Mobile Communication Fundamentals

Introduction issues in mobile communications:-

Mobile Computing is a technology that allows transmission of data, voice and video via a computer or any other wireless enabled device without having to be connected to a fixed physical link. The main concept involves −

• Mobile communication
• Mobile hardware
• Mobile software

Mobile communication

The mobile communication in this case, refers to the infrastructure put in place to ensure that seamless and reliable communication goes on. These would include devices such as protocols, services, bandwidth, and portals necessary to facilitate and support the stated services. The data format is also defined at this stage. This ensures that there is no collision with other existing systems which offer the same service.

Connective

Since the media is unguided/unbounded, the overlaying infrastructure is basically radio wave-oriented. That is, the signals are carried over the air to intended devices that are capable of receiving and sending similar kinds of signals.

Mobile Hardware

Mobile hardware includes mobile devices or device components that receive or access the service of mobility. They would range from portable laptops, smartphones, tablet Pc's, Personal Digital Assistants.

Hadware

These devices will have a receptor medium that is capable of sensing and receiving signals. These devices are configured to operate in full- duplex, whereby they are capable of sending and receiving signals at the same time. They don't have to wait until one device has finished communicating for the other device to initiate communications.

Above mentioned devices use an existing and established network to operate on. In most cases, it would be a wireless network.

Mobile software

Mobile software is the actual program that runs on the mobile hardware. It deals with the characteristics and requirements of mobile applications. This is the engine of the mobile device. In other terms, it is the operating system of the appliance. It's the essential component that operates the mobile device.

Mobile Software

Since portability is the main factor, this type of computing ensures that users are not tied or pinned to a single physical location, but are able to operate from anywhere. It incorporates all aspects of wireless communications.

Cellular Approach

With limited frequency resource, cellular principle can serve thousands of subscribers at an affordable cost. In a cellular network, total area is subdivided into smaller areas called “cells”. Each cell can cover a limited number of mobile subscribers within its boundaries. Each cell can have a base station with a number of RF channels.

Frequencies used in a given cell area will be simultaneously reused at a different cell which is geographically separated. For example, a typical seven-cell pattern can be considered.

Total available frequency resources are divided into seven parts, each part consisting of a number of radio channels and allocated to a cell site. In a group of 7 cells, available frequency spectrum is consumed totally. The same seven sets of frequency can be used after certain distance.

The group of cells where the available frequency spectrum is totally consumed is called a cluster of cells.

Two cells having the same number in the adjacent cluster, use the same set of RF channels and hence are termed as “Co-channel cells”. The distance between the cells using the same frequency should be sufficient to keep the co-channel (co-chl) interference to an acceptable level. Hence, the cellular systems are limited by Co-channel interference.

Cellular Network

• Hence a cellular principle enables the following.
• More efficient usage of available limited RF source.
• Manufacturing of every piece of subscriber's terminal within a region with the same set of channels so that any mobile can be used anywhere within the region.

Shape of Cells

For analytical purposes a “Hexagon” cell is preferred to other shapes on paper due to the following reasons.

• A hexagon layout requires fewer cells to cover a given area. Hence, it envisages fewer base stations and minimum capital investment.
• Other geometrical shapes cannot effectively do this. For example, if circular shaped cells are there, then there will be overlapping of cells.
• Also for a given area, among square, triangle and hexagon, radius of a hexagon will be the maximum which is needed for weaker mobiles.

In reality cells are not hexagonal but irregular in shape, determined by factors like propagation of radio waves over the terrain, obstacles, and other geographical constraints. Complex computer programs are required to divide an area into cells. One such program is “Tornado” from Siemens.

Operating Environment

Due to mobility, the radio signals between a base station and mobile terminals undergo a variety of alterations as they travel from transmitter to receiver, even within the same cell. These changes are due to −

• Physical separation of transmitter and receiver.
• Physical environment of the path i.e. terrain, buildings, and other obstacles.

Slow Fading

• In free space conditions (or) LOS, RF signal propagation constant is considered as two i.e. r = 2. This is applicable for static radio systems.
• In mobile environment, these variations are appreciable and normally ‘r’ is taken as 3 to 4.

Rayleigh Fading

The direct line of sight in mobile environment, between base station and the mobile is not ensured and the signal received at the receiver is the sum of a number of signals reaching through different paths (multipath). Multipath propagation of RF waves is due to the reflection of RF energy from a hill, building, truck, or aero plane etc.; the reflected energy undergoes a phase change also.

If there are 180 out-of phase with direct path signals, they tend to cancel out each other. So the multipath signals tend to reduce the signal strength. Depending upon the location of the transmitter and receiver and various reflecting obstacles along the path length, signal fluctuates. The fluctuations occur fast and it is known as “Rayleigh fading”.

In addition, multipath propagation leads to “pulse widening” and “Inter symbol Interference”.

Doppler Effect

Due to the mobility of the subscriber, a change occurs in the frequency of the received RF signals. Cellular mobile systems use following techniques to counter these problems.

• Channel coding
• Interleaving
• Equalization
• Rake receivers
• Slow frequency hopping
• Antennae diversity

Co-Channel Interference and Cell Separation

We assume a cellular system having a cell radius “R” and Co-channel distance “D” and the cluster size “N”. Since the cell size is fixed, co-channel interference will be independent of power.

Co-chl interference is a function of “q” = D/R.

Q = Co-chl interference reduction factor.

Higher value of “q” means less interference.

Lower value of “q” means high interference.

“q” is also related to cluster size (N) as q = 3N

q = 3N = D/R

For different values of N, q is −

N = 1 3 4 7 9 12
Q = 1.73 3 3.46 4.58 5.20 6.00

Higher values of “q”

• Reduces co-channel interference,
• Leads to higher value of “N” more cells/cluster,
• Less number of channels/cells,
• Less traffic handling capacity.

Lower values of “q”

• Increases co-channel interference,
• Leads to lower value of “n” fewer cells / cluster,
• More number of channels / cells,
• More traffic handling capacity.

Generally, N = 4, 7, 12.

C/I Calculations and ‘q’

The value of “q” also depends on C/I. “C” is the received carrier power from the desired transmitter and “I” is the co-channel interference received from all the interfering cells. For a seven-cell reuse pattern, the number of co-channel interfering cells shall be six in number.

I = m2b∑Mz1 I_m

Loss of signal is proportional to (distance) –r

R – Propagation constant.

c α R-r

R = Radius of cell.

I α 6 D-r

D= Co-channel separation distance

C/I = R – r / 6D –r = 1/6 × Dr / Rr = 1/6 (D/R) r

C/I = 1/6 q r since q = D/R and q r = 6 C/I

Q = [6 × C/I]1/r

Based upon the acceptable voice quality, the value of C/I has been found to be equal to 18 dB.

Assuming,

• A seven-cell reuse pattern
• Omni directional antennae

Value of ‘q’ can be typically around 4.6.

Value r is taken as 3.

This is an ideal condition, considering the distance of the mobile units from the interfering cells to be uniformly equal to ‘D’ in all cases. But practically mobile moves and distance ‘D’ reduces to ‘D-R’ when it reaches the boundary of the cell, and C/I drops to 14.47 dB.

Hence ‘freq’ reuse pattern of 7 is not meeting C/I criteria with omni directional antennae.

If N = 9 (or) 12,

N = 9q = 5.2C/I = 19.78 dB

N = 12q = 6.0C/I = 22.54 dB

Hence, either 9 or 12 cell pattern is to be with omni directional antennae, but traffic handling capacity is reduced. Hence they are not preferred.

In order to use N = 7 (or lower), directional antennas are used in every cell site. A cell having 3 sectors is very popular and will be like the figure shown below.

Antenna’s font – back coupling phenomenon reduces number of potential interferers.

For example if N = 7.

With omni directional antennae, number of interfering cells shall be six. With directional antennae & 3 sectors the same is reduced to two. For N = 7 and three sectors, the C/I improves from 14.47 dB to 24.5 dB even in worst conditions. Then C/I meets the requirement of 18dB. For N = 7 and six sectors, the C/I improves to 29 dB.

For Urban applications, N = 4 and a three sector cell is used so that more number of carriers per cell are obtained than N = 7. Also the C/I becomes 20 dB in worst cases.

DAMPS Uses 7/21 cell pattern

GSM Uses 4/21 cell pattern

Advantages of sectoring

• Decrease co-channel interference
• Increase system capacity

Disadvantages of sectoring

• Large number of antennas at the base station.
• Increase in the number of sectors/cell reduces the trunking efficiency
• Sectoring reduces the coverage area, for a particular group of channels.
• Number of ‘Hand offs’ increases.

Hand Off

When the mobile unit travels along a path it crosses different cells. Each time it enters into a different cell associated with f = different frequency, control of the mobile is taken over by the other base station. This is known as ‘Hand off’.

Hand off is decided based on −

• Received signal strength information if it is below a threshold value.
• Carrier to interference ratio is less than 18 dB.

Adjacent Channel Interference

A given cell/sector uses a number of RF channels. Because of imperfect receiver filters, which allow nearby frequencies to leak into pass band, adjacent channel interference takes place.

It can be reduced by keeping the frequency separations between each RF channel in a given cell as large as possible. When the reuse factor is small, this separation may not be sufficient.

A channel separation, by selecting RF frequencies, which are more than 6 channels apart, is sufficient to keep adjacent channel interferences within limits.

For example, in GSM which follows 4/12 pattern, N = 4

Sectors = 3/cell

Channel Interface diagram

IA will use RF Carr. 1, 13, 25,………..

IB will use RF Carr 5, 17, 29,…………

IC will use RF Carr. 9, 21, 33,……….. and so on.

Trunking

Cellular radios rely on trunking to accommodate a large number of users in a limited radio spectrum. Each user is allocated a channel on need/per call basis and on termination of the cell, the channel is returned to the common pool of RF channels.

Grade of Service (GOS)

Because of trunking, there is a likelihood that a call is blocked if all the RF channels are engaged. This is called ‘Grade of Service’ “GOS”.

Cellular designer estimates the maximum required capacity and allocates the proper number of RF channels, in order to meet the GOS. For these calculations, ‘ERLANG B’ table is used.

Cell Splitting

When the number of users reaches a saturation in a start-up cell (initial design) and no more spare frequency is available, then the start-up cell is split, usually in four smaller cells and traffic increases by four and more number of subscribers can be served.

After ‘n’ splits, the traffic will be −

T2 = T0 × 42

Power will be reduced −

P2 = P0 – n × 12 db

Hence cell splitting improves the capacity and lowers the transmission power.

Cellular

Air Interface in GSM Network:-

GSM stands for global service for messaging. this is the mostly used service for transmission of data and call generation.
in this we use  components for connecting the call.
BTS,BSC,MSC,VLR,HLR etc are the main parts of the GSm network.

GSM Interface

What is BTS

BTS is used for generating the call it provide path to the user to connect the call.when a path will be found then the bts transfer the call to the bsc. there can be number of bts connected with a single bsc.

What is BSC

BSC stands for Base Switching Centre. BSC and BTS are simmilar to each othr they both are used for providing the path to the suscriber,bsc is used for to remove paging problem. when call found the path it will be transfered to the msc for connecting.
there are several no of bsc those are connected with a single msc in a zone. the main work of bsc is to gave the information about the user to the msc.it gives ISDN ,TMSI etc to the suscriber for connecting the call.

Mobile Services Switching Center (MSC)

The central component of the Network Subsystem is the MSC. The MSC performs the switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others. Every MSC is identified by a unique ID.

Home Location Register (HLR)

The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription in the form of SIM, then all the information about this subscription is registered in the HLR of that operator.

Visitor Location Register (VLR)

The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.

Authentication Center (AUC)

The Authentication Center is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and ciphering of the radio channel. The AUC protects network operators from different types of fraud found in today's cellular world.

Equipment Identity Register (EIR)

The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where its International Mobile Equipment Identity (IMEI) identifies each MS. An IMEI is marked as invalid if it has been reported stolen or is not type approved.

CDMA 

Code Division Multiple Access (CDMA) is a sort of multiplexing that facilitates various signals to occupy a single transmission channel. It optimizes the use of available bandwidth. The technology is commonly used in ultra-high-frequency (UHF) cellular telephone systems, bands ranging between the 800-MHz and 1.9-GHz.

Code Division Multiple Access system is very different from time and frequency multiplexing. In this system, a user has access to the whole bandwidth for the entire duration. The basic principle is that different CDMA codes are used to distinguish among the different users.

Techniques generally used are direct sequence spread spectrum modulation (DS-CDMA), frequency hopping or mixed CDMA detection (JDCDMA). Here, a signal is generated which extends over a wide bandwidth. A code called spreading code is used to perform this action. Using a group of codes, which are orthogonal to each other, it is possible to select a signal with a given code in the presence of many other signals with different orthogonal codes.

How Does CDMA Work?

CDMA allows up to 61 concurrent users in a 1.2288 MHz channel by processing each voice packet with two PN codes. There are 64 Walsh codes available to differentiate between calls and theoretical limits. Operational limits and quality issues will reduce the maximum number of calls somewhat lower than this value.

In fact, many different "signals" baseband with different spreading codes can be modulated on the same carrier to allow many different users to be supported. Using different orthogonal codes, interference between the signals is minimal. Conversely, when signals are received from several mobile stations, the base station is capable of isolating each as they have different orthogonal spreading codes.

The following figure shows the technicality of the CDMA system. During the propagation, we mixed the signals of all users, but by that you use the same code as the code that was used at the time of sending the receiving side. You can take out only the signal of each user.

CDMA Capacity

The factors deciding the CDMA capacity are −

• Processing Gain
• Signal to Noise Ratio
• Voice Activity Factor
• Frequency Reuse Efficiency

Capacity in CDMA is soft, CDMA has all users on each frequency and users are separated by code. This means, CDMA operates in the presence of noise and interference.

In addition, neighboring cells use the same frequencies, which means no re-use. So, CDMA capacity calculations should be very simple. No code channel in a cell, multiplied by no cell. But it is not that simple. Although not available code channels are 64, it may not be possible to use a single time, since the CDMA frequency is the same.

Centralized Methods

• The band used in CDMA is 824 MHz to 894 MHz (50 MHz + 20 MHz separation).
• Frequency channel is divided into code channels.
• 1.25 MHz of FDMA channel is divided into 64 code channels.

Processing Gain

CDMA is a spread spectrum technique. Each data bit is spread by a code sequence. This means, energy per bit is also increased. This means that we get a gain of this.

P (gain) = 10log (W/R)

W is Spread Rate

R is Data Rate

For CDMA P (gain) = 10 log (1228800/9600) = 21dB

This is a gain factor and the actual data propagation rate. On an average, a typical transmission condition requires a signal to the noise ratio of 7 dB for the adequate quality of voice.

Translated into a ratio, signal must be five times stronger than noise.

Actual processing gain = P (gain) - SNR

= 21 – 7 = 14dB

CDMA uses variable rate coder

The Voice Activity Factor of 0.4 is considered = -4dB.

Hence, CDMA has 100% frequency reuse. Use of same frequency in surrounding cells causes some additional interference.

In CDMA frequency, reuse efficiency is 0.67 (70% eff.) = -1.73dB

Advantages of CDMA

CDMA has a soft capacity. The greater the number of codes, the more the number of users. It has the following advantages −

• CDMA requires a tight power control, as it suffers from near-far effect. In other words, a user near the base station transmitting with the same power will drown the signal latter. All signals must have more or less equal power at the receiver
• Rake receivers can be used to improve signal reception. Delayed versions of time (a chip or later) of the signal (multipath signals) can be collected and used to make decisions at the bit level.
• Flexible transfer may be used. Mobile base stations can switch without changing operator. Two base stations receive mobile signal and the mobile receives signals from the two base stations.

• Transmission Burst − reduces interference.

Disadvantages of CDMA

The disadvantages of using CDMA are as follows −

• The code length must be carefully selected. A large code length can induce delay or may cause interference.
• Time synchronization is required.
• Ra dual transfer increases the use of radio resources and may reduce capacity.
• As the sum of the power received and transmitted from a base station needs constant tight power control. This can result in several handovers.

CDMA Advantage/ Disadvantage Diagram

GPRS:-

General Packet Radio Service (GPRS) is a Mobile Data Service accessible to GSM and IS-136 mobile phones users. This service is packet-switched and several number of users can divide the same transmission channel for transmitting the data. This tutorial helps you to understand GPRS Basics and should be your starting point.

GPRS has opened a wide range of unique services to the mobile wireless subscriber. Some of the characteristics that have opened a market full of enhanced value services to the users. Below are some of the characteristics:

• Mobility - The ability to maintain constant voice and data communications while on the move.
• Immediacy - Allows subscribers to obtain connectivity when needed, regardless of location and without a lengthy login session.
• Localization - Allows subscribers to obtain information relevant to their current location.

Using the above three characteristics varied possible applications are being developed to offer to the mobile subscribers. These applications, in general, can be divided into two high-level categories:

• Corporation
• Consumer

These two levels further include:

• Communications - E-mail, fax, unified messaging and intranet/internet access, etc.
• Value-added services - Information services and games, etc.
• E-commerce - Retail, ticket purchasing, banking and financial trading, etc.
• Location-based applications - Navigation, traffic conditions, airline/rail schedules and location finder, etc.
• Vertical applications - Freight delivery, fleet management and sales-force automation.
• Advertising - Advertising may be location sensitive. For example, a user entering a mall can receive advertisements specific to the stores in that mall.

IEEE 802.11 

The 802.11 standard is defined through several specifications of WLANs. It defines an over-the-air interface between a wireless client and a base station or between two wireless clients.

There are several specifications in the 802.11 family −

• 802.11 − This pertains to wireless LANs and provides 1 - or 2-Mbps transmission in the 2.4-GHz band using either frequency-hopping spread spectrum (FHSS) or direct-sequence spread spectrum (DSSS).
• 802.11a − This is an extension to 802.11 that pertains to wireless LANs and goes as fast as 54 Mbps in the 5-GHz band. 802.11a employs the orthogonal frequency division multiplexing (OFDM) encoding scheme as opposed to either FHSS or DSSS.
• 802.11b − The 802.11 high rate WiFi is an extension to 802.11 that pertains to wireless LANs and yields a connection as fast as 11 Mbps transmission (with a fallback to 5.5, 2, and 1 Mbps depending on strength of signal) in the 2.4-GHz band. The 802.11b specification uses only DSSS. Note that 802.11b was actually an amendment to the original 802.11 standard added in 1999 to permit wireless functionality to be analogous to hard-wired Ethernet connections.
• 802.11g − This pertains to wireless LANs and provides 20+ Mbps in the 2.4-GHz band.

Here is the technical comparison between the three major WiFi standards.

Feature	WiFi (802.11b)	WiFi (802.11a/g)
PrimaryApplication	Wireless LAN	Wireless LAN
Frequency Band	2.4 GHz ISM	2.4 GHz ISM (g) 5 GHz U-NII (a)
Channel Bandwidth	25 MHz	20 MHz
Half/Full Duplex	Half	Half
Radio Technology	Direct Sequence Spread Spectrum	OFDM (64-channels)
Bandwidth	<=0.44 bps/Hz	≤=2.7 bps/Hz
Efficiency	Avg	High
Modulation	QPSK	BPSK, QPSK, 16-, 64-QAM
FEC	None	Convolutional Code
Encryption	Optional- RC4m (AES in 802.11i)	Optional- RC4(AES in 802.11i)
Mobility	In development	In development
Mesh	Vendor Proprietary	Vendor Proprietary
Access Protocol	CSMA/CA	CSMA/CA