipath channels). Implementation more complex than FDMA. Not practicable for analog transmission. Remarks: Modern MA method. Applied in the hybrid MA scheme of GSM and TD-CDMA. 2.4 Space Division Multiple Access Principle: The directional properties of the mobile radio channels are exploited to separate the users in the angular / spatial domain. This MA scheme requires the utilization of directional antennas at the TXs and/or the RXs. base station sector 2 sector 1 mobile station 2 mobile station 3 sector 3 mobile station 1 Fig. 2.3. SDMA
2.4 Space Division Multiple Access 23 The other mentioned MA schemes FDMA, TDMA and CDMA rely on properties of the user signals in the spectral and temporal domain. In contrast to that, SDMA is based on the directional properties of the mobile radio channels. User separation at receiver: In the spatial domain by using directional antennas. Advantages: User bandwidth B u equals the cell bandwidth B/r, which mitigates the impact of adverse channel properties, see also advantages of TDMA, Section 2.3. The user separation is done in the angular domain by utilizing the directional properties of the mobile radio channels. Practicable in analog and digital transmission systems. Disadvantages: Antennas necessary, which allow to resolve directions. Less robust than TDMA and FDMA. The implementation is rather expensive, e.g. branches, complex signal processing... several antennas, several RF The separation of the user signals is not always possible in the angular domain. Remarks: Modern MA scheme. The separation of the user signals is not always possible in the angular domain. Applicable in analog and digital communications. Fundamentals of mobile radio communications 21.05.03