Multiplexing Module W.tra.2 Dr.M.Y.Wu@CSE Shanghai Jiaotong University Shanghai, China Dr.W.Shu@ECE University of New Mexico Albuquerque, NM, USA 1
Multiplexing W.tra.2-2 Multiplexing shared medium at link layer Basic multiplexing Time, Frequency, Spacing Advanced multiplexing Spread spectrum & frequency hopping Code division Speech coding Voice to digital signals (application layer) End of module W.tra.2
W.tra.2-3 Access to shared channel Non-deterministic way: Random access Used for Packet Radio, not circuit-based Deterministic way: Multiplexing Basic multiplexing is used to control access to the shared channel based on the orthogonalization of signals Time, Frequency, Space Advanced multiplexing: spread spectrum multiple access Frequency hopping, direct sequence (CDMA)
W.tra.2-4 Duplexing & simplexing Duplexing can be done using frequency or time domain techniques FDD (Frequency division duplexing) Forward band & reverse band are two distinct bands of frequencies for every user Hardware capable of working at dual channels TDD (Time division duplexing) Forward & reverse links are at the same frequency band, but distinct time slots Hardware only deals with a single band Timing makes propagation delay sensitive Used in cordless phone with short range
W.tra.2-5 Multiple access classification Narrowband multiplexing Transmission bandwidth = carrier bandwidth Available radio spectrum is divided into large number of narrowband channels FDMA/FDD TDMA/FDD, or TDMA/TDD Wideband multiplexing Transmission bandwidth > > carrier bandwidth Multiple users are assigned into a wideband system CDMA
Multiplexing W.tra.2-6 Multiplexing shared medium at link layer Basic multiplexing Time, Frequency, Spacing Advanced multiplexing Spread spectrum & frequency hopping Code division Speech coding Voice to digital signals (application layer) End of module W.tra.2
Time division multiple access W.tra.2-7 TDMA shares the available bandwidth in the time domain: time sub-channels advantages Only one carrier in the medium at any time over whole bandwidth Throughput high even for many users disadvantages Precise synchronization; guard spaces = time gaps t c k 1 k 2 k 3 k 4 k 5 k 6 f
W.tra.2-8 TDMA TDMA High synchronization overhead (compared to FDMA) Synchronization between different senders Receivers have to adjusting the frequency and listen at exactly the right point in time Flexible to balance load (assign more slots) Not require FDD Is not continuous, but in burst, resulting in low battery consumption Simple handoff
W.tra.2-9 TDMA Example 1: Consider Global System for Mobile (GSM), which is a TDMA/FDD system that uses 25 MHz for the forward link, which is broken into radio channels of 200 KHz. If 8 speech channels are supported on a single radio channel, and if no guard band is used, find the number of simultaneous users that can be accommodated in GSM N = 25x10 6 /(200x10 3 /8) = 1000 Thus, GSM can accommodate about 1000 simultaneous users.
W.tra.2-10 Frequency division multiple access Separation of the whole spectrum into smaller frequency bands: frequency sub-channels A channel gets a certain band of the spectrum for the whole time advantages: no dynamic coordination necessary works also for analog signals disadvantages: guard spaces needed to eliminate ACI c k 1 k 2 k 3 k 4 k 5 k 6 f t
W.tra.2-11 FDMA FDMA Symbol time is large as compared to avg delay spread, so less ISI FDMA channels are relatively narrow 30KHz in AMPS FDMA accommodates analog FM as well Commonly used for radio stations Static channel assignments No coordination between sender & receiver Lack of flexibility to balance loads Used in cellular network: dynamic assignment
W.tra.2-12 FDMA FDMA in cellular network BS: base station MS: mobile station Frequency division duplexing (FDD) Downlink frequency: BS MS High-frequency transmission suffers attenuation, uses more power for compensating transmission losses Uplink frequency: MS BS Always lower than the downlink frequency due to limited power at an MS
W.tra.2-13 FDMA Example 1: A US AMPS cellular operator is allocated 12.5 MHz (B t is 12.5 MHz) for each simplex band, B guard is 10 KHz, and B c is 30 KHz, find the number of channels available in an FDMA system. B t : transmission bandwidth B c : carrier bandwidth N = (12.5x10 6 2(10x10 3 ))/(30x10 3 ) = 416 In USA, each cellular carrier is allocated 416 channels.
TDMA and FDMA W.tra.2-14 In practice, use Hybrid approach: combine TDMA and FDMA Cellular phone network is an example GSM uses 200 KHz channels (FDMA) divided into eight time slots (TDMA). A single handset uses one slot in two channels for sending and receiving Used in Europe TDMA standard IS-136 uses 30 KHz channels (FDMA), with each channel divided into six time slots (TDMA). A single handset uses one timeslot for sending and the other for receiving Used in AT&T wireless
Multiplexing W.tra.2-15 Multiplexing shared medium at link layer Basic multiplexing Time, Frequency, Spacing Advanced multiplexing Spread spectrum & frequency hopping Code division Speech coding Voice to digital signals (application layer) End of module W.tra.2
Spread spectrum multiple access W.tra.2-16 SSMA is an advanced multiplexing Belong to wideband multiplexing transmission bandwidth > > carrier bandwidth A pseudo-noise (PN) sequence converts a narrowband signal to a wideband noise-like signal before transmission Bandwidth efficient? Not efficient when used by a single user Efficient in a multiple-user environment Two types: Frequency hopped multiple access (FH) Direct sequence multiple access (DS), also called CDMA
Frequency hopped multiple access W.tra.2-17 Carrier frequencies are varied in a PN sequence within a wideband channel. advantages Robust against frequency selective interference Protection of tapping Immune to fading disadvantages PN generator coordination t c k 1 k 2 k 3 k 4 k 5 k 6 f
FHMA W.tra.2-18 At any time, a frequency hopped signal only occupies a single, relative narrow channel Either FM or FSK can be used FHMA & FDMA comparison Two types Fast frequency hopping system Changing rate of F c > symbol rate Slow frequency hopping system Changing rate of Fc <= symbol rate Used in Bluetooth, HomeRF user data modulator transmitter B c F c FHMA narrow vary narrowband signal modulator frequency synthesizer FDMA narrow fixed spread transmit signal Fc hopping sequence
W.tra.2-19 Code division multiple access The narrowband message k signal is multiplied by 1 a very large bandwidth signal called the spreading signal Also called Direct Sequence multiplexing All channels use the same spectrum at the same time advantages: bandwidth efficient no coordination necessary good protection against interference and tapping disadvantages: complex hardware k 2 k 3 k 4 k 5 k 6 t c f
W.tra.2-20 CDMA XOR of the signal with pseudo-random number (chipping sequence) many chips per bit (e.g., 128) result in higher bandwidth of the signal t b 0 1 t c 0 1 1 0 1 0 0 1 1 0 1 0 1 user data XOR chipping sequence = resulting signal user data X spread spectrum signal modulator transmit signal 0 1 1 0 1 0 1 1 0 0 1 0 1 0 t b : bit period t c : chip period chipping sequence radio carrier transmitter
CDMA W.tra.2-21 Many users share the same frequency Either TDD or FDD may be used Like many people talk at the same time but in different languages. Increasing the number of users raises the noise floor No absolute limit, though performance gradually degrades for all users Need precise power control To overcome near-far problem Assure each provides the same signal level to the base station receiver.
W.tra.2-22 CDMA A unique code assigned to each user; i.e., code set partitioning used mostly in wireless broadcast channels (cellular, satellite, etc) All users share same frequency, but each user has own chipping sequence (i.e., code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence
CDMA: encode/decode W.tra.2-23 Code: 1 1 1 1 1 1 1-1
W.tra.2-24 CDMA If codes are orthogonal, multiple users can coexist and transmit simultaneously with minimal interference Consider two codes C i and C j (i <> j), if C i @ C j, = 0, the codes are orthogonal. where @ denotes inner product, i.e. summation of bit-by-bit product Example: C i and C j are orthogonal C i = 1 1 1 1 1 1 1-1 C j = 1 1 1 1 1 1 1 1 C i @ C j = 1 1 + 1 1 + 1 + 1 1 1 = 0
CDMA: two-senders W.tra.2-25 CodeA: 1 1 1 1 1 1 1-1 CodeB: 1-1 1 1 1 1 1 1
Multiplexing W.tra.2-26 Multiplexing shared medium at link layer Basic multiplexing Time, Frequency, Spacing Advanced multiplexing Spread spectrum & frequency hopping Code division Speech coding Voice to digital signals (application layer) End of module W.tra.2
W.tra.2-27 Speech coding Transmit speech With the highest possible quality Using the least possible channel capacity Meet complex requirement of hardware for encode/decode Two types of speech coding Waveform coders Source independent Code a variety of signals equally well Less complex in implementation, but high bit rate Source coders Efficient bit rate, but complex hardware Signal specific, depending on a priori knowledge about signal to be coded
W.tra.2-28 Waveform coders Waveform coders in time domain Analog (voice or others) signals are digitized by a device: Codec (code-decoder) PCM (Pulse Code Modulation) 1. Sampling Based on sampling theorem, if the original voice signal has a limited bandwidth (without any component with a frequency > B), the sample frequency > 2B 2. Quantization A fixed number of amplitude levels are used 3. Binary encoding
PCM W.tra.2-29 PCM, used in telecommunication from very beginning, such as T1. 1. Sampling The Codec makes 8000 samples per second 1/8000 second = 125 μs Produce PAM (Pulse Amplitude Modulation) pulses, with their amplitudes proportional to that of the original signal 2. Quantization Take 2 8 =256 different levels (discrete) 3. Binary encoding Each sample produces a 8-bit binary number
T1 as PCM W.tra.2-30 T1 in Public Switched Telephone Network DS-0 rate, from PCM channels 8 bits x 8000 samples/sec = 64kbps PCM + TDMA T-carrier PCM --- Digitalization TDM --- Multiplexing The T-1 carrier (or DS1 format) consists of 24 voice channels multiplexed byte-by-byte together T-1 frame 24 channels x 8 bits/channel + 1 framing bit = 193bits DS-1 rate: 193 bits x 8000 frames/sec = 1.544Mbps
From voice to radio W.tra.2-31 In today s wireless communication PCM Digital Modulation Analog voice signal (baseband) Data Carrier modulated signal (broadband)
Multiplexing W.tra.2-32 Multiplexing shared medium at link layer Basic multiplexing Time, Frequency, Spacing Advanced multiplexing Spread spectrum & frequency hopping Code division Speech coding Voice to digital signals (application layer) End of module W.tra.2