Unit III : Basic Radio Propagation and Multiple Access Techniques

Que 1. Explain in brief Reflection, Diffraction and Scattering in wireless communication.

When a signal reaches the receiver from the transmitter in a single path, without suffering any reflections, diffractions, or scattering, this is known as propagation along the line-of sight (LOS)path. An LOS component has the shortest time delay among all the received signals and is usually the strongest signal received.

In non-line-of-sight (NLOS) propagation, a signal transmitted into a wireless medium reaches the receiver via one or more indirect paths, each having different attenuations and delays. When a transmitted signal travels through communication paths other than the LOS path to reach the receiver, it is said to have undergone NLOS propagation. NLOS propagation is responsible for coverage behind buildings and other obstructions. The main NLOS propagation mechanisms are reflection, scattering, and diffraction.

In a wireless communication system, a transmitted signal can reach the receiver via a number of propagation mechanisms are as follows

1. Reflections
2. Diffraction
3. Scattering

1. REFLECTION

It occurs when the EM wave strikes an object that has large dimensions as compared to the wavelength of the propagating wave and bounce back in the same medium.
Example: walls of building, clear ground.

If there is a large amount of reflection, it is possible for signal strength to become weak or to suffer from interference.

The multipath reflection using wide beam antennas is essential in mobile broadband communication. It is difficult to achieve single path reflection due to unavailability of object which can provide necessary angle of incidence to obtain desired propagation.

Reflection occurs when a wave hits the interface between two dissimilar media, so that all of or at least part of the wave front returns into the medium from which it originated.

Reflection creates multipath, which can degrade the strength and quality of the received signal as well as cause data corruption or cancel signals.

2. Diffraction

Diffraction occurs when the signal encounters an edge or a corner, whose size is larger than the wavelength of the signal, e.g., an edge of a wall.

It is also referred as bending. The loss to the EM signal due to this depends on frequency of EM wave and sharpness of the bend.

Diffraction occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities (edges). The waves that encounter the object, waves bend around the object, taking a longer and different path.

The secondary waves resulting from the obstructing surface are present throughout the space and even behind the obstacle, giving rise to a bending of waves around the obstacle, even when a line of-sight path does not exist between transmitter and receiver. At high frequencies, diffraction, like reflection depends on the geometry of the object, as well as the amplitude, phase, and polarisation of the incident wave at the point of diffraction.

Although the received field strength decreases rapidly as a receiver moves deeper into an obstructed (shadowed) region, the diffraction field still exists and often has sufficient signal strength to produce a useful signal. Diffraction is one of the main ways that it is possible to provide cellular coverage in cities with lower frequencies (less than 3GHz).

3. Scattering

If there were microscopic irregularities in the surface we would get diffuse instead of specular reflection. Radio wave scattering occurs when signals hit a rough irregular surface. The signals, instead of being reflected, gets scattered in many directions. Scattering can also be caused by dust, humidity, rain, vegetation etc. Scattering reduces the quality of received signals.

Scattering is when an RF wave encounters an object and scatters into multiple waves. If a signal has been scattered then the integrity and strength of the signal will suffer tremendously. Scattering is more unpredictable than others.

Que 2. Differentiate between Reflection, Diffraction and Scattering.

Phenomenon	Reflection	Diffraction	Scattering
Occurrence	Reflection occurs when light bounces off a surface	Diffraction is the spreading of light when it passes through a narrow opening or around an object.	Scattering occurs when signals hit a rough irregular surface
Cause by	Plane surface (Object that has large dimensions as compared to the wavelength of the propagating wave)	Sharp irregularities	microscopic irregularities in the surface
Example	walls of building, clear ground	an edge of a wall	dust, humidity, rain, vegetation etc.
Path	Depend upon angle of incident	Depend upon edge of an object	Depend upon irregularities of an object
Multipath propagation	Less	Moderate	Large
Signal Strength	More	Less	Lowest
Object size compared to incident signal wavelength	Large enough	Large	Small
Medium	take place in the same medium.	take place in the same medium.	take place in the same medium.
Signal losses	Less	Moderate	More
Predication of return signal after strike of an object	Possible	Less possibilities	Unpredictable

Que 3. Explain in brief fading in wireless communication.

1. Fading in wireless communication is defined as the fluctuation in the strength of the signal received at the receiver. These are basically unwanted variations introduced at the time when the signal propagates from an one end to another end by taking multiple paths.
2. In Wireless Communication, fading refers to the attenuation of the transmitted signal power due to various variables during wireless propagation. These variables can be atmospheric conditions such as rainfall and lightning, geographical position, time, radio frequency etc.
3. The channel between transmitter and receiver can also be time varying or fixed depending upon whether the transmitter/receiver are fixed or moving with respect to each other which can cause fading to occur.
4. This multipath propagation is nothing other than the outcome of several atmospheric reflections. So, when waves of a signal from an antenna transmitted at the same time then multipath propagation causes the reception of waves in different time spans at the receiver. The waves when propagate through different paths suffers variation in height and ionization density in the ionospheric region of the atmosphere.
5. We are also aware of the fact that the actually received signal is the vector sum of the waves received at that instant at the receiver. These received signals from different paths at the receiver either get added constructively or destructively. This causes the variation in the overall strength of the received signal is known as fading.
6. It is noteworthy that due to fading the strength of the signal shows variation from a few dBs to around 10 or 20 dB. This depends on the condition that whether fading is normal or severe. Fading is a gradual phenomenon and signal can be recovered. However, the fade-out is a phenomenon which is a condition of sudden ionospheric disturbance or ionospheric storms in sunspot cycles, etc. that leads to complete fading of the signal.

In wireless communication many factors generating Fading effect. Followings are the list of components (factors) of fading.

• Path Loss
• Fluctuation of the received signal power
• Fluctuations in signal phase
• Variations of Angle of arrival of received signal
• Reflection and diffractions from various object
• Received power variation created by multi path
• Frequency Shift (Doppler shift)

7. In above diagram, In Path loss is just gradually decreases in signal strength with increase in distance but not much of fluctuation. In the shadowing, some fluctuation of the signal strength but the fluctuation frequency over the distance is not that high. In Multipath, the fluctuations of signal strength and the fluctuation frequency is also pretty high. A real received signal is the result of summation of all the factors and it shows the effect of fading.

Fading has different types are as follows.

Que 4. Explain the concept of Frequency Division Multiple Access(FDMA).

1. FDMA is the process of dividing one channel or bandwidth into multiple individual bands each for use by a single user. Frequency division multiple access (FDMA) assigns individual channels to individual users. It can be seen from Figure 1 that each user is allocated a unique frequency band or channel.

2. These channels are assigned on demand to users who request service. During the period of the call, no other user can share the same channel. In FDD systems, the users are assigned a channel as a pair of frequencies; one frequency is used for the forward channel, while the other frequency is used for the reverse channel.

The features of FDMA are as follows:

• The FDMA channel carries only one phone circuit at a time.
• If an FDMA channel is not in use, then it sits idle and cannot be used by other users to increase or share capacity. It is essentially a wasted resource.
• After the assignment of a voice channel, the base station and the mobile transmit simultaneously and continuously.
• The bandwidths of FDMA channels are relatively narrow (30 kHz in AMPS) as each channel supports only one circuit per carrier. That is, FDMA is usually implemented in narrowband systems.
• The symbol time of a narrowband signal is large as compared to the average delay spread. This implies that the amount of inter-symbol interference is low and, thus, little or no equalization is required in FDMA narrowband systems.
• The complexity of FDMA mobile systems is lower when compared to TDMA systems, though this is changing as digital signal processing methods improve for TDMA.
• Since FDMA is a continuous transmission scheme, fewer bits are needed for overhead purposes (such as synchronization and framing bits) as compared to TDMA.
• FDMA systems have higher cell site system costs as compared to TDMA systems, because of the single channel per carrier design, and the need to use costly bandpass filters to eliminate spurious radiation at the base station.
• The FDMA mobile unit uses duplexers since both the transmitter and receiver operate at the same time. This results in an increase in the cost of FDMA subscriber units and base stations.
• FDMA requires tight RF filtering to minimize adjacent channel interference.

Advantages of FDMA: AS FDMA systems use low bit rates(large symbol time)compared to average delay spread, it offers the following advantages-

• Reduces the bit rate information and the use of efficient numerical codes increases the capacity
• It reduces the cost and lowers the inter symbol interference (ISI)
• Equalization is not necessary
• An FDMA system can be easily implemented. A system can be configured so that the improvements in terms of speech encoder and bit rate reduction may be easily incorporated.
• Since the transmission is continuous , less number of bits are required for synchronization and framing.

Disadvantages of FDMA: Although FDMA offers several advantages, it has a few drawbacks as well, which are listed below-

• It does not offer significantly from analog systems; improving the capacity depends on the signal-to-interference reduction, or a signal-to-noise ratio (SNR).
• The maximum flow rate per channel is fixed and small.
• Guard bands lead to a waste of capacity.
• Hardware implies narrow-band filters, which cannot be realized in VLSI and therefore increases the cost.
• Near-far problem: Signals closer to the receiver are received with less attenuation than signals farther away. Given the lack of complete orthogonality, the transmissions from the more remote mobile units may be more difficult to recover. Thus, power control techniques are very important in a CDMA system.

Que. Explain the concept of Time Division Multiple Access (TDMA).

Time division multiple access (TDMA) systems divide the radio spectrum into time slots, and in each slot only one user is allowed to either transmit or receive.

As shown in Figure, A particular user gets time slots which are non-continuous in nature. The nexttime slot is assigned to him/her only when the other users sharing the same spectrum have each done one burst of transmission at least.

Collection of time slots which are assigned to unique users is called as a Frame. In other words, a user gets only one time slot per frame to transmit. In TDMA/TDD, half of the time slots in the frame would be used for forward link and the other half would be used for reverse link. In TDMA/FDD systems, since two separate physical channels exist, transmit and receive frames are different and on different carrier frequencies.

In a TDMA frame, the preamble contains the address and synchronization information that both the base station and the subscribers use to identify each other.

Guard times are utilized to allow synchronization of the receivers between different slots and frames. Different TDMA standards have different TDMA frame structures.

TDMA Features:

1. TDMA systems divides the radio spectrum into time slots, and in each time slot only one user is allowed to either transmit or receive. TDMA effectively shares a single carrier frequency with several users.
2. Transmission for any user is non-continuous, bursty in nature. This reduces the battery consumption if the subscriber’s transmitter can be switched off when not in use.
3. In TDMA, the handoff process is much simpler as transmission mode is discontinuous. The mobile can work in slotted mode and is able to monitor the RSSI of other Base Stations when it is idle and not transmitting/receiving anything. Thus MAHO (Mobile Assisted Hand Off) can be implemented easily.
4. TDMA uses different time slots for transmission and reception, thus duplexers are not required. Even if FDD is used, a switch rather than a duplexer inside the subscriber unit is all that is required to switch between transmitter and receiver using TDMA.
5. A TDMA frame consists of data for many users. Hence, guard times are necessary to separate users. TDMA system requires high synchronization overhead bits due to bursty transmission. In each TDMA frame, the preamble contains the address and synchronization information. Thus, TDMA systems have larger overheads as compared to FDMA.
6. TDMA could allocate varied number of time slots per frame to different users.
7. In TDMA, the allotted spectrum is not divided into narrow channels. Hence, during propagation, the TDMA channel bandwidth is comparatively more than the coherence bandwidth of the medium. This results in frequency selective fading. Hence, Equalization is necessary on the receiver side.

Advantages of TDMA :
Here is a list of few notable advantages of TDMA −

• Permits flexible rates (i.e., several slots can be assigned to a user, for example, each time interval translates 32Kbps, a user is assigned two 64 Kbps slots per frame).
• Can withstand gusty or variable bit rate traffic. Number of slots allocated to a user can be changed frame by frame (for example, two slots in the frame 1, three slots in the frame 2,one slot in the frame 3, frame 0 of the notches 4, etc.).
• No guard band required for the wideband system.
• No narrowband filter required for the wideband system.

Disadvantages of TDMA :
The disadvantages of TDMA are as follow −

• High data rates of broadband systems require complex equalization.
• Due to the burst mode, a large number of additional bits are required for synchronization and supervision.
• Call time is needed in each slot to accommodate time to inaccuracies (due to clock instability).
• Electronics operating at high bit rates increase energy consumption.
• Complex signal processing is required to synchronize within short slots.

Que 6. Explain the concept of Code Division Multiple Access (CDMA).

1. In code division multiple access (CDMA) systems, 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 orders of magnitudes greater than the data rate of the message.
2. All users in a CDMA system, as seen from Figure, use the same carrier frequency and may transmit simultaneously. Each user has its own pseudorandom codeword which is approximately orthogonal to all other codeword’s.
3. The receiver performs a time correlation operation to detect only the specific desired codeword. All other code words appear as noise due to de-correlation. For detection of the message signal, the receiver needs to know the codeword used by the transmitter. Each user operates independently with no knowledge of the other users.
4. In CDMA, the power of multiple users at a receiver determines the noise floor after decorrelation. If the power of each user within a cell is not controlled such that they do not appear equal at the base station receiver, then the near-far problem occurs. The near-far problem occurs when many mobile users share the same channel. In general, the strongest received mobile signal will capture the demodulator at a base station.
5. In CDMA, Figure shows Spread spectrum multiple access in which each channel is assigned a unique PN code which is orthogonal or approximately orthogonal to PN codes used by other users. Stronger received signal levels raise the noise floor at the base station demodulators for the weaker signals, thereby decreasing the probability that weaker signals will be received.
6. To combat the near-far problem, power control is used in most CDMA implementations. Power control is provided by each base station in a cellular system and assures that each mobile within the base station coverage area provides the same signal level to the base station receiver. This solves the problem of a nearby subscriber overpowering the base station receiver and drowning out the signals of far away subscribers.
7. Power control is implemented at the base station by rapidly sampling the radio signal strength indicator (RSSI) levels of each mobile and then sending a power change command over the forward radio link. Despite the use of power control within each cell, out-of-cell mobiles provide interference which is not under the control of the
   receiving base station.

8. CDMA is a spread-spectrum multiple-access technique. A spread-spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power. A spreading code is a pseudo-random code that has a narrow ambiguity function, unlike other narrow pulse codes.
9. In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. Data for transmission is combined by bitwise XOR (exclusive OR) with the faster code. The figure shows how a spread-spectrum signal is generated.

Que 7. Explain features of CDMA.

Features (attributes) of CDMA are as follows:

1. Frequency usage – Many users of a CDMA system share the same frequency. Either TDD or FDD may be used.
2. Soft Capacity Limit – Increasing the number of users in a CDMA system raises the noise level in a linear manner. 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, and improves as the number of users is decreased.
   Hence, unlike TDMA or FDMA, CDMA has a soft capacity limit.
3. Multipath Resistance – In CDMA systems, channel data rates are very high. Hence the symbol (chip) duration is very short and usually much less than the channel delay spread. Since PN sequences have low auto correlation, multipath which is delayed by more than a chip will appear as noise. A RAKE receiver can be used to improve
   reception by collecting time delayed versions of the required signal.
4. Soft Handoff – Since CDMA uses co channel cells, it can use macroscopic spatial diversity to provide soft handoff. Soft handoff is performed by the MSC, which can simultaneously monitor a particular user from two or more base stations. The MSC may choose the best version of the signal at any time without switching frequencies.
5. Self-Jamming – Self Jamming is a problem in CDMA system. Self-Jamming arises from the fact that the spreading sequences of different users are not exactly orthogonal. Hence in the de-spreading of a particular PN code, there is some level of cross correlation. There will be non-zero contributions at the receiver from the transmissions of other users in the system.
6. Near Far effect – The near far problem occurs at a CDMA receiver if an undesired user has a high detected power as compared to the desired user. Signals close to the receiver are received with less attenuation than signals farther away.
7. Fading effects – Multipath may be substantially reduced because signal is spread over a large spectrum.
8. Economical – CDMA is the most cost-effective technology that requires fewer cell sites and no costly frequency reuse pattern. The average power transmitted by CDMA mobile station averages 6 to 7 mill watts(mW), which is significantly lower than the average power transmitted by FM and TDMA phones. Transmitting less power means
   that the average battery life will be longer.

Que 8. Compare the multiple access techniques FDMA, TDMA and CDMA.

Parameter / Approach	FDMA	TDMA	CDMA
Concept	It is a technology by which the total bandwidth available to the system is divided into frequencies. This division is done between non overlapping frequencies that are then assigned to each communicating Pair	In TDMA the division of calls happens on time basis. The system first digitizes the calls, and then combines those conversations into a unified digital stream on a single radio channel.	Every bit of a conversation is been tagged with a specific and unique code.
key resources	Frequency	Active Time period	Codes
Sharing of resources	In FDMA each user is allotted a frequency, through which communication can be done all the time	In TDMA total available frequency is given to user for a particular time period	Sharing of resources is done through codes (PN sequence)
bandwidth	here entire band of frequencies is divided into multiple RF channels/carriers. Each carrier is allocated to different users.	here entire bandwidth is shared among different subscribers at fixed predetermined or dynamically assigned time intervals/slots	here entire bandwidth is shared among different users by assigning unique codes
Signal Separation	Filtering in the frequency domain	Synchronization in the time domain	Code plus special recievers
Transmission Scheme	Continuous	Discontinuous	Continuous
System flexibility	Flexible System	Less flexible than TDMA& CDMA	Less flexible than CDMA, More Less flexible than CDMA
System complexity	Less Complex than TDMA & CDMA	Less Complex than CDMA, More Flexible than FDMA	Complex system
Cell capacity	Limited	Limited	No absolute limit on channel capacity but it is an interference limited system
Advantages	simple, robust	flexible, fully digital	flexible, soft handover
Disadvantages	Inflexible	Guard space needed; synchronization is difficult	complex receivers
Comment	used with TDMA and SDMA	used with SDMA and FDMA	used with TDMA and FDMA

Also you can refer : Unit-2 Basic Cellular Concept