Mobile Communications Mobile Communications Multiple Access SDMA TDMA FDMA CDMA Mobile Communication Multiple Access 1 Multiple Access o All users of a cell in cellular networks use the same frequency spectrum spectrum multiplexing schemes: o SDMA (Space Division Multiple Access) o FDMA (Frequency Division Multiple Access) o TDMA (Time Division Multiple Access) TDMA (Time Division Multiple Access) o CDMA (Code Division Multiple Access) Mobile Communication Multiple Access 2 SDMA (Space Division Multiple Access) o Is used in all cellular networks o Re-use of frequency bands in non-neighbouring cells o Re use of frequency bands in non neighbouring cells o A cell is assigned frequency bands in such a way that there is no interference with neighbouring cells o Example: 4-cell repetition scheme o Example: 4-cell repetition scheme Mobile Communication Multiple Access 3
SDMA (Space Division Multiple Access) o 7-cell repetition scheme repetition schemes exist for K = 3, 4, 7 and multiples p The bigger K, the: • smaller the number of channels per cell smaller the number of channels per cell • lesser the interference Mobile Communication Multiple Access 4 SDMA (Space Division Multiple Access) o Additionally the cells can be subdivided into sectors by use of directed antennas use of directed antennas Mobile Communication Multiple Access 5 SDMA (Space Division Multiple Access) Cell planning larger cells for rural areas smaller cells for densly smaller cells for densly populated areas Mobile Communication Multiple Access 6
FDMA (Frequency Division Multiple Access) o Multiple access through frequency division o Example: GSM 900 provides 125 x 200Khz channels within 25 Mhz o Example: GSM 900 provides 125 x 200Khz channels within 25 Mhz o Assignment of individual channels by a control channel ... 891,2 Mhz channel 45 891 0 Mhz 891,0 Mhz channel 44 890,8 Mhz channel 43 890,6 Mhz ... Mobile Communication Multiple Access 7 TDMA (Time Division Multiple Access) o traditional technology in fixed networks o is often used together with FDMA o is often used together with FDMA o prerequisites: o voice coding o data compression o data compression o access on a frequency channel is only allowed during predefined time slots o example GSM TDMA frame: example GSM TDMA frame: mobile station A 6 7 0 1 2 3 4 5 6 7 0 1 TDMA frame 4,615 ms Mobile Communication Multiple Access 8 CDMA (Code Division Multiple Access) o spread spectrum (because the frequency spectrum is spread) o all mobile stations use the same channel, they are distinguished o all mobile stations use the same channel, they are distinguished from each other by codes o advantages: o no coordination and synchronization necessary o no coordination and synchronization necessary o uniform usage of the whole spectrum by each of the users o different CDMA methods: o Direct Sequence (DS) Direct Sequence (DS) o Frequency Hopping (FH) o Time Hopping (TH) Mobile Communication Multiple Access 9
DS-CDMA 1 bit stream (19,2 Kbit/s) 0 0 0 1 1 chip stream (1 23 Mc/s) chip stream (1,23 Mc/s) 1 0 0 1 1 code generator (1,23 Mc/s) code generator (1 23 Mc/s) 1 0 1 1 0 0 1 1 1 0 0 1 1 1 0 0 1 1 0 1 1 0 0 1 0 0 1 1 0 1 1 0 0 Mobile Communication Multiple Access 10 DS-CDMA Power Levels from MS Received Power Levels at BTS C A C C C D C D o exact adjustment of transmit power levels are constantly necessary Mobile Communication Multiple Access 11 DS-CDMA Example A Data 0 1 1 A Key 1 0 0 0 0 1 0 1 0 0 1 0 0 0 1 0 1 1 A Data XOR A Key 0 1 1 0 0 1 0 1 0 1 1 0 0 0 1 0 1 1 A Signal A Signal 0 0 B Data 1 0 1 0 0 0 0 0 1 1 0 1 0 0 0 0 B Key 1 1 1 0 1 0 1 0 0 0 0 0 0 0 1 0 1 1 0 1 0 0 0 0 0 0 0 B Data XOR B Key B Data XOR B Key 1 1 1 1 1 1 1 1 1 1 1 1 B Signal Composite A+B Signal Mobile Communication Multiple Access 12
DS-CDMA Example A Key ey 1 0 0 1 0 0 0 0 0 1 0 1 0 0 1 0 1 0 1 0 0 1 0 0 0 1 0 0 0 0 1 0 1 1 1 1 Composite A+B Composite A+B Signal (A+B)*A Key Integrator A Data A Data 1 1 1 1 0 0 * - Operator: (A+B)*1 = (A+B) , (A+B)*0 = (A+B) Mobile Communication Multiple Access 13 DS-CDMA Example 0 1 0 1 0 0 0 0 0 0 0 1 1 0 1 0 0 0 0 1 1 0 1 0 0 0 0 0 0 0 B Key B Key 1 1 1 1 1 1 Composite A+B Composite A+B Signal (A+B)*B Key Integrator B Data 0 1 0 Mobile Communication Multiple Access 14 DS-CDMA Example Composite A+B p Signal False Key 0 0 0 0 0 1 0 0 0 0 0 1 1 1 1 1 1 1 (A+B)* False Key ( ) y Integrator Mobile Communication Multiple Access 15
FH-CDMA o frequency Hopping (FH-CDMA), e.g. Bluetooth o carrier frequency of the transmitted signal is not constant biut o carrier frequency of the transmitted signal is not constant biut varies over time o in one time slot the carrier remains constant. In the following one it “hops” to a different frequency it hops to a different frequency. Frequency Frequency Hopping CDMA . . . 2403-2404 2402-2403 time T Mobile Communication Multiple Access 16 FH-CDMA o there are several different hopping algorithms o cyclic hopping o cyclic hopping o pseudo random hopping o there is sometimes a distinction between slow frequency hopping and fast frequency hopping and fast frequency hopping o with S-FH (slow FH) for each data packet the frequency remains constant o with F-FH (fast FH) for each data packet the frequency with F FH (fast FH) for each data packet the frequency changes irrespectively of bits and packets, one bit can be distributed among several hops o the larger the number of frequency bands, the less probable h l h b f f b d h l b bl collisions become o with F-FH a collision within one frequency band may not significant, it may be corrected by the other bands o with S-FH there is normally an error correction on a higher level Mobile Communication Multiple Access 17 TH-CDMA o with time hopping CDMA the data are sent in short bursts with pseudo random intervals between them pseudo random intervals between them o time line is separated into equal frames which are themselves divided into time slots. A sender occupies one slot randomly Mobile Communication Multiple Access 18
Multiple Access summary Mobile Communication Multiple Access 19
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