T325: Technologies for digital media
Block III – Part 3: Access and modulation
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• Time division multiple access (TDMA)
• Code Division Multiple Access (CDMA)
• Channelization codes
• Scrambling codes
• OFDMA
• Modulation and symbols
• Quadrature methods and quadrature
amplitude modulation
Outline
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• The key Access technology in GSM system is the Time
division multiple access (TDMA)
• Operator’s band of frequencies for uplink and downlink is
divided into a set of channels for use within a cell
• Each channel is shared by a few users simultaneously using
time-division multiplexing
• The sharing of frequency channels by several users in GSM
means that TDMA is combined with frequency division
multiple access (FDMA)
Time division multiple access
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• In the uplink and downlink
directions, communication on
each of the frequency channels
is divided (in GSM) into
‘frames’ of 4.615 ms duration.
• Frames are further subdivided
into slots, with eight time slots
per frame.
• A slot lasts about half a
millisecond (546.5 µs), and
accommodates 114 bits of data.
• Slots are allocated to
individual users.
Time division multiple access
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• In TDMA, users DO NOT literally have simultaneous
access.
• Users rapidly take turns to have exclusive use of the
particular frequency channel they have been allocated to in
a cell.
• These turns come round every 4.615ms (the duration of the
frame), and the turn lasts for approximately half a
millisecond (the duration of the slot).
• The user’s communication channel (the channel which
carries the user’s data) in this case consists of their recurring
slot, which is in the same relative position in succeeding
frames.
• However, 1 in 13 frames is reserved entirely for control
data.
Time division multiple access
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• Note: Time slot & GSM channel , Should not to be
confused with a frequency channel
• Each frequency channel carries eight GSM channels.
• In GSM, the user has exclusive access to their GSM
channel even if a call consists of silence.
• As mentioned above, a frame lasts 4.615ms. How many
frames are there per second?
Time division multiple access
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• A slot in a frame carries 114bits. For a single user, what is
the data rate in bits per second of their GSM channel if no
frames are ‘robbed’ for control data?
• There are 114bits associated with each time slot. Hence,
per GSM channel the rate is
• 216.7 x 114bit/s= 24 700 bit/s in one direction. (The same
rate is also available in the other direction.)
• 1 in 13 frames is reserved for control data. Hence, what is
the actual data rate for a GSM channel?
• One frame in 13 is reserved for control data; the previous
answer needs to be reduced by a factor of 12/13 to arrive at
the actual data rate per GSM channel:
Time division multiple access - Activity
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• In GPRS, higher data rates are achieved by allocating a
user more than one GSM channel.
• A single GSM channel has a capacity of 22.8 kbit/s
• In practice, there are often constraints that limit the
number of GSM channels that can be allocated to one
user.
• EDGE (2.75 G technology) increases the data rate further
by using multilevel signaling.
• Multilevel signaling in EDGE enables 3 bits of data to be
transmitted in the time it takes to transmit 1 bit in GSM
and GPRS.
Time division multiple access - GPRS and EDGE
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• If the signaling medium could adopt four different states,
each state could represent a combination of 2 bits of data.
If the medium could adopt 16 states, then each one could
represent a combination of 4 bits of data (24 = 16)
• Each state in a signaling medium is referred to as a
symbol, and the duration of a symbol is the symbol
period.
Time division multiple access
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• The number of symbols per second is the symbol rate.
• Symbol rate = 1/ symbol period.
• Generally, using multilevel signalling does not increase the
bandwidth requirement of the channel carrying the signal.
• However, using many levels generally increases vulnerability
to noise compared with fewer states.
Time division multiple access
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• Additional refinement of EDGE: availability of extra
coding schemes compared with GSM and GPRS, with
different coding rates  Coding rate can be more carefully
matched to the radio-channel conditions
• In favorable circumstances EDGE can attain around 300 kbit/s!
• GSM (and GPRS and EDGE) works within a finite resource,
in the sense that, per cell, there are only so many GSM
channels available.
• What if demands exceed the capacity in a particular
location?
 Replace the cell with several smaller ones.
Time division multiple access
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• CDMA was an alternative air interface to TDMA in 2G
(mainly in the USA).
• In 3G, CDMA is used exclusively.
• One of the major advantages it offers is a less definite
limit on capacity.
• As more and more users operate in a cell, the
quality that each user experiences deteriorates
somewhat, but there is not the same absolute limit
on the number of channels that TDMA imposes.
Code Division Multiple Access (CDMA)
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• CDMA enables adjacent cells to operate on the same
frequency band, which offers many advantages,
including ‘soft’ handover from cell to cell, whereby an
existing connection does not need to be broken before a
new one is established with another cell.
• On the other hand, CDMA is complex and requires an
amount of processing power
• Could not easily be incorporated into handheld devices
when the GSM system was being developed.
• This is one reason why TDMA was adopted for GSM
• There are two main sorts of code in CDMA, namely
channelisation codes and scrambling codes.
Code Division Multiple Access (CDMA)
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• Suppose the base station
transmits simultaneously to two
users, A and B.
• If the data is sent unmodified
and simultaneously, then the
result is the bottom part of the
figure
• This is ordinary addition, not
binary addition
A and B send data simultaneously
CDMA: channelisation codes
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• Decoding the superposed signal (provided there are two sets
of data):
• If result = 2  A = 1 and B = 1
• If result = 0  A = 0 and B = 0
• BUT if result = 1  (A = 1 and B = 0) or (A = 0 and B = 1)
• The idea with CDMA is to encode the 1s and 0s in such a
way that the receiver can always undo the superposition of
coded data and get back to the original data streams.
 Different signals, intended for different recipients, can
be sent simultaneously, but retrieved independently at
the receiver
CDMA: channelisation codes
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• Coding creates separate channels of
communication  It makes the data streams
Orthogonal
• Orthogonality: when multiple signals are
conveyed through a shared medium, the signals
should not interfere with each other in a way that
prevents data recovery.
• TDMA, CDMA and OFDMA enable signals to remain
orthogonal (even when they are sharing the same
transmission medium)
CDMA: channelisation codes
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• The CDMA encoding consists of replacing the 1s
and 0s of a user’s binary data stream by patterns
of shorter chips prior to transmission.
• The chips, like binary data itself, can exist in
either of two states: represented as 1 and -1,
rather than 1 and 0.
• The pattern of chips used to encode the 1s and 0s
is different for each stream of data
• Individual users receive more than one stream of data,
and each stream would have its own code.
CDMA: channelisation codes
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Wherever there is a 1 in the bipolar data, it is replaced by
the chip sequence
Code chips for A
Data for A in binary form (a)
and bipolar form (b)
CDMA: channelisation codes
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Encoding of A’s data:
(a) binary data;
(b) bipolar data;
(c) is (b) encoded with A’s code
CDMA: channelisation codes
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• The transmission of A’s and B’s data is accomplished by
transmitting a superposition of the two encoded versions
Code chips for A
Code Chips for B
CDMA: channelisation codes
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• How to reconstruct A’s and B’s data from the superposed
version of their encoded data which arrives at A’s and B’s
receivers?
• By carrying out a mathematical operation
called correlation.
• A’s receiver ‘knows’ that the code used to encode the data
intended for A was 1, -1,1, -1.
• Using this code during correlation enables A’s data to be
extracted uncorrupted from the signal sent by the base station.
• the superposed set of chips received is 2, -2, 0, 0
• To extract A’s data, we multiply, in order, each chip in the
received signal by the corresponding chip in A’s code.
• Example: the first chip of the signal is 2. The first chip of A’s code
is 1. We multiply them and store the result; that is, 2 x 1= 2.
CDMA: channelisation codes
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• You can see that the result of the correlation process has
been to recover a bipolar form of A’s original data, scaled
by a factor of 4.
Recovered data (a) and original data (b)
CDMA: channelisation codes
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• Replacing bits by chips  Spreading the frequency
spectrum of the transmitted signal over a wider band of
frequencies.
• This gives enhanced immunity to certain types of noise,
which is known as processing gain.
• Processing gain is related to the number of chips used per
bit.
• If we had used 8-chip codes instead of 4-chip codes, the
processing gain would have been doubled.
• Processing gain accounts for the robustness of CDMA
in the presence of noise.
CDMA: channelisation codes
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• The codes create orthogonal channels for users A and B
 As if the data had been transmitted through dedicated
channels
• Codes such as those used above are generally referred to as
channelization codes in CDMA.
• In the downlink in CDMA, channelization codes are used to
define independent channels of communication to different
users.
• In the uplink, the scrambling code distinguishes users 
Channelisation codes are nevertheless still needed in the
uplink.
CDMA: channelisation codes
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• For CDMA to work as a practical system, we
require not only that individual users be able to
extract their data, but also that, if no data is
transmitted, none should be recovered by the
correlation process.
CDMA: channelisation codes
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• Suppose we have a user C who has been assigned the
code: 1,1,-1, -1
• There is no data for C in the encoded stream
• What happens when the correlation process is carried out
using this code on the stream in which there is only data
for A and B?
• In the first period, A’s and B’s superposed data consisted
of the chip sequence 2, -2, 0, 0. Carrying out correlation
using C’s chip sequence on this interval gives
• (2 x1) + (-2 x1) + (0 x -1) + (0 x -1) = 0
 C has no data in the transmitted chips, and correlation
yields no data for C.
CDMA: channelisation codes
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• All the channelization codes in CDMA must be
mutually orthogonal.
• Two codes are said to be orthogonal  their
correlation result should be ZERO
• Why orthogonality?
• It is the orthogonality of these codes that ensures that
streams of data can be recovered from the composite
signal uncontaminated by the other streams of data also
present in the composite signal.
• Orthogonality also ensures that correlation yields
nothing when there is no relevant contribution to the
composite signal
• Walsh codes: set of mutually orthogonal codes
Orthogonality of channelization codes
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• In practice, the number of chips used in W-CDMA varies from 4 to
256 (and up to 512 for downlink only).
• The maximum number of chips is also the maximum number of
codes available in practice.
• Thus, for the uplink there is a maximum of 256 parallel codes available
per user and for the downlink there is a maximum of 512 parallel codes
available per cell.
• The number of chips in a Walsh code is always an integer power of 2.
• The codes are allocated to users by the RNC and allocated according
to the desired data rate.
• Shorter codes are associated with higher data rates because
• They use fewer chips to represent a bit of data, and
• The chip rate is constant for all codes.
CDMA: channelisation codes
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• Walsh codes can lose their orthogonality if they
become unsynchronized.
• Example: if the codes for B and C were put out of
synchronism by an amount equal to the duration of 1 chip.
• The orthogonality of B and C depends on the synchronization
of the codes.
CDMA: channelisation codes - Synchronisation
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• Synchronization between data streams cannot always
be guaranteed.
• It can be achieved in the downlink from the base station,
since individual data streams originate from the same place
• Each user’s equipment needs to be synchronized with the
data stream as it arrives, so that correlation can be properly
carried out by the user’s equipment.
• Using synchronisation channels (designated SCH) in
the transmission from the base station, the receiving equipment
is able to interpret these synchronization channels without
initially being synchronised to them
CDMA: channelisation codes – Synchronization
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• Channelisation codes have the property that even codes of
different length retain their orthogonality.
• 4-chip code will be orthogonal to a 64-chip code (if
synchronized).
• Restrictions apply when long and short codes coexist.
• The particular long code chosen would restrict the allowed
shorter codes that could be used at the same time.
• only certain short codes are orthogonal to certain long
codes.
CDMA: channelisation codes – Synchronisation
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• Replacing each bit by several chips increases the
bandwidth of the signal carrying the data.
• The signal energy in the modulated wave is spread over a
wider span of frequencies.
• The signal now makes more fluctuations in a given length
of time.
• CDMA, therefore, is an example of spread spectrum
transmission.
• Spreading the spectrum of a signal increases its resilience
to noise.
• The degree of spectrum spreading can be quantified via
the spreading factor.
Spreading factor, frames and data rate
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• The spreading factor is equal to the number of chips used to
represent a single bit of data in the coding process.
• The spreading factor is numerically equal to the processing
gain.
• In UMTS, the chip rate is fixed at 3.84 Mchip/s.
• In the commonest variant of UMTS, the downlink occupies
5MHz and the uplink occupies 5MHz
• As in GSM, data is chunked into frames at the physical level.
• In UMTS, the frames are 10ms long.
 there are 100 frames per second,
 number of chips per frame is (3.84*106)/100 = 38 400 chips.
Spreading factor, frames and data rate
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• If a spreading factor of 4 is used, then a group of 4 chips
represents a single bit of data.
Spreading factor, frames and data rate
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• Larger spreading factors there are fewer bits per frame
and, hence, lower data rates.
• A frame is subdivided into 15 equal-length slots
• The length of a single slot, 2560 chips, comfortably
encompasses even the longest spreading codes, which are
512 chips long, and does so an integer number of times (5
times).
• A slot, therefore, always contains a whole number of
encoded bits of data.
• Spreading factor is sometimes expressed in terms of chip
rate and data rate as follows:
Spreading factor, frames and data rate
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• Channelization codes are also named : orthogonal
variable spreading factor (OVSF) codes
• Variable because the possibility of varying the spreading
factor
• The length of spreading code used cannot be changed
within a frame
• In voice communication, the spreading factor is usually
much higher than in data.
• The much greater spreading factor (and, hence, processing
gain) in voice communication gives much greater resilience
to noise .
Spreading factor, frames and data rate
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• CDMA uses orthogonal channelisation codes to keep
streams of data separate.
• Orthogonality of channel-coded data depends on
synchronisation, and synchronisation cannot always be
guaranteed  there is a need for additional coding
process!
• The additional coding process, in both the uplink and the
downlink, uses Scrambling codes
• Scrambling codes are different from the channelisation
codes, although, like channelisation codes, they are used
to create orthogonal (or almost orthogonal) streams of
data.
CDMA: Scrambling codes - Identifying the source
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Downlink
Uplink
• Adjacent cells use the same
frequency in UMTS
• users can receive signals from
more than one Node B.
• channelisation codes are not
unique to a given source, the
Node Bs cannot reliably
determine which user a signal
arrives from.
Why the scrambling codes?
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• Scrambling codes uniquely identify sources of signals in
either direction
• User equipment can distinguish one Node B from another
through the Node B’s different scrambling codes;
• A Node B can distinguish one piece of user equipment
from another through the different scrambling codes
allocated to user equipment.
• Scrambling codes in UMTS can be either 256 chips long
or 38 400 chips long (number of chips in a frame).
• Scrambling codes have a pseudo-random character that
makes them look like noise  known as pseudo-noise
codes or pseudo-random codes.
CDMA: Scrambling codes - Identifying the source
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• Because encoding does not
increase the number of chips,
scrambling codes are not
spreading codes.
• At the receiver, scramblingencoded data is decoded
through the process of
correlation.
• Received chip sequences are
multiplied by the scrambling
code.
• The scrambling code needs to
be in the right relationship with
the received chips for this to
work.
CDMA: Scrambling codes - Identifying the source
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• Channelization codes are used in the uplink prior
to the application of the scrambling code.
• Channelization codes spread the signal bandwidth,
which gives processing gain.
• The need to distinguish between different sorts of data
transmitted by user equipment: traffic and control data
• In the uplink direction, control data and traffic are
differentiated by being assigned to different
channels, each with its own channelization code.
• In the downlink direction, the two sorts of data are
time-division multiplexed within the same
channel.
CDMA: Scrambling codes - Identifying the source
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• In Figure 3.22, at the left, we have A’s and B’s data which
are to be transmitted by the imaginary communication
system. Data is represented as Xs and Os because its form
is not important.
CDMA: Scrambling codes – Properties
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• A’s and B’s data are encoded with the respective Walsh
codes (that is, channelisation codes).
• This new form is represented with coloured stripes.
• These two encoded versions are synchronised.
• Because a shared medium is used, they are superposed, or
added.
• At the receiver, correlation of the received signal with A’s
code (which has to be synchronised with the encoded
data) leads, theoretically, to perfect recovery of A’s data.
• B’s data could also be recovered perfectly using B’s
code.
CDMA: scrambling codes – Properties
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Synchronized
streams with
walsh codes used
Nonsynchronized
streams with
walsh codes
used
CDMA: Scrambling codes – Properties
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• Gold codes: a set of scrambling codes named after the
person who devised them.
CDMA: scrambling codes – Properties
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CDMA: scrambling codes – Properties
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CDMA: scrambling codes – Properties
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• Synchronisation achieves two main outcomes:
• Enables receiving devices to work out where the frame
boundary is; that is, when frames start
• Enables a receiving device to work out the scrambling code of
the Node B it is receiving a signal from.
• Synchronisation takes place using two special
synchronisation channels transmitted from the Node B:
• Primary synchronisation channel (P-SCH)
• Secondary synchronisation channel (S-SCH)
• Each of these transmits fixed data, using a channelisation
code but no scrambling code.
Synchronisation of user equipment to Node B
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• The primary synchronisation channel transmits a
fixed data word repeatedly once per slot
• Each frame contains an identical set of slots
• In every cell the Node B transmits the same code word in
the primary synchronisation channel
Synchronisation of user equipment to Node B
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1. When the user equipment is switched on, it scans the
radio channels in the 3G band, based on a stored list of
preferred frequencies.
• Priority is given to the user equipment’s home network;
that is, the one with which the user is registered as a
customer.
2. Once the user equipment has found a frequency on
which transmissions are taking place, it looks for the
primary synchronisation channel and uses the fact that
the fixed data word is transmitted once per slot to align
itself to the slot boundary.
Synchronisation of user equipment to Node B
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3. The UE then looks for the secondary synchronisation channel.
• The secondary synchronisation channel transmits a sequence
which occupies a whole frame.
• The sequence is repeated in all frames.
• Within a frame, the sequence is divided into portions across the 15
slots.
• Unlike the case with the primary synchronisation code, the
sequence in the secondary synchronisation channel varies from cell
to cell. However, there are only 64 possible sequences, so the user
equipment is able to work out which sequence is in use.
• Needs to know where the slot boundaries are, (discovered from the
primary synchronisation channel.)
• With the sequence established, the location of the frame boundary is
known.
Synchronisation of user equipment to Node B
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• One of the objectives of 3G was to achieve higher data
rates than were possible in 2G and its derivatives, such as
GPRS and EDGE.
• The various 3G systems have themselves given rise to
derivatives designed to give higher data rates than
were achievable with the original specifications.
• The most notable of these are
• HSDPA(high-speed downlink packet access)
• HSUPA (high-speed uplink packet access)
Achieving high data rates in UMTS
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• In the pursuit of higher wireless data rates, the trend has
been towards the use of wider bandwidths of
electromagnetic spectrum.
• Provided spectrum is being used efficiently, there is not
much alternative to using more spectrum if the data rate is to
be increased significantly.
• CDMA tends to have difficulties with wide bandwidths (up
to 20 MHz envisaged ‘beyond 3G’)
• Interest has focused on OFDMA, which holds out the
prospect of less computationally complex implementations.
• OFDMA is already used in the 802.11g Wi-Fi standard, which
uses channel widths of 20 MHz.
OFDMA
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• OFDMA is based on orthogonal FDM (OFDM).
• The essence of OFDM is the simultaneous use of many
wireless carrier signals, known as subcarriers, each with a
different frequency, and with the frequencies chosen in a
way that gives particular properties.
• Separate subcarriers, or groups of subcarriers, can be
allocated to different streams of data, giving a type of
FDM.
• However, the way the frequencies are chosen in OFDM
differentiates it from ‘ordinary’ FDM.
OFDMA
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• Sometimes in OFDM all the subcarriers are allocated
to a single data stream.
• This can be advantageous over the use of a single
carrier.
• You can probably appreciate that
if, instead of having just a single
carrier at 1GHz, you also had
additional carriers at 1.1 and
1.2GHz, and could use them
simultaneously, you could
enhance the rate of transmission
of data by spreading the data
across three channels rather than
restricting it to one.
OFDMA
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• Modulation: is the process
of modifying a radio wave to
enable it to carry data.
• The radio wave which is
modulated is called the
carrier wave.
• Modulation methods based
on changes of amplitude can
be problematic because the
circuitry that generates and
transmits varying-amplitude
carrier waves tends to be
inefficient .
Modulation and Symbols
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• There are benefits in using multilevel signalling, as
more data can be conveyed per symbol.
• In the context of modulation, this means modulating the
carrier in a way that gives more than two discernible
states.
• The more states that there are, the more bits can be
represented by each state.
Symbols and multilevel modulation
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• Sine and Cosine Orthogonality
• Binary phase-shift keying is sometimes regarded as a special type
of amplitude modulation (amplitude changes by a factor of -1)
• This way of thinking about amplitude change is useful in
connection with quadrature amplitude modulation (QAM)
techniques.
Quadrature methods and quadrature amplitude
modulation
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• Two sine waves different in phase by 90º are orthogonal,
where as sine waves different in phase by 180º are not
orthogonal.
• sine waves and cosine waves are orthogonal  changes of
amplitude or phase on a sine wave do not affect the amplitude
and phase states of a cosine wave that is superposed on it; and
vice versa.
Quadrature methods and quadrature amplitude
modulation
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Quadrature methods and quadrature amplitude
modulation
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• The orthogonality of sine and cosine waves means that
they can separately represent data.
• The data can be extracted by correlation.
• Hence, in quadrature modulation methods we transmit a
superposition of a modulated sine wave carrier and a
modulated cosine carrier.
• At the receiver, correlation could consist of multiplying
the received superposition of carriers by a synchronised
sine wave and by a synchronised cosine carrier wave
(Similar to correlation in CDMA channelization codes).
Quadrature methods and quadrature amplitude
modulation
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• The I and Q axes represent the orthogonal carrier
waves, and graduations on these axes represent available
amplitudes.
• One wave is referred to as the ‘in-phase’ (I) component
and the other as the ‘quadrature’ (Q) component.
• Putting the axes at right angles indicates the
orthogonality of these carriers.
• This modulation method is known as quadrature phaseshift keying (QPSK).
4-QAM constellation diagram
Constellation diagrams
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• In practice, the four blobs in QPSK are usually rotated
relative to the axes
• Each blob now represents a particular combination of “I”
and “Q” signal states.
• Sometimes, rather than absolute “I” and “Q” states, the
blobs represent changes of state data is conveyed by a
change of symbol, rather than by a symbol itself.
Constellation diagrams
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• All the blobs are equidistant from the origin (the place
where the axes cross) there is no change of peak-to-peak
amplitude in the I and Q waves .
• Because this modulation scheme does not cause the
inefficiencies associated with changes of signal
amplitude, a form of QPSK is used in the uplink in
UMTS.
Constellation diagrams
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• One of the drawbacks of 16-QAM (and 64-QAM) is that
the relative closeness of the constellation points increases
the likelihood of one point being mistaken for another
when the symbols are subject to noise interference (as
they usually are in radio transmission).
• Hence, 64-QAM and 16-QAM work best in areas of
relatively strong signal, and relatively low noise.
• When conditions are not favorable to multiple-state
modulation methods, transmitters tend to fall back on
ones with fewer states that are more resistant to noise.
• For instance, in a noisy environment, 64-QAM usually
falls back to 16-QAM, and then to QPSK.
Constellation diagrams
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• How many additional bits of data are represented by each blob
in 16-QAM compared with
• (a) QPSK?
• (b) BPSK?
Sol:
• (a) In 16-QAM there are 16 blobs, or states. Hence, each blob
can represent 4bits of data(2^4 = 16). In QPSK there are four
blobs; hence, each blob represents 2 bits of data (2^2 = 4).
Thus, each blob in 16 QAM carries two more bits of data than
does each blob in QPSK.
• (b) In BPSK there are only two blobs, so each blob represents
1 bit of data. Hence, each blob in 16-QAM represents three
more bits of data than does each blob in BPSK.
Constellation diagrams - Activity
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