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The purpose of this assignment is to motivate students to understand the digital modulation schemes used with multi-carrier modulation employed in the downlink of LTE cellular networks. you also develop skills in information gathering and technical documentation. In summary, the learning outcomes in this project include:


a. Develop capabilities in requirements of real-world case study.
b. Ability to reformulate ill-formulated questions in a solvable form.
c. Develop skills to critically review, consolidate, analyze, synthesize knowledge of overall system.
d. Ability to understand and apply technical skills to real information technology applications.
e. Prove the ability to apply content learned during their studies to solve practicallyrelevant problems.
f. Develop research and lifelong learning skills.
g. Acquisition of teamwork and interpersonal communication skills.
h. Develop the skill to write a professionally-looking technical reports

The Importance of Modulation in Wireless Communications

Modulation forms the most important part to any wireless communications. The kind of modulation applied in any communication is of significant importance due to the current society as a result of the limitation in the availability of spectrum. The transition from analog to digital communication carried with it numerous advantages among them improved communication quality, increased carrying capacity of information, availability of information swiftly as well as increased security of data among other advantages.  

Despite these advantages, a range of restrictions has met the applications of digital communication applications in the industry. Such restrictions as permissible power, availability of bandwidth and system's inherent noise levels from the bulk of the hiccups met by the developers of digital communication systems as they go about their advancements. These factors are found to mainly affect the efficiency of the spectrum which ends up lowering the rate of transfer of information in any allotted bandwidth. Other factors that play a major role in this category include reliability of the system as a result of increased demand of services by users, increased rates of data, larger frequencies of carriers as well as higher mobility.

The various digital communication techniques are associated with such advantages as greater capacity for the transmission of high data quantity that is immune to high noise levels. Another benefit yet is the ease of detection of its state of transmission at the point of reception while in a noisy medium.  Trade-offs is usually made in cases where a digitally transmitted signal is initiated in the form of an analog waveform.  This is done so due to the possibility of the loss of information during the process of quantization which is a requirement for the conversion of analog signals to digital signals.

Due to the limitation in such resources as time slots and bandwidth in uplink-downlink transmission, it is fundamental to consider the type of digital modulation technique chosen for a particular task.  How a modulation technique performance is a question of the power efficiency as well as the bandwidth.  Power efficiency is the measure of the ability of a modulation technique to preserve the bit error probability of the digitally transmitted message as the lowest power levels possible. Bandwidth efficiency refers to the ability of a modulation technique to contain data within a limited range of bandwidth.

The main modulation schemes used in LTE include QPK and QAM. QAM (Quadrature Amplitude Modulation) digital modulation involves the sending of two different signals at the same time on the same carrier frequency. In this modulation technique, the amplitude of the signal is allowed to vary with the phase. QAM is a modulation technique of higher order which is a derivative of the combination between PSK and ASK. There are various techniques into which QAM can be divided including 8 QAM, 16 QAM, and 64 QAM among other techniques [4].

Advantages and Limitations of Digital Communication

Quadrature Amplitude Modulation is mainly applied in applications that require higher data delivery, for example, cable modem systems and also in digital communication systems. A shift from 8 PSK to QAM is usually advisable in case of higher data requirements beyond 8 PSK as it is possible to achieve a greater distance between the adjacent points in the Q and I plane through even distribution of the points.  

Due to the differences in the amplitudes, the demodulator has to properly detect both the phase and the amplitude. This is the main complexity that is associated with this modulation technique. The most common forms of QAM modulation are 16 QAM, 64 QAM, 128 QAM and 256 QAM. The higher the order of the M-ary of the QAM, the greater the room for more points available within the constellation. This allows for a higher capability of transmission of more bits per symbol thereby allowing the transmission of data in relatively smaller bandwidth.

The end result is, therefore, higher bit error rate since the transmitted signal is more vulnerable to noise and any other forms of distortion. More power is needed in the transmission of this signal so as to effectively spread out the symbols hence reduced power efficiency in comparison to other modulation techniques. Important to note is that QAM of higher orders is technically able to transmit more data hence more spectrally efficient in their transmission even though their reliability id lower than lower order QAM.  

PSK (Phase Shift Keying) is yet another modulation technique. This technique comprises of BPSK and QPSK among other techniques. The technique involves the transmission of data by converting the phase of the carrier wave. BPSK is the simplest of all the PSK modulation modes and it makes use of two phases which are separated from one another by 180? phase shift thereby referred to as 2-PSK. The phase of the waveform is shifted by the signal to one of the two states so as to represent the binary symbol of either 0 or 1 respectively.  

BPSK signal is passed through a correlator which is composed of integrator and multiplier in which the signal is demodulated. In the correlator, the coherent reference signal alongside the incoming signal is multiplied and the output fed into the integrator. The output is then compared against the 0 threshold which allows for decision making based on the decision rule.  BPSK is mainly applied in satellite communication due to its robustness and the simplicity in its implementation.

Power Efficiency and Bandwidth Efficiency

QPSK (Quadrature Phase Shift Keying) is a higher order PSK and deploys four-level phase state to transmit 2 symbols or bits at the same time. It achieves this by selecting any of the four carrier phase shifts which could be equally spaced. The four carrier phase shifts include 0, n/2, n and 3n/2 with each of the phases of a carrier corresponding to a specific pair of bits if message 00, 01, 10, 11. From this property, QSPK is able to transmit double information using the same bandwidth hence more efficient than BPSK. Just like the case of BPSK, the incoming signal is multiplied with the reference signal both in the quadrate and in-phase channels in the demodulator.  QSPK is applied in satellite communication for example in cellular phone systems, video conferencing and other digital communication which occur over RF carrier.

  • Ability to work well with a network of single frequency
  • Has a better way of improving spectral efficiency as compared to other sidebar broadband
  • It does not require any other conventional filter types since it is fully fine-tuned with subchannel filters
  • Eliminates chances of time sync errors which result due to the low sensitivity of the system
  • Eliminates errors caused by multipath propagation since it works against intent symbol interface
  • Makes use of Fast Fourier Transmission that enhances efficient transmission
  • Allows recovery of symbols which may be lost as a result of frequency selectivity of the channel
  • Maximization of decoding likelihood is possible with reasonable complexity
  • Excellent protection against impulsive parasitive noise and cochannel interference
  • High computational efficiency by use of techniques of FFT in the implementation of the modulation and demodulation functions

An OFDM signal is composed of modulated carriers which are closely spaced. Upon application of any form of modulation, the sidebands on the carrier spread out on either side hence transmission of the signal. The wholes signal is supposed to be received by the receiver so that the correct demodulation of the data can occur. In this regard, closely transmitted signals are usually spaced so as to enable the receiver to separate them by the use of a filter. To enhance effective separation of the two signals, a guard band is usually placed between them.

Chances of overlaps is not an exception in OFDM but still, the signals can still be received by the interface between them is orthogonal to each other. This orthogonal nature is achievable by having the spacing of the carrier being reciprocal of the period of the symbol. An in-depth understanding of how OFDM technology supports the different modulation types can be achieved by a comprehension of how the OFDM works at the receiver. The receiver is the bank of demodulation in which each of the carriers is translated down to a DC. The resultant signal is then integrated over the symbol period so as to regenerate date from the carrier. This demodulator as well demodulates other carriers. A whole number of cycles would be realized in the symbol period since the carrier spacing is equivalent to the reciprocal of the symbol period. This can be defined as a region of no interface contribution.

The receiving and transmitting systems of the OFDM technology must be linear in order to achieve the no interference contribution. Any form of non-linearity would result into an interface that would them cause distortion of inter-modulation. As a result, unwanted signals would be generated hence impaired orthogonal transmission and interference.

Quadrature Amplitude Modulation (QAM)

The best modulation scheme is chose based on a number of factors among them the signal to Noise Ratio, power efficiency, the available bandwidth, and cost effectiveness and quality of the service it offers [6]. The overall performance of a modulation scheme is determined by establishing its probability of errors resulting from noise and interface induced in the channel. From research and experimental outcomes, it has been established that modulation techniques which are able to transmit more bits per symbol are more prone to error resulting from noise.

The design of any communication system is a factor of application orientation and significantly relies on the signal type involved. This is as a result of the differences in the nature of the complexity involved in each of the modulation techniques. The complexity crops in at the design process and involves modulation and demodulation of the various components of the communication systems. From experimental results, it has been found that higher order QAM have the ability to transmit greater data volumes even though they are less reliable.

Higher order QAM are known to have higher rates of errors compared lower order counterparts. However, through the use of techniques for correcting errors, the errors in higher order QAM can be corrected. Such correction techniques for error include turbo coding and convolution coding both of which are important in improving the performance of the system. Still, it has been found out that phase shift keying schemes of lower ends can be used in functions that require performances of low error and minimal power.

These schemes are however only usable for bandwidths of low efficiency. Schemes of the higher end are best suited for functions that need higher frequencies of bandwidth and low efficiency of power. In conclusion, each modulation technique produces different harmonic content thereby influencing the application. A modulation technique would be more suitable than another depending on the type of application to which it is to be put. The suitability of the application of a specific modulation scheme depends on the properties of the modulation scheme in question.

References

Jerry D. Gibson, Mobile Communications Handbook, Third Edition, 3rd ed., Jerry D. Gibson, Ed. New York, USA: CRC Press, 2016.

Sajal Kumar Das, Mobile Terminal Receiver Design: LTE and LTE-Advanced, 3rd ed., Sajal Kumar Das, Ed. Bangladesh, China: John Wiley & Sons, 2017.

Stefania Sesia, LTE - The UMTS Long Term Evolution: From Theory to Practice, 2nd ed., Stefania Sesia, Ed. London, UK: John Wiley & Sons, 2010.

Houman Zarrinkoub, Understanding Lte with MATLAB: From Mathematical Modeling to Simulation and Prototyping, 4th ed., Houman Zarrinkoub, Ed. California, USA: CreateSpace Independent Publishing Platform, 2014.

Tzi-Dar Chiueh, OFDM Baseband Receiver Design for Wireless Communication, 5th ed., Tzi-Dar Chiueh, Ed. Kansas, USA: John Wiley & Sons, 2008.

André Bourdoux, Digital Compensation for Analog Front-Ends: A New Approach to Wireless Transceiver Design, 3rd ed., André Bourdoux, Ed. New York, USA: John Wiley & Sons, 2015.

Professor Loutfi Nuaymi, WiMAX: Technology for Broadband Wireless Access, 4th ed., Professor Loutfi Nuaymi, Ed. Hong Kong, China: John Wiley & Sons, 2017.

Chung G. Kang, MIMO-OFDM Wireless Communications with MATLAB, 7th ed., Chung G. Kang, Ed. Beijing, China: John Wiley & Sons, 2010.

Tzi-Dar Chiueh, Baseband Receiver Design for Wireless MIMO-OFDM Communications, 2nd ed., Tzi-Dar Chiueh, Ed. New York, USA: John Wiley & Sons, 2012.

Christopher Cox, An Introduction to LTE: LTE, LTE-Advanced, SAE and 4G Mobile Communications, 6th ed., Christopher Cox, Ed. London, UK: John Wiley & Sons, 2012.

Antti Toskala, WCDMA for UMTS: HSPA Evolution and LTE, 3rd ed., Antti Toskala, Ed. Oslo, Australia: John Wiley & Sons, 2016.

Pramod Viswanath, Fundamentals of Wireless Communication, 4th ed., Pramod Viswanath, Ed. New York , USA: Cambridge University Press, 2015.

Richard van Nee, OFDM for Wireless Multimedia Communications, 4th ed., Richard van Nee, Ed. Chicago, USA: Artech House, 2010.

Ahmad R.S. Bahai, Multi-Carrier Digital Communications: Theory and Applications of OFDM, 3rd ed., Ahmad R.S. Bahai, Ed. Kansas, USA: Springer Science & Business Media, 2014.

William Shieh, OFDM for Optical Communications, 5th ed., William Shieh, Ed. London, England: Academic Press, 2015.

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