The Development Of Wireless Communication System Biology Essay

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From the past few decades the number of mobile users has been increasing in a dramatic way. As they increased, their requirement is also becoming very high according to their needs. The early mobile communication system was limited within the national boundaries only. Therefore at that time they attracted only a small number of users. The systems equipment which they had to depend on was very expensive, power hungry, inefficient etc. The 'first Generation' mobile communication system was introduced and commercially brought to the market in the early 80s and it was an analog telecommunication's standard. Since this system makes use of analog rather than digital signal, it was less effective in terms of transmitting information, which results more likely to suffer interference problems and the signals could not cover long distance transmission. As a result, using a mobile phone with analog signal became very difficult.

The next step taken towards the development of wireless system was 2G or second generation. The system is known as GSM (Global System for Mobile communication) and the use of digital technology in mobile phone system was introduced. This became very effective in the densely populated areas within cities in terms of handling large number of calls avoid risk of interference, call drops, handoff etc. The GSM system is able to utilize any of the three frequency bands at 900, 1800 and 1900 MHz.

Following the footsteps towards the development of wireless communication system is known as 3rd generation or 3G, which offers wide range of options such as broadband mobile communication with voice, video, graphics, audio and other information with high-speed transmission of data.

OVERVIEW

LTE or Long Term Evolution was first introduced by 3GPP (3rd Generation Partnership Project). 3GPP is basically a collaboration between groups of telecommunications association known as the Organizational Partners. There are basically six organizational partners; the pioneers of 3GPP are from Asia, Europe and North America. The organizations are ARIB (Association of Radio Industries and Businesses, Japan), ATIS (Alliance for Telecommunications Industry Solutions, USA), CCSA (China Communications Standards Associations, China), ETSI (European Telecommunications Standards Institute, Europe), TTA (Telecommunications Technology Association, Korea), TTC (Telecommunication Technology Committee, Japan). LTE can be considered as the up gradation of cellular 3G services. The main aim of LTE is to bring technical benefits to cellular networks. It is designed to meet high speed data requirement, high capacity of voice support, increase user capacity, increase network coverage, etc.

The requirements for LTE are to support 100 mbps at downlink, 50 mbps at uplink within 20 MHz bandwidth and the bandwidth is scalable from 1.25 MHz to 20 MHz. This flexibility is useful when different network operators operate in different bandwidth allocations. In 3G networks, the improvement of spectral efficiency is also expected, allowing carriers so that more data and voice services are provided over a given bandwidth. The major parts of LTE are Single Carrier Frequency Division Multiple Access (SC-FDMA) and Orthogonal Division Multiple Access (OFDMA). In communication system, OFDMA was well utilized to achieve high spectral efficiency and is used as downlink model because it has high PAPR than SC-FDMA. SC-FDMA is introduced newly and became very efficient and handy in uplink multiple access scheme. The main challenges of this main multiple access schemes are to achieve good robustness efficient Bit Error Rate (BER), high spectral efficiency, low delays, low computational complexity, low error probability, low Peak to Average Power Ratio (PAPR).

In this thesis paper the performance of OFDMA and SC-FDMA of LTE physical layer would be performed by simulating the model using different modulation schemes which are BPSK, QPSK, 16QAM and 64QAM on the platform of BER, PAPR, PSD and error probability.

CHAPTER 1

INTRODUCTION

Due to the increasing number of mobile subscribers over the past few decades, the requirement of the users and the network operators has increased simultaneously. LTE is designed to meet carrier needs for high-speed data and media transport as well as high capacity voice support. The aim of this thesis is to evaluate the performance of LTE physical layer by taking measurement and by simulation using AWR Microwave Office. The basic means of LTE PHY is of conveying both data and control information between an enhanced base station and mobile user equipment. In cellular applications LTE PHY introduces some advanced technology which are Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple Input and Multiple Output (MIMO) for data transmission and the two multiple access schemes that are used in LTE PHY are Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier- Frequency Division Multiple Access (SC-FDMA). OFDMA is used on the downlink and SC-FDMA is used on the uplink.

The modulation techniques that are used are BPSK, QPSK, 16- QAM and 64- QAM. The performance of the LTE physical layer will be assessed by the parameters such as Signal to Noise Ratio (SNR), BER, Power Spectral Density (PSD), bit error probability and PAPR.

CHAPTER 2

TYPES OF COMMUNICATION

2.1 Point-to-Point Communication:

This type of communication is normally used for long distance and is occurred between two endpoints.

2.2 Point-to-Multipoint Communication:

This communication type is made by using one transmitter and it is done from one point to many points. Data is sent to every point by the transmitter e.g. video conferencing.

2.3 Simplex:

The signal is transmitted in only one direction in this form of communication.

2.4 Half Duplex:

In this type of communication, data is transmitted to one user at a time but in both directions. It is considered to be sharing of time within two users e.g. Walkie Talkie.

2.5 Full Duplex:

Full duplex is a two ways form of communication as in information can be sent and received at the same time by both the users e.g. telephone.

CHAPTER 3

LTE PERFORMANCE DEMANDS

The LTE PHY is designed to meet the following goals:

Support scalable bandwidths of 1.25, 2.5, 5.0, 10.0 and 20.0 MHz

Peak data rate that scales with system bandwidth

Downlink (2 Ch MIMO) peak rate of 100 Mbps in 20 MHz channel

Uplink (single Ch Tx) peak rate of 50 Mbps in 20 MHz channel

Supported antenna configurations

Downlink: 4x2, 2x2, 1x2, 1x1

Uplink: 1x2, 1x1

Spectrum efficiency

Downlink: 3 to 4 x HSDPA Rel. 6

Uplink: 2 to 3 x HSUPA Rel. 6

Latency

C-plane: <50 - 100 msec to establish U-plane

U-plane: <10 msec from UE to server

Mobility

Optimized for low speeds (<15 km/hr)

High performance at speeds up to 120 km/hr

Maintain link at speeds up to 350 km/hr

Coverage

Full performance up to 5 km

Slight degradation 5 km - 30 km

Operation up to 100 km should not be precluded by standard

CHAPTER 4

MULTIPLE ACCESS SCHEMES

4.1 OFDM:

OFDM was first introduced in 1950s. [1] Practically it was almost impossible to demodulate and generate such a signal at that time. Through DSP (Digital Signal Processor) it was possible to generate and demodulate such a signal. OFDM became a very handy candidate in the area of modern broadband wireless network. This development results in factors such as achieve high spectral efficiency, impulse noise rejection and tolerance to multipath propagation and frequency selective fading. When a signal is transmitted it is necessary to ensure that the transmitted signal reaches its destination and the received signal is more like accurate. The exact information, which is transmitted, is very difficult to achieve at the receiver, this is due to factors such as multi path interference, noise, attenuation etc.

Multipath interference occurs when multiple signals arrive at the receiver at different time and this leads to Inter Symbol Interference which is caused by reflection of obstacles like buildings, trees, houses, hills etc. OFDM allows the use of many parallel subcarriers and enables longer signal duration, which makes the signal more robust to multi path dispersion. OFDM spreads energy of an impulse noise over transmitted burst where the noise level is slightly increased rather than losing few symbols. Therefore the probability of rising of bit error rate is reduced.

Frequency Division Multiplexing is a technology, which uses multiple frequencies to transmit multiple signals in parallel simultaneously. OFDM is based and is similar to FDM with a benefit of much more spectral efficient which is achieved by spacing the sub-channels much closer together.

Figure 4.1: Comparison of OFDM and OFDMA subcarriers allocation [2] 

The Figure [4.1] above gives us a clear view for the comparison of the OFDM and OFDMA subcarriers allocation. In the figure, carrier space is divided into groups namely NG. NE carriers are contained in each group and thus create NE subcarriers. For each subchannel are the type of coding and modulation separately set. The subcarriers depend on the channel condition to be allocated to different users. Operators assigning to these users the best-suited subcarriers find this characteristic very helpful. It leads towards the efficient usage of resources.

4.2 OFDMA:

OFDMA is newly introduced in the modern technology and offers new key features that are being utilized in mobile wireless broadband networks. OFDMA is basically a multi user OFDM that on the same channel allows multiple access. This leads to facilitate accommodation of many users at the same channel at the same time. In OFDMA the available frequency band is divided into number of orthogonal frequency subcarriers where the data is first converted into parallel bit streams and using conventional modulation scheme it is modulated on each subcarrier. OFDMA offers shorter and constant delay and allows low data rate from many users. One of the best features of OFDMA is it reduces multipath fading because each user is modulated over several orthogonal frequencies rather than a fixed frequency for entire connection period. It also increases the capacity of each user because of using several frequencies. In terms of interference reduction OFDMA is very dependable and has improved non line of sight capabilities that are essential in mobile environment. Through OFDM higher through put is accomplished and offers better frequency re-use. This also makes the sale planning much easier.

Since many bits are transported in parallel, the overall resulting data rate can be much higher than the transmission speed of each subcarrier. In terms of practical radio environment, due to the signals arrive at different times from different directions, it is very important to minimize the effect of multipath fading. The process of OFDMA is as follows: As shown in the diagram above, the input bits are grouped first and are transmitted over different frequencies (subcarriers). According to theory, using a separate chain hardware block, each subcarrier signal could be generated. The resulting signal of the output of these blocks have to be summed up and then be sent over the air which is not practicable due to high number of subcarriers. A mathematical approach is taken instead where each subcarrier is transmitted on a different frequency. The IFFT mathematical function is used to transform frequency domain to time domain. From the receiver end the received signal is first demodulated and amplified. Through FFT function the signal is converted into the frequency domain from the time signal and this constructs the frequency or amplitude diagram at the transmitter. A detector at the center frequency of each subcarrier is used so that the bits can be generated which were used originally to create the subcarrier.

One of the main drawbacks of OFDMA is it has high PAPR (Peak to Average Power Ratio) that is peak signals power is much greater than average signal power for which it is used as the downlink model of LTE technology.

4.2.1 OFDMA and The LTE Generic Frame Structure:

For the 3GPP LTE downlink, the best selection of multiplexing scheme is OFDMA. Though complexity regarding resource scheduling is involved here, it is more effective compared to packet-oriented approaches while it comes to efficiency and latency. For predetermined period of time users are assigned a specific number of subcarriers in OFDMA. In the LTE specifications, these are classified as PRBs. PRBs not only have a time domain but also have a frequency domain. At the 3GPP base station (eNodeB) a scheduling function handles the allocation of PRBs.

Within the context of LTE, for explaining OFDMA adequately, studying the PHY layer generic frame structure is essential. The generic frame structure is employed along with FDD while alternative frame structures are being used with TDD.

As shown in the given table, the duration for LTE frames is 10 msec. The frames are divided into 10 subframes that are 1.0 msec long. The subframes are then divided into two slots and the duration is 0.5 msec. Each slot contains 6 or 7 OFDM symbols, which depends either on the normal or extended cyclic prefix that is employed.

Table 4.2.1: Available Downlink Bandwidth is Divided into Physical Resource Blocks [3] 

The estimated available subcarriers are depended on the overall transmission bandwidth within the system. Parameters for system bandwidths are defined by the LTE specifications. As shown in the diagram below, the system bandwidths range from 1.25 MHz to 20 MHz. A PRB, for each slot, consists of 12 consecutive subcarriers and the duration is 0.5 msec. The base station scheduler assigns a PRB as the smallest element of resource allocation.

Figure 4.2.1-1: Downlink Resource Grid [4] 

The downlink signal that is transmitted is actually made of NBW subcarriers with a duration of Nsymb symbols of OFDM. As described in the figure given below, a resource grid represents the transmitted downlink signal. A single subcarrier within the grid is represented by each box for one symbol period. It is classified as a resource element. In MIMO applications, it is noteworthy that every transmitting antenna has a resource grid.

LTE, unlike packet-oriented networks, does not use a PHY preamble so that channel estimation, carrier offset estimate, timing synchronization etc. Alternatively, as of the table below, the PRBs integrates special reference signals. During the 1st and 5th OFDM symbols in each slot, reference signals are transported while using the short CP. On the other hand, during the 1st and 4th OFDM symbols, it is done so when the long CP is in use.

Figure 4.2.1-2: LTE Reference Signals are Interspersed Among Resource Elements [5] 

It must be noted that in every 6th subcarrier, reference signals are transmitted. In addition, reference symbols are spread over in time and frequency both. The channel response that bears the reference symbols on subcarriers is directly computed. On the remaining subcarriers, interpolation is employed so that the channel response can be estimated.

4.3 SC-FDMA:

Single Carrier- Frequency Division Multiple Access can be considered as a modified form of OFDM, the performance and complexity is almost the same. It also deals with multiple users to share a communication resource. The basic structure of SC-FDMA is more like OFDMA but in addition it uses Discrete Fourier Transform (DFT) block and on the receiver end it uses Fast Fourier Transform (FFT) block. It is used as an uplink model in the LTE technology. It is also able to provide high data rate on the uplink. SC-FDMA has the additional benefit of Low Peak to Average Power Ratio (PAPR) makes it suitable for uplink transmission by user terminal. Data is transmitted over the air interface by SC-FDMA in many subcarriers but adds an additional processing step that is shown in the Figure [4.3.1] below.

Figure 4.3.1: OFDMA [6] 

As compared to OFDM, instead of putting bits together, the information of each bit over all the subcarriers are spread by SC-FDMA. The process how it is done is as follows: a number of bits are grouped together and then the bits are sent into a Fast Fourier Transformation function. The output of the process creates subcarriers for the following IFFT.

At the receiver end, the signal is amplified, demodulated and is managed by the FFT function. At the transmitter sight, the effect of the additional signal processing is removed by the IFFT function and the result of the IFFT is a time domain signal. The recreation of the original bits is achieved by feeding a time domain signal through a single detector block. Therefore instead of detecting bits on many different subcarriers, on a single carrier, a single detector is used. The whole process is clarified in figure [4.3.2] below.

Figure 4.3.2: SC-FDMA [7] 

4.4 TDMA:

In Time Division Multiple Access, a frequency channel is distributed in to numerous time slots. For different time slots, the same frequency channel is accessed by several users. Each user is allotted a time slot, which is separate, and it is for a specific period. The transmission of signal is completed in rapid succession. TDMA consists of many receivers connected with one transmitter instead of one transmitter connecting only one receiver. TDMA is applicable for 2nd generation (2G) and 3rd generation (3G).

4.5 FDMA:

In Frequency Division Multiple Access, in many channels or sub-bands are the frequency band divided which is allocated to a network. For the whole call duration, each user assigns one channel and it is to be noted that each frequency band is capable of carrying either voice conversation or digital data. FDMA is used by Advance Mobile Phone Services (AMPS) that is 1G analogue cellular system. This scheme is inefficient because of its wastage of its assigned channel or sub-band at the time of the unused period of call.

4.6 CDMA:

Spread spectrum technology is used in Code Division Multiple Access. The whole spectrum of the system in CDMA is used by all of the users, which is established on codewords. On a sequence of pseudo noise codes is the bandwidth distributed that is available. The code sequence within a large bandwidth signal multiplies each signal. On the same frequency, all users transmit simultaneously in CDMA. To the other users, each user's codeword is orthogonal and at the receiver side, the operation of correlation is employed so that the information can be retrieved for a specific codeword.

4.7 SDMA:

In Space Division Multiple Access, the transmitted energy is controlled in the direction of particular user in space. [8] Spot beam antennas are employed so that energy for each user is radiated which uses the same frequency separately. The same electromagnetic spectrum is used over multi paths for transmission by SDMA.

CHAPTER 5

MIMO CONCEPT

MIMO stands for Multiple Input Multiple Output, which is an essential part of LTE technology. MIMO enables us to increase spectral efficiency and throughput. The basic concept of MIMO is the use of multiple antennas at the transmitter and receiver side. The basic configuration of MIMO is considered as 2x2, which are 2 antennas at the base station for transmission and 2 antennas at the receiver side.

5.1 Downlink MIMO:

On the downlink resource block, different streams of data is transmitted simultaneously by Spatial multiplexing. The data streams can belong to one single user (SU-MIMO) or to different users (multi user MU-MIMO). The advantage of MU-MIMO is it increases the overall capacity of the system and SU-MIMO increases the data rate of single user. If the mobile radio channel allows, then only spatial multiplexing is possible.

Figure 5.1: Spatial Multiplexing [9] 

The Figure [5.1] above describes how the process is accomplished. On the left hand side of the diagram, there are datas, which are to be transmitted. Nt is the number of transmit antennas and Nr is the number of receiver antennas, which is a 2x2 matrix for the scenario of LTE baseline. The transmitting antennas use the same frequency but different data is transmitted out of both antennas. From the diagram it can be observed that the datas are divided into 2 parts in the transmitter section and sent by the 2 antennas A and B. The receiver antennas receive them through A and B and at the end through processing the original data is achieved. MIMO uses an advanced digital signal processing technology that reduces the risk of interference between frequencies. Therefore the quality of bit error rate and data rate improves by using multiple Tx/Rx antennas and it is also very effective in terms of efficiency of the spectrum.

5.2 Uplink MIMO:

MU-MIMO can be used for the uplink and on the same resource block, multiple user terminal may transmit. The big advantage is the scheme requires only one transmit antennas at UE side. To keep the cost of UE low, antenna subset selection can be utilized in terms of 2 or more transmit antennas.

CHAPTER 6

TRANSMISSION IMPAIRMENTS

The loss of information to be lost in a signal, which is transmitted through a transmission media, can be considered as transmission impairments. Since the signals travel through a transmission media, the imperfection causes signal impairment. In easy words, it means the transmitted signal; at the beginning of the medium is not the same as the signal at the receiver end. It is not practically possible even if an ideal transmission media send data and the receiver get the same data. The major problem caused in transmission medium are-

Attenuation

Noise

Fading

6.1 Attenuation:

Attenuation can be considered as the energy loss of a transmitted signal through a given medium. The received signal must be strong enough so that the receiver can detect and interpret the signal. The receiver might not be ale to identify the signal if the attenuation is too high. Attenuation increases with frequency, if the frequency is too high then the attenuation is also high. In general, attenuation is caused by line joints (which can also cause noise), this is because it takes energy to turn a signal around a corner.

6.2 Noise:

During transmission, undesired signals get inserted and affect the real signal. On other word, it can be also be described as unwanted energy from different sources other than the transmitter. There are 4 types of noise-

6.2.1 Thermal noise/ White noise:

It occurs due to thermal agitation of electrons. It is present in all electronic devices, function of temperature and is uniformly distributed across frequency spectrum. Thermal noise places and upper bound on system performance and cannot be eliminated. The mathematical term to describe it is as follows: N= KTW (define).

6.2.2 Intermodulation noise:

This is where signals share the same transmission medium at different frequencies. Intermodulation noise may result in signals that are sum or difference or multiples of original frequencies. It also occurs when there is nonlinearity in the transmitter, receiver or intervening transmission system. Component malfunction or excessive signal strength can cause nonlinearity.

6.2.3 Impulse noise:

In communication system, it may be caused by flaws, by lightening. It is the non-continuous noise, consisting of irregular pulses or noise spikes of short duration and high amplitudes.

6.2.4 Cross talk:

This is caused where there are unwanted coupling between signal paths. It occurs due to electric coupling between nearby twisted pairs, multiple signals on a coaxial cable or unwanted signals picked up by microwave antennas.

6.2.5 Inter Symbol Interference (ISI):

Interference with one symbol with a subsequence symbol in a signal is caused by ISI. It is a type of unwanted signal distortion that causes previous transmissions to have an effect on the current transmission.

6.2.6 AWGN Noise:

In every communication channels additive white gaussian noise AWGN is a common factor. Basically it is the statistically random radio noise characterized by a wide frequency range with regards to a signal in a communications channel.

6.3 Fading in Wireless Communication:

One of the major cause of signal fading is multipath effect, i.e. when a signal is transmitted from a transmitter, to reach the receiver it uses different path and are affected by reflection, diffraction, scattering and absorption. At the receiver end, all these multipath signals are added together. Since they use different paths, some signals are shorter while some are longer. The one that travels in the direction of the line of sight should be the shortest. There are 2 types of channel fading:

Doppler spread:

The doppler spread is caused when a transmitter is moving away from a receiver, for which the received signal frequency is low relative to the transmitted signal. There are number of reasons that can cause relative movement between a receiver and a transmitter. For example, the movement of a cellphone, the movement of some background objective, which causes the change of path length etc.

Delay spread:

Since the transmitted signals follow different path to the receiver destination, they correspond to different transmission times. Therefore the receiver receives multiple copies of signals for an identical signal pulse from the transmitter at different times. The signals that follow shorter path reaches early at the receiver end and the ones, which follow the longer paths, reach after. This situation causes the spread of the original signal in the time domain and the spread is called delay spread.

6.3.1 Flat Fading:

When the bandwidth of a transmitted signal is lower than the radio channel bandwidth, then the received signal experiences flat fading. The effect of flat fading channel can be considered as a decrease of the signal to noise ratio. The flat fading channels are also known as amplitude varying channels or narrow band channels due to the signal is narrow with respect to the channel bandwidth. Because due to the fluctuation in the channel gain caused by multipath, the strength of the transmitter changes. On the other hand, the spectrum characteristics stay as it is. This is illustrated in Figure [6.3.1].

Figure 6.3.1: Flat Fading [10] 

6.3.2 Frequency Selective Fading:

Frequency Selective Fading occurs when bandwidth of the signal is higher than the coherence bandwidth of the channel. Different frequency component of the signal therefore experience de- correlated fading. Such channels are dispersive, giving rise to inter symbol interference, which can be combated using equalizers as described in Figure [6.3.2] below.

Figure 6.3.2: Frequency Selective Fading [11] 

6.3.3 Fast fading:

Fast fading occurs when the delay constraint of the channel is higher relative to the coherence time of the channel (roughly speaking symbol duration). In the regime, over the period of use, the phase and the amplitude change imposed by the channel is varied. Error-correcting coding and bit interleaving helps combat fast fading.

6.3.4 Slow fading:

Slow fading arises when the delay constraint of the channel is small relative to the coherence time of the channel. Over the period of use, the phase and the amplitude change imposed by the channel is roughly constant. In slow fading channel, during the entire duration of transmission, a deep fade therefore lasts and cannot be mitigated.

CHAPTER 7

DIGITAL MODULATION

The 3 categories where most communication system falls are-

Bandwidth efficiency:

This is where data within a limited bandwidth can be accommodated by a modulation scheme.

Power efficiency:

Power efficiency is where the information can be sent reliably at the lowest practical power level.

Spectral efficiency:

It is one of the crucial terms in every types of communication. The designer aims to increase the spectral efficiency so far as it is possible.

One of the main challenges for the designers is to achieve good bandwidth with low bit error rate. With the use of digital modulation facilities such as more information capacity, compatibility with digital data services, higher data security, better quality communications and quicker system availability can be achieved. In order to transmit the information signal the carrier signal is usually in sinusoidal form through digital modulation, its frequency, phase, amplitude or combination of these can be varied over time. There are different types of modulations, which are described below:

7.1 Amplitude Shift Keying (ASK):

ASK is a type of modulation where the frequency and phase remain constant and the amplitude of a high frequency carrier signal is varied in proportion to the instantaneous amplitude of the modulating message signal. The diagram below gives a clear demonstration of a transmitted carrier signal with some amplitude for logic 1 and with 0 amplitude for logic 0. Figure 7.1 presents a clear understanding of the process.

Figure 7.1: Amplitude Shift Keying (ASK) [12] 

7.2 Frequency Shift Keying (FSK):

As is elaborated in Figure [7.2] below, in this type of modulation frequency is varied while the amplitude and phase remains constant. The diagram below gives us a clear understanding about FSK where logic 0 represents lower frequency and logic 1 represents upper frequency.

Figure 7.2: Frequency Shift Keying (FSK) [13] 

7.3 Phase Shift Keying (PSK):

As we can see from Figure [7.3] below, PSK variation occurs in the phase of sinusoidal signal and the frequency and the amplitude of the carrier signal remains same.

Figure 7.3: Phase Shift Keying (PSK)

7.4 Binary Phase Shift Keying (BPSK):

It is a digital modulation scheme where 2 different phases of a reference signal are modulated to convey data. It has 2 theoretical phase angles, +90° and -90°. Therefore phase separation between an adjacent point makes it immune to noise, improves bit error rate performance and interference. In this modulation each modulated symbol represents a single phase. The Figure [6.4] below gives us a clear view.

Figure 7.4: Binary Phase Shift Keying (BPSK)

7.5 Quadrature Phase Shift Keying (QPSK):

In this type of modulation, the QPSK uses 4 phases at 0°, 90°, -90° and 180°. It is beneficial over BPSK in terms of high spectral efficiency and it uses 4 symbols at a time for modulation rather than using 2 symbols. In comparison with BPSK, QPSK is more bandwidth efficient, but in terms of power efficient, they are same. Below is a constellation diagram [7.5] and a constellation table [7.5].

Figure 7.5: Quadrature Phase Shift Keying (QPSK) Constellation

Table 7.5: QPSK Signal Space Characteristics

7.6 Quadrature Amplitude Modulation (QAM):

In this modulation, 2 amplitude-modulated signals (AM) are combined into a single channel by which the effective bandwidth gets doubled. In wireless applications and in digital system, pulse amplitude modulation (PAM) is used with QAM. There are 2 carriers in a QAM signal, and are separated in phase by 90 degrees using the same frequency bands i.e. 1 quarter of a cycle, and the term quadrature arises from there. The 2 signals are called I and Q. The signals can be represented by a sine and a cosine wave. At the source, these modulated carriers are combined for transmission. The carriers are separated at the destination. This has a great advantage, each orthogonal carrier can be modulated independently because they differ by a 90° phase shift and occupy the same frequency band. It is very affective in terms of bit error rate. The 4 different types of phases are 16 QAM, 32 QAM, 64 QAM and 256 QAM. QAM is basically a combination of amplitude modulation and phase shift keying for which the efficiency of transmission for radio communication system has increased. The drawback of QAM is that it is more susceptible to noise due to its symbols are very close to each other and therefore the rate of interference increases. Below is a constellation diagram for 16 QAM and 64 QAM, Figure [7.6].

Figure 7.6: 16-QAM and 64-QAM

7.7 Adapative Modulation:

LTE takes a greater advantage of adaptive modulation if a channel is affected by fading noise and variations; this intelligent technique is used to select a proper modulation scheme for the channel. The specialty of this technique is if signal's conditions become bad then it switches from one modulation scheme to another which suits best for the signal. The amount of deviation in throughput and spectral efficiency also varies if a modulation scheme changes. BPSK and QPSK has low throughput as compared to 64 QAM. In order to achieve high spectral efficiency and high transmission throughput, it is important to use higher modulation schemes. On the other hand, in the channel, the lower modulation schemes are less vulnerable to noise and interference.

CHAPTER 8

LTE PHYSICAL LAYER

8.1 Introduction:

To meet the increasing demand of consumers for enhanced broadband services, LTE technology is designed to achieve lower latency, high data rate, increased network capacity, spectral efficiencies, cellular network bandwidth etc.

8.2 Generic Frame Structure:

Generic frame structure is an element, which is shared by the LTE DL and UL. It has already been mentioned that both modes of operation, FDD and TDD are defined in the LTE specifications. In this section, specification of FDD is exclusively illustrated. The generic frame structure is applicable on both the DL and UL for FDD operation.

Figure 8.2: LTE Generic Frame Structure [14] 

LTE transmissions are set into frames. In duration, these frames are 10 msec. Frames are composed of 20 slot periods of 0.5 msec. Sub-frames, in duration, are 1.0 msec and comprise of two slot periods.

8.3 LTE Downlink:

The specification of LTE PHY has been designed so that bandwidths from 1.25 MHz to 20 MHz can be accommodated. As the basic modulation scheme, OFDM was selected, as it is robust while severe multipath fading is present. Downlink multiplexing is effectuated via OFDM.

Physical channels are supported by the DL, information and physical signals are conveyed from higher layers within the LTE stack. The PHY layer uses these signals exclusively. Transport channels are mapped by physical channels, these are the service access points (SAPs) for the L2/ L3 layers. Physical signals and channels use various types of coding parameters and modulation, it depends on the assigned task.

8.3.1 Modulation Parameters:

For the DL, OFDM is used as the modulation scheme. The basic subcarrier spacing is 15 kHz; for some MB- SFN scenarios, a reduced subcarrier spacing of 7.5 kHz is available. The table [8.3.1] below gives a brief idea of OFDM modulation parameters.

Table 8.3.1-1: DL OFDM Modulation parameters [15] 

Either short or long Channel Prefix (CP) is used relying on the channel delay spread. While using short CP, as shown in the table [8.3.1-2] below, the remaining six symbols in a slot have a bit shorter CP than the first OFDM symbol. This is done so that slot timing of 0.5 msec is maintained.

Table 8.3.1-2: Cyclic Prefix Duration [16] 

It is noteworthy that the CP is represented in absolute terms e.g. 6.67 μsec for long CP and in relation to standard time units, Ts which is used in the specification documents of LTE. Ts = 1 / (15000 x 2048) seconds, correlating to the 30.72 MHz sample clock for the 2048 point FFT used along with the 20 MHz system bandwidth.

8.3.2 Downlink Multiplexing:

The basic multiplexing scheme, which is related to the LTE downlink, is, OFDMA. It is a new-to-cellular technology, which has been discussed already. As was described previously, in order to form physical resource blocks (PRBs), 12 adjacent subcarriers comprise of groups together on a slot-by-slot basis. A PRB that is the smallest unit of bandwidth, is allotted by the base station scheduler.

8.3.3 Physical Channels:

There are 3 different types of physical channels for the LTE downlink. All of the physical channels have one common characteristic i.e. they all carry information from higher layers in the LTE stack. This differs from physical signals, which carry information within the PHY layer exclusively. The 3 physical channels for LTE DL are:

Physical Downlink Shared Channel (PDSCH)

Physical Downlink Control Channel (PDCCH)

Common Control Physical Channel (CCPCH)

Algorithms have been defined by each physical channel for:

Bit scrambling

Modulation

Layer mapping

CDD Precoding

Resource element assignment.

8.3.3.1 Physical Downlink Shared Channel (PDSCH):

In order to transport data and multimedia, the PDSCH is used. It is thereby designed for very high data rates. Modulation options contain QPSK, 16QAM and 64QAM. PDSCH also uses Spatial multiplexing. Spatial multiplexing is an exclusive feature in PDSCH. In the given table [8.3.3.1], layer mapping for PDSCH is discussed.

Table 8.3.3.1: PDSCH Layer Mapping [17] 

8.3.3.2 Physical Downlink Control Channel (PDCCH):

UE- specific control information is conveyed by the PDCCH. Robustness in considered instead of maximum data rate. The only modulation format available is QPSK.

8.3.3.3 Common Control Physical Channel (CCPCH):

Cell- wide control information is carried by the CCPCH. Robustness is the main matter of consideration here as well. QPSK is the only modulation format that is available. The CCPCH is transmitted closer to the center frequency so fas as it is possible. On the 72 subcarriers, which are active in the centre of the DC subcarrier, CCPCH is transmitted exclusively. Resource elements (k, l) is mapped by control information. Here l refers to the subcarrier when k refers to the OFDM within the slot.

8.3.4 Physical Signals:

Physical signals use designated resource elements. Physical signals derive from physical channels as in they do not convey information from/ to higher layers. There are 2 types of physical signals:

1. Reference signals used to determine the channel impulse response (CIR)

2. Synchronization signals that convey network timing information

8.3.4.1 Reference Signals:

Reference signals are the production of an orthogonal sequence and a pseudo-random numerical (PRN) sequence. In total, the number of possible unique reference signals is 510. Within a network, a specific reference signal is allocated to each cell. The signal acts as a cell specific identifier.

Figure 8.3.4.1: Resource Element Mapping of Reference Signals [18] 

As we can see from the Figure [8.3.4.1] above, transmitted reference signals go within the first and third from- last OFDM symbol of each slot on equally spaced subcarriers. It is important that UE gets the appropriate CIR from each transmitting antenna. As a result, in the cell, the other antenna ports are idle while one antenna port is active in transmitting a reference signal.

Every sixth receives the reference signals. Subcarriers are estimated by CIR, which do not carry reference signals are generally computed via interpolation. The change of the subcarriers bearing reference signals PRF hopping is taken into consideration.

8.3.4.2 Synchronization Signals:

Synchronization signals use the pseudo- random orthogonal sequences that are similar to reference signals. These depend on the use by UE at the time of the cell search procedure and accordingly are classified as primary and secondary synchronization cells. Both types of signals centered around the DC subcarrier are transmitted on the 72 subcarriers during the 0th and 10th slots of a frame.

8.3.5 Transport Channels:

LTE PHY includes transport channels, which, for higher layers, act as service access points (SAPs). The different types of transport channels include:

8.3.5.1 Broadcast Channel (BCH):

These are in fixed format and they must be broadcast over entire coverage area of cell.

8.3.5.2 Downlink Shared Channel (DL-SCH):

DL-SCH supports dynamic link adaption by coding, varying modulation and transmit power and also support Hybrid ARQ (HARQ). It is suitable for use with beamforming and transmission over entire cell coverage area. It also provides support for discontinuous receive (DRX) for power save and dynamic and semi-static resource allocation.

8.3.5.3 Paging Channel (PCH):

PCH Support for UE DRX. It is mapped to dynamically allocated physical resources. PCH is the requirement for broadcast over entire cell coverage area.

8.3.5.4 Multicast Channel (MCH):

MCH provides support for MB-SFN and semi-static resource allocation. MCH is one the requirements for broadcast over entire cell coverage area.

8.3.6 Mapping Downlink Physical Channels to Transport Channels:

As shown in the given figure, physical channels are mapped by transport channels. Supported transport channels have been listed above. The following functions are provided by these transport channels:

1. Structure for transmitting data from/ to higher layers

2. Mechanism by which the PHY can be configured by higher layers

3. Status indicators (packet error, CQI etc.) to higher layers

4. Support for higher-layer peer-to-peer signaling

8.3.7 Downlink Channel Coding:

For the DL physical channels, different types of coding algorithms are used. Modulation is restricted to QPSK for the common control channel (CCPCH). The PDSCH uses not more than 64 QAM modulation. Paramount requirement is the coverage for control channels. Although a final determination related to code has not yet been made, with the CCPCH, convolutional coding has been selected to use. In order to achieve the highest possible downlink data rates, higher-complexity modulation is applied on the PDSCH. The PDSCH, depending on channel conditions, uses QPSK, 16QAM, or 64QAM. Therefore over latency, coding gain is emphasized and for the PDSCH, rate 1/ 3 turbo coding has been selected.

8.3.8 OFDMA Transmitter and Receiver:

In OFDMA transmitter, the available spectrum is divided into number of orthogonal subcarriers. [19] The spacing of the subcarrier is for LTE system is 15 KHz along with the OFDM symbol duration of 66.67μs. The high bit-rate data stream is passed through modulator and adaptive modulation schemes like BPSK, QPSK, 16-QAM, 64-QAM are employed. Serial to parallel converter converts the modulated symbols multilevel sequence into frequency components (subcarriers). These components are parallel. These complex data symbols are then converted by the IFFT stage into time domain and eventually OFDM symbols are generated. Between OFDMA symbols, a guard band is employed so that the Intersymbol Interference is cancelled at receiver. This guard band in LTE is named Cyclic Prefic (CP). The delay spread or the channel impulse response should not be greater than the CP duration. Though the ISI is not dealt by the receiver, yet it has to consider the impact of the channel for every single subcarrier experiencing frequency dependent phase and amplitude changes. Generally, two types of CP are used in LTE by the OFDMA. They are the normal CP that is used for high frequencies basically in urban areas and the extended CP using for lower frequencies in rural areas. At the receiver, the subcarriers are converted from parallel to serial sequence before removing the CP. The OFDM symbols are further converted by the FFT stage into frequency domain and demodulation and equalizer follow it.

Figure 8.3.8: Transmitter and Receiver of OFDMA [20] 

8.4 LTE Uplink:

For the uplink, SC-FDMA is used by the LTE PHY as the basic transmission scheme. SC-FDMA is a multi-carrier scheme essentially re-using many of the functional blocks, which are included in the UE OFDM receiver signal chain. One of the principle advantages of SC-FDMA is a lower PAPR, which is approximately 2 dB.

8.4.1 Modulation Parameters:

In FDD applications, the generic frame structure is used by the uplink, which is similar to the downlink. PRB width (12 subcarriers) and the same subcarrier spacing of 15 kHz are used as well. Downlink modulation parameters, both normal and extended CP length, are similar in every aspect to the uplink parameters as shown in the figures above. But subcarrier modulation is much different to it.

In the uplink, mapping data onto a signal constellation, which can be QPSK, 16QAM, or 64QAM, depends on channel quality. Unlike OFDM, uplink symbols are constantly fed into a serial/ parallel converter instead of using the QPSK/ QAM symbols directly to modulate subcarriers. Then the symbols are sent into an FFT block. The output is a detached frequency domain representation of the QPSK/QAM symbol sequence.

Then the discrete fourier terms are mapped to subcarriers at the output of the FFT before they are converted back into the time domain known as IFFT. Before transmitting signals, the final step is to append a CP. It is noteworthy that while in the time domain the SC-FDMA signal has a lower PAPR, individual subcarrier amplitudes varies more than an equivalent OFDM signal in the frequency domain.

8.4.2 Multiplexing:

UE assigns uplink PRBs via the DL CCPCH by the base station scheduler. Uplink PRBs are made of 12 contiguous subcarriers that form a group together. The duration is of one slot time.

8.4.3 Uplink Physical Channel:

Uplink physical channels are used so that information can be transmitted which originates in layers above the PHY. UL physical channels are listed below:

8.4.3.1 Physical Uplink Shared Channel (PUSCH):

Resources for the PUSCH by the UL scheduler are assigned on a sub-frame basis. Subcarriers may be leaped from sub-frame to sub-frame. They are assigned in multiples of twelve PRBs. The PUSCH may use modulation that are QPSK, 16QAM or 64QAM.

8.4.3.2 Physical Uplink Control Channel (PUCCH):

The PUCCH is never transmitted along with PUSCH data. Control information is conveyed by PUCCH, which includes ACK/NACK, channel quality indication (CQI), HARQ and uplink scheduling requests. The transmission of PUCCH is frequency hopped for added reliability at the lot boundary as is shown in the Figure [8.4.3.2] below.

Figure 8.4.3.2: PUCCH is Hopped at Slot Boundary [21] 

8.4.4 Uplink Physical Signals:

These types of signals are employed within the PHY. Information is not conveyed by uplink physical signals from higher layers. There are 2 types of UL physical signals that have been defined namely, the Reference Signal and the Random Access Preamble.

8.4.4.1 Uplink Reference Signal:

UL reference signals are of 2 types. Coherent demodulation is facilitated by the demodulation signal. It is same as the assigned resource by size and is transmitted in the 4th SC-FDMA symbol of the slot. On the other hand, frequency dependent scheduling is facilitated by a sounding reference signal. Both types of UL reference signals are established on Zadhoff-Chu sequences.

8.4.4.2 Random Access Preamble:

The PHY and higher layer are involved in the procedure of the random access. The transmission of the random access preamble initiates the cell search procedure by the UE at the PHY layer. When is succeeded, the base station receives a random access response. The Figure [8.4.4.2] below contains a preamble, a cyclic prefix and a guard time while there is no transmitted signal. The whole format of the random access preamble is shown below.

Figure 8.4.4.2: Random Access Preamble Format [22] 

The timing parameters for the generic frame structure are:

TRA : 30720 TS

TGT : 3152 TS

TPRE : 24576 TS

Where TS = period of a 30.72 MHz clock

Zadoff-Chu sequences derive from random access preambles. Preambles that are transmitted on block of 71 contiguous subcarriers are assigned for random access and it is done by the base station. The number of possible preamble sequences per cell is 64 in FDD applications.

The accurate frequency that is used for transmitting the random access preamble is selected by higher layers from available random access channels in the UE. The other informations that are provided by higher layers to the PHY include:

Preamble format

Available random access channels

Power ramp step size

Initial transmission power

Maximum number of retries

8.4.5 Uplink Transport Channels:

UL transport channels, as with the DL, for higher layers act as service access points. The basic characteristics of UL transport channels are:

8.4.5.1 Uplink- Shared Channel (UL-SCH):

UL-SCH supports dynamic link adaptation, varying coding, modulation and/ or Tx power and possible use for beam forming. It also provides support for HARQ and also for dynamic and semi-static resource allocation.

8.4.5.2 Random Access Channel (RACH):

RACH is a possible risk of collision and it supports for transmitting limited control information.

8.4.6 Mapping Uplink Physical Channels to Transport Channels:

Physical channels are mapped by transport channels. The process is given in the diagram [8.4.6] below.

Figure 8.4.6: Mapping of UL Transport Channels to UL Physical Channels [23] 

8.4.7 Coding:

Like the DL-SCH, the same rate 1/3 turbo encoding scheme is used by the UL-SCH (one internal server and two 8- state constituent encoders).

8.4.8 MB-SFN:

In a single cell or multi-cell mode, in any mode, MBMS (Multimedia Broadcast Multicast Services) can be performed. The DL-SCH is mapped by MBMS traffic in single cell transmissions. In order to form an MB-SFN, transmissions from cells in multi-cell mode are synchronized carefully.

For cellular broadcast, one of the elegant applications of OFDM is MB-SFN. On a common frequency, cells that are closely coordinated, broadcast identical transmissions. The receiver receives signals arriving from adjacent cells and deal with it just as multipath delayed signals. In this way, energy coming from multipath transmitters is combined by UE and it is done without any receiver complexity.

The relative delay is small between the two signals when the UE is at a cell boundary. However, the amount of delay is larger when the UE is far away from one base station and relatively close to a second base station. That is why transmissions of MB-SFN is supported with the use of a longer CP and a subcarrier spacing of 7.5 kHz. Common reference signal is also used by MB-SFN networks from all transmitters inside of the network so that channel estimation is facilitated.

The result of the transmission of the MB-SFN scheme is that UE can roam within the cells and handoff procedure is not required. The received signal is dealt same as with a conventional single channel transmission of OFDM though signals from different cells vary in transmission time and in strength.

8.4.9 SC-FDMA Transmitter and Receiver:

In SC-FDMA, according to the QPSK, 16-QAM, or 64-QAM modulations, the data is mapped into signal constellation. It depends upon the conditions of channel as is with the OFDMA. However the subcarriers are not modulated directly by the QPSK/QAM. The symbols pass through a serial to parallel converter, following by a DFT block, producing separate frequency domain representation of the symbol of the QPSK/QAM as shown in the diagram [8.4.9-1].

Figure 8.4.9-1: SC-FDMA Transmitter

DFT element follows pulse shaping; it is noncompulsory and sometimes the output signal from DFT needs to be shaped. In the actual signal bandwidth extension is occurred when the pulse shaping is active. In subcarrier mapping block, the subcarriers are then mapped with the discrete fourier symbols that are from the output of DFT block. After these frequency domain modulated subcarriers are mapped, for time domain conversion, they pass through IDFT. The remaining transmitter operation is same as OFDMA. In the transmitter of SC-FDMA, a vital role is played by the sub-carrier mapping. Every N DFT output on a single subcarrier available on M subcarriers is mapped. In this case, for available bandwidth, the total number of subcarriers is M. There are two methods for achieving the subcarrier mapping. They are the localized subcarrier mapping and the distributed subcarrier mapping. In localized subcarrier mapping, M adjacent subcarriers are assigned from the modulation symbols. But in distributed mapping, the symbols are spread equally through the entire channel bandwidth. Localized subcarrier mapping are also signified as localized SC-FDMA (LFDMA) whereas distributed subcarrier mapping are signified as distributed SC-FDMA (DFDMA). Zero amplitude in transmitter is assigned by the IDFT to the subcarriers that are unoccupied and the process is done in both of the modes of subcarrier mapping. In SC-FDMA, the IFDMA is more efficient as in the signal can be modulated by the transmitter in time domain without the use of DFT and IDFT. If Q = MxN for the distributed mode with equidistance between subcarriers then it is called Interleaved FDMA (IFDMA). [24] Number of subcarriers is M while number of users is Q and number of subcarriers for each user is N. In localized mapping, on N consecutive subcarriers are N-discrete frequency signals mapped. But the case is a bit different in distributed mapping. In distributed mapping, N-discrete frequency signals are mapped equally spaced subcarriers. These processes are shown in the diagrams [8.4.9(a)] and [8.4.9(b)] below.

Figure 8.4.9(a): Distributed FDMA

Figure 8.4.9(b): Localized FDMA

SC-FDMA receiver is shown in the figure below. Conventional OFDMA is similar to it alongside additional blocks of subcarrier demapping, optional shaping filter and IDFT. The filter relates to the spectral shaping that is employed in the transmitter. N-discrete signals are resulted by M-mapped subcarrier demapping. At last, the SC-FDMA signal is converted to the signal constellation by IDFT.

Figure 8.4.9-2: SC-FDMA Receiver

In LTE uplink transmission, there are some added signals that carry data e.g. reference signal, control signal, random access preamble etc. These signals are described as sequence signaling, having continual amplitude with zero autocorrelation. Unlike data carrying signals, these are excluded from SC-FDMA modulation scheme.

CHAPTER 9

ANALYSIS OF THE SIMULATION DESIGN

In this chapter, the model of OFDMA and SC-FDMA transmitter and receiver are designed using AWR Microwave Office. The transmitter and receiver design are made using different modulation scheme which are BPSK, QPSK, 16-QAM and 64-QAM. According to the theoretical values, there are some losses in the system in comparison with the practical world. Therefore to simulate the background noise of the channel, Additive White Gaussian Noise (AWGN) is used. The modulation schemes are used to analyze the Bit Error Rate (BER), Signal to Noise Ratio (SNR), Constellation diagram for both OFDMA and SC-FDMA.

9.1 OFDMA TRANSMITTER AND RECEIVER MODEL:

9.1.1 BPSK:

9.1.2 QPSK:

9.1.3 16-QAM:

9.1.4 64-QAM:

9.2 SC-FDMA TRANSMITTER AND RECEIVER MODEL:

9.2.1 BPSK:

9.2.2 QPSK:

9.2.3 16-QAM:

9.2.4 64-QAM:

9.3 MODULATION PARAMETERS USED IN SIMULATION:

For transmission bandwidth 20 MHz :

Sub-carrier Spacing- 15 KHz

FFT Size- 2048

Sampling Frequency- 30.72 MHz

No. of Occupied Sub-carrier- 1201

No. of OFDM symbols/slot- 7 for Normal CP and 6 for Extended CP

CP Lengths (us/sample)- Normal- (4.69/144) x 6, (5.21/160) x 1

Extended- 16.67/1024

BER:

The BER is ratio of error bits and total number of bits transmitted during time interval.

BER = Error Bits / Number of Transmitted Bits

SNR:

The SNR is the ratio of bit energy (Eb) to the noise power spectral density (N0) and it is expressed in dB.

SNR=Eb /N0

BER vs SNR Process:

The BER is articulated in terms of SNR for any modulation scheme. Through the comparison of the transmitted signal and received signal is BER measured and the error counts are computed over total number of bits that are transmitted.

CHAPTER 10

SIMULATION PERFORMANCE

10.1 BER vs SNR of OFDMA and SC-FDMA:

The BER vs SNR of OFDMA and SC-FDMA are shown in figure [10.1-1] and figure [10.1-2] separately.

BER for OFDMA:

Figure 10.1-1: BER vs SNR of OFDMA with Adaptive Modulation

Consider the value for BER = 0.001

Modulation Scheme

Bit per Symbol

SNR (dB)

BPSK

1

6.66

QPSK

2

6.66

16-QAM

4

10.45

64-QAM

6

14.64

Table 10.1-1: OFDMA, BER vs SNR

BER for SC-FDMA:

Figure 10.1-2: BER vs SNR of SC-FDMA with Adaptive Modulation

Consider the value for BER = 0.001

Modulation Scheme

Bit per Symbol

SNR (dB)

BPSK

1

6.66

QPSK

2

6.66

16-QAM

4

10.44

64-QAM

6

14.64

Table 10.1-2: SC-FDMA, BER v SNR

10.2 IQ Constellation diagram for OFDMA and SC-FDMA:

IQ Constellation for OFDMA:

Figure 10.2-1: OFDMA, IQ Constellation

IQ Constellation for SC-FDMA:

Figure 10.2-2: SC-FDMA, IQ Constellation

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