Multiple Access o All users of a cell in cellular networks use the same frequency spectrum spectrum multiplexing schemes: Mobile Communications Mobile Communications o SDMA (Space Division Multiple Access) 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) SDMA TDMA FDMA CDMA Mobile Communication Multiple Access 1 Mobile Communication Multiple Access 2 SDMA (Space Division Multiple Access) SDMA (Space Division Multiple Access) o Is used in all cellular networks o 7-cell repetition scheme 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 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 3 Mobile Communication Multiple Access 4
SDMA (Space Division Multiple Access) SDMA (Space Division Multiple Access) o Additionally the cells can be subdivided into sectors by Cell planning use of directed antennas use of directed antennas larger cells for rural areas smaller cells for densly smaller cells for densly populated areas Mobile Communication Multiple Access 5 Mobile Communication Multiple Access 6 FDMA (Frequency Division Multiple Access) TDMA (Time Division Multiple Access) o Multiple access through frequency division o traditional technology in fixed networks 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 is often used together with FDMA o is often used together with FDMA o Assignment of individual channels by a control channel 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: ... 891,2 Mhz mobile station A channel 45 891,0 Mhz 891 0 Mhz channel 44 890,8 Mhz channel 43 6 7 0 1 2 3 4 5 6 7 0 1 890,6 Mhz ... TDMA frame 4,615 ms Mobile Communication Multiple Access 7 Mobile Communication Multiple Access 8
CDMA (Code Division Multiple Access) DS-CDMA o spread spectrum (because the frequency spectrum is spread) 1 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 bit stream (19,2 Kbit/s) 0 1 1 0 0 o advantages: o no coordination and synchronization necessary o no coordination and synchronization necessary chip stream (1 23 Mc/s) chip stream (1,23 Mc/s) o uniform usage of the whole spectrum by each of the users 1 1 1 0 0 o different CDMA methods: code generator (1,23 Mc/s) code generator (1 23 Mc/s) o Direct Sequence (DS) Direct Sequence (DS) o Frequency Hopping (FH) o Time Hopping (TH) 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 9 Mobile Communication Multiple Access 10 DS-CDMA DS-CDMA Example A Data 0 1 1 Power Levels from MS A Key 1 0 0 0 0 1 0 1 0 0 1 0 0 0 1 0 1 1 Received Power Levels at BTS C A 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 C C 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 C D C D B Data XOR B Key B Data XOR B Key 0 1 0 1 0 0 0 0 0 0 0 1 0 1 1 0 1 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 B Signal o exact adjustment of transmit power levels are constantly Composite A+B necessary Signal Mobile Communication Multiple Access 11 Mobile Communication Multiple Access 12
DS-CDMA Example 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 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 1 1 1 1 B Key B Key 1 1 1 1 1 1 Composite A+B Composite A+B Composite A+B Composite A+B Signal Signal (A+B)*B Key (A+B)*A Key Integrator Integrator A Data A Data 1 1 1 1 0 0 B Data 0 1 0 * - Operator: (A+B)*1 = (A+B) , (A+B)*0 = (A+B) Mobile Communication Multiple Access 13 Mobile Communication Multiple Access 14 DS-CDMA Example 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 Composite A+B p Signal 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. False Key 0 0 0 0 0 1 0 0 0 0 0 Frequency 1 1 1 1 1 1 1 Frequency Hopping CDMA ( (A+B)* False Key ) y . . . Integrator 2403-2404 2402-2403 time T Mobile Communication Multiple Access 15 Mobile Communication Multiple Access 16
FH-CDMA TH-CDMA o there are several different hopping algorithms o with time hopping CDMA the data are sent in short bursts with o cyclic hopping o cyclic hopping pseudo random intervals between them pseudo random intervals between them o pseudo random hopping o time line is separated into equal frames which are themselves o there is sometimes a distinction between slow frequency hopping divided into time slots. A sender occupies one slot randomly 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 Mobile Communication Multiple Access 18 Multiple Access summary Mobile Communication Multiple Access 19
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