Wimax wireless radio frequency system.

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CHAPTER 1

Introduction

The wireless technology are emerging technology that is leading the current business- market and making lots of profit within the telecommunication industries. Today there are numerous wireless technologies used for various applications. Some of the most widely applicable wireless communication technologies are listed below:

  1. AM/FM. (Voice transmission)
  2. Infrared. (Data transmission, Low speed, LOS required)
  3. Bluetooth. (V2.0 faster than infrared, NLOS, Frequency hopping)
  4. ZigBee. (Similar to Bluetooth with greater range and throughput)
  5. Wi-Fi.(IEEE 802.11 WLAN network, OFDM,TDM)
  6. WiMAX. (IEEE 802.16 WMAN Network, OFDM,OFDMA)

Wireless radio transmissions are based on transmission of radio waves that propagates through air. Radio waves that lie between the frequency ranges of 30 MHz to 20 GHz are generally used for data transmission. The frequency range that is lower than 30 MHz generally support data transmission/communication, but these frequency bands are typically used for voice transmission over long distance, example: Frequency Modulation (FM), Amplitude Modulation (AM) radio broadcasting as these waves reflect on the ionosphere surface of the Earth's atmosphere to extend the communication. Radio waves that propagate over 20 GHz frequency band cannot be used for long-distance communication because it may easily get contaminated by the moisture present in the atmosphere.

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Due to the higher demand and utilization of digital network, the telecommunication industries are forced to design the new communication network with higher capacity and load. These industries are changing themselves due to the demand for the services to provide in greater range and quality such as voice and video conferencing, online application, multimedia content etc. The dependency on computer network these days are far greater than before and the internet has led to the higher demand for the connection to be provided on the fly, which in turns leads to the increase in the needs for higher capacity and reliability broadband wireless telecommunication systems.

Availability of Broadband introduces high performance connectivity to over almost all of the internet users' worldwide, thus introducing new wireless broadband standards and technologies that will rapidly full fill the need of wireless coverage. Digital wireless communications are emerging technologies that are facing stunning expansion during the last few decades. The new method of obtaining high capacity wireless network is because of none other than the large application of cellular mobile technology, WLAN (Wireless Local Area Network) and the exponential growth of Internet. (1997, S. Sampei)

Worldwide Interoperability for Microwave Access, known as WiMAX, is a wireless networking standard that is specially designed for addressing interoperability based on IEEE 802.16 products. WiMAX also defines a wireless metropolitan area network (WMAN), where the users get connected to the main station of broadband wireless provider. It incorporates the feature of both line of sight (LOS) as well as non-line of sight (NLOS) communication and is alternative technology deployed to replace cables, DSL or T1, E1 etc. One of the most advantageous features of WiMAX based product is that, any products based on WiMAX technology can be shared with some other technologies that provide broadband access and can be implemented in most of the possible areas. The figure below shows the deployment of WiMAX.

BS= Base Station

MSS= Mesh Subscriber Station

RS=Relay station

SS= Subscriber Station

(S.Yousefi, M.Mazoochi, S.Bashirzadeh, “Architecture for Large Scale Deployment of WiMAX Networks” International Conference on Communications and Mobile Computing, IEEE Press 2009.)

It is believed that soon WiMAX will replace all other broadband connection across the globe and is most suitable for the places where it is difficult to deploy other technologies like DSL and cable and is the best choice for the solution of last mile internet access on the go. On the other hand, where wired network are more sophisticated and costly for the maintenance, WiMAX again becomes the best choice for implementation. In this way, with the help of WiMAX, rural, urban areas can easily gets connected to the developed areas or city. WiMAX itself can even be used to deliver backhaul for mobile carrier, universities, and Wi-Fi hot-spots. WiMAX offers a best solution for such challenge as it is cost-effective, easily deployable, high data rate and more above these is its feature of mobility.

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WiMAX has become one of the largest competitors to 3G cellular systems and the mobile data application which can be achieved by IEEE 082.16e is the highest data rate as compared to other technologies till date. Although there is another emerging technology named 3G LTE which is believed to be one of the serious competitors to WiMAX in near future. More detail comparison of WiMAX with other wireless technologies are discussed in later chapters.

Digital modulation

Digital modulation is a type of modulation scheme that transforms digital signals into waveform that is suitable for the transmission channel. Digital modulation can be divided into two major parts. One of them uses a constant amplitude carrier and the information is carried out as the phase of frequency varies, such as FSK, PSK. The other one is known as amplitude shift keying that carries information on the variation of carrier amplitude.

Different modulation schemes are being used these days for effective wireless communication. From the fast few years the entire digital modulation scheme has transacted from simple amplitude modulation (AM) or frequency modulation (FM) to digital techniques like QPSK, FSK, PSK, MSK, QAM etc.

Objective:

The main objective of this thesis is to study the OFDM physical layer and implement the WiMAX transmitter and receiver with basic parameters required for transmitting and receiving the data using orthogonal frequency division multiplexing (OFDM) technique on the SDR (Software Defined Radio). The detail study of SDR (Software Defined Radio) is discussed in later chapter.

Structure of thesis:

Chapter 1: Deals with the introduction of all the necessary backbone required to help understanding the thesis.

Chapter 2: Describes the fundamentals of WiMAX and OFDM, their characteristics, advantages and disadvantage.

Chapter 3: Describes the approach that is used for the simulation i.e. SDR (Software Defined Radio), GNU Radio Companion.

Chapter 4: Discuss Implementation/Simulation of project for the result and Analysis.

Chapter 5: Conclusion and Future work.

Chapter 2

IEEE 802.16 (The WiMAX standard):

The WiMAX stands for Worldwide Interoperability for Microwave Access. WiMAX can be used as a wireless metropolitan area network (Wireless MAN) technology to connect with the IEEE 802.11 standard (also known as Wi-Fi) hot spots to the Internet so as to provide a wireless extension to cable and DSL for long distance broadband access. IEEE 802.16 has a service range of up to 30 miles which allows users to connect directly with the base station without direct line of sight requirement. This technology provides shared data rates of up to 70 Mbps, which can be considered to be enough bandwidth to support more than 50 businesses running simultaneously and supports more than thousand homes that are running at 1-Mbps DSL connectivity as well. The initial WiMAX standard, IEEE 802.16 operates in the frequency range of 10- to 66-GHz. Later 802.16a added support to operate in the frequency range of 2 to 11-GHz, for which most of the parts has already been unlicensed internationally, and the requirement of domestic license for few of them is still remaining to acquire. Because of licensing issue most of the business will probably deploy 802.16a standard. The WiMAX is an enhanced and improved version and is one of the latest technologies that will overlap the limitations of the 802.11 standard by providing extended bandwidth, range and security. In some circumstances, WiMAX is often aimed to provide the connectivity between network endpoints without having the requirement of direct line of sight. In most of the cases as we know that the spectrum that falls less than 5 to 6 GHz is needed to provide reasonable Non-Line of Sight performance and cost effectiveness for point-to-multipoint deployments. WiMAX makes an efficient use of multipath signals without altering the laws of physics. The data rate that is supported by the 802.11 can be easily supported by 802.16, but the only issue that arises is about the interference which is lessened. WiMAX can operate both on licensed as well as non-licensed frequencies that provide the suitable environment of deployment for the wireless service providers. For the deployment of the WiMAX technology, two main parts are required, (2008, G.S.V. Radha Krishna Rao, G.Radhamani)

  1. WiMAX tower
  2. WiMAX receiver
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A WiMAX tower is similar to the tower of cell phone, which transmits signal in all direction. The receiver that falls within the range can easily gets connected with or without the need of line of sight requirement. It is assumed that a single WiMAX tower can provide a very large coverage area of 3000 Sq mi which is equivalent to 8000 sq km. (2008, G.S.V. Radha Krishna Rao, G.Radhamani)

A WiMAX receiver is similar to that of cell phone that have an embedded antenna or it may be of small piece of box, PCMCIA (Personal computer memory card international association) card that can easily be plugged in to the laptop or may be built in to the laptop in the same way as Wi-Fi receiver is. (2008, G.S.V. Radha Krishna Rao, G.Radhamani)

A WiMAX tower station has a direct wired internet connection of very high bandwidth. One WiMAX tower can be easily connected to another WiMAX tower using a LoS microwave link. Because of this feature of connecting one WiMAX tower with another WiMAX tower which is also referred to as a backhaul, allows WiMAX provider to cover very large area in remote places where the other service provider have not been able to provide wired connection like DSL, Cable, T1 etc. A typical scenario of WiMAX transmitter and receiver is shown in the fig below. (2008, G.S.V. Radha Krishna Rao, G.Radhamani)

A WiMAX operation comprises a WiMAX Base Station that provides coverage for entire metropolitan area. Edge network and WiMAX base station can be connected by wireless point-to-point link or by fibre optics link whatever available. WiMAX with point to multipoint wireless connection is effective method for providing last mile broadband access which is not only cost-effective but have high bandwidth to support numerous users at a time. Fixed WiMAX service can be provided to the subscriber by means of end devices (receiver) that are normally mounted on the roof or can either be indoor units. In case of providing service for residential user, RJ-45 or RJ-11 connections are typically included to deliver high speed internet connection without requiring any other additional devices except laptop (PC) or telephone. In order to provide service for business types, T1/E1 interface along with 10/100 Base T Ethernet connection would be desirable. Wireless distribution system can be enabled within the building periphery by means of wireless LAN (Wi-Fi), once the WiMAX terminal gets combined with the existing wireless router. Because of limited spectrum in the lower frequency bands, WiMAX will have limited capacity and will require the alteration of Base Station at a distance of 2 to 3 km. However in the remote areas with low density population, WiMAX can take full advantage and can provide full coverage up to 75 sq km in 3.5 GHz frequency band. (2008, G.S.V. Radha Krishna Rao, G.Radhamani)

The WiMAX quality of service (QoS) is accomplished as it is designed to support variety of traffic such as applications that require very high data rate example: voice and video streaming, applications that require low data rate example: web surfing. Network like WiMAX cannot operate without certain quality of service. While running applications that require very high or low data rate some delays can be acceptable but introduction of too much delay makes application useless. Thus according to IEEE 802.16 group an acceptable delay for VoIP is considered between 120 ms - 150 ms. Keeping all this in mind IEEE 802.16 is designed to be flexible enough and efficient to respond to the end user applications with varied bandwidth and latency requirements. (2008, G.S.V. Radha Krishna Rao, G.Radhamani)

OFDM-256 PHY used in fixed WiMAX could not be used in mobile WiMAX 802.16e as it is specially designed to address mobility so as to meet the requirement for mobile applications. Since the fixed and mobile WiMAX physical layer are fundamentally incompatible and these two versions will be used for different applications. More detail study of WiMAX physical layer is included in later topic. Transition of IEEE 802.16 to IEEE 802.16e-2005 is tabulated below.

Standard

802.16

802.16-2004

802.16e-2005

Year

2001

2004

2005

Frequency band

10GHZ-66GHz

2GHz-11GHz

2GHz-11GHz for fixed.

2GHz-6GHz for mobile.

Application

Fixed LOS

Fixed NLOS

Fixed and mobile NLOS

MAC Architecture

Point-to-multipoint, mesh

Point-to-multipoint, mesh

Point-to-multipoint, mesh

Transmission scheme

Single carrier

Single carrier, 256 OFDM or 2048 OFDM

Single carrier, 256 OFDM or scalable OFDM with 128, 512, 1024 or 2048 subcarriers.

Modulation

QPSK, 16 QAM, 64 QAM.

QPSK, 16 QAM, 64 QAM.

QPSK, 16 QAM, 64 QAM.

Gross data rate

32Mbps-134.4Mbps

1Mbps-75Mbps

1Mbps-75Mbps

Multiplexing

Burst TDM/TDMA

Burst TDM/TDMA/OFDMA

Burst TDM/TDMA/OFDMA

Duplexing

TDD and FDD

TDD and FDD

TDD and FDD

Channel-Bandwidths

20 MHz, 25 MHz, 28 MHz

1.75MHz, 3.5MHz, 7MHz, 14 MHz, 1.25 MHz, 5MHz, 10MHz, 15MHz, 8.75 MHz

1.75MHz, 3.5MHz, 7MHz, 14MHz, 1.25 MHz, 5MHz, 10 MHz, 15 MHz, 8.75 MHz

Air-interface designation

Wireless MAN-SC

WirelessMAN-SCa

WirelessMAN-OFDM

WirelessMAN-OFDMA

WirelessHUMAN

WirelessMAN-SCa

WirelessMAN-OFDM

WirelessMAN-OFDMA

WirelessHUMAN

Implementation

none

256-OFDM for Fixed.

Scalable OFDMA for Mobile.

Table 2.1:- IEEE 802.16 Transition

Relationship with other wireless standard:

WiMAX Vs. 3G LTE

The new WiMAX standard delivers the downlink speed of more than 128 Mbps and uplink speed of 56 Mbps in 20 MHz bandwidth. WiMAX update IEEE 802.16m is expected to offer bandwidth of at least of 1 Gbps for fixed and 100 Mbps for mobile users. The key strength to choose WiMAX is that it uses the approach similar to that of cell phone thus no line of sight is required. The signal broadcasted by the WiMAX is not prone to interference, it is more secure with good quality of service and the most important is its reliability. The WiMAX network includes two key components: a base station to transmit the wireless signal and the subscriber that receives the signal on WiMAX enabled devices. The Mobile WiMAX standard incorporates the Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple Input/Multiple Output (MIMO) smart antenna technology. These technologies help to provide more data into the available airwaves to increase throughput and/or coverage. MIMO is particularly beneficial in high interference environments, like urban centres.

3G LTE on the other hand is a GSM technology (UMTS Cellular technology) that is currently used by the carriers to provide 3G mobile broadband. It incorporates the use of same technology as in WiMAX i.e. OFDMA/ MIMO and SC-FDMA technology. It delivers the downlink speed of 100 Mbps and uplink speed of 50 Mbps in 20 MHz bandwidth. It supports at least 200 active users in every 5 MHz cell. LTE-advanced is expected to offer 1 Gbps for fixed and 100 Mbps for mobile users. HSPA (High Speed Packet Access) which is a combination of HSDPA, HSUPA and HSPA+ are now being deployed. 3G LTE is generally known as 3.99G as it is does not offers full 4G functionality. It uses system architecture Evolution (SAE) so as to reduce the latency time and to route the data to its destination directly.

As it is seen that both WiMAX and 3G LTE are similar technologies are still competing to stand. In case of business environment, WiMAX has already been implemented and is in market where as LTE is still in testing phase. The operator can establish WiMAX network with very low cost as compared to 3G LTE. 3G LTE consumes less power than WiMAX. Latency is one if the key factor in online services like online gaming and videoconferencing, in case of WiMAX is 50 ms but for LTE is only 10 ms. 3G LTE can be easily upgraded to 4G as it is fully integrated with similar infrastructure whereas WiMAX has still not seen any sort of upgrade, so telecom industry can easily set-up the 3G LTE technology without concerning the future development of LTE. In terms of mobility, a mobile target is required with a speed lower than 120 km/h in case of WiMAX but LTE can cope up to 350 km/h.

WiMAX vs. Wi-Fi

IEEE 802.11 which is also known as Wi-Fi (Wireless fidelity) or WLAN is a wireless standard capable of offering data rate up to 54 Mbps at 5.2 GHz frequency band. IEEE 802.11b which operates in the frequency band of 2.4 GHz supports data rate of up to 11 Mbps. These technologies provide a coverage area of 100 m and have a fixed channel bandwidth of 20 MHz. (2004, T. Cooklev)

IEEE 802.16 standard is a solution for last mile wireless broadband internet access that Wi-Fi has not been able to provide. WiMAX architecture is specially designed for metropolitan area network (MAN) for which the base station of WiMAX will be able to provide access to thousands of subscribers. Wi-Fi on the other hand having very less coverage area provides only local area network (LAN). WiMAX installation requires very high cost as compared to Wi-Fi thus Wi-Fi becomes more suitable for those who cannot afford things at high cost. There can be numerous similarities and dissimilarities between Wi-Fi and WiMAX; however it is considered that WiMAX has taken birth from the Wi-Fi with very high data rate and range to provide exceptional quality of service. A more detailed comparison of WiMAX with other wireless and cellular technology is tabulated below. (2004, T. Cooklev)

Wi-Fi

WiMAX

UMTS

HSDPA

Standard

802.11

802.16

Channel width

20 MHz

Variable

20 MHz

Variable

28 MHZ

Fixed

5 MHz

Spectrum

2.4 - 5.2 HGz

2-11 GHz

10-66 GHz

2 GHz

Data rate

2-54 Mbps

70 Mbps

240 Mbps

1-14 Mbps

Range

100 m

1-7 Km

12-15 Km

50 Km

multiplexing

TDM

FDM/ TDM

FDM/TDM

FDM

Transmission

OFDM

OFDM/ OFDMA

SC

WCDMA

Mobility

Pedestrian

Vehicular (802.16e)

No

Vehicular

Advantages

Throughput and costs.

Throughput and range.

Mobility and Range.

Disadvantages

Short range.

Interference issues, synchronization issue.

Low data rate and expensive.

Table 2.2:- WiMAX Vs other wireless standard

Rate

Wi-Fi

WiMAX

UMTS

Mobility

Fig 2.2: - Comparison with other wireless standard.

Features of WiMAX

WiMAX being wireless broadband solution offers various features along with flexibility. Some of the salient features of WiMAX that cannot be neglected are as follows.

  • OFDM based physical layer.
  • OFDMA.
  • High data rate.
  • Quality of service.
  • Mobility support.
  • Scalability.
  • Adaptive modulation and coding (AMC).
  • Link layer retransmission.
  • Support for advanced antenna technology.
  • Security.
  • IP based architecture.

(2010, M.A. Mohamed, F.W. Zaki, R.H. Mosbeh)

WiMAX Technical overview

An air interface standard for wireless broadband was developed by IEEE 802.16 group in 1998. The group's initially intended to develop of a line-of-sight based point-to-multipoint wireless broadband system that operates in the 10GHz-66GHz millimetre wave band. Later on the IEEE 802.16 group produced 802.16a to include non-line-of-sight (NLOS) applications in the frequency band of 2-11 GHz by means of Orthogonal Frequency Division Multiplexing (OFDM) based physical layer. The revisions made further has resulted to a new standard in 2004, called IEEE802.16-2004, which replaces the entire previous version developed so far and introduce itself as a foundation of the first WiMAX technology. Further the WiMAX is enhanced in various aspects of signal processing and transmission techniques.

Starting from IEEE 802.16 up to IEEE 802.16e-2005 a number of enhancements have been made (tabulated in table 2.1). From this we can conclude that IEEE 802.16 is a bunch of standards that can be deployed depending on the requirements, it can be achieved by defining a limited number of system profiles and certification profiles. The WiMAX forum has proposed two different system profiles, among them one is based on IEEE 802.16-2004 i.e. fixed system profile and other one based on IEEE 802.16e-2005 i.e. mobility system profile. The WiMAX forum has defined five fixed certification profiles and fourteen mobility profiles so far. To date, there are two fixed WiMAX profile against which equipment have been certified.

With the completion of the IEEE 802.16e-2005 standard, the WiMAX group has advanced their step towards developing mobile WiMAX system profile based on the newer profile. This uses scalable OFDMA as the physical layer. It is also found that the current mobility certification profiles are based on TDD. Although TDD is preferred more, but FDD profile may be needed in the future to comply with regulatory pairing and interoperator coexistence requirements in certain bands.

Air interface for IEEE 802.16-2004 specification that operates on frequency band of 2-11 GHz is defined by the WiMAX standard. This air interface defines two different layer of WiMAX, one of them is medium access control (MAC) layer and other one is physical layer (PHY).

Medium Access Control (MAC) layer

MAC layer of the IEEE 802.16 was specially designed for point-to-multipoint BWA application based on CSMA/CA scheme. The WiMAX MAC layer which is situated above the PHY layer is one of the major parts that play a vital role for providing an interface between the transport layer and physical layer. It takes packets called MAC service data units, organize into MAC protocol data unit and transfer to the physical layer for wireless transmission. The reverse process is done in the receiver end. The IEEE 802.16-2004 and IEEE 802.16e-2005 MAC layer is designed to have various sub layers to interface with different higher layer protocols like internet protocol, Ethernet, ATM TDM voice, etc.

Due to the requirement of services in different environment the MAC layer is designed to support multiple physical layer specification and services. Since the base station is responsible for controlling different independent sectors, MAC layer works with point to multipoint network topology. MAC layer supports various algorithms, among them Access and bandwidth allocation algorithms controls numerous terminals that may be shared by numerous users. Thus the MAC protocol is responsible for defining the transmission between base station (BS) and a subscriber station (SS) on the channel. MAC protocol in the case of downstream channel is quite simple that uses time division multiplexing (TDM) to multiplex the data whereas in case of upstream channel MAC protocol uses time division multiple access (TDMA) technique to ensure the efficient use of the bandwidth as there will be multiple subscriber stations competing to access the medium. There will be multiple users running various applications independently and thus the service is varied according to the number of users that uses voice or video conferencing, web surfing etc. to support these variety of services MAC layer accommodates both continuous and bursty traffic by ensuring the amount of data rate and delay required by each services.

Transport efficiency Issues are also addressed by the 802.16 MAC. By making the efficient use of bandwidth to provide maximum data rate and system capacity, the modulation and the coding scheme are also specified in profile that can be varied accordingly for each subscriber stations. The bandwidth request and bandwidth grant mechanism is designed to be scalable, efficient, and self- correcting, enabling multiple type of service flows to support wide range of applications. Automatic repeat request (ARQ) is yet another great feature that improves the transmission performance and provides support for mesh architecture that allows direct communication between the subscriber stations which in turns improve the scalability of system rather than using only point to multipoint network architecture. The MAC layer also supports for automatic power control, security, encryption and decryption mechanisms.

Physical (PHY) layer

The physical layer of WiMAX is designed and influenced from the physical layer of Wi-Fi (IEEE 802.11a).The design is based on IEEE 802.16-2004 and IEEE 802.16e-2005. Since the aspect of Wi-Fi and WiMAX are different due to their purpose and application but are similar in construction. Like Wi-Fi, WiMAX is also based on the principle of OFDM which is one of the most suitable modulation techniques for line of sight communication with very high data rate. The IEEE 802.16 standard defines four different physical layers that any of which can be used with the MAC layer to provide a reliable end-to-end communication link. The physical layers defined by IEEE 802.16 are:

  • A single carrier (SC) modulation:
  • A single carrier (SCa) modulation:
  • A 256-point FFT OFDM :
  • A 2048-point FFT OFDMA:

A single carrier (SC) modulation scheme is designed to use beyond the frequency band of 11 GHz and requires a line of sight condition. While the single carrier (SCa) air interface is designed to use in the frequency band of between 2-11 GHz for point-to-multipoint operation. The 256 point FFT OFDM operates in Non-line of sight condition between the frequency bands of 2-11 GHz for point to multipoint operation and is used in fixed WiMAX. Similarly the 2048-point FFT OFDMA operates in similar way as 256-point FFT OFDM operates but the advantage over 256-point FFT OFDM is that 2048-Point FFT OFDMA is used in mobile WiMAX. (2007, A.Roca)

There are various functional stages involved in the WiMAX physical layer operation. These operations can be considered as a basic and necessary stages required for the digital signal processing of the data. At first the physical layer includes Forward Error Correction (FEC), and includes channel encoding, rate matching (puncturing or repeating), interleaving, and symbol mapping. Once done with the first stage, normally next stage relates to the creation of OFDM symbol. In order to create OFDM symbol, the incoming data are mapped with proper modulation scheme into the appropriate sub-channel and sub-carrier. Channel state information can be tracked and estimated at the receiver end by inserting the pilot symbol in the pilot sub-carrier. Since the signal should be transmitted in time domain thus, the OFDM symbol is converted from frequency domain into time domain by means of Inverse Fast Fourier Transform (IFFT). Once the time domain signal is achieved, it is then converted into analog form for proper transmission over air. Fig 2.3 shows the logical component required to transmit WiMAX OFDM signal. Similarly reverse operation is done at the receiver end to receive the transmitted data.

OFDM Basic:

OFDM is derived from the concept of Multi Carrier Modulation (MCM) transmission technique which helps to eliminate or minimizes Inter symbol interference (ISI). Multi carrier modulation technique operates on a principle of dividing the input bit stream into several parallel bit stream so that each stream can be modulated on separate carriers called subcarriers or tones. Each subcarrier is separated by a guard band so that they do not overlap with each other. OFDM can also be considered as a special case of spectrally efficient version of MCM technique that utilizes orthogonal subcarriers and overlapping spectrums. Because of the orthogonality nature of the subcarriers, the bandpass filters are not required in OFDM. Therefore, the efficient utilization of available bandwidth can be achieved without causing the Inter Carrier Interference (ICI). Orthogonality in OFDM can be achieved by performing Fast Fourier Transform (FFT) algorithm on the input stream. OFDM provides high data rate with long symbol duration due to the combination of multiple low data rate subcarriers, this helps reducing or eliminating the effect of Inter Symbol Interference (ISI). In addition to this, the use of Cyclic Prefix (CP) in OFDM symbol even help to reduce the effect of ISI but the presence of CP can cause low SNR or data rate. (2000, R.V.Nee, R.Prasad). During the design of OFDM it is important to choose the size of the FFT very carefully. Selecting the FFT size greater than required would cause the reduction of subcarrier spacing which in-turn increase the symbol time. “This makes it easier to protect against multipath delay spread” (2007, J.G. Andrews, A.Ghosh, R.Muhamed). Due to the reduced subcarrier space, the system cannot be prone to inter carrier interference.

OFDM system implementation:

The concept of OFDM was evolved already around 50's and 60's as an efficient Multi Carrier Modulation technique. Although it is quite very old concept, the system was not implemented at that time because the algorithm like FFT/IFFT was not digitally implemented on devices. Later in 1965 when Cooley and Turkey reintroduces the FFT calculation and embedded on semiconductor devices makes the OFDM come into existence as a revolutionary technology.

(1965, J.W. Cooley, J.W. Tukey)

The most powerful yet faster mathematical calculation for digital signal processing algorithm known as Discrete Fourier Transform (DFT) and Inverse Discrete Fourier Transform (IDFT) has made possible for the digital implementation of OFDM system. The DFT and IDFT operations are used to transform the time domain signal into frequency domain and vice-versa. The method of these transformations is considered as mapping of data into orthogonal subcarriers.

To transform frequency domain signal into time domain the frequency domain data are correlated by the IDFT along with its orthogonal basis function that are sinusoids at certain frequency. This method of correlation is referred to as an equivalent method of mapping the input data onto the sinusoidal basis functions. Since the Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) blocks are mathematically equivalent to the DFT and IDFT thus these blocks are practically used for the implementation of OFDM system.

For the transmission of data from the OFDM transmitter, the source symbols are supposed as it is in frequency domain. These symbols are passed on to the IFFT block which performs the transformation of frequency domain signal into time domain. Let us suppose if we choose the number of subcarriers for the system as “N” then, the basis functions for the IFFT is equal to the N orthogonal sinusoids having different frequencies and the number of symbol received by the IFFT is equal to the N as well. The amplitude and phase of the sinusoid is determined by each of N complex value for that subcarrier. The single OFDM symbol is then represented by the summation of all N sinusoids coming from the IFFT. The OFDM symbol length is defined by N times T (N*T) where T represents the input symbol period of IFFT. Hence the modulation of data into N orthogonal subcarriers in such a simple way is provided by the IFFT block. The basic OFDM transmitter and receiver that employ the FFT and IFFT block is shown in the fig below.

OFDM transmitter

Data in Baseband OFDM signal

OFDM receiver

Data out Baseband OFDM signal

Fig. 2.4:- Basic OFDM transmitter and receiver

Similarly at the receiver end the reverse operation of transforming time domain signal into frequency domain is obtained by the FFT. (2007, A.H.Mohammad)

OFDM system design consideration

The design of an OFDM system must be focused to decrease the data rate at given subcarriers so that the symbol duration increases which in turns effectively reduce the multipath effect. With providing the higher value of cyclic prefix (CP) will result in good reduction of multipath effect but it simultaneously increase energy loss. Thus in order to obtain reasonable system design these two parameters must be chosen very carefully

OFDM system depends on the following four requirements:

Bandwidth:

The amount of bandwidth available plays a vital role for selecting the number of subcarriers. With the availability of higher bandwidth allows obtaining higher value of subcarriers along with required CP length.

Bit rate:

The proposed OFDM system should be capable of providing the required data rate.

Tolerable delay spread:

In order to determine optimum value of CP length it is necessary to know the value of maximum tolerable delay spread in advance.

Doppler values:

Due to the mobile nature of the user it is obvious to introduce the effect of Doppler shift which should be considered properly.

System Design Parameters:

The parameters required to design an OFDM system are evaluated from the OFDM system design requirements. These design parameters are listed below.

Number of subcarriers/Tones:

As it is already discussed in previously, with higher value of subcarriers helps eliminating multipath effect but simultaneously decrease the chance of synchronization between transmitter and receiver.

Symbol duration and CP length:

Improper selection of the ratio of symbol duration and CP length may increase multipath effect and simultaneously increase the loss of bandwidth due to inappropriate value of CP.

Subcarrier spacing:

The available bandwidth and number of subcarriers is used to decide the subcarrier spacing but this must be chosen in such a way that the proper synchronization is established between the transmitter and receiver.

Modulation type per subcarrier:

The modulation scheme depends on the performance requirement which can be supported by the use of adaptive modulation.

FEC coding:

The channel robustness can be achieved by selecting the proper forward error correction (FEC) code.

(2000, R.V.Nee, R.Prasad)

Merits and demerits of OFDM:

The advantage of OFDM system can be listed as follows,

  • Overlapping spectra resulting in high spectral efficiency.
  • Implementation of FFT makes the system simple to design.
  • High data rate transmission.
  • Highly flexible, can support different modulation scheme.
  • Transmitter helps combating the channel effect resulting in low receiver complexity.
  • Maximum likelihood detection can be possibly used with sensible complexity.

Beside this there are few disadvantages of OFDM system which are listed below,

  • Since the OFDM system is very susceptible to frequency offset and timing thus the demodulation with an offset in frequency can cause high bit error rate (BER).
  • The number of subcarriers is directly proportional to the peak to average power ratio; as a result it is very difficult to implement DAC/ADC.
  • High synchronization between transmitter and receiver is required.

Chapter 3

Introduction to SDR (Software Defined Radio)

A software-defined radio is an integrated system of a software and hardware. It is a radio communications system which uses the software for signal processing and finally implemented on hardware. Such system typically consists of PC/Laptop, a radio frequency front-end, and an ADC and/or DAC.

Several radio communication methods can be easily implemented through modifiable software or firmware which operates on programmable processing technologies and this can be achieved by means of software defined radio (SDR). Field Programmable Gate Array (FPGA), Digital Signal Processor (DSP), General Purpose Processor (GPP) and Programmable System on Chip (SOC) or other application specific programmable processors are incorporated in the SDR so that the new wireless features and capabilities can be added to existing system without the need of new hardware devices by just amending the software in the computer. The basic architecture of SDR (Software Defined Radio) is shown in the fig below. This architecture is often called as software communication architecture.

The Universal Software Radio Peripheral (USRP) is a hardware device that is connected to a PC via high speed USB 2.0 interface. It is specially designed to use with GNU Radio. The combination of various available daughter boards forms a complete hardware to make a complete Software-Defined Radio. The USRP houses two dual-channel Analog- to-Digital conversion codec, a USB controller and a field programmable gate array (FPGA). Basically the SDR (Software Defined Radio) includes an antenna that can be connected directly to the analog-to-digital (ADC) converter as a receiver. Since the bandwidth and resolution and such converters are limited and thus to compensate this radio front end is required. The radio front end simply amplifies the radio signal and modulates it within the frequency band of ADC. The USRP acts as an antenna. The combined system of GNU Radio and USRP is shown in the fig below.

GNU Radio Basic:

The GNU Radio is a software project that was introduced by Eric Blossom in 1998. The software consists of huge collection of predefined signal processing blocks written in C++ language and with the help of coding in python these blocks are interconnected with each other in a meaningful way so that the user can get the desired output.. The GNU Radio has three different types of signal processing blocks,

  • Data processing blocks having both input and output, such as, modulators, filters, etc.
  • A source blocks that do not have input but only output, such as, signal source, files source, etc.
  • A sink blocks that do not have output but have only input, such as, file sink, FFT sink, etc.

The predefined signal processing blocks are provided with all necessary parameters to adjust as per required by the users. The users can do some mathematical calculations in order to give proper values to the parameters. Sometimes the provided signal processing blocks may not be enough and may not full fill the requirement of the user applications and in such case it is possible for the users to write their own code and build the new signal processing block. This is one of the great advantages of the GNU Radio software. These created blocks can be implemented using C++ language and is made accessible from the python programming using the SWIG. The SWIG (Simplified Wrapper Interface Generator) acts as a glue to connect the program written in C++ language with other high level language such as pearl, python etc. The block diagram of the GNU Radio component is shown in the fig 3.3 below,

Python Flow Graph

(Created by interconnecting the signal processing blocks)

SWIG (Port C++ blocks to python)

GNU Radio signal processing blocks

USB Interface

Generic RF Front-end

Fig 3.3: Block diagram of GNU Radio component

The GNU Radio companion (GRC):

The GNU radio companion (GRC) is a graphical version of GNU radio where the user can just drag and drop the existing pre-defined signal processing blocks for required simulation. The graphical user interface of GRC has made the simulation tool much easier and popular among the people who are highly devoted to SDR (Software Defined Radio). The software can be loaded by simply giving the command “grc” on terminal window (Discussed in later topic). The software is designed in much flexible way so that the user can add, delete or modify any blocks during the operation. The components of GNU Radio companion are as follows,

  • Flow graph
  • Signal processing blocks
  • Parameters
  • Sockets
  • Connection
  • Variables

USRP (Universal Software Radio Peripheral)

The creation of SDR (Software Defined Radio) in any computer is allowed by the the Universal Software Radio Peripheral (USRP), which is connected via the USB version 2.0 interface. The USRP is a hardware device that consists of mother board and daughter board over which the antenna is mounted and can be used for various radio frequency bands. Presently the daughter board operating from DC to 5.9 GHz are available. The schematic design, logical structure or architecture of USRP is an open source. Typically the USRP board setup requires at least one mother board and one daughter board. The basic block diagram of USRP is shown in fig 3.4.

The motherboard of the USRP consists of analog to digital converters (ADC), digital to analog converter (DAC), millions of logical gate, Field Programmable Gate Array (FPGA) and a programmable USB 2.0 controller. The USRP motherboard can support up to four daughter boards for which two of them is used for transmitting signal while other two is used for receiving signals via the RF front-end which is embedded on the daughter boards.

The single USRP device can be used for transmitting and receiving signal simultaneously in real time thus, it can also be considered as a transceiver. The local oscillators and clocks of USRP are all fully coherent which allows it to behave as a MIMO (Multiple Inputs, Multiple Outputs) system. The processing of high sampling rate takes place FPGA of USRP whereas the processing of lower sampling rate takes place in the PC. The onboard digital down converter (DDC) carry out the process of mixing, filtering and decimating the incoming received signal into the FPGA. The onboard digital up converter (DUC) is used to carry out the process of interpolating the base band signals at defined samples/sec prior translating into required output frequency.

USRP Features

  • Can support four 12-bit 64 M sample/sec ADC and four 14-bit 128 M sample/sec DAC.
  • Can support four DDCs and DUCs with programmable decimation and interpolation rate for each of them respectively.
  • Capable of process the signal at a frequency range of 16 MHz wide.
  • High-speed USB 2.0 interface (60 MB/s) with computer.
  • The standard architecture of USRP can support various RF daughter boards.
  • Can be used as a MIMO system.

Getting started with GNU Radio

It is necessary to find out the system requirement before getting started with the GNU Radio software. GNU Radio can be used both in Linux as well as windows operating system environment. Installing GNU Radio in windows is quite difficult and time consuming as it requires third party software called “Cygwin”. Cygwin is software that provides a Linux like environment in windows so that GNU Radio can be used easily. Beside this, for the GNU Radio to operate properly in windows it requires thousands of library files to be installed correctly and is time consuming. Any one of the missing library file could result in malfunction of the software. Thus it is recommended to use GNU Radio in Linux environment rather than using it in windows environment.

For those who do not want to get rid of the windows operating system there is another option to use GNU Radio software i.e. on virtual machine. The virtual machine can be created easily in any system using the software called “VMware workstation”. The VMware Workstation is desktop software that allows running multiple 32-bit and 64-bit compatible desktop and server operating systems at the same time on a single PC, in fully networked, portable virtual machines-with no rebooting or hard drive partitioning required. With one operating system installed in host machine we can use different operating system in the guest machine at a same time. The only thing to keep in mind is about the hardware requirement for running the workstation and multiple operating systems because the hardware resource is shared during this operation.

Like physical computers, the virtual machines running under Workstation perform better if they have faster processors and more memory.

The terms host and guest describe physical and virtual machines:

  • Host - The physical computer on which the Workstation software is installed is called the host computer, and its operating system is the host operating system.
  • Guest - The operating system running inside a virtual machine is called a guest operating system.

Once the PC gets loaded with VMware, the next step goes with the installation of Linux operating system on virtual machine. For this project Linux Ubuntu 10.10 is used so it discuss all about installing Linux Ubuntu 10.10. Linux Ubuntu operating system is distributed freely and is it is also free to download and use. It can be easily downloaded from the given website.

http://www.ubuntu.com/download/ubuntu/download.

Once it is downloaded it can be installed easily on virtual machine that we have created earlier. After installing Ubuntu we can proceed with the installation of GNU Radio companion. Here is the installation process of GRC on Linux Ubuntu 10.10 version.

We just need to open the terminal and type the command

@ubuntu:~$ sudo apt-get install gnuradio

@ubuntu:~$ sudo apt-get install gnuradio-companion

Once the installation is complete we can just type the command “grc” in terminal window in order to start the GNU Radio companion.

There is another way to install the software which is described below:

  1. Open System à Administration àSynaptic package manager.
  2. Go to Settings à Repositories à Other Software and click Add.
  3. Enter the following in the APT line and click Add Source.
    • deb http://gnuradio.org/ubuntu stable main
    • deb-src http://gnuradio.org/ubuntu stable main
    • deb http://mirrors.kernel.org/ubuntu jaunty main universe
  4. Click close and go back to main window and click reload. (New added package cannot be seen until it is reloaded)
  5. After reloading, we can findGNU radiofrom the list, now right click onGNU radioand clickmark for installation.Same procedure can be followed forGNU radio-companion. Once selected for installation, the package manager will automatically highlight the installation package with green colour.
  6. Clickapplyfor any dialog box appearing during installation.
  7. Once the installation completes without any disruption, we have our GNU radio companion ready to use.
  8. Now by just opening the terminal window and giving the command“grc”and press enter will open the software.

Chapter 4

GNU Radio implementation

The implementation of GNU radio is quite simple as using other normal software. As it has already been discussed about the GNU Radio features and advantages, this chapter discuss how the SDR (Software Defined Radio) using GNU Radio companion has been implemented to achieve the project goal. Before the implementation of any complex transmitter and receiver design, the project begins with the simple flow graph design for analysing sine wave using spectrum analyser. After getting familiar with some basic signal processing blocks and gradually after some sort of research and study the project march towards the design of dial tone generator (discussed in chapter 3), the design of BPSK transmitter and move on to the DQPSK transmitter and receiver with Bit Error Rate (BER) calculation, constellation analysis/plot so that the project can further go ahead with OFDM implementation.

In order to get familiar with the GNU radio software, the project moves with the very first stage of analysing sine wave in spectrum analyser and the FFT plot. The GRC flow graph is shown in the fig 4.1 below.

In the flow graph, signal source is used as a sine wave generator. This block is used to generate various type of signal such as constant, sine, cosine, square, triangle and saw tooth with desired frequency and amplitude. Here the variable slider is used so that the frequency can be varied from minimum of 0 to 10 KHz. The amplitude of the signal source is constant and is set to 1, but it can also be varied using another variable slider. The signal source is then passed on to the scope sink and FFT sink via the throttle block. The throttle block here acts as an accelerator that accelerates the processing time by reducing the CPU congestion. The scope plot and FFT plot of the signal set at various frequencies is shown in the fig 4.2 and 4.3 respectively.

The plot of the signal is taken at the frequency set to 1.5 KHz where as the spectrum is taken at various frequency varying from 0 to 10 KHz.

After analysing the result for above signal the result comes accurately. For the verification of the output following equation can be used to calculate the time period,

f=1t

Where, f = frequency

t = time period

Since the frequency is set to 1.5 KHz, using the above equation we get t = 0.66 ms.

After figuring this out, the project moved towards the dial tone generator. The dial tone generator requires a set of signals with different frequency and amplitude and a noise. Dial tone generator is shown in the fig 4.4 below.

In the above fig, the two sinusoidal signal sources along with the noise source with different frequency and amplitude are added together by means of an “Add” block. As we can see that the frequency of one signal source is set to 350 Hz with amplitude of 100 mv and the frequency of other signal source is set to 450 Hz with amplitude of 100 mv. The reason for giving different value of frequency is nothing but just to generate the tone which sounds similar to the real dial tone that we hear in telephone set. Addition of a very small amount of noise gives the beauty to the tone and appears as it is real tone. The signal is then connected to “Scope sink” for analysing the waveform and “Audio sink” to listen the generated tone via “Throttle” block. The audio sink uses the audio device driver installed in the system, thus it is necessary to install the audio device driver properly to listen the generated tone. The “Variable Slider” is used to vary the amplitude and frequency of signal and noise source. As soon as the simulation is started the software automatically generates a python code (*.py file), this gives the software more flexibility for the users who does not know python programming, the user can still analyse the result with the help of the output graph produced.

BPSK:

Since in BPSK a pair of signal is used to represent binary symbol 1 and 0, which can be represented by the equation.

S1t=2EbTbCos2πfct 0 ≤ t ≤ Tb

S2t=2EbTbCos2πfct+π  0 ≤ t ≤ Tb

=-2EbTbCos2Ï€fct

Where,   Eb= Transmitted signal Energy per bit, fc= Carrier Frequency and Tb= Bit duration.

As defined in above two equations, the pair of sinusoidal wave only differs in phase by 180o and is thus referred to as antipodal signal and the constellation plot is as shown in fig below.

-Eb     Eb

Fig 4.5:- BPSK constellation

Thus the concept of BPSK signal generation is that a carrier wave is multiplied by the input binary sequence which can be achieved easily in GNU radio companion.

Product Modulator

Binary data Sequence   BPSK Signal S (t)

S(t)=2TbCos2Ï€fct

Fig 4.6:- BPSK transmitter

The BPSK flow graph in GNU Radio companion is shown in the fig 4.7 below. For the transmission of BPSK signal, the vector source is used that can transmit sequence of binary data of 0's and 1's which is multiplied by the carrier wave whose carrier frequency can be varied from 500 Hz to 10 KHz. The multiplication is done by means of multiply block and the result is analysed from the scope sink.

From fig 4.8 we can see that the phase of transmitted signal is changed as the transition of incoming bits are changed from 0 to 1.

DQPSK:

This section will discuss the digital modulation technique called DQPSK (Differential Quadrature Phase Shift Keying) as this has been used and implemented during this project. The constellation point in the figure below is used to convey number of bits. For higher modulation techniques such as QAM-16, QAM-64 can transmit more bits thus having more constellation point. The disadvantage of having more constellation point is that they are more close to each other and it is very difficult to determine the certainty of the correct sent bits. For the DQPSK, it is little more complicated as compared to BPSK. Since in DQPSK, the information carried by the transmitted signal is contained in the phase. In particular, the phase of the carrier takes on one of four equally spaced values, such as p/4, 3p/4, 5p/4 and 7p/4. For this set of value we can define the transmitted signal as

Sit=2EsTCos2Ï€fct+2i-1Ï€4, for i=1, 2, 3, 4

Where, Es= Energy of symbol.

fc= Carrier Frequency.

T=Symbol time.

The in-phase and Quadrature component (I &Q) of the signal is then given by

1t=2TCos2Ï€fct

2t=2TSin2Ï€fct

From this equation the four constellation point can be represented by the following coordinates.

±Es2,    ±Es2

I

Q

Fig 4.9:- QPSK with gray coding.

The implementation of DQPSK in GNU Radio companion along with its flow graph is shown in the fig 4.10 below.

The fig above shows the DQPSK transmitter and receiver. The flow graph is used to analyse the transmitted and received bits along with BER calculation. Here random source is used to transmit random bits of 0's and 1's, which is encoded before modulation. With the minimum value of 0 and maximum value of 2 the Random Source block generates the sequence of 01110101........ With the option Repeat=Yes will just repeat the sequence so as to give the continuous random output. After the modulation, practically the receiver receives the transmitted signal along with the noise, thus the source of white noise has also been added. The constellation sink is used to analyse the constellation plot of DQPSK. The constellation plot is shown in fig 4.11 below. At the receiver end, the transmitted signal is demodulated, encoded and then passed on to the BER block for BER calculation and to the spectrum analyzer to analyze the received bits.

From fig 4.11, it is found that the constellation plot is very fine small dot which is as accurate as it should be theoretically. From fig 4.12 the constellation is scattered because of the presence of noise, as we can see that the noise amplitude has set to 100m. The fig 4.13 is the plot of transmitted data and received data. It is found that the received bits are exactly at the same point as transmitted bits, i.e. they are in phase with each other as expected. This plot was taken in the absence of noise. The transmitted bits are in the form of dots (Blue coloured) and the received bits are in link line (Green coloured). In presence of noise the received bits gets shifted by certain time value and the BER can be calculated using the Error Rate block. The plot of received vs. transmitted bits is shown in the fig below,

Till now the flow graph is used for transmitting digital random binary data, now the same flow graph is also used for transferring image and observes the output at different SNR settings. To achieve this, the random source is replaced by the file source where we can give the destination of the image file manually and similarly at the receiver end we added the file sink and give the location to save the transmitted image. The flow graph is shown in the fig 4.15 below,

Upon execution of the flow graph, the image is received at different value of SNR that was set manually. The received image file at different SNR values are shown in the fig below,

SNR= 9.629 dbSNR= 9.421 db

SNR= 9.396dbSNR=7.95db

SNR= 6.935 dbSNR =4.436 db

Fig 4.16: Image received at different SNR value

It is observed that at higher value of SNR the image received is clear and rendered properly, as the SNR value is decreased gradually the quality of image received decreases gradually. At the SNR value of 4.436 db we can see that there is no image received at all.

OFDM implementation/simulation:

For the transmission of OFDM signal, GNU Radio companion has pre defined OFDM modulator and demodulator thus we can easily use these blocks to analyse the result. The flow graph of OFDM is shown in the fig below,

Conclusion:

During this project, the GNU Radio was explored and the main objective achieved during this project was learning the SDR (Software Defined Radio), advantage, disadvantage and its reason of implementation in wireless network. Although I was beginner and new to GNU Radio, I explored a lot and start designing with simple flow graph which can be helpful for those who wants to learn something new in GNU Radio and accelerate to other complex radio system quickly. Since the software was build in Linux platform, the installation process goes little bit different from windows operating system. After getting started with the GNU Radio software the design of DQPSK transmitter and receiver was done. Upon execution of DQPSK flow graph, I observe that the transmitted bits are received without any errors and the constellation plot was exact too. After adding some noise to the receiver the bit error rate was calculated using the Error Rate block and the received bits gets shifted because of phase delay. For better evaluation of the flow graph, transmission of text file and image was also conducted and observed that the transmission and reception was successful as long as there is no noise present at the receiver end. While transmitting image in the presence of noise, the received image size did not vary but was not rendered completely and distracted image was received. <