Effect of Combined Adaptive Modulation Technique on The Performance Of Wireless Communication System
ABSTRACT
Modern reliable high speed wireless data communication requires special adaptive technology to combat various negative effects that occurs in a communication channel such as multipath phenomena, fading, path loss, Doppler shift etc. Adaptive modulation and transmitter antenna diversity are the key techniques to overcome the channel driven problems. In our study we had investigated the three combined adaptive modulation schemes – adaptive modulation and selective transmitter diversity, adaptive modulation and transmit adaptive array and adaptive space time modulation. We had compared the effect of fading gain, instantaneous Signal to Noise Ratio and constellation size on system performance for the three different schemes. With the increase of fading gain the Bit Error Rate performance decreases. But with the increase of Signal to Noise ratio and constellation size the system performance increases significantly. Among all three combined schemes the adaptive modulation and transmit adaptive array is better because this scheme requires the complex valued Channel State Information to specify the antenna weighting factors, whereas others only require the predicted channel power.
CHAPTER 1
1.1 Overview of Communication
Communication is the transfer of information from one place to another place [1]. As a matter of fact communication is just a transfer of message and even the way two people converse is a form of communication. Distribution of the data, message from one place to another location with high reliability and security is the major role of communication system.
1.1.1 Elements of Communication System
Communication System consists of the following components which acts together to accomplish information transfer or exchange. This includes input transducer, transmitter, channel, receiver and output transducer [1-3].
Figure1.1: Basic block diagram of communication system
The above diagram represents the block diagram of communication system. Now let us explaineach block in details.
1.1.2 Main Blocks of Communication Systems
Input Transducer:
· The input message by a source must be converted by a transducer to form suitable for the particular type of communication.
· In electrical communication, speech waves are converted to voltage variation by a microphone.
Transmitter:
· The transmitter processes an input signal to produce a transmitted signal suited to the characteristics of the transmission channel.
· Signal processing for transmission always involves modulation. In addition to modulation, other functions performed by the transmission are amplification, filtering and coupling the modulated signal to the channel.
Channel:
The channel can have different forms
· The atmosphere or free space
· Coaxial cable
· Fiber optic
· Wave guide etc
Receiver:
The receiver function is to extract the desired signal from the received signal at the channel output and to convert it to a form suitable for the output transducer. Speaker acts as a transducer to convert the received electric signal to voice signal.
Output Transducer:
The function of the output transducer is to convert the electric signal at its input into the form desired by the user.
1.1.3 Classification of Communication Systems
Communication system can be categorized into two different types – Analog Communication & Digital Communication, based on types of signal it transmits between communication transmitter and receiver.
Table 1.1: Different aspects of Analog and Digital Communication
| Topics | Analog Communication | Digital Communication |
| Features | 1. This form of communication uses analog messages.
2. An analog message is a physical quantity that varies with time usually in a smooth and continuous fashion. 3. Since the information resides in a time varying waveform, an analog communication system should deliver this wave form with a specific degree of reliability or fidelity. |
1. This form of communication uses digital messages.
2. A digital message is a ordered sequence of symbols selected from a finite set of discrete elements. 3. Since the information resides in discrete symbols a digital communication system should deliver these symbols with a specified degree of accuracy in a specified amount of time. |
| Advantages | 1. Smaller bandwidth is required.
2. Synchronizing problem is relatively easier. 3. Infinite amount of signal resolution. 4. Analog has high density compare to digital communication. |
1.Inexpensive digital circuits
2. Privacy preserved (data encryption). 3. Can merge different date and transmit over a common digital transmission system. 4.Error correction is possible by coding |
| Topics | Analog Communication | Digital Communication |
| Disadvantages | 1. Expensive analog components L & C.
2. No privacy i.e. security is high. 3. Cannot merge data from different sources. 4. No error correction capability |
1. Larger bandwidth is required.
2. Synchronizing problem is relatively difficult. 3. It is unreliable as the messages cannot be recognized by signatures. 4. The establishment of Digital Communication causes degradation of the environment in some cases and also misuse of efficiency. |
| Application | 1. Data Transmission, Shannon Hartley Theorem.
2. Smart Transducer, Communication Security. 3.Multiplexing , Vocoder ,Channel, Flash ADC. |
1.Digital Transmission, Orthogonal Frequency Division Multiplexing
2. TWAIN, GSM, 3.Digital Watermarking 4.Direct digital synthesizer, Visix, Error detection and correction 5.Robotic welding 6.Paralleled power wave 7.Networking terminology (LAN,WAN,MAC,Ethernet) |
Communication system can also be categorized another two different types – wired and wireless communication.
Table 1.2: Different aspects of wired and wireless communication
| Topics | Wired communication | Wireless communication |
| Features | 1. Radio waves are the forms of electromagnetic radiation the energy is conveyed by waves of magnetic and electric field.
2. In a wire these waves are induced and guided by an electric current passing along with the electrical conductor but this is not the only way of propagation. 3. The radio waves are produced by radio transmitter which consists of a radio wave source connected to some form of antenna. |
1. The transmission
Channel is the main issue of communication system and conventionally it is the set of hard wired cables that connect all the lines of the wire. 2. In wireless system the cables are replaced by the free space, but only at the cost of requiring the erection of antennas that allow the line of sight communication. |
| Advantages | 1. Compressive sensing.
2. Secondary surveillance radar. 3. Sound ranging is passive method 4. Sound ranging equipment tends to be small. 5. It offers more security as data is not sent through air. 6. No worry of batteries. 7. Cheap long distance communication quick. |
1. Freedom from land acquisition.
2. Ease of communication over different terrain. |
| Topics | Wired communication | Wireless communication |
| Disadvantages | 1.Limited range
2. More signal loss occurs and the signal travels down the wire. 3.A terrestrial wireless link is less secure and less reliable 4.Needs more time to send data than wireless 5.Not comfortable for user to send information |
1.Bandwidth allocation is extremely limited
2.Atmospheric effects 3.Transmission needs to be clear 4.Interference and efficiency propagation 5.Restrictive costs |
| Application | 1.Telephone Network
2.Cable Television 3.Internet Access 4.Fiber optic Communication 5.Waveguide (Electromagnetism) |
1.Cellular Telephone (Phones and Modems)
2.Wi-Fi , Wireless microphones, remote controls IrDA 3.RFID (Radio Frequency Identification) 4.Wireless USB, Wireless sensing network, Wireless LAN |
1.2 Modern Wireless Communication Systems
1.2.1 GSM
GSM stands for Global System for Mobile communication [2]. It is a 2nd generation mobile phone system. At present, this is the most popular standard for mobile telephony system and according to GSM Association – This standard is used by 80% of the global mobile market. In this standard both the signaling and speech channels are digital. Which makes this standard different from the previous standard’s [4-6].
Table 1.3: History of GSM
Services by GSM:
According to specifications, many services are offered by GSM. The most basic service is telephony. Speech is digitally encoded and transmitted as bit stream. It also offers a variety of data services. GSM users can send & receive data to & from various other systems like – POTS, ISDN, PSPDN, CSPDN at a rates up to 9600 bps. In contrast with its analog counterparts, GSM provides a unique feature – SMS. Two modes are available for SMS – point to point & cell broadcast. A lot of other services are currently available for the users of GSM.
GSM Network Architecture:
Figure 1.2: GSM architecture
Several building blocks composed the network architecture. The layout of a generic GSM network is shown in Figure-1.2. From the figure we can see that the whole network is composed of three subsystems – Mobile Station, Base Station Subsystems & Network Subsystem. The first Part is carried by the user, second part controls the radio link and the third part involves with switching.
v Mobile Station:
The mobile equipment (the terminal) & the Subscriber Identity Module (SIM) make the Mobile Station (MS). SIM provides operator mobility to the user. Identification of mobile equipment is done by a unique 16 digit number called IMEI. For SIM card identification another unique number is used called IMSI. IMEI & IMSI are independent from each other.
v Base Station Subsystem:
In this subsystem, there are two parts. The Base Transceiver System (BTS) & the Base Station Controller (BSC). BTS is responsible for defining a call & handles the radio link protocols with Mobile Stations. Requirements for BTS deployments are ruggedness, reliability, portability and minimum cost. While radio resources for one or more BTS is managed by the BSC. It also deals with the process of radio-channel setup, frequency hopping and handovers.
v Network Subsystem:
Mobile services Switching Center (MSC) is the main part of network subsystem. It works as a switching node and some extra functionality that are needed to handle a mobile subscriber, such as registration, authentication, location updating & call routing to roaming subscribers is also provided by MSC. For the extra functionality VLR, HLR, EIR & AuC are used together with MSC.
Technical Aspects of GSM system:
Different carrier frequency ranges are used by GSM. Mostly used are either 900 MHz or 1800 MHz Due to limitations of frequency spectrum, some other ranges are also used like 850 MHz, 1900 MHz, 400 MHz, 450 MHz etc. Regardless of carrier frequency, frequency is divided into time-slot for the use of individual phones. By doing this we get 8 full-rate or 16 half-rate voice channels per Hertz of frequency. Time slots are then grouped to form a Time Division Multiple Access (TDMA) frame. The data rate is 270.833 Kbit/sec. and frame duration is 4.615 ms. The modulation scheme that is selected for GSM is Gaussian Minimum Shift-Keying (GMSK) considering a lot of factors like spectral efficiency, complexity of the transmitter and limited spurious emission. The speech coding technique for GSM is a Regular Pulse Excited – Linear Predictive Coder (RPE – LPC) with a Long Term Predictor Loop. The length of each speech sample frame is 20 milli seconds. Each frame is encoded as 260 bits with a total bit rate of 13 Kbps. In case of Channel coding, GSM uses conventional coding and block interleaving. And finally come security. GSM is a very secured network. Here authentication of receiver is done in two ways. It uses a secret a key between SIM card and Authentication Center (AuC) which is ideally non-breakable.
In conclusion, it can be said that GSM is the most popular and widely accepted mobile communication standard. Though it is a complex standard but this complexity gives us a standard level of integrated service and quality along with high level of security.
1.2.2 CDMA
In code division multiple access (CDMA) system, the narrowband massage signal is multiplied by a very large bandwidth signal called the spreading signal [4-6]. All CDMA users use the same carrier frequency and may transmit simultaneously which we see in Figure 1.3. Each user has its own pseudorandom codeword. The receiver performs a time correlation operation to detect only the specific desired codeword. All other codeword appear as noise. Each user operates independently with no knowledge of the other users.
Figure 1.3: CDMA system
There are three ways to spread the bandwidth of the signal:
v Frequency hopping: The signal is rapidly switched between different frequencies within the hopping bandwidth pseudo-randomly, and the receiver knows before hand where to find the signal at any given time.
v Time hopping: The signal is transmitted in short bursts pseudo-randomly, and the receiver knows beforehand when to expect the burst.
v Direct sequence: The digital data is directly coded at a much higher frequency. The code is generated pseudo-randomly, the receiver knows how to generate the same code, and correlates the received signal with that code to extract the data.
Key Features of CDMA System:
The key features of a CDMA system (Particularly IS-95) are given below:
Diversity: To mitigate the effect of fading, some form of diversity is required in cellular systems. CDMA system provides following types of diversity:
Ø Time diversity, provided by symbol interleaving, error detection & correction coding.
Ø Frequency diversity provided by the 1.25 MHz wideband signal.
Ø Space (Path) diversity, provided by dual cell-site receive antennas, multipath rake receivers, and multiple cell sites (soft handoff)
Power Control: CDMA is interference limited multiple access system. Because all users transmit on the same frequency, internal interference generated by the system is the most significant factor in determining system capacity and call quality. The transmit power for each user must be reduced to limit interference, however, the power should be enough to maintain the required Eb/No (signal to noise ratio) for a satisfactory call quality. Maximum capacity is achieved when Eb/No of every user is at the minimum level needed for the acceptable channel performance. As the MS moves around, the RF environment continuously changes due to fast and slow fading, external interference, shadowing, and other factors. The aim of the dynamic power control is to limit transmitted power on both the links while maintaining link quality under all conditions. Additional advantages are longer mobile battery life and longer life span of BTS power amplifiers.
Soft Handoff: Since all cells in CDMA use the same frequency, it is possible to make the connection to the new cell before leaving the current cell. This is known as a “make-before-break” or “soft” handover. Soft handovers require less power, which reduces interference and increases capacity.
CDMA System Capacity:
The theoretical capacity of a CDMA system (IS-95) in terms of calls per 1.25 MHz channel per cell is provided by:
(1.1)
Where
Np = Capacity in terms of calls/1.25 MHz channel/cell
W/R= Ratio of the spreading code to the maximum information rate.
U= Voice activity gain
S = Sectors per cell.
F = Frequency reuse factor
Eb/N0= Minimum ratio of bit energy to noise power
Advantages of CDMA:
- Increased cellular communications security.
- Simultaneous conversations.
- Increased efficiency, meaning that the carrier can serve more subscribers.
- Smaller phones.
- Low power requirements and little cell-to-cell coordination needed by operators.
- Extended reach – beneficial to rural users situated far from cells.
Disadvantages of CDMA:
- Due to its proprietary nature, all of CDMA’s flaws are not known to the engineering community.
- CDMA is relatively new, and the network is not as mature as GSM.
- CDMA cannot offer international roaming, a large GSM advantage.
1.2.3: Third Generation (3G)
3G stands for Third Generation. It is a telecommunication standard developed by the International Telecommunication Union – Radio Communication (ITU – R). Its specifications are set to facilitate a global wireless infrastructure. Its standard name is IMT – 2010. IMT – 2000 is a general name used for all 3G systems. More advanced capabilities are included here and it provides a clear direction for smooth transformation from 2G to 3G [2].
The key features of the IMT – 2000 systems are [5]:
· High degree of commonality of design worldwide.
· Compatibility of services within IMT – 2000 and fixed networks (e.g. PSTN)
· High quality of service for voice and data.
· Small terminal at subscriber end for worldwide use including Pico, Micro, Macro and satellite cells.
· Worldwide roaming capabilities.
· Capability for multimedia applications and a wide range of services and terminals.
Services provided by 3G cellular systems:
v High bearer rate capabilities, including
· 2Mbps for fixed environment
· 384 Kbps for indoor / outdoor and pedestrian environment.
· 144 Kbps for vehicular environment.
v Standardization Work: The following table gives an idea about standardization works at different region
Table 1.4: Regional Standardization of 3G
| Europe (ETSI: European Telecommunications Standardization Institute) = > UMTS (W-CDMA) | Japan (ARIB: Association of Radio Industries & Business) = > W-CDMA | USA (TIA: Telecommunications Industry Association) = >
CDMA 2000 |
v Approved Radio Interfaces:
The following figure gives an idea about the radio interfaces approved for to be used in 3G system. Different techniques that are used for multiplexing purposes are customized to be used in 3G system
Figure 1.4: Approved radio interface of 3G
Comparison between different 3G systems:
Table 1.5: A comparison of different standard of 3G systems in 3 different regions
| Parameter | W – CDMA (Europe) | W – CDMA (Japan) | CDMA 2000 (USA) |
| Multiple Access | WB DS-CDMA | WB DS-CDMA | WB DS-CDMA |
| Duplex Method | FDD / TDD | FDD / TDD | FDD |
| Channel Bandwidth | 1.2/5/10/20 MHz | 1.25/5/10/20 MHz | 1.25/5/10/20 MHz |
| Chip Rate (Mcps) | 1.024*(1,4,8,16) | 1.024*(1,4,8,16) | 1.2288*(1,3,6,12) |
| Frame Length | 10 ms | 10ms | 20.5ms |
| Inter – BS synch. | Asynch. | Asynch./Synch. | Synch. |
| Data Modulation | QPSK/BPSK (FDD)
QPSK/QPSK (TDD) |
QPSK/BPSK (FDD)
QPSK/BPSK (TDD) |
QPSK/BPSK |
| Spread Modulation | QPSK/QPSK | QPSK/QPSK | QPSK/QPSK |
| MultiRate Concept | VSF+ Multicode +
Multislot (TDD) |
VSF+ Multicode +
Multislot (TDD) |
VSF/Multicode |
| Tx. Power Control | CLPC, 1.6 Ks/s
OLPC, 1.6 Ks/s |
CLPC, 1.6 Ks/s
OLPC, 1.6 Ks/s |
CLPC, 0.8 Ks/s
OLPC, 0.8 Ks/s |
| Spreading Codes | Short/Long | Short/Long | Short/Long |
| Coherent Detection | With Pilot Symbol | With Pilot Symbol | With Pilot Symbol |
| Voice Codecs | Variable or fixed rate | Variable or Fixed rate | Variable Rate (EVRC) |
1.2.4 Wi-Fi
Introduction: Wi-Fi is a shorted form of “Wireless Fidelity” [2]. Currently this is one of the most popular wireless communication standard. In its early stages, it was used solely to wirelessly connect laptop computers to the internet via Local Area Network (LAN). But due to further advances in technology, the scenario has changed totally. Now-a-days it is used to connect not only computer but also almost all sort of non-computer electronic devices such as – Home theater receivers, Portable gaming devices, DVD players, Digital camera and even GPS receiver.
Wireless Standard:
The official name for the specification is <href=”#11″>IEEE 802.11, and it is comprised of more than 20 different standards, each of which is denoted by a letter appended to the end of the name. The most familiar standards are 802.11b and 802.11g (Wireless B and G) which are used in the majority of commercial Wi-Fi devices. Both of these standards operate in the <href=”#2.4″>2.4 GHz band, and the only major difference between the two is the <href=”#rate”>transfer rate. Some consumer electronics, however, use a different standard—<href=”#a”>Wireless A. These devices operate within the <href=”#5″>5 GHz range and have transfer rates equivalent to 802.11g. However, since they operate on different frequencies, devices using the 802.11a standard cannot communicate with B and G-enabled devices.
Comparison of standards:
Table 1.6: An overview of the three most popular current 802.11 standards
| Standard | Frequency | Data Transfer Rate Typical (Max) | Range (indoor) |
| 802.11a | 5 GHz | 25 (50) Mb/sec | about 10 m (30 ft) |
| 802.11b | 2.4GHz | 6.5 (11) Mb/sec | 30 m (90 ft) |
| 802.11g | 2.4 GHz | 25 (54) Mb/sec | 30+ m (90+ ft) |
Advantages of Wi-Fi:
The major advantages of WI-Fi are given below
· It provides unparalleled mobility and flexibility to the user. For example, if anyone had a Wi-Fi enabled mp3 player, he can listen to the music from the local server of the LAN or any computer attached to the network. Even he can listen to the internet radio without facing the hassle of wire.
· Setting up a Wi-Fi network is a very easy and quick process. Many modern routers are “Plug-and-Play” devices. Just connect and start using services even without installing any software’s.
· Fast data transfer rate is the main advantage of Wi-Fi. The standard 802.11g is currently the fastest commercially available Wi-Fi protocol in the market with transfer speed up to 54Mbps.
Limitations of Wi-Fi:
Wi-Fi has some limitations also. They are pointed out below
· Though it is easy to setup, securing the network requires some more cautious effort. Otherwise, it will be an easy target of the hacker. Encryption of data does not come automatically. It has to be done when the network is running.
· The frequency spectrum for Wi-Fi lies primarily within the 2.4 GHz spectrum. Which make it susceptible to the interference that comes from neighboring equipments such as – Bluetooth devices, cordless telephones, Micro wave oven and other household devices.
· Though it supports high data rate, but this data rate is still beyond the limit of some of the today’s high-end media. High-Definition audio and video files are bandwidth and timely-delivery-intensive. But Wi-Fi cannot do this consistently and flawlessly.
1.2.5 WiMAX
WiMAXis an acronym for Worldwide Interoperability for Microwave Access. It is Based on Wireless Metropolitan Area Network (MAN) technology [4-6]. It is optimized for the delivery of IP centric services over a wide area. This is a scalable wireless platform for constructing alternative and complementary broadband networks. And also WiMAX is a certification that denotes interoperability of equipment built to the IEEE 802.16 or compatible standard. This technology can provide Broadband Wireless Access (BWA) up to 30 miles (50 kilometers) for fixed stations, and 3-10 miles (5-15 kilometers) for mobile stations. In a typical cell radius deployment of three to ten kilometers, WiMAX forum certified system can be expected to deliver up to 40 Mbps per channel, for fixed and portable access applications.
Services:
WiMAX can provide two form of wireless services:
Non-line-of-sight: service is a WiFi sort of service. Here a small antenna on your computer connects to the WiMAX tower. In this mode, WiMAX uses a lower frequency range – 2 GHz to 11 GHz (similar to WiFi).
Line-of-sight: service, where a fixed dish antenna points straight at the WiMAX tower from a rooftop or pole. The line-of-sight connection is stronger and more stable, so it’s able to send a lot of data with fewer errors. Line-of-sight transmissions use higher frequencies, with ranges reaching a possible 66 GHz.
Features of WiMAX:
Some key features of WiMAx are given below:
· OFDM-based physical layer
· Very high peak data rates
· Scalable bandwidth and data rate support
· Adaptive modulation and coding (AMC)
· Link-layer retransmissions
· Support for TDD and FDD
· WiMAX uses OFDM
· Flexible and dynamic per user resource allocation
· Support for advanced antenna techniques
· Quality-of-service support
· Robust security
· Support for mobility
· IP-based architecture
WiMAX Standards:
Following is the chart of comparison of various IEEE 802.16 Standard related to WiMAX
Table 1.7: Different aspects of three different WiMAX standards
| 802.16 | 802.16a | 802.16e | |
| Spectrum | 10-66 GHz | 2-11 GHz | <6 GHz |
| Configuration | Line of Sight | Non- Line of Sight | Non-Line of Sight |
| Bit Rate | 32 to 134 Mbps
(28 MHz Channel) |
? 70 or 100 Mbps
(20 MHz Channel) |
Upto 15 Mbps |
| Modulation | QPSK, 16-QAM, 64-QAM | 256 Sub-carrier OFDM using QPSK, 16-QAM, 64-QAM,
256-QAM, |
Same as 802.16a |
| Mobility | Fixed | Fixed | ? 75MPH |
| Channel Bandwidth | 20, 25, 28 MHz | Selectable
1.25 to 20 MHz |
5 MHz (planned) |
| Typical Cell Radius | 1-3 miles | 3-5 miles | 1-3 miles |
| Completed | Dec, 2001 | Jan, 2003 | 2nd Half of 2005 |
WiMAX Architecture:
A wireless MAN based on the WiMAX air interface standard is configured in much the same way as a traditional cellular network with strategically located base stations using a point-to-multi-point architecture to deliver services over a radius of up to several miles, depending on frequency, transmit power, and receiver sensitivity. In areas with high population densities, the range will generally be capacity limited rather than range limited, owing to limited bandwidth. The base stations are typically backhauled to the core network by means of fiber or point-to-point microwave links to available fiber nodes or via leased lines from an existing wireline operator. The range and NLOS capability make the technology equally attractive and cost effective in a wide variety of environments. The technology was envisioned from the beginning as a means of providing wireless last mile broadband access in the MAN with performance and services comparable to or better than traditional DSL, cable, or T1/E1 leased line services.
Figure 1.5: WiMAX Network Architecture
The technology is expected to be adopted by different incumbent operator types, for example, wireless internet service providers (WISPs), cellular operators (CDMA and WCDMA), and wire line broadband providers. Each of these operators will approach the market with different business models based on their current markets and perceived opportunities for broadband wireless as well as different requirements for integration with existing (legacy) networks. As a result, 802.16 network deployments face the challenging task of needing to adapt to different network architectures while supporting standardized components and interfaces for multi-vendor interoperability.
1.2.6 Fourth Generation (4G)
4G is the short name for fourth-generation wireless, the stage of broadband mobile communications that will supersede the third generation [4-6]. It is a fourth generation cellular communication system based on fourth generation mobile technology. In this system, networks operate on internet technology and combine all other applications & technologies such as Wi-Fi with this. This is a fully IP – based wireless communication system with high level of network security. It can provide high data speed of up to 100 Mbps (Outdoor) & 1Gbps (indoor). One of the key features of this system is that it can provide any services, anytime, anywhere at an affordable cost.
Objectives:
· Efficient and High network capacity
· Nominal date rate: 100 Mbps-1 Gbps
· Smooth hand-off across heterogeneous network
· Seamless connectivity
· Global roaming across multiple networks
· High quality of service for multimedia support (real time audio, high speed data, HDTV video content, mobile TV, etc)
· Interoperation ability with the existing wireless standards
· All IP system, packet switched network
Table 1.8: Comparison between 3G & 4G
| 3G (including 2.5G, sub3G) | 4G | |
| Major Requirement Driving
Architecture |
Predominantly voice driven – data was always add on | Converged data and voice over IP |
| Network Architecture | Wide area cell-based | Hybrid – Integration of Wireless LAN (Wi-Fi, Bluetooth) and wide area |
| Speeds | 384 Kbps to 2 Mbps | 20 to 100 Mbps in mobile mode |
| Frequency Band | Dependent on country or continent (1800-2400 MHz) | Higher frequency bands (2- 8 GHz) |
| Bandwidth | 5-20 MHz | 100 MHz (or more) |
| Switching Design Basis | Circuit and Packet | All digital with packet voice |
| Access Technologies | W-CDMA, 1xRTT, Edge | OFDMA and MC-CDMA (Multi Carrier CDMA) |
| Forward Error Correction | Convolutional rate 1/2, 1/3 | Concatenated coding scheme |
| Component Design | Optimized antenna design, multi-band adapters | Smarter Antennas, software multi-band and wideband radios, Software-Defined Radio |
| IP | A number of air link protocols,
Including IP 5.0 |
All IP (IP6.0) |
Figure 1.6: 4G Network Architecture
4G Key Components:
v Access Schemes
To add advantages in scalability new access schemes like OFDMA, Single carrier FDMA, and
MC-CDMA has been proposed as part of the next generation UMTS, 802.16e and 802.20 standards.
v IPv6
Using Ipv6 Removes the need for Network Address Translation (NAT). It enables a number of applications with better multi-cast, security and route optimization capabilities. Provides support to a great number of wireless enabled devices. It also provides more available address space and number of addressing bits. This also enables 4G coding schemes innovation
v Multi-Antenna Systems
By using multi – antenna systems we can use MIMO (Multiple-input and multiple-output) multiplexing Which is used to send data via various routes across a network in order to increase date capacity. MIMO increases the peak data rates and average throughput of data systems.
v Software-Defined Radio (SDR)
4G devices will constitute all collection of wireless standards. This can be realized by using SDR technology. SDR is one form of open wireless architecture (OWA).
1.3 Motivation and objective of the present work
In wireless communication, the channel though which we transmit the information is very important. During Transmission through the channel the signal suffers some unavoidable phenomena like fading, path loss, noise and relative movement. To overcome those phenomena, adaptive modulation schemes can be applied. Another phenomena is the time varying nature of the channel, which requires the adjustment of modulation schemes, coding techniques and transmit power levels according the channel instantaneous SNR. If the channel SNR can be estimated then the desired modulation scheme can be adjusted. So it very important to model and predict the future channels. In our thesis paper, first we investigate the combined adaptive transmission techniques with transmitter diversity. Then we study the capacity of Rayleigh fading channel under different adaptive transmission techniques. At last we study the long range channel prediction.
1.4 Organization of the paper
This thesis consists of 3 chapters – chapter 1 gives an introduction of various types of communication systems. In chapter 2 we shall discuss the features of different communication channels and then importance of channel modeling is elaborated. Finally the adaptive transmission, combined adaptive modulation with transmitter diversity, long range prediction etc are explained. In chapter 3 we have given our simulated results for different combined adaptive modulation schemes.
CHAPTER 2
COMMUNICATION CHANNEL MODELS
2.1 Physical Communication Channels
Communication channels are the transmission medium through which the transmission of information across a communication network is accomplished [7]. It is the center to the operation of communication system. Its properties determine both the information carrying capacity of the system and the quality of service offered by the system. It is used to convey an information signal from one or several senders (transmitters) to one or several receivers. Depending on mode of transmission we can classify two basic groups of communication channels.
Communication channel
Channels based on guided propagation Channels based on free space propagation
Examples: Telephone channels Examples: Broadcast channels
Coaxial cables Mobile radio channels
Optical fibers Satellite channels
2.1.1 Channels Based on Guided Propagation
Telephone channels
A telephone network uses circuit switching to establish an end to end communication link on temporary basis [7]. Communication link is established between a speaker at one end of the link and a listener at the other end. The telephone channel supports only the transmission of electrical signals. Appropriate transducers are used at the transmitter and receiving ends of the system. A microphone is placed near the speaker’s mouth to convert sound waves into electrical signal and the electrical signal is converted back into acoustic form by using a moving–coil receiver placed near the listener’s ear. The telephone channel is a bandwidth limited channel as sharing of channel among a multitude of user at one time.
The telephone channel is built using twisted pairs for signal transmission. A twisted pair consists of two solid copper conductors, each of which is encased in a polyvinylchloride (PVC) sheath. Typically, each pair has a twist rate of 2 to 12 twists per foot and a characteristic impedance of 90 to 110 ohms. Twisted pairs are usually made up into cables, with each cable consisting of many pairs. Twisted pairs are naturally susceptible to electromagnetic interference (EMI), the effects of which are mitigated through twisting the wires.
Coaxial cables
A Coaxial cable consists of an inner conductor and an outer conductor, separated by a dielectric insulating material [7]. The inner conductor is made of a copper wire encased inside the dielectric material. As for the outer conductor, it is made of copper, tinned copper, or copper coated steel. A coaxial cable has a characteristic impedance of 50 to 75 ohms. Compared to twisted pair cable it is less affected by EMI and has higher bandwidth. It has a bit rate up to 20 Mb/s with 10 Mb/s being the standard.
Applications:
· Coaxial operate as a multiple access medium by using high impedance tap.
· Used as a transmission medium for local area network in an office environment.
· Used in cable television system, also known as community antenna television (CATV) system.
Optical fiber
An optical fiber is a dielectric wave guide that transports light signals from one place to another just as a twisted-wire pair or a coaxial cable transports electrical signals [7]. It consists of a central core within which the propagating electromagnetic field is confined and which is surrounded by a cladding layer, which itself surrounded by a thin protective jacket. The core and cladding are both made of pure silica glass, whereas the jacket is made of plastic.
Characteristics:
· Enormous potential bandwidth ranging from GHz to THz
· Low transmission loss, as low as 0.1db/km.
· Immunity to electromagnetic interference
· Small size and weight
· Ruggedness and flexibility
Applications:
· Used in long distance communication as low transmission loss.
· To transmit telephone signals Internet communication, and cable television signals.
· To supply a low level of power (around one watt) to electronics situated in a difficult electrical environment (high powered antenna elements).
Twisted pair cable coaxial cable
Optical fiber cable
Figure 2.1: Different Channels based on guided propagation
2.1.2 Wireless Communication Channels
Wireless broadcast channels
Wireless broadcast channel support the transmission of radio and television signals [7]. The information bearing signal, representing speech, music, or pictures, is modulated onto a carrier frequency that identifies the transmitting station. The transmission originates from an antenna that acts as the transition or matching unit between the source of the modulated signal and electromagnetic waves in free space. The antenna is designed to excite the waves in required directions. The transmitting antenna is mounted on a tower to provide an unobstructed view of the surrounding area. Radio waves are bent around the earth’s surface by the virtue of the phenomenon of diffraction.
At the receiving end, an antenna is used to pick up the radiated waves, establishing a communication link to the transmitter. Most radio receivers are of super-heterodyne type. This technique consists of down converting the received signal to some convenient intermediate frequency (IF) band, and then recovering the original information – bearing signal by means of appropriate decoder.
Mobile radio channel
Mobile radio channel extends the capacity of the public telecommunication network by introducing mobility into the network by the virtue of its ability to broadcast. The term mobile radio is usually meant to encompass terrestrial situations where a radio transmitter or receiver is capable of being moved, regardless of whether it actually moves or not. This channel is applied where there is no “line-of-sight” path for communication; rather, radio propagation takes place mainly by the way of scattering from the surfaces of the surrounding buildings and by diffraction over and around them. The result is multipath phenomena in that the various incoming radio waves reach their destination from different direction with different time delays. The multitude propagation paths with different electrical length are combined in different ways. The received signal strength may vary with the variation of the receiver. So the mobile radio channel can be viewed as a linear time varying channel that is statistical in nature.
Satellite channel
A satellite channel adds invaluable dimension to the public telecommunication network by providing broad-area coverage in both a continental and an intercontinental sense [7]. The satellites are placed in geostationary orbit. For the orbit to be geostationary it requires that the satellite orbits the earth in 24 hours and it is placed in orbit directly above the equator on an eastward heading. A satellite communication system, a message signal is transmitted from an earth station via an uplink to a satellite, amplified in a transponder on board the satellite, and then retransmitted from the satellite via a downlink to another earth station. The frequency band for satellite communication is 6 GHz for the uplink and 4 GHz for the downlink.
Characteristics:
· Broad coverage area.
· Reliable transmission links.
· Wide transmission bandwidth.
· Less inexpensive microwave equipments.
· Low attenuation due to rainfall.
Application:
· For communication.
· Forecast the weather and give alert for imminent disaster.
· To provide GPS service.
· For scientific research and in military.
Wireless broadcast channels Mobile radio channel
Satellite channel
Figure 2.2: Channels based on free space propagation
2.2 Communication Channel Models
A channel can be modeled physically by characterizing the physical processes which modify the transmitted signal. For example in wireless communications the channel can be modeled by calculating the reflection off every object in the environment [2]. A sequence of random numbers might also be added in to simulate external interference or electronic noise in the receiver.
Statistically a communication channel is usually modeled as a triple consisting of an input alphabet, an output alphabet, and for each pair (i,o) of input and output elements a transition probability p(i,o). Semantically, the transition probability is the probability that the symbol o is received given that ‘i’ was transmitted over the channel.
Statistical and physical modeling can be combined. For example in wireless communications the channel is often modeled by a random attenuation (known as fading) of the transmitted signal, followed by additive noise. The attenuation term is a simplification of the underlying physical processes and captures the change in signal power over the course of the transmission. The noise in the model captures external interference and/or electronic noise in the receiver. If the attenuation term is complex it also describes the relative time a signal takes to get through the channel. The statistics of the random attenuation are decided by previous measurements or physical simulations.
Table 2.1: Examples of digital and analog channel models
| Digital Channel | Analog Channel |
| 1. Binary symmetric channel (BSC), a discrete memory-less channel with a certain bit error probability | 1. Noise model, for example
Ø Additive white Gaussian noise (AWGN) channel, a linear continuous memory-less model Ø Phase noise model |
| 2. Binary bursty bit error channel model, a channel “with memory”
3. Binary erasure channel (BEC), a discrete channel with a certain bit error detection (erasure) probability 4. Packet erasure channel, where packets are lost with a certain packet loss probability or packet error rate. 5. Arbitrarily varying channel (AVC), where the behavior and state of the channel can change randomly |
2.Interference model, for example cross-talk (co-channel interference) and inter-symbol interference (ISI)
3.Distortion model, for example a non-linear channel model causing inter-modulation distortion (IMD) 4.Frequency response model, including attenuation and phase-shift 5.Modeling of underlying physical layer transmission techniques, for example a complex-valued equivalent baseband model of modulation and frequency response 6.Radio frequency propagation model, for example Ø Long-distance path loss model Ø Fading model, for example Rayleigh fading, Rician fading, log-normal shadow fading and frequency selective (dispersive) fading Ø Doppler shift model, which combined with fading results in a time-variant system Ø Ray tracing models, which attempt to model the signal propagation and distortions for specified transmitter-receiver geometries, terrain types, and antennas. |
2.2.1 Binary symmetric channel
A binary symmetric channel (or BSC) is a common communications channel model used in coding theory and information theory [2]. In this model, a transmitter wishes to send a bit (a zero or a one), and the receiver receives a bit. It is assumed that the bit is usually transmitted correctly, but that it will be “flipped” with a small probability (the “crossover probability”). This channel is used frequently in information theory because it is one of the simplest channels to analyze.
Figure 2.3:Binary symmetric channel [2]
The BSC is a binary channel; that is, it can transmit only one of two symbols (usually called 0 and 1). (A non-binary channel would be capable of transmitting more than 2 symbols, possibly even an infinite number of choices.) The transmission is not perfect, and occasionally the receiver gets the wrong bit.
This channel is often used by theorists because it is one of the simplest noisy channels to analyze. Many problems in communication theory can be reduced to a BSC. On the other hand, being able to transmit effectively over the BSC can give rise to solutions for more complicated channels.
Definition
A binary symmetric channel with crossover probability p denoted by BSCp, is a channel with binary input and binary output and probability of error p; that is, if X is the transmitted random variable and Y the received variable, then the channel is characterized by the conditional probabilities
Pr( Y = 0 | X = 0 ) = 1 ? p
Pr( Y = 0 | X = 1) = p
Pr( Y = 1 | X = 0 ) = p
Pr( Y = 1 | X = 1 ) = 1 ? p
It is assumed that 0 ? p ? 1/2. If p > 1/2, then the receiver can swap the output (interpret 1 when it sees 0, and vice versa) and obtain an equivalent channel with crossover probability 1 ? p ? 1/2.
2.2.2 Binary erasure channel
Figure 2.4: Binary erasure channel [2]
The channel model for the binary erasure channel showing a mapping from channel input X to channel output Y (with known erasure symbol). The probability of erasure is pe
A binary erasure channel (or BEC) is a common communications channel model used in
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