UNIT-3

Equalization and Multiple Access

Equalization Techniques: Transversal Filters, Adaptive Equalizers,

Zero Forcing Equalizers, Decision Feedback Equalizers, and related
algorithms; Multiplexing and Multiple Access: FDMA, TDMA,
CDMA, OFDMA, SC- FDMA, IDMA Schemes and Hybrid Method
of Multiple Access Schemes, RAKE Receiver; Multiple Access for
Radio Packet Systems: Pure ALOHA, Slotted ALOHA, CSMA and
their versions; Packet and Pooling Reservation Based Multiple Access
Schemes.

Equalization Techniques
Equalization is a process used in communication systems to mitigate the effects of channel
impairments, such as inter-symbol interference (ISI), caused by multipath propagation or other
distortions. The goal of equalization is to reconstruct the transmitted signal as accurately as
possible at the receiver.

 Compensates for distortion introduced by the communication channel.

 Reduces ISI, ensuring reliable and accurate data transmission.
 Improves signal-to-noise ratio (SNR) and system performance.

Types of Equalization Techniques

Linear Equalizers

Apply a linear filter to the received signal.

 Advantages:
o Simple to implement.
o Works well for low ISI and noise levels.
 Disadvantages:
o May amplify noise in high-ISI environments.
 Examples:
o Zero-Forcing (ZF) Equalizer:
 Eliminates ISI completely but may amplify noise in poorly conditioned
channels.
−1
 W=H , where H is the channel matrix.
o Minimum Mean Square Error (MMSE) Equalizer:
 Balances ISI reduction and noise amplification.
 Minimizes the mean square error between transmitted and received
signals.

Wireless and Mobile Communication GNIOT DR. ANIL KUMAR DUBEY

Non-Linear Equalizers

Use non-linear techniques to achieve better performance in complex channels.

 Advantages:Effective for high-ISI and high-noise environments.
 Disadvantages:Higher computational complexity.
 Examples:
 Decision Feedback Equalizer (DFE):
o Uses past detected symbols to cancel ISI.
o Combines a feedforward filter (linear) and a feedback filter (non-linear).
 Maximum Likelihood Sequence Estimation (MLSE):
o Searches for the most likely transmitted symbol sequence.
o Implements algorithms like the Viterbi algorithm for sequence detection.

Adaptive Equalizers
Dynamically adjust equalizer coefficients based on real-time channel conditions.

 Advantages:Suitable for time-varying channels.

 Disadvantages:May require training sequences, increasing overhead.
 Algorithms:
o Least Mean Squares (LMS):
 Iteratively updates equalizer weights to minimize error.
 Simple and widely used.
o Recursive Least Squares (RLS):
 Provides faster convergence than LMS but at higher computational cost.
o Kalman Filter:
 Optimally estimates channel parameters in dynamic environments.

Adaptive equalization is a technique that can be used with either linear or non-linear equalizers
to improve their performance in a changing channel environment. An adaptive equalizer is an
equalizer that automatically adapts to the time-varying properties of the communication
channel.
Adaptive Equalization assumes the channel is time-varying and tries to design an equalizer
filter whose filter coefficients are varying in time according to the change of channel and tries
to eliminate intersymbol interference and additive noise at each time. The implicit assumption
of adaptive equalization is that the channel is varying slowly.
Adaptive equalization works by adjusting the equalization filter in real-time to compensate for
the distortion introduced by the channel. The equalization filter can be adapted based on the
received signal characteristics and feedback from the receiver.
Adaptive equalization uses algorithms to estimate the channel response and then adaptively
adjusts the filter coefficients to minimize the error between the received signal and the
transmitted signal.
The desired output signal received by the equalizer is given by:

Wireless and Mobile Communication GNIOT DR. ANIL KUMAR DUBEY

here , x(t) = original message
f(t) = combined impulse response
n(t) = baseband noise

if the impulse responses of the equalizer heq(t) , then the output of the equalizer is:

There are two operating modes of adaptive equalization:

1. Training Mode: In the training mode, the equalizer is trained to learn the characteristics
of the channel by using a known training sequence. The training sequence is typically a
predetermined sequence of symbols that is transmitted by the transmitter and known by
the receiver. The receiver uses this known sequence to estimate the channel
characteristics and adapt the equalizer coefficients accordingly.
2. Tracking mode: In the tracking mode, the equalizer coefficient is adjusted based on the
decisions made on the received signal. The receiver makes the decision and uses that
decision to update the equalization coefficient. or we can say that the equalizer uses the
received signal to estimate the channel characteristics and adjust the filter coefficients to
compensate for any changes in the channel.
Advantages of Adaptive Equalization
 Improves signal quality
 Reduces interference
 Increases spectral efficiency

Wireless and Mobile Communication GNIOT DR. ANIL KUMAR DUBEY

  Enables longer distances
 Reduces power consumption
 Adapts to different channel conditions
 Robustness to channel changes: Adaptive equalizers can adjust to changes in the channel,
making them more robust to variations in the wireless environment.

Multiple Access Techniques

If there is a dedicated link between the sender and the receiver then data link control layer is
sufficient, however if there is no dedicated link present then multiple stations can access the
channel simultaneously. Hence multiple access protocols are required to decrease collision and
avoid crosstalk.

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 In wireless communication systems, it is often desirable to allow the subscriber to send
information simultaneously from the mobile station to the base station while receiving
information from the base station to the mobile station.

A cellular system divides any given area into cells where a mobile unit in each cell
communicates with a base station. The main aim in the cellular system design is to be able
to increase the capacity of the channel, i.e., to handle as many calls as possible in a given
bandwidth with a sufficient level of quality of service.

There are several different ways to allow access to the channel. These includes mainly the
following −

 Frequency division multiple-access (FDMA)

 Time division multiple-access (TDMA)
 Code division multiple-access (CDMA)
 Space division multiple access (SDMA)

Depending on how the available bandwidth is allocated to the users, these techniques can be
classified as narrowband and wideband systems.

Narrowband Systems

Systems operating with channels substantially narrower than the coherence bandwidth are called
as Narrow band systems. Narrow band TDMA allows users to use the same channel but allocates
a unique time slot to each user on the channel, thus separating a small number of users in time on
a single channel.

Wideband Systems

In wideband systems, the transmission bandwidth of a single channel is much larger than the
coherence bandwidth of the channel. Thus, multipath fading doesn’t greatly affect the received
signal within a wideband channel, and frequency selective fades occur only in a small fraction of
the signal bandwidth.

Frequency Division Multiple Access (FDMA):

FDMA is a type of channelization protocol. This bandwidth is divided into various frequency
bands. Each station is allocated a band to send data and that band is reserved for the particular
station for all the time which is as follows.

Wireless and Mobile Communication GNIOT DR. ANIL KUMAR DUBEY

 Figure – FDMA

The frequency bands of different stations are separated by small bands of unused frequency
and unused frequency bands are called as guard bands that prevent the interference of stations.
It is like the access method in the data link layer in which the data link layer at each station
tells its physical layer to make a bandpass signal from the data passed to it. The signal is
created in the allocated band and there is no physical multiplexer at the physical layer.

Advantages of FDMA
 FDMA uses simple hardware resources and is easy to set up.
 It efficiently handles smaller groups of users.
 The system isn’t overly complicated.
 All stations can transmit continuously without waiting their turn.
 It lowers the amount of data transmitted, which can increase capacity.
 It reduces interference between symbols, improving communication quality.

Wireless and Mobile Communication GNIOT DR. ANIL KUMAR DUBEY

 Disadvantages of FDMA
 FDMA works only with analog signals.
 It lacks flexibility, so existing traffic patterns must change gradually.
 Transponders need extensive bandwidth.
 It doesn’t support high traffic capacity.
 RF filters must meet strict adjacent channel rejection standards, which can increase costs.
 The maximum bit rate per channel is small and remains fixed.

Time Division Multiple Access (TDMA):

TDMA is the channelization protocol in which bandwidth of channel is divided into various
stations on the time basis. There is a time slot given to each station, the station can transmit
data during that time slot only which is as follows.

Figure – TDMA

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 Each station must aware of its beginning of time slot and the location of the time slot. TDMA
requires synchronization between different stations. It is type of access method in the data link
layer. At each station data link layer tells the station to use the allocated time slot.

Advantages of TDMA
 As cell sizes decrease, TDMA requires substantial investment in space, support, and base-
station hardware.
 It can transmit data at speeds ranging from 64 kbps to 120 Mbps.
 TDMA separates users based on time, ensuring no interference from simultaneous
transmissions.
 It supports services like fax, voiceband data, SMS, multimedia applications, and video
conferencing.
 TDMA extends battery life by allowing devices to transmit only part of the time during
conversations.
 It effectively handles both data transmission and voice communication needs.

Disadvantages of TDMA
 If all time slots in the current cell and the next cell are occupied, users allocated specific
slots may not connect to a call.
 Frequency/slot allocation in TDMA can be complex.
 High data rates in TDMA require equalization.
 Network and spectrum planning in TDMA is complex and time-consuming, needing
expertise and resources.
 TDMA focuses on organization and range planning.

Code Division Multiple Access (CDMA) :

In CDMA, all the stations can transmit data simultaneously. It allows each station to transmit
data over the entire frequency all the time. Multiple simultaneous transmissions are separated
by unique code sequence. Each user is assigned with a unique code sequence.

Wireless and Mobile Communication GNIOT DR. ANIL KUMAR DUBEY

 Figure – CDMA
In the above figure, there are 4 stations marked as 1, 2, 3 and 4. Data assigned with respective
stations as d1, d2, d3 and d4 and the code assigned with respective stations as c1, c2, c3 and
c4.

Advantages of CDMA
 CDMA has a very high spectral capacity, supporting many users within a wide bandwidth.
 It doesn’t require synchronization between users.
 CDMA channels are hard to decode, improving cellular communication security.
 It provides better secure transmission capabilities.
 Dropouts only occur when the user is twice the distance from the base station.

Disadvantages of CDMA
 CDMA faces channel pollution when a user’s phone connects to multiple cell sites, but only
one has strong signal.
 CDMA isn’t as mature as GSM, since it’s newer.
 CDMA requires time synchronization.
 Performance of the CDMA system decreases as the number of users increases.
 CDMA equipment tends to be more expensive due to its complexity.

Spread Spectrum Multiple Access

Spread spectrum multiple access (SSMA) uses signals which have a transmission bandwidth
whose magnitude is greater than the minimum required RF bandwidth.

There are two main types of spread spectrum multiple access techniques −

 Frequency hopped spread spectrum (FHSS)

 Direct sequence spread spectrum (DSSS)

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 Frequency Hopped Spread Spectrum (FHSS)
This is a digital multiple access system in which the carrier frequencies of the individual users
are varied in a pseudo random fashion within a wideband channel. The digital data is broken into
uniform sized bursts which is then transmitted on different carrier frequencies.

Direct Sequence Spread Spectrum (DSSS)

This is the most commonly used technology for CDMA. In DS-SS, the message signal is
multiplied by a Pseudo Random Noise Code. Each user is given his own code word which is
orthogonal to the codes of other users and in order to detect the user, the receiver must know the
code word used by the transmitter.

FDMA TDMA CDMA

FDMA stands for Frequency TDMA stands for Time CDMA stands for Code
Division Multiple Access. Division Multiple Access. Division Multiple Access.
In this, sharing In this, only the sharing In this, there is sharing of both
of bandwidth among different of time of satellite i.e. bandwidth and time among
stations takes place. transponder takes place. different stations takes place.
There is no need of any There is no need of any Codeword is necessary.
codeword. codeword.
In this, there is only need of In this, guard time of the In this, both guard bands and
guard bands between the adjacent slots are guard time are necessary.
adjacent channels are necessary. necessary.
Synchronization is not required. Synchronization is Synchronization is not
required. required.
The rate of data is low. The rate of data is The rate of data is high.
medium.
Mode of data transfer is Mode of data transfer is Mode of data transfer is digital
continuous signal. signal in bursts. signal.
It is little flexible. It is moderate flexible. It is highly flexible.

Random Access Protocol

In this, all stations have same superiority that is no station has more priority than another
station. Any station can send data depending on medium’s state( idle or busy). It has two
features:
 There is no fixed time for sending data
 There is no fixed sequence of stations sending data
The Random access protocols are further subdivided as:
ALOHA
It was designed for wireless LAN but is also applicable for shared medium. In this, multiple
stations can transmit data at the same time and can hence lead to collision and data being
garbled.

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Pure ALOHA
When a station sends data it waits for an acknowledgement. If the acknowledgement doesn’t
come within the allotted time then the station waits for a random amount of time called back-
off time (Tb) and re-sends the data. Since different stations wait for different amount of time,
the probability of further collision decreases.
Vulnerable Time = 2* Frame transmission time
Throughput = G exp{-2*G}
Maximum throughput = 0.184 for G=0.5

Slotted ALOHA
It is similar to pure aloha, except that we divide time into slots and sending of data is allowed
only at the beginning of these slots. If a station misses out the allowed time, it must wait for the
next slot. This reduces the probability of collision.

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Vulnerable Time = Frame transmission time
Throughput = G exp{-*G}
Maximum throughput = 0.368 for G=1

CSMA
Carrier Sense Multiple Access ensures fewer collisions as the station is required to first sense the
medium (for idle or busy) before transmitting data. If it is idle then it sends data, otherwise it
waits till the channel becomes idle. However there is still chance of collision in CSMA due to
propagation delay. For example, if station A wants to send data, it will first sense the medium. If
it finds the channel idle, it will start sending data. However, by the time the first bit of data is
transmitted (delayed due to propagation delay) from station A, if station B requests to send data
and senses the medium it will also find it idle and will also send data. This will result in collision
of data from station A and B.

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 CSMA Access Modes

 1-Persistent: The node senses the channel, if idle it sends the data, otherwise it continuously
keeps on checking the medium for being idle and transmits unconditionally(with 1 probability)
as soon as the channel gets idle.
 Non-Persistent: The node senses the channel, if idle it sends the data, otherwise it checks the
medium after a random amount of time (not continuously) and transmits when found idle.
 P-Persistent: The node senses the medium, if idle it sends the data with p probability. If the
data is not transmitted ((1-p) probability) then it waits for some time and checks the medium
again, now if it is found idle then it send with p probability. This repeat continues until the
frame is sent. It is used in Wifi and packet radio systems.
 O-Persistent: Superiority of nodes is decided beforehand and transmission occurs in that order.
If the medium is idle, node waits for its time slot to send data.

CSMA/CD
Carrier sense multiple access with collision detection. Stations can terminate transmission of data
if collision is detected. For more details refer – Efficiency of CSMA/CD.

CSMA/CA
Carrier sense multiple access with collision avoidance. The process of collisions detection
involves sender receiving acknowledgement signals. If there is just one signal(its own) then the
data is successfully sent but if there are two signals(its own and the one with which it has
collided) then it means a collision has occurred. To distinguish between these two cases,
collision must have a lot of impact on received signal.

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 Packet and Pooling Reservation Based Multiple Access Schemes
Reservation-based multiple access schemes are designed to manage communication resources
efficiently in scenarios with multiple users sharing a common channel. By reserving time slots,
frequency bands, or codes for specific users or data packets, these schemes aim to reduce
contention and improve overall system performance.

Packet Reservation Multiple Access (PRMA) :

PRMA is a protocol designed for systems where multiple users send data packets over a shared
channel, combining features of TDMA (Time Division Multiple Access) and random access.

Pooling-Based Reservation Multiple Access (Pooling-Based PRMA)

Pooling-based schemes extend PRMA by introducing shared pools of slots, where users can
dynamically reserve slots based on their traffic needs.

Wireless and Mobile Communication GNIOT DR. ANIL KUMAR DUBEY