index.md (2671B)
1 +++ 2 title = "Physical: Modulation & multiplexing" 3 +++ 4 5 # Physical: Modulation & multiplexing 6 modulation schemes — send bits as signals 7 multiplexing — share channel among users 8 9 Baseband transmission (directly convert bits to signal): 10 11 - send symbols representing bits 12 - symbol rate — rate at which signal changes (also “baud rate”) 13 - bit rate — symbol rate times bits per symbol 14 - NRZ (non-return-to-zero): +1V=“1”, -1V=“0” 15 - others: 16 17  18 19 - to decode symbols, signals need enough transitions 20 - problem is, after enough zeros you can’t tell when the next symbol starts 21 - Manchester encoding — add in clock signal by XORing with data signal 22 - high-to-low transition is 1 23 - low-to-high is a 0 (just the clock) 24 - NRZI — transition means 1, no transition means 0 25 - long runs of 1 is no problem, 0 still is 26 - 4B/5B — every 4 bits mapped to 5-bit pattern using fixed table 27 - scrambling — XOR with pseudorandom data, XOR again to decrypt 28 29 Passband transmission (regulate amplitude/frequency/phase of carrier signal to mean bits) 30 31 - Amplitude Shift Keying — two different amplitudes mean 0 or 1 32 - Frequency SK — two different tones mean 0 and 1 33 - Phase SK — wave is shifted 0 or 180 degrees to mean 0 or 1 34 - Quadrature Phase SK — shifts of 45/135/225/315 degrees to transmit two bits at once (four different levels) 35 - combine to represent more bits: 36 - Quadrature Amplitude Modulation (QAM-16): phase (angle with X axis on constellation diagram) and amplitude (distance from origin on constellation diagram) is modulated, can transmit 4 bits 37 - QAM-64 — more combinations 38 - constellation diagrams: 39 40  41 42 Multiplexing 43 44 - FDM (Frequency Division Multiplexing) — place channels on specific frequency bands 45 - WiFi and Bluetooth change frequencies — "frequency hopping" 46 - TDM (Time Division Multiplexing) — shares channel over time, users take turns on a fixed schedule (round-robin style) 47 - CDMA (Code Division Multiple Access) — narrow band signal is spread out over wider frequency band 48 - each bit time divided into *m* short intervals (‘chips’), typically 64 or 128 49 - each station is assigned *m*-bit code (‘chip sequence’) 50 - to transmit 1 bit, a station sends its chip sequence 51 - to transmit 0 bit, a station transmits negation of chip sequence. 52 - if more stations transmit at the same time, bits add linearly 53 - to recover, compute normalised inner product (station chip sequences must be known in advance) 54 55 