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Home , CdmaOne, Communication channel, GSM, Multiplexing, Spectral efficiency

Multiple Access

Hamid Bahrami

Electrical & Computer Engineering System Block Diagram Duplexing l Duplexing: transmit and receive at the same time l Example: , how about walkie-talkie

Terminal Terminal A B Simplex

Terminal Terminal A B Half-duplex

Terminal Terminal A B Full-duplex Duplexing l Frequency division duplexing (FDD) l Multiplexes the Tx and Rx in one time slot in which transmission and reception is on 2 different frequencies l It provides simultaneous transmission channels for mobile/ (forward and reverse channels) l At the base station, separate transmit and receive antennas are used to accommodate the two separate channels l At the mobile unit, a single (with duplexer) is used to enable transmission and reception l To facilitate FDD, sufficient frequency isolation of the transmit and receive frequencies is necessary Duplexing l Time division duplexing (TDD) l Multiplexes the Tx & Rx in one frequency at different time slots Time Division Duplexing

T R T R Amplitude Time

l Ex: in a simple 2-way where a button is pressed to talk and released to listen l Only possible for digital transmission l Multiplexing (channelization) is the process of simultaneously transmitting several using a single Frequency Division Multiplexing l The available is divided into non-overlapping frequency slots and each message is assigned a frequency slot within the available band l Signals are transmitted by different frequency bands and then added together to form a l The signals are narrowband and frequency limited

fN-1 Frequency Band N fN-2

f3 Frequency Band 2 f2 Frequency f1 Frequency Band 1 f0 Time Time Division Multiplexing l Digital signals from several sources are multiplexed in time and transmitted over a single communication channel l The communication channel is divided into frames; each is further segmented into slots; each user is assigned a slot (or channel) within each time frame l Only for digital communication

Slot Slot Slot Slot s s . . . s s s ...... N 1 2 k 1 2 . . . N 1 2

Sync word Information or data word FRAME Division Multiplexing (CDM)

l Multiple signals are transmitted simultaneously on the same time and same frequency l Each signal is assigned a distinct code sequence l The code sequences (spreading ) are orthogonal (or almost orthogonal)

! Example of orthogonal codes

Code 1 = {1, 1, 1, 1}

Code 2 = {1, 1, -1, -1}

Code 3 = {1, -1, -1, 1}

Code 4 = {1, -1, 1, -1} FDM/TDM/CDM l Multiplexing in space using multiple antenna at the Tx and Rx Wavelength Division Multiplexing l Multiple Access l Multiple access: used to allow many mobile users to share simultaneously a finite amount of l Frequency division multiple access (FDMA) l Time division multiple access (TDMA) l Code division multiple access (CDMA) l Space Division Multiple Access (SDMA) l Demand Access Multiple Access (DAMA) l Random Access Multiple Access (RAMA) l Hybrid Multiple Accesses Multiple Access l To transmit multiple signals in a point-to-point communication system, we use multiplexing. l In a multi-user system with multiple receivers (a point-to-multipoint or broadcast system), a system with multiple (a multipoint-to-point or multi-access system), or a system composed of many users communicating directly with each other (a multipoint-to-multipoint system), we can use the same concept to transmit these multiple signals. This is called multiple access. Multiple Access l Multiple access: used to allow many users to share simultaneously a finite amount of radio spectrum l Frequency division multiple access (FDMA) l Time division multiple access (TDMA) l Code division multiple access (CDMA) l Space Division Multiple Access (SDMA) l These are sometimes called channelization protocols. These are contentionless protocols. Good for circuit switch. l Random Access Multiple Access (RAMA): Less coordinated and controlled. Contention based protocols. Good for packet switch. l Hybrid Multiple Accesses Multiple Access

Multiple Access Protocol

Contentionless Contention (Scheduling Access) (Random Access)

CDMA

Fixed Demand Repeated Random Random Access Assigned Assigned Access w/reservation

FDMA Polling ALOHA Implicit TDMA Token Passing Slotted ALOHA Explicit Frequency Division Multiple Access l Individual frequencies are assigned to individual users on demand for the duration of calls l Frequency distances are far enough à no interference l When the call is finished à the channel is released and available for a new call l If the transmission path deterioratesà switches the system to another channel

Guard band (at the edges & between) to minimize crosstalk

1 2 ! n

B

FRAME Frequency Division Multiple Access l The FDMA channel carries only one phone circuit at a time l If a channel is not in use, then it sits idle and cannot be used by other users à the waste of bandwidth l After the assignment of a voice channel, the base station and the mobile transmit simultaneously and continuously l The bandwidths of FDMA channels are relatively narrow à narrowband systems l The symbol time of a signal is large as compared to the average delay spread à little or no equalization Frequency Division Multiple Access l Low complexity à but it is changing l Fewer bits are needed for synchronization as frame bits l Higher cell sit system cost ß the need of bandpass filters and single channel per carrier design l Duplexers ß both tx and rx operate at the same time l Require tight RF filter to minimize the adjacent channel interference l Orthogonal FDMA (OFDMA) is the multiple access scheme for systems (LTE and IEEE 802.16/WiMAX) Frequency Division Multiple Access l Guard band Bguard: allocated at the edge of the allocated spectrum band l Channel bandwidth: Bc l The total spectrum allocation: Bt l The number of channels: N B - 2B N = t guard BC Channel Channel ...... Channel 1 2 Ns

B B g c MHz Bs Practice l Design a Frequency Division Multiplex (FDM) signal set consisting five voice channels, each in the frequency range 300 to 3400 Hz. The multiplexed composite is to occupy the spectral region from 30k to 50 kHz. Decide the allocating frequency bands for each channel by using the maximum guard band frequency.

The voice channel bandwidth is 3100 Hz. The maximum guard bandwidth is (3100+ Guard)´5 £ 50000-30000Þ Guard £ 900Hz Channel 1: 30000 ~ 33100 Hz and Guard band: 33100 ~ 34000 Hz Channel 2: 34000 ~ 37100 Hz and Guard band: 37100 ~ 38000 Hz Channel 3: 38000 ~ 41100 Hz and Guard band: 41100 ~ 41000 Hz Channel 4: 42000 ~ 45100 Hz and Guard band: 45100 ~ 46000 Hz Channel 5: 46000 ~ 49100 Hz and Guard band: 49100 ~ 50000 Hz Frequency Division Multiple Access l Example: AMPS (Advanced System) l Developed by and introduced in 1978. l , Analog, 800MHz FM band l Each AMPS channel has a one way bandwidth of 30KHz (60KHz each duplex channel) l 416 channels in 824-849MHz band (uplink) and 416 channels in 869-894MHz band (downlink) Example 9.2, page 452 l If a US AMPS (advanced mobile phone system) cellular operator is allocated 12.5 MHz for each simplex band, and if

Bguard is 10 KHz, and Bc is 30 kHz, find the number of channels available in an FDMA system.

12.5´106 - 2´10 ´103 N = = 416 30 ´103 Time Division Multiple Access l Divide the radio spectrum into time slots l In each slot, only one user is allowed to either transmit or receive l Buffer-and-burst method à non-continuous transmission (digital communication)

One TDMA Frame Control Bits Information Data Trail Bits

Slot 1 Slot 2 Slot 3 Slot N

Trail Bits Sync. Bits Information Data Guard Bits Time Division Multiple Access l Shares a signal carrier frequency with several users, where each user makes use of non-overlapping time slots l Data transmission is not continuous, but occurs in bursts à low battery consumption l Handoff process is simple ß discontinuous transmission l Duplex is not required l Adaptive equalization is necessary ß high data rate l The guard time should be minimized l High synchronization Time Division Multiple Access l IS-54 l The original TDMA format for AMPS l By dividing a 30-kHz channel into 3 time slots, enabling 3 different users to occupy it at same time l Provides a 3-fold increase in traffic capacity relative to AMPS, given the same bandwidth allocation l A second phase of the IS-54 provides for 6 (instead of 3) TDMA user channels in each 30 kHz radio channel l IS-136 (also called called D-AMPS) l Enhanced TDMA with special control channels to allow short message service, battery life extension, other features, 6 timeslots, three users occupy in rotation Time Division Multiple Access l GSM (Global System for Mobile) l Pan-European digital cellular standard to replace six incompatible analog cellular systems then in use in different geographic areas in early 80’s l Employing TDMA and each radio channel carries 8 timeslots (8 speech channels) and the radio channel bandwidth (for all 8 channels) is almost 270Kbps. l Freq. band 850MHz or 1900MHz (in many countries 900 and 1800MHz) l AT&T network is mainly a GSM based network. Time Division Multiple Access l Efficiency of TDMA l A measure of the percentage of transmitted data that contains information as opposed to providing overhead for the access scheme

æ bOH ö h = ç1- ÷´100% è bT ø

bOH = Nrbr + Ntbp + Ntbg + Nrbg

bT = Tf R

bOH =overhead bits per frame Tf: frame duration Nr = # of reference burst per frame R: System Nt = # of trail (preamble) burst per frame br = # of overhead bits per reference burst bp = # of overhead bits per preamble bg = # of equivalent bits in each guard time interval Time Division Multiple Access l Number of channels in TDMA system

m(B - 2B ) N = t guard BC

m: the maximum number of TDMA users supported on each radio channel

l Example 9.4: If GSM uses a frame structure where each frame consists of eight time slots, and each time slot contains 156.25 bits, and data is transmitted at 270.833 kpbs in the channel. Find (1) the time duration of a bit (2) the time duration of a slot, (3) the time duration of a frame, and (4) how long must a user occupying a single time slot wait between two successive transmissions. Example 9.4

1 The time duration of a bit : T = = 3.692µs b 270.833

The time duration of a slot :Tslot =156.25´Tb = 0.577ms

The time duration of a frame :Ts = 8´Tslot = 4.615ms Waiting time : 4.615ms Code Division Multiple Access l It is classified as a multiple access. Another possibility is FHMA. l The narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal l The spreading signals a pseudonoise code sequence that has a rate which is greater than the data rate Chinese English Principles OF

CDMA Arabic English Major

Hindu Code Division Multiple Access l Many users of a CDMA system share the same frequency, either TDD or FDD l Soft capacity limit à increasing the number of users raises the floor in a linear manner l Multiple might be substantially reduced ß the signal is spread over a large spectrum l Channel data rates are high à the symbol duration is short and less then the channel delay spread l RAKE receiver can be used l Self-jamming is a problem ß not perfectly orthogonal l Near-far problem Code Division Multiple Access l Reverse Link (from mobile unit to base station) l Near-far problem l The power of each user do not appear equal at the base station l Many mobile users share the same channel l Power control: l To maximize the total user capacity l To minimize power consumption of portable units l Forward Link (from base station to mobile unit) l Link does not suffer much from near-far problem since all cell signals can be received at the mobile with equal power l When at excessive intercell interference, the power control can be applied by increasing the power to the mobile Code Division Multiple Access l Advantages l Voice Activities Cycles (35% talking, 65% listening) l Improved call quality l No Equalizer Needed (only correlator needed) l No hard handoff l No guard time l Less fading l No frequency management needed l Capacity advantage l Enhanced privacy l Coexistence l Simple system planning Code Division Multiple Access l IS-95 (Interim Standard 95) also called cdmaOne l , a San Diego-based company l IS-95 was used as a standard l Each channel bandwidth was 1.2288 MHz (much larger than narrowband) l Offer greater traffic capacity than GSM l CDMA 2000 () l Supports data rate of up to 153Kbps l Channel bandwidth is 1.25MHz l and Sprint Nextel networks CDMA

Data signal Transmitted Signal ak(t)dk(t) Baseband d (t) x Modulator xk(t) k BPF

ak(t) PN Code ~ Generator Acos(wct)

Chip Clock

l The data symbols dk(t) are spread into ak(t)dk(t) l Then spread signal is modulated

stkkkkck()=+ 2 Pa(tt) d( ) cos2( pf ft ) (CDMA) Channel

s (t) y (t) k hk(t) k

L jy kl htkkl()= å bd[t -t kl ] e l=1

¥ ytk ()= ò-¥ h(t ) s(t -t k ) dt L ¥ jy kl =+2cosPadkklkå bwfdtò-¥ (tt--ttkk) k( ) ( ck t) [ t - t kl] ed l=1 L =+2cosPakklkå bwq(tt--ttkl) d k( kl ) ( ckl t) l=1 CDMA Receiver

r(t)

y(t ) T Decision Demodulator b ˆs ò0 (×)dt Device kl

at()- T 2Ptkck cos(wq+ ) kd l Detection accomplished by de-modulating & de-spreading l This involves the correlation of the received signal with the delayed version of the spreading signal (despreading operation) l In other words, the received signal is multiplied again by a synchronized version of the PN code Performance Analysis for Multiple users

ò

K rtkk()=+å y(t) nt () k =1 KL =++åå 2Pakklkbpq(tt--ttkl) d k( kl ) cos( 2 ft c kl) nt ( ) kl==11

K rtkk()=+å y(t) nt () k =1 KL =++åå 2Pakklkbpq(tt--ttkl) d k( kl ) cos( 2 ft c kl) nt ( ) kl==11 Performance Analysis l Assuming the first user is desired

(1)iT+ b zraftdt11= ò (tt) ( )cos( 2p c ) iTb

Tb zra11= ò0 (tt) ( )cos( 2p ftdtc )

KL Tb zPaadtt11ikklkk=+åå 2bwwqò0 (tt--ttkl) (t) ( kl ) cos( cckl) cos( ) kl==11

Tb + ò0 n(tt) a1 ( )cos(wc t) dt Space Division Multiple Access (SDMA) l Separating the signals of multiple users using Hybrid FDMA/CDMA (FCDMA) l Available spectrum is divided into subbands l Each subband is then considered as a CDMA system l It is the principle of multi-carrier CDMA (MC-CDMA) Hybrid Direct Sequence/Frequency Hopping Multiple Access (DS/FHSS) l Signal of each user is a DS signal whose center frequency hops periodically in a pseudo-random fashion. l Avoids near-far problem, soft hand-off is not possible Other Hybrid Techniques l Time Division CDMA (TCDMA) l Different spreading codes are assigned to different cells l Within each cell, only one user is allocated a particular time slot such that only one user is transmitting in each cell at one slot l Time Division Frequency Hopping (TDFH) l At the start of a new TDMA frame, the user hops to a new channel l This avoids severe fades or erasure in any particular channel l The user is allowed to hop according to a predefined sequence l TXs are made to transmit on different frequencies at different times Orthogonal Frequency Division Multiplexing (OFDM) l Similar to FDM by using three principles l Multirate, multisymbol and multicarrier l Distribute the data over a large number of carrier allows smaller transmission rate, e.g., bigger symbol duration l Reduce the effect of ISI l No/Less frequency selective fading l Possible guard interval between symbols l Orthogonal relationship between the signals allow to overlap each other without interference l No cross talk between carriers l No need for inter-carrier guard band OFDM Case Study – Tx

Wireless

Data à Serial to Parallel à à IFFT à Parallel to Serial à Guard à Mod

http://upload.wikimedia.org/wikipedia/commons/4/4e/OFDM_transmitter_ideal.png OFDM Case Study – Rx

Wireless Local Area Network

Demod à Guard removal à Serial to Parallel à FFT à DeMod à Parallel to Serial à Data Orthogonal Frequency Division Multiple Access (OFDMA) Multicarrier DS-CDMA l Statement of problems l Variety of services, including voice, data, image and l DS-CDMA suffers ISI and Multi-user Interference (MUI) l OFDM has the potential benefit l Multicarrier DS-CDMA l Marries the best of the OFDM and DS-CDMA l 4G Random Access l In (PR), many users attempt to access a single channel in an uncoordinated (or minimally coordinated manner). l Collision can happen and is detected by the basestation (access point or AP). The AP broadcasts an ACK or a NACK signal for successful and unsuccessful reception of user’s data. l The users use a contention technique to transmit on a common channel. l PR is easy to implement but has low and may cause delay. It is only used for transmission of data (less sensitive to delay) and not in voice systems. Random Access Protocols l Pure ALOHA: a user accesses a channel as soon as a packet is ready to be transmitted. After transmission the user wait for ACK. (Developed in University of Hawaii in early 70’s) l In case of collision (when NACK is received), the terminal waits for a random period of time and then retransmits the same packet. l There is almost no timing in the system. l Very easy to implement but large delay especially when the number of users is large. Random Access Protocols l Pure ALOHA Sending a packet

Waiting for ACK

No ACK Randomly received? selected a delay

Yes

Continue the next packet Random Access Protocols l Slotted ALOHA: Time is divided into equal time- slots. All users have synchronized clocks. l If a user has a packet to transmit, its transmission is done only at the beginning of each time-slot. The rest of the protocol is similar to ALOHA. l This prevents partial collision and improves the (reduces delay). l Still for large number of users, the delay can be high. Random Access Protocols l Slotted ALOHA Random Access Protocols l In pure and slotted ALOHA protocols the users don’t listen to the channel. l By listening to the channel before transmission, greater efficiency is achieved. l Carrier Sense Multiple Access (CSMA): A users with a packet to transmit, constantly monitor the channel. If the channel is idle, the user transmits. l CSMA with Collision Detection (CD): The user continues listening to the channel during its own transmission. If senses a collision, it stops transmission. l CSMA with Collision Avoidance (CA): If the channel is busy, the user waits (backs off) for a random amount of time (why random?). Random Access Protocols l Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) l Inter-Frame Space l DIFS: distribution coordination function IFS l SIFS: short IFS DIFS SIFS DIFS SIFS DIFS

Terminal A Packet A

Packet B Terminal B

Packet C Terminal C

ACK AP ACK

Backoff Intervel Packet Arrival Residual Backof Time Random Access Protocols l Reservation Protocol: The transmission time is divided into slots. Unlike previous contention based random access protocols, here a user can reserve slots for transmission. l One can think of it as a middle ground between multiple access schemes and other random access protocols. l Specially good for transmission of high priority packets and for transmission without (continuous transmission). l Reservation and some other protocols (such as Polling) are sometimes called controlled-access protocols. l WiFi uses CSMA/CA with reservation. Capture Effect and Hidden Terminal l Often the closest transmitter is able to capture a receiver because of small . This is called near-far (or capture) effect. l Good because many packets of this terminals arrive at the receiver despite collision. l Bad because a strong transmitter may make it impossible for the receiver to detect a much weaker transmitter which is attempting to communicate to the same receiver (hidden terminal). Example: IEEE 802.11 Wireless LAN

• 802.11b • 802.11a – 2.4-5 GHz unlicensed spectrum – 5-6 GHz range – up to 11 Mbps – up to 54 Mbps • 802.11g – 2.4-5 GHz range – up to 54 Mbps • 802.11n: multiple antennae – 2.4-5 GHz range – up to 200 Mbps

v all use CSMA/CA for multiple access v all have base-station and ad-hoc network versions 80211. Wireless LAN Architecture

v wireless host communicates with base station § base station = access point (AP) v Basic Service Set (BSS) (aka “cell”) in infrastructure mode hub, switch or router contains: AP § wireless hosts

BSS 1 § access point (AP): base station AP § ad hoc mode: hosts only

BSS 2 IEEE 802.11 Multiple Access

• avoid collisions: 2+ nodes transmitting at same time

• 802.11: CSMA - sense before transmitting – don’t collide with ongoing transmission by other

• 802.11: no collision detection as in ! – difficult to receive (sense collisions) when transmitting due to weak received signals (fading) – can’t sense all collisions in any case: hidden terminal, fading – goal: avoid collisions: CSMA/C(ollision)A(voidance) IEEE 802.11 MAC Protocol: CSMA/CA

802.11

1 if sense channel idle for DIFS then sender receiver transmit entire frame (no CD) 2 if sense channel busy then DIFS start random backoff time timer counts down while channel idle data transmit when timer expires if no ACK, increase random backoff interval, repeat 2 SIFS 802.11 receiver ACK - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) IEEE 802.11 MAC Protocol: CSMA/CA

idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames • sender first transmits small request-to-send (RTS) packets to BS using CSMA – RTSs may still collide with each other (but they’re short) • BS broadcasts clear-to-send CTS in response to RTS • CTS heard by all nodes – sender transmits data frame – other stations defer transmissions

avoid data frame collisions completely using small reservation packets!