1. Multiple radio access schemes for wireless networks are primarily used
for exchanging control information between a BS and a MS.
Users can also receive signals transmitted by other users in the system. In
fact, many users access the traffic channels when the reverse (uplink)
path from MS to BS is to be established.
There are three basic ways to have many channels within an allocated
bandwidth: frequency, time, or code. They are addressed by three
multiple division techniques :
(FDMA) - frequency division multiple access.
(TDMA) - time division multiple access.
(CDMA) - code division multiple access.
Two other variants known as :
(OFDM) - orthogonal frequency division multiplexing .
(SDMA) - space division multiple access.
Multiple Division Techniques
for Traffic Channels
2. A MS must distinguish which signal is meant for itself among many signals being
transmitted by other users or BSs, and the BS should be able to recognize the
signal sent by a particular user.
Multiple-access techniques are based on the orthogonalization of signals.
If a system employs different carrier frequencies to transmit the signal for each
user, it is called a FDMA system.
If a system uses distinct time slots to transmit the signal for different users, it is a
TDMA system.
If a system uses different code to transmit the signal for each user, it is a CDMA
system.
To provide simultaneous two-way communications (duplex communications), a
forward channel (downlink) from the BS to the MS and a reverse channel
(uplink) from the MS to the BS are necessary.
Two types of duplex systems are utilized:
frequency division duplexing (FDD) divides the frequency used.
time division duplexing (TDD) divides the same frequency by time.
FDMA mainly uses FDD, while TDMA and CDMA systems use either FDD or TDD.
A number of channels can be simultaneously used to transfer data at a much
higher rate, and such an effective technique is known as OFDM.
Concepts and Models for Multiple
Divisions
3. FDMA is a multiple-access system that has been widely adopted in
existing analog systems for portable and automobile wireless telephones.
The BS dynamically assigns a different carrier frequency to each active
user (MS).
A frequency synthesizer is used to adjust and
maintain the transmission and reception
frequencies.
There is a pair of channels for the communication between the BS and the
MS. The paired channels are called forward channel (downlink) and
reverse channel (uplink). Different frequency bandwidths are assigned to
different users. This implies that there is no frequency overlapping
between the forward and reverse channels.
A protecting bandwidth is used between the forward and reverse
channels, and a guard band Wg between two adjacent channels is used to
minimize adjacent channel interference between them. The frequency
bandwidth for each user is called sub-band Wc. If there are N channels in a
FDMA system, the total bandwidth is equal to N · Wc.
FDMA
5. TDMA splits a single carrier wave into several time slots and distributes the slots
among multiple users, the communication channels essentially consist of many
units, i.e., time slots, over a time cycle, which makes it possible for one frequency
to be efficiently utilized by multiple users, given that each utilizes a different time
slot.
This system is widely used in the field of digital portable and automobile
telephones and mobile satellite communication systems.
A TDMA system may be in either of two modes: FDD (in which the
forward/reverse or uplink/downlink communication frequencies differ) and TDD
(in which the forward/reverse communication frequencies are the same).
For a TDMA system, there is guard time between the slots so that interference
due to propagation delays along different paths can be minimized.
A wideband TDMA enables high-speed digital transmissions, in which selective
frequency fading due to the use of multiple paths can become a problem. This
requires that bandwidth be limited to an extent such that selective fading can be
overcome, or appropriate measures such as adaptive equalization techniques
could be adopted for improvement. A high-precision synchronization circuit also
becomes necessary on the MS side to carry out intermittent burst signal
transmission.
TDMA
8. In a CDMA system, different spread-spectrum codes are selected and assigned to
each user, and multiple users share the same frequency.
A CDMA system is based on spectrum-spread technology, which makes it less
susceptible to the noise and interference by substantially spreading over the
bandwidth range of the modulated signal.
CDMA system, received signals at the BS from a far away MS could be masked by
signals from a close-by MS in the reverse channel.
A CDMA system is usually quantified by the chip rate, which is defined as the
number of bits changed per second.
Two basic types of CDMA implementation methodologies:
1) Direct sequence (DS), and 2) frequency hopping (FH). a FH method, a
pseudorandom sequence is used to change the radio signal frequency across a
broad frequency band in a random fashion.
CDMA
9. Spread Spectrum : is a transmission technique wherein data occupy
a larger bandwidth than necessary. Bandwidth spreading is
accomplished before transmission through the use of a code that is
independent of the transmitted data. The same code is used to
demodulate the data at the receiving end.
Originally designed for military use to avoid jamming (interference
created intentionally to make a communication channel unusable),
spread spectrum modulation is now also used in personal
communication systems due to its superior performance in an
interference dominated environment.
CDMA
10. Direct Sequence Spread Spectrum (DSSS) : the radio signal is multiplied by a
pseudorandom sequence whose bandwidth is much greater than that of the
signal itself, thereby spreading its bandwidth.
a pseudorandom sequence directly phase modulates a (data-modulated) carrier,
thereby increasing the bandwidth of the transmission and lowering the spectral
power density (i.e., the power level at any given frequency). The resulting RF
signal has a noise like spectrum and in fact can be intentionally made to look like
noise to all but the intended radio receiver. The received signal is despread by
correlating it with a local pseudorandom sequence identical to and in
synchronization with the sequence used to spread the carrier at the radio
transmitting end.
CDMA
11. Frequency Hopping Spread Spectrum (FHSS) : A spread spectrum
modulation technique implies that the radio transmitter frequency hops from
channel to channel in a predetermined but pseudorandom manner.
The RF signal is dehopped at the receiver end using a frequency synthesizer
controlled by a pseudorandom sequence generator synchronized to the
transmitter’s pseudorandom sequence generator.
A frequency hopper may be fast hopped, where there are multiple hops per data
bit, or slow hopped, where there are multiple data bits per hop.
Multiple simultaneous transmission from several users is possible using FH, as
long as each uses different frequency hopping sequences and none of them
“collides” (no more than one unit using the same band) at any given instant of
time.
CDMA
12. Walsh Codes : In CDMA, each user is assigned one or many
orthogonal waveforms derived from one orthogonal code. Since
the waveforms are orthogonal, users with different codes do not
interfere with each other.
CDMA requires synchronization among the users, since the
waveforms are orthogonal only if they are aligned in time. An
important set of orthogonal codes is the Walsh set . Walsh
functions are generated using an iterative process of constructing a
Hadamard matrix
starting with H0 = [0].
The Hadamard matrix
is built by using the
function :
CDMA
13. Near-Far Problem : The near-far problem stems from a wide range
of signal levels received in wireless and mobile communication
systems.
Out-of-band radiation of the signal from the MS1 interferes with the
signal from the MS2 in the adjacent channel. This effect, called
adjacent channel interference, becomes serious when the
difference in the received signal strength is high. For this reason,
the out-of-band radiation must be kept small.
The tolerable relative adjacent channel interference level can be
different depending on the system characteristics. If power control
technique is used, the system can tolerate higher relative adjacent
channel interference levels. The near-far problem becomes more
important for CDMA systems where spread spectrum signals are
multiplexed on the same frequency using low cross correlation
codes.
CDMA
15. Power Control : Power control is simply the technique of
controlling the transmit power in the traffic channel so as to affect
the received power and hence the CIR.
While power control can often be effective for traffic channels,
there are some disadvantages :
a) Since battery power at a MS is a limited resource that needs to be
conserved, it may not be possible or desirable to set transmission
powers to higher values.
b) Second, increasing the transmitted power on one channel,
irrespective of the power levels used on other channels, can
cause inequality of transmission over other channels.
c) As a result, there is also the possibility that a set of connections
using a pure power control scheme can suffer from unstable
behavior, requiring increasingly higher transmission powers.
d) Finally, power control techniques are restricted by the physical
limitations on the transmitter power levels.
CDMA
16. The basic strategy in OFDM is to split high-rate radio channels into
multiple lower rate sub-channels that are then simultaneously
transmitted over multiple orthogonal carrier frequencies.
The transmitter of OFDM converts high-speed data streams into n
parallel low-speed bit streams, which are then modulated and
mixed with inverse discrete Fourier transform (IDFT); then guard
time is inserted to reduce inter-symbol interferences (ISI). The
reverse actions are taken at the receiver side.
In all these systems, the information is first modulated before
being transmitted over a channel.
Figure 7.21 illustrates the modulation operation of the OFDM
transmitter.
Figure 7.22 shows the demodulation steps of the OFDM receiver,
with explicit use of the discrete Fourier transform (DFT).
OFDM
18. In SDMA, the omni-directional communication space is divided into
spatially separable sectors. This is possible by having a BS use smart
antennas, allowing multiple MSs to use the same channel simultaneously.
The communication characterized by time slot, carrier frequency, or
spreading code can be used as shown in Figure 7.23.
Use of a smart antenna maximizes the antenna gain in the desired
direction, and directing antenna gain in a particular direction leads to
range extension, which reduces the number of cells required to cover a
given area. Moreover, such focused transmission reduces the
interference from undesired directions by placing minimum radiation
patterns in the direction of interferers.
As the BS forms different beams for each spatially separable MS on the
forward and reverse channels, noise and interference for each MS and BS
is minimized. This enhances the quality of the communication link
significantly and increases overall system capacity. Also, by creating
separate spatial channels in each cell intra-cell reuse of conventional
channels can be easily exploited. Currently, this technology is still being
explored and its future looks quite promising.
SDMA
21. AM : Amplitude modulation (AM) is the first method ever used to transfer
voice information from one place to another. The amplitude of a carrier
signal with a constant frequency is as varied as the information signal
required to transmit.
The total power of the transmitted wave varies in amplitude in
accordance with the power of the modulating signal.
The bandwidth of an AM scheme—that is, the amount of space that it
occupies in the Fourier domain—is twice that of the modulating signal.
This double sideband nature of AM halves the number of independent
signals that can be sent using a given range of transmission frequencies.
By suppressing one sideband before transmission, single sideband (SSB)
modulation doubles the number of transmissions that can fit into a given
transmission band.
At the receiver end, the carrier signal is filtered out, rebuilding the
information signal (speech, data, etc.). When a carrier is amplitude
modulated with a pure sine wave, up to one-third (33.3%) of the overall
signal power is contained in the sidebands.
The other two-thirds of the signal power are contained in the carrier,
which does not contribute to the transfer of data. This makes AM an
inefficient mode of communication.
Modulation Techniques
23. FM : Frequency modulation (FM) is a method of integrating the
information signal with an alternating current (ac) wave by varying
the instantaneous frequency of the wave.
The carrier is stretched or squeezed by the information signal, and
the frequency of the carrier is changed according to the value of
the modulating voltage.
In FM, the total wave power does not change when the frequency
alters. To recover the signal, the receiver rebuilds the information
wave by checking how the known carrier signal has modified the
information.
An FM system provides a better signal-to-noise ratio (SNR) than an
AM system, which implies that it has less noise content. Another
advantage is that it needs less radiated power. However, it does
require a larger bandwidth than AM.
Modulation Techniques ( FM )
25. FSK : Frequency shift keying (FSK) is used for modulating a digital
signal over two carriers by using a different frequency for a “1” or a
“0”. The difference between the carriers is known as the frequency
shift.
One obvious way to generate a FSK signal is to switch between two
independent oscillators according to whether the data bit is a “1”
or a “0.” This type of FSK is called discontinuous FSK since the
waveform generated is discontinuous at the switching time.
The phase discontinuity poses several problems, such as spectral
spreading and spurious transmissions. A common method of
generating an FSK signal is to frequency modulate a single-carrier
oscillator using the message waveform.
This type of modulation is similar to FM generation, except that the
modulating signal is in binary.
FSK has high signal-to-noise ratio (SNR) but low spectral efficiency.
It was used in all early low bit-rate modems.
Modulation Techniques ( FSK )
27. PSK : Phase shift keying (PSK) is a method of transmitting and
receiving digital signals in which the phase of a transmitted signal is
varied to convey information.
In digital transmission, the phase of the carrier is discretely varied
with respect to a reference phase and according to the data being
transmitted.
when encoding, the phase shift could be 0◦ for encoding a “0” and
180◦ for encoding a “1,” thus making the representations for “0”
and “1” apart by a total of 180◦.
This kind of PSK is also called binary phase shift keying (BPSK) since
1 bit is transmitted in a single modulation symbol.
PSK has a perfect SNR but must be demodulated synchronously,
which means a reference carrier signal is required to be received at
the receiver to compare with the phase of the received signal,
which makes the demodulation circuit complex.
Modulation Techniques ( PSK )
29. QPSK : Quadrature phase shift keying (QPSK) takes the concept of
PSK a step further as it assumes that the number of phase shifts is
not limited to only two states.
The transmitted carrier can undergo any number of phase changes.
This is indeed the case in quadrature phase shift keying. With QPSK,
the carrier undergoes four changes in phase and can thus represent
four binary bit patterns of data, effectively doubling the bandwidth
of the carrier. The following are the phase shifts with the four
different combinations of input bits .
Normally, QPSK is implemented using I/Q modulation with I (in-
phase) and Q (quadrature) signals summarized with respect to the
same reference carrier signal (in other words, from the same local
oscillator). A 90◦ phase offset is placed in one of the carriers.
Modulation Techniques ( QPSK )
30. We can consider each of the two binary sequences to be a BPSK
signal. The two binary sequences are separately modulated by the
two quadrate signals. The summation of the two modulated
waveforms is the QPSK waveform, and the phase shift also has four
states corresponding to every two adjacent input bits. Figure 7.29
shows the constellations of BPSK and QPSK.
Modulation Techniques ( QPSK )
Figure 7.29
Signal constellations of
BPSK and QPSK.
31. π/4QPSK : In π/4QPSK, the input sequence is encoded by the
changes in the amplitude and direction of the phase shift and not in
the absolute position in the constellation.
In QPSK and BPSK, the input sequence is encoded in the absolute
position in the constellation.
π/4QPSK uses two QPSK constellations offset by ±π/4. Signaling
elements are selected in turn from the two QPSK constellations.
Transitions must occur from one constellation to the other one.
This ensures that there will always be a phase change for each
symbol. Therefore, π/4QPSK can be non-coherently demodulated,
which simplifies the design of the demodulator.
π/4QPSK is popular in most second-generation systems, such as
North American Digital Cellular (IS-54) and Japanese Digital Cellular
(JDC).
Modulation Techniques ( π/4QPSK )
33. QAM : Quadrature amplitude modulation (QAM) is simply a combination
of AM and PSK, in which two carriers out of phase by 90◦ are amplitude
modulated. If the baud rate is 1200 Hz, 3 bits per baud, a signal can be
transmitted at 3600 bps. We modulate the signal by using two measures
of amplitude and four possible phase shifts. Combining the two, we have
eight possible waves (Table 7.2).
Mathematically, there is no limit
to the data rate that may be
Supported by a given baud rate in
a perfectly stable, noiseless
transmission environment.
In practice, the governing factors
are the amplitude (and,
consequently, phase) stability, and
the amount of noise present, in
both the terminal equipment and
the transmission medium
(carrier frequency, or communication channel) involved.
Modulation Techniques (QAM )
34. 16QAM : 16QAM involves splitting the signal into 12 different phases
and 3 different amplitudes for a total of 16 different possible
values, each encoding 4 bits.
16QAM is used in applications including microwave digital radio,
DVB-C (digital video broadcasting—cable), and modems. 16QAM or
other higher-order QAMs (64QAM, 256QAM) are more bandwidth
efficient than BPSK, QPSK, or 8PSK and are used to gain high-speed
transmission. However, there is a tradeoff, and the radio becomes
more complex and is more susceptible to errors caused by noise
and distortion.
Error rates of higher-order QAM systems degrade more rapidly
than QPSK as noise or interference is introduced. A measure of this
degradation would be a higher BER (Bit Error Rate).
Modulation Techniques (16QAM )
