UNIT 3

Cellular Concepts

3.1 Evolution of Wireless systems

     Wireless Communication is the communication without electric conductor between two places.

     To communicate radio waves are used.

     It has multiple applications like radios, wireless network, Cellular telephones.

     It is used from 1890 for radio telegraphy.

Revolution of Wireless

Photo phone

     First conversation without wire happened in 1880.

     Photo phone is invented by Alexander Graham Bell and Charles Sumner Tainter.

     Using a beam of light an audio is sent by the photo phone.

     Sunlight and clear line of sight were the requirement of photo phone  to operate and transmit the signal respectively.

     Then practical application of photo phone found in fiber optic cable.

Electric Wireless Technology

      In 19th century electromagnetic induction is used for telegraphy before practical use of radio.

      Thomas Edison invented telegraph using running train.

Radio Waves

      In 1984 wireless telegraph is invented using radio by Guglielmo Macroni.

      It was a short range phenomenon.

      Maconi build an application with large distance coverage.

Revolution

     Revolution started in 1990when digital wireless networks affected society by devices like cell phones, telephony, computer networks independent of wire.

     It is due to transition of digital to analog RF Technoloy.

     This helped to increase voice related traffic , digital data delivery and streaming media.

3.2 Introduction to cellular telephone system

Cellular telephone systems, also referred to as Personal Communication Systems (PCS), are extremely popular and lucrative worldwide: these systems have sparked much of the optimism about the future of wireless networks. Cellular telephone systems are designed to provide two-way voice communication at vehicle speeds with regional or national coverage. Cellular systems were initially designed for mobile terminals inside vehicles with antennas mounted on the vehicle roof. Today these systems have evolved to support lightweight handheld mobile terminals operating inside and outside buildings at both pedestrian and vehicle speeds.

3.3 Frequency reuse

      It is process of combining analog and digital signal to send over shared medium.

      It divides the capacity of communication channel into multiple channels

     Multiplexing is divided into space division, frequency division and time division multiplexing.

Space Division Multiplexing

     In wired medium, separate point to point conductors are used to each channel.

     In wireless medium multiple elements of antennas are used such that it forms phased array antenna.

     Multiple output, multiple input and single input multiple output are the examples of this.

Frequency Division Multiplexing

     In this multiple signals are send in distinct frequency in single medium.

     The signals are electrical.

     Radio, television broadcasting are the examples of FDM.

     The service provides can send several channels or signals continuously to all subscribers even the customer has single cable connection.

Time Division Multiplexing

      In this for separation of data streams the time is used instead of frequency and space.

      It consists of group of bits in sequence one after another.

      Each sequence is associated with each receiver.

      Carriers sense multiple access and multidrug are the eaxples of time division multiplexing.

3.4 Channel Assignment

It means assigning channels or spectrum bands to radio interfaces for communication.

There are two channel assignment strategies in cellular system.

A. Fixed channel assignment:

1. In fixed channel assignment each cell is permanently allocated predetermined group of channels. Any call attempt within cell can only be served by unused channels in that particular cell.
2. If all channels are occupied, the call is blocked and subscriber does not receive service.
3. Borrowing technique where a cell is allowed to borrow channels from a neighbouring cell if all channels are already occupied is always used with this type of strategy. Mobile Base station (MSC) monitors the function of base station including borrowing ensuring that borrowing does not interfere with any call in progress in donor cell.

B. Dynamic channel assignment:

1. In dynamic channel assignment strategy, voice channels are not allocated permanently.
2. Entire pool of frequency channels lies with MSC and each time a call request is made, the serving base station requests a channel from the MSC. Switch then allocates a channel to the requested cell following a algorithm.
3. MSC allocates frequency channels on dynamic basis if that frequency channel is not presently in use in the cell or any other cell which falls within the minimum restricted distance of frequency reuse to avoid co-channel interference.
4. It reduces chances of blocking which increases trunking capacity of system as all available channels are accessible to all cells.
5. In this MSC has to collect real time data on channel occupancy, traffic distribution, radio signal strength indication of all channels on continuous basis, thus increasing the computational load on MSC.

3.5 Handoff strategies

Processing handoffs is an important task in any cellular radio system. Handoffs must be performed successfully and as infrequently as possible and be imperceptible to the user. In order to meet these requirements, system designer must specify an optimum signal at which to initiate a handoff.

This margin is given by cannot be too large or too small. If is large unnecessary handoffs which burden the MSC may occur and if is too small, there may be may be insufficient time to complete a handoff before a call is lost to meet these conflicting requirements. Figure is given which explains the different steps of handoff required to meet.

Following figure demonstrates the case where a handoff is not made and the signal drops below the minimum acceptable level to keep the channel active. This dropped call event can happen when there is an expansive delay by the MSC in assigning a handoff or when the threshold is set too small for the handoff time in the systems. Excessive delay may occur during high traffic conditions or due to no channels are available on any of the nearby base stations.

In deciding when to handoff, it is important to ensure that the drop in the measured signal is not due to momentary fading and the mobile is actually moving away from the serving base station. But when there is a drop in the measured signal level due to momentary fading and not due to the actual moving mobile away from the base station, that is situation if the handoff of a call takes place then such handoffs must be prevented by ensuring that the base station monitors the signal level for a certain period of time before the handoff is initiated.

3.6 Cell Splitting

• Cell splitting is the process of subdividing a congested cell in to smaller cells, each with its own base station and corresponding reduction in antenna height and transmitter power.
• Cell splitting increases the capacity of a system since it increases number of times that channels are reused.
• In cell splitting original cell is split in to smaller cells. New cell radius is half of the original radius.
• In this the cell boundaries need to be revised so that the local area which was earlier considered as a single cell can now contain number of smaller cell ,these new cells are called microcells
• Dynamic cell splitting: This technique is based on utilizing the allocated spectrum efficiency in real time. In this of splitting techniques cells are not splitted permanently depending on requirement of traffic the splitting of the cells are carried out.
• The algorithm for dynamically splitting cell sites is a tedious job since we cannot afford to have single cell unused during cell splitting at heavy traffic hours. Proof:
• When the cell radius is reduced by a factor, it is also desirable to reduce the transmitted power. The transmit power of the new cells with radius half that of the old cells can be found by examining the received power PR at the new and old cell boundaries and setting them equal.
• This is necessary to maintain the same frequency re-use plan in the new cell layout as well. Assume that PT1 and PT2 are the transmit powers of the larger and smaller base stations respectively. Then, assuming a path loss index n=4, we have power received at old cell boundary = PT1/R4 and the power received at new cell boundary = PT2/(R/2)4. On equating the two received powers, we get PT2 = PT1 / 16. In other words, the transmit power must be reduced by 12 dB in order to maintain the same S/I with the new system lay-out.
• At the beginning of this channel splitting process, there would be fewer channels in the smaller power groups.
• As the demand increases, more and more channels need to be accommodated and hence the splitting process continues until all the larger cells have been replaced by the smaller cells, at which point splitting is complete within the region and the entire system is rescaled to have a smaller radius per cell.
• If a cellular layout is replaced entirety by a new layout with a smaller cell radius, the signal-to-interference ratio will not change, provided the cluster size does not change. Some special care must be taken, however, to avoid co-channel interference when both large and small cell radii coexist.
• It turns out that the only way to avoid interference between the large-cell and small-cell systems is to assign entirely different sets of channels to the two systems.
• So, when two sizes of cells co-exist in a system, channels in the old cell must be broken down into two groups, one that corresponds to larger cell reuse requirements and the other which corresponds to the smaller cell reuse requirements.
• The larger cell is usually dedicated to high speed users as in the umbrella cell approach so as to minimize the number of hand-offs.

3.7 Propagation Mechanism: Free space loss

      Signal is propagated in multiple ways.

      Ground, signal, line of sight etc. ways can be used to propagate  signals.

Ground Propagation.

      The radio waves are transmitted using ground propagation in which waves travel from lowest portion.

      The waves are low frequency signals, to spread waves antennas are used.

      The waves can use curvature of planet.

Sky Propagation

      Waves are transmitted using sky propagation.

      The waves are propagated in the upward direction in atmosphere i.e. in ionosphere.

      Then waves are reflected back to the earth

      It is used for larger distance.

Line of sight.

      It called as line of sight because the waves are transferred from antenna to antenna.

      It is also called as visual propagation in which very high frequency signals are used..

      The antennas should be of same size, in limited size and should be facing each other such that curvature of earth should not affect signal.

3.8 Reflection

• Reflections Occurs when a wave impinges upon a smooth surface.
• Dimensions of the surface are large relative to l.
• Reflections occur from the surface of the earth and from buildings and walls.

3.9 Diffraction

• Diffraction Occurs when the path is blocked by an object with large dimensions relative to l and sharp irregularities (edges).
• Secondary “wavelets” propagate into the shadowed region.
• Diffraction gives rise to bending of waves around the obstacle.

3.10 Scattering

• Scattering Occurs when a wave impinges upon an object with dimensions on the order of l or less, causing the reflected energy to spread out or“scatter” in many directions.
• Small objects such as street lights, signs, & leaves cause scattering

3.11 Fading and Multipath: Small scale multipath propagation

• Multi path causes large and rapid fluctuations in a signal
• These fluctuations are not the same as the propagation path loss.

Multipath causes three major things in wireless communication

• Rapid changes in signal strength over a short distance or time.
• Random frequency modulation due to Doppler Shifts on different multipath signals.
• Time dispersion caused by multipath delays
• These are called “fading effects
• Multipath propagation results in small-scale fading.

3.12 Impulse response model of multipath channel

Mobile radio channel may be modeled as a linear filter with time varying impulse response in continuous time. To show this, consider time variation due to receiver motion and time varying impulse response h(d, t) and x(t), the transmitted signal.

The received signal y(d, t) at any position d would be

y(d, t) = x(t) ∗h(d, t) = x(τ) h(d, t −τ) dτ

For a causal system: h(d, t) = 0, for t < 0 and for a stable system ¸

∞−∞|h(d, t)|dt<∞

Applying causality condition in the above equation,

h(d, t −τ ) = 0 for t −τ <0⇒ τ > t,

i.e., the integral limits are changed τ

(d, t) =¸ tx(τ ) h(d, t −τ ) dτ.−∞

Since the receiver moves along the ground at a constant velocity v, the position of the receiver is

d = vt,

i.e.,y(vt, t) =¸ tx(τ ) h(vt, t −τ ) dτ.−∞

Since v is a constant, y(vt, t) is just a function of t. Therefore the above equation can be expressed as

y(t) =¸ tx(τ) h(vt, t −τ) dτ = x(t) ∗h(vt, t) = x(t) ∗h(d, t)

It is useful to discretize the multipath delay axis τ of the impulse response into equal time delay segments called excess delay bins, each bin having a time delay width equal to

( τi+1 −τi) = ∆τ and τi = i∆τ for i ∈ {0, 1, 2, ..N −1},

Where N representsthe total number of possible equally-spaced multipath components, including the first arriving component.

The useful frequency span of the model is 2/∆τ . The model may be used to analyze transmitted RF signals having bandwidth less than 2/∆τ.

If there are N multipaths, maximum excess delay is given by N ∆τ.

{y(t) =x(t) ∗h(t,τi)|i=0,1,...N−1}

Bandpass channel impulse response model is x(t) →h(t,τ) =Re{ hb(t,τ) ejωct → y(t) =Re{r(t) ejωct}

Baseband equivalent channel impulse response model is given by

c(t) →2 hb(t, τ) →r(t) = c(t) ∗2 hb(t, τ)

Average power is x2(t) = 1 c(t)2

The baseband impulse response of a multipath channel can be expressed as

N−1 hb(t, τ) = . Ai(t, τ) exp[j(2πfcτi(t) + ϕi(t, τ))]δ(τ−τi(t))

i=0 where ai(t, τ ) and τi(t) are the real amplitudes and excess delays, respectively, of the ith multipath component at time t.

The phase term 2πfcτi(t) + ϕi(t, τ ) in the above equation represents the phase shift due to free space propagation of the ith multipath component, plus any additional phase shifts which are encountered in the channel. If the channel impulse response is wide sense stationary over a small-scale time or distance interval, then

hb(τ) =N−1 .aI exp[jθi] δ(τ−τi) i=0

For measuring hb(τ ), we use a probing pulse to approximate δ(t) i.e., p(t) ≈ δ(t −τ)

Power delay profile is taken by spatial average of |hb(t, τ )| over a local area. The received power delay profile in a local area is given by

p(τ) ≈k|hb(t;τ)|

3.13Multiple Access Techniques-TDMA

      It is the Time Division Multiple Access.

      TDMA is used where there is no need of continuous transmission. Due to this it can be used instead of FDMA.

      In this time slot allocated to users is non- overlapping.

      The single frequency is shared by the entire user.

      The data transmission is in bursts instead of continuous.

      No need to use duplexers as there is different time slots are there for different transmission.

      The bandwidth supply is on demand basis. It is used by reassigning the time slot.

3.14FDMA

• It sub-divide frequency into a several frequency band with non-overlapping.

• FDMA allows user to transmit signals simultaneously to satellite transponder with the help of assigning a specific frequency to every user among a channel.
• Each transaction of signal has own unique radio channel. Channel are probably 30 KHz or less used to transmit or receive channels.
• Frequency allocation made by national policies. Uplink (i.e. from mobile station to base station). Downlink (i.e. from base station to mobile station) uses frequencies bands like. Uplink and downlink with frequencies mentioned below :

Uplink (UL) and downlink (DL) can be described with the help of one relation among them as follows:

fd = fu + 45 MHz

• For Example :

If UL frequency is :

fu = 890 MHz + n∙ 0. 2 MHz

Then

fd = 935 MHz + n ∙ 0. 2 MHz

• Fixed assigned frequency makes scheme very inflexible limits to no. Of sender turn into disadvantage of FDMA.

3.15CDMA

      It is Code Division Multiple Access.

      It is a multiple access system. In this single channel is used by multiple senders to send the information simultaneously.

      In this entire available spectrum is used by all users instead of blocks allocated.

      For voice and data communication CDAMA is recommended.

      As compare to TDMA, CDMA has more air space capacity.

      CDMA very finely handles hands –on base station.

References

1. Theodore S Rappaport, “Wireless Communications Principles and Practice” Second Edition,      Pearson Education

2. John C. Bellamy, “Digital Telephony”, Third Edition; Wiley Publications 

3. ThiagarajanVishwanathan, “Telecommunication Switching Systems and Networks”; PHI Publications 

4. Wayne Tomasi, “Electronic Communications Systems”; 5th Edition; Pearson Education

5. Vijay K Garg, Joseph E Wilkes, “Principles and Applications of GSM” Pearson Education 

6. Vijay K Garg, Joseph E Wilkes, “IS-95CDMA and CDMA 2000 Cellular/PCS Systems Implementation” Pearson Education 

7. Mischa Schwartz, “Mobile Wireless Communications”, Cambridge University Press