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Adapted from T.S. Rappaport’s

Wireless Communications

Chapter 9

Multiple access techniques for

wireless communications

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9.1 Introduction to multiple access

• Sharing of spectrum is required to achieve high capacity

• Duplexing* FDD: need a duplexer

- To minimize interference between forward and reverse links on each channel, the frequency split is made as grate as possible within the freq. Spectrum.

- But, must not be too great in order to allow inexpensive duplexers and a common trnsceiver antenna to be used in each subscriber unit.

* TDD: a time latency, only used for a small coverage

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• Multiple access techniques (commonly used)* FDMA

* TDMA

* CDMA

* Packet Radio

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• Narrowband systems:* Relate the bandwidth of signal channel to the expected coherence

bandwidth of the radio propagation channel.

• Wideband systems:* Transmission bandwidth of signal channel is much larger than the

coherence bandwidth of the radio channel.

* Multipath fading does not greatly affect the received signal within the wideband channel.

* Frequency selective fades occur in only a small fraction of the signal bandwidth.

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9.2 FDMA

• Features

* usually implemented in narrowband systems

* carries only one phone user at a time per channel

* If an FDMA channel is not in use, then it sits idle and cannot be used by other users to increase or share capacity.

* Since FDMA is a continuous transmission scheme, fewer bits are needed for overheaded purposes (e.g., synchronization and framing) as compared to TDMA.

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* The symbol time is large as compared to the average delay spread; Consequently, the amount of inter-symbol interference is low

little or no equalization is required

* complexity is lower

* Need to use duplexers

* Also require tight RF filtering to minimize adjacent channel interference

* the cell site system costs are higher because of the single channel per carrier design and need to use costly bandpass filters.

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• Nonlinear effects in FDMA

* Many channels share the same antenna at the BS

* When operated near saturation for maximum power efficiency, the power amplifiers are nonlinear.

* Can cause signal spreading, and generate

inter-modulation frequencies

adjacent-channel interference

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• # of channels supported in a FDMA system

(9.1)

where Bguard is the guard band allocated at the edge of the spectrum.

c

guardt

B

BBN

2

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9.3 TDMA

• Transmission in a buffer-and-burst method, not continuously.

• Digital data and digital modulation must be used

• In TDMA/FDD systems, it intentionally induce several time slots of delay between the forward and reverse time slots of a particular user, so the duplexer is not required.

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• Features:* Number of time slots per frame depends on modulation

techniques, available bandwidth, etc.

* Data transmission is not continuous, but in burst

low battery consumption, since the subscriber transmitter can be turned off when not in use.

* Simpler handoff process, since the subscriber is able to listen for other base stations during the idle time slots. (MAHO)

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• Features: (Cont.)* Duplexers are not required. Even if FDD is used, a switch

rather than a duplexer is all that is required.

* Adaptive equalization is usually necessary, since the transmission rates are generally very high as compared to FDMA channels.

* High synchronization are guard slots overhead.

* It is possible to allocate different numbers of time slots per frame to different users.

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• # of channel slots in a TDMA system

(9.5)

where m is the number of time slots on each channel.

c

guardtot

B

BBmN

)2(

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• The preamble contains the address and synchronization information that both the base station and the subscribers use to identify each other.

• Guard times (bits) are utilized to allow synchronization of the receivers between different slots and frames.

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9.4 Spread Spectrum Multiple Access

• Conceptual view

• Immune to multi-path interference

Robust multiple access capability in a multiple

users environment

• Types of spread spectrum multiple access* Frequency hopping

* direct sequence (also called CDMA)

A narrowband signal

wideband noise-like signal

Pseudo-noise (PN) sequence

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FH

• carrier frequencies of the individual users are varied in a pseudo random within a wideband channel

• Digital data is broken into uniform sized bursts which are transmitted on different carrier frequencies.

• The instantaneous bandwidth of any one transmission burst is much smaller than the total spread bandwidth.

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• Pseudorandom change of the carrier frequencies of the user randomizes the occupancy of a specific channel at any given time, thereby allowing for multiple access over a wide range of frequencies.

• in the receiver, a locally generated PN code is used to synch the receiver instantaneous frequency with that of the transmitter

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• At any given point in time, a frequency hopped signal only occupies a signal, relatively narrow channel since narrowband FM or FSK is used.

• A frequency hopped system provides a level of security, especially when a large number of channels are used, since an unintended (or an intercepting) receiver that does not know the pseudorandom sequence of frequency slots must retune rapidly to search for the signal it wishes to intercept.

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CDMA

• The narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal.

• The spreading signal is a pseudo-noise code sequence that has a chip rate which is order of magnitudes greater than the data rate of the message.

• Each user has its own pseudorandom codeword which is approximately orthogonal to all other codewords.

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• receiver performs a time correlation operation to detect only the specific desired codeword

• For detection of the message signal, the receiver needs to know the codeword used by the XMIT.

• Each user operates independently with no knowledge of the other users.

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• In CDMA, the power of multipath users at a receiver determines the noise floor after decorrelation.

• If the power of each user within a cell is not controlled such that the near-far problem occurs.

• To combat the near-far problem, power control is used in most CDMA implementations.

This solves the problem of a nearby subscriber overpowering the base station receiver and drowning out the signals of far away subscribers.

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• Features of CDMA:

* Many users share the same frequency. Either TDD or FDD may be used.

* Has a soft capacity limit. Increasing the number of users in a CDMA system raises the noise floor in a linear manner.

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• Thus, there is no absolute limit on the number of users in CDMA. Rather, the system performance gradually degrades for all users as the number of users is increased.

* Inherent frequency diversity will mitigate the effects of both multipath fading and small-scale fading.

* It provide soft handoff.

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9.4.3 hybrid spread spectrum techniques

• Hybrid FDMA/CDMA* The required bandwidth not be contiguous.

* Different users can be allocated different subspectrum bandwidths depending on their requirements.

* Per Figure 9.6 (P. 460)

• Hybrid direct sequence/Frequency Hopped* Consists of a direct sequence modulated signal whose center frequ

ency is made to hop periodically in a pseudorandom fashion.

* Per Figure 9.7 (P. 460)

* Avoids the near-far problem

* But, not adaptable to the soft handoff.

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• Time division CDMA

* Different spreading codes are assigned to different cells.

* only one user per cell is allotted a particular time slot.

* it avoids the near-far effect since only one user transmits at a time within a cell.

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• Time division frequency hopping

* Subscriber can hop to a new frequency at the start of a new TDMA frame, thus avoiding severe multipath fading or co-channel interference.

* Has been adopted for the GSM standard, where the hopping sequence is predefined and the subscriber is allowed to hop only on certain frequencies which are assigned to a cell.

* The use of TDFH can increase the capacity of GSM by several fold.

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9.5 Space Division Multiple Access (SDMA)

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• Controls the radiated energy for each user in space. That is, serves different users by using spot beam antennas (Beamformer).

• Per Figure 9.8(P. 461)

• Different areas covered by the antenna beam may be served by the same frequency (in TDMA or CDMA) or different frequencies (in FDMA)

• Sectorized antennas are primitive application of SDMA. But, the adaptive antennas (or smart antennas) are the focus of many current research. (mainly for the base station architecture.)

• Why does the reverse link present the most difficult in cellular systems?

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9.6 Packet Radio Multiple Access

• Many users attempt to access a single channel in an uncoordinated (or minimally coordinated) manner.

• Collisions are detected at the base station

• ACK or NACK signal is broadcast to alert the desired user

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• Very easy to implement but has low spectral efficiency and may induce delays.

• ALOHA, developed for early satellite systems, are the best example of contention techniques.

* Transmit whenever they have data to send

* Then, listen to the acknowledgement feedback to determine if transmission has been successful or not.

* If collision occurs, waits a random amount of time, and then retransmits the packet.

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* The performance of contention techniques can be evaluated by the throughput and average delay.

* Slotted ALOHA: time is divided into equal time slots of length greater than the packet duration.

* This prevents partial collisions.

* Figure 9.10 (P. 465) shows that slotted ALOHA double the channel utilization of pure ALOHA.

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• CSMA (carrier Sense Multiple Access)

* By listening to the channel before engaging in transmission, greater efficiencies may be achieved.

* Two important parameters: detection delay and propagation delay.

* Several variations of CSMA:

- 1-persistent CSMA: as soon as the channel is idle, it transmits the message with probability one.

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- Non-persistent CSMA: after receiving a NACK, the terminal waits a random time before retransmission. This is popular for wireless LAN, where the packet transmission interval is much greater than the propagation delay.

- P-persistent CSMA: when the channel is idle, the packet is transmitted in the first available slot with probability P or in the next slot with probability 1-P.

- CSMA/CD: when collision d is detected during the transmission, it is aborted in midstream.

- Data sense multiple access: used in CDPD.

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• Reservation protocols* Reservation ALOHA: certain packet slots are assigned with priorit

y, and is possible for users to reserve slots for the transmission of packets.

* Packet reservation multiple access (PRMA): as a means of integrating bursty data and human speech.

• Capture effect in packet radio* results from near-far effect

• Table 9.2 (P. 469) shows multiple access techniques for different traffic types.

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9.7 Capacity of Cellular Systems

• Radio capacity is determined by the required carrier-to-interference ratio (C/I) and the channel bandwidth Bc

• Considering the forward channel interference problem, the co-channel reuse ratio

(9.14)

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• The carrier-to-interference

(9.15)

• Considering only the first tier six co-channel cells,

(9.16)

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• Assuming the mobile is at the cell edge D0=R, then the minimum carrier-to-interference ratio that still provides acceptable signal quality at the receiver is given by

(9.17)

• Thus, the ratio capacity

(9.21)

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• For n=4

(9.22)

• Typically the minimum required C/I is about 21dB for narrowband digital system and 18 dB for narrowband analog FM systems.

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• In a digital cellular system, C/I can be expressed as

(9.24)

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9.7.1 Capacity of Cellular CDMA

• The capacity of CDMA is interference limited, while it is bandlimted in FDMA and TDMA.

• Therefore, any reduction in the interference will cause a linear increase in the capacity of CDMA.

• A sectorized antenna system is often used.

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• Another way is to operate in a discontinuous transmission mode (DTX). That is, the transmitter is turned off during the periods of silence in speed. (voice activity ~=3/8)

• At a base station, the signal-to-noise ratio is

(9.28)

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• The SNR can be further represented in terms of Eb/N0 given by

(9.29)

where

R is the baseband information bit rate,

W is the total RF bandwidth,

S is the signal power of each user.

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• Taking the background thermal noise ratio account,

(9.30)

• Then, the number of users is

(9.31)

where W/R is the processing gain in CDMA.

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• When adding the voice activity and sectorized antenna,

(9.33)

• The other commonly used technique to reduce the interference is power control.

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9.7.2 Capacity of CDMA with Multiple Cells

• A particular base station is unable to control the power of users in neighboring cells, and these users add to the noise floor and decrease capacity on the reverse link.

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• While a single cell CDMA system offers ideal frequency reuse (f=1), the actual frequency reuse factor of a CDMA in multi-cells is given by

(9.34)

Where N0 is the total interference power received from (N-1) in-cell users.

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where w1 and w2 represent the user distribution.

• The typical of f could be in the range of 0.316 to 0.707, depending on the path loss exponent and user distribution.

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9.7.3 Capacity of Space Division Multiple Access

• For interference limited CDMA operating in an AWGN channel, with perfect power control and omni-directional antenna, the average bit error rate,

(9.50)

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• An ideal adaptive antenna system (smart antenna) is able to form a beam for each user in the cell of interest, and the base station tracks each user in the cell as it moves.

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• For a single cell, the average bit error rate

(9.54)

where

D is the directivity (beam) of the antenna, typically in the range of 3dB to 10 dB, N is the antenna elements, K is the number of users.

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• For a multiple-cell environment,

(9.55)

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