Wireless Communications
Module 4:
Diversity, Equalization, and Multiple Access
Syllabus
• 4.1 Diversity (3 hours): Receiver diversity – selection
combining, maximal ratio combining. Transmitter diversity –
Alamouti scheme for 2x2 MIMO.
• 4.2 Equalization (3): Equalization – Linear and non-linear
equalization, Zero forcing, MMSE equalizers. LMS algorithm.
Adaptive Equalization.
• 4.3 Multiuser Systems (2): Uplink and Downlink, Multiple
Access, Frequency-Division Multiple Access (FDMA),
Time-Division Multiple Access (TDMA), Code-Division Multiple
Access (CDMA), Orthogonal Frequency-Division Multiple
Access (OFDMA).
Equalization
• Equalization defines any signal processing
technique used at the receiver to alleviate the
Inter Symbol Interference problem caused by
delay spread.
• Signal processing can also be used at the
transmitter to make the signal less susceptible
to delay spread.
• It may be a simple linear filter or a complex
algorithm.
Equalization
Equalization
• Equalizer design must typically balance ISI
mitigation with noise enhancement, since both
the signal and the noise pass through the
equalizer, which can increase the noise power.
• Nonlinear equalizers suffer less from noise
enhancement than linear equalizers but
typically entail higher complexity
• Moreover, equalizers require an estimate of the
channel impulse or frequency response to
mitigate the resulting ISI.
Equalization
• Since the wireless channel varies over time, the
equalizer must learn the frequency or impulse
response of the channel (training) and then
update its estimate of the frequency response
as the channel changes (tracking).
• The process of equalizer training and tracking
is often referred to as adaptive equalization,
since the equalizer adapts to the changing
channel.
Equalization
• Equalizers are typically implemented digitally.
• Figure 11.3 is a block diagram of an equivalent
lowpass end-to-end system with a digital
equalizer.
• The input symbol dk is passed through a
pulse-shaping filter g(t) and then transmitted
over the ISI channel with equivalent lowpass
impulse response c(t).
Equalization
Equalization
• We define the combined channel impulse
response
• and the equivalent lowpass transmitted signal is
thus given by
• The pulse shape g(t) improves the spectral
properties of the transmitted signal & the
channel c(t) is introduced by transmission
medium.
Equalization
• At the receiver front end, equivalent lowpass
white Gaussian noise n(t) with PSD N0 is
added to the received signal for a resulting
signal w(t).
• This signal is passed through an analog
matched filter gm∗ (-t) to obtain the equivalent
lowpass output y(t), which is then sampled via
an A/ D converter.
• The purpose of the matched filter is to
maximize the SNR of the signal before
sampling and subsequent processing.
Equalization
• The equalizer output provides an estimate of
the transmitted symbol.
• This estimate is then passed through a decision
device that rounds the equalizer output to a
symbol in the alphabet of possible transmitted
symbols.
Equalization
• During training, the equalizer output is passed
to the tap update algorithm to update the tap
values, so that the equalizer output closely
matches the known training sequence.
• During tracking, the round-off error associated
with the symbol decision is used to adjust the
equalizer coefficients.
LMS Algorithm
LMS Algorithm
Multiuser Systems
• In multiuser systems the system resources
must be divided among multiple users.
• In order to support multiple users, the signal
space dimensions of a multiuser system must
be allocated to the different users.
• Allocation of signaling dimensions to specific
users is called multiple access.
• Multiple access methods are applied to the two
basic multiuser channels: downlink channels
and uplink channels.
Multiuser Channels: The Uplink and Downlink
• A “multiuser” channel is any channel that must
be shared among multiple users.
• There are two different types of multiuser
channels, the uplink channel and the downlink
channel.
Multiuser Channels: The Uplink and Downlink
Multiuser Channels: The Uplink and Downlink
• An uplink channel, also called a multiple
access channel or reverse channel, has many
transmitters sending signals to one receiver,
where each signal must be within the total
system bandwidth B.
• However, in contrast to the downlink, in the
uplink each user has an individual power
constraint Pk associated with its transmitted
signal sk(t).
FDMA
FDMA
• In FDMA the system signaling dimensions are
divided along the frequency axis into
nonoverlapping channels, and each user is
assigned a different frequency channel;
• The channels often have guard bands between
them to compensate for imperfect filters,
adjacent channel interference, and spectral
spreading due to Doppler.
Time-Division Multiple Access (TDMA)
• In TDMA, the system dimensions are divided
along the time axis into nonoverlapping
channels, and each user is assigned a different
cyclically repeating timeslot.
• These TDMA channels occupy the entire
system bandwidth, which is typically wideband,
so some form of ISI mitigation is required.
• The cyclically repeating timeslots imply that
transmission is not continuous for any user.
Time-Division Multiple Access (TDMA)
Code Division Multiple Access
Code Division Multiple Access
• 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(symbol) rate
which has orders of magnitudes greater than
the data rate of the message.
Code Division Multiple Access
Code Division Multiple Access
• Each user has its own pseudorandom codeword
which is approximately orthogonal to all other
codewords.
• The receiver performs a time correlation operation
to detect only the specific desired codeword. All
other codewords appear as noise due to
decorrelation.
• For detection of the message signal, the receiver
needs to know the codeword used by the transmitter.
Code Division Multiple Access
▪ To compat near—far problem, power control
is used in most CDMA implementations.
▪ Multipath fading is reduced.
▪ Soft handoff is performed by MSC
▪ If spreading sequences of different users are
not exactly orthogonal then self jamming
occurs
