FBMC vs OFDM Waveform Contenders for 5 G Wireless Communication System

The multicarrier transmission techniques have been the most beguiling one for the development of wireless communication systems like 4G, Long Term evolution and now the successor 5G. For 5G or fifth generation wireless communication, Orthogonal Frequency Division Multiplexing (OFDM) and Filter Bank Multi carrier modulation (FBMC) are the dominant waveform contenders. In this research paper, the disadvantages of OFDM have been addressed and it has shown that filter bank multicarrier (FBMC) could be a more effective solution. The comparative analysis of FBMC and OFDM has been performed based on Power Spectral Densities, sub channels, computational complexity and prototype filter comparison simulated using MATLAB.


Introduction
The evolution of internet and the growing demand of high data rate users have given Dynamic spectrum access networks a widespread attention in the recent years.In the DSA networks, multiple nodes contend to employ a shared frequency spectrum.The OFDM technique with cyclic prefix (CP-OFDM) that uses an orthogonal set of subcarriers is by far the most widespread case of multicarrier systems which have been anticipated for the purpose of sharing the various subsets of these subcarriers amid nodes [1] [2] installed for DSA.But, the tight timing and synchronization requirement in OFDM is problematic to attain in practice when the nodes belong to various administrative units.Hence, mutual interference amid the signals of various users is resulted in case of an absence of synchronization.Furthermore, the CP employed is chastely redundant in terms P. Kansal, A. K. Shankhwar DOI: 10.4236/wet.2017.8400560 Wireless Engineering and Technology of information and significantly diminishes the bandwidth efficiency.The disadvantages of OFDM technique were overcome by a multicarrier communication system called as FBMC first proposed by Saltzberg [3] that delivers an improved spectral shaping of subcarriers than OFDM systems.This is then achieved by the careful designing of the prototype filter, which abridges equalization in the lack of CP and also promises an additional effectual spectral utilization by diminishing interference across subcarriers [4].By engaging Offset Quadrature Amplitude Modulation (OQAM), the full capacity of the transmission bandwidth can be attained in FBMC systems.Hence, the main aim of this research paper is to compare the multicarrier techniques OFDM and FBMC techniques for 5G wireless systems.By using MATLAB all the simulations have been performed for comparing the two multicarrier techniques in terms of the Power Spectral Densities, computational complexity and prototype filter comparisons of FBMC and OFDM [5].
This research paper is organized into the following sections-Section 1 gives the introduction of the research paper and highlights the idea behind the comparison of FBMC and OFDM techniques, Section 2 gives the main differences between the two multicarrier techniques with the help of their respective block diagrams, Section 3 discusses about the transmultiplexer configuration of FBMC technique giving the details about the input and outputs, the filter banks used in transmitter and receiver side and the offset quadrature amplitude modulation pre/post processing, Section 4 is about the simulations performed on MATLAB for comparing the FBMC and OFDM techniques in terms of Power spectral densities, computational complexities of the techniques in terms of magnitude response and the magnitude responses of the prototype filters for both techniques, finally Section 5 gives the conclusion of the research paper.Section 5 is followed by Acknowledgements, nomenclature used in the paper and the references.FFT plus CP out is substituted by the analysis filter bank (AFB).The main differences between OFDM and FBMC techniques have been summarized in Table 1 on the properties-cyclic prefix, sidelobes, synchronization, Doppler effect, MIMO systems, Spectrum sensing and computational complexity.The basic principles of OFDM and FBMC are explicated below.

Orthogonal Frequency Division Multiplexing (OFDM)
In OFDM multicarrier system, the frequency spectrum of the subcarriers is overlapped with the least frequency spacing and the orthogonality is attained amid the various subcarriers.In Figure 1, the input stream is subdivided into parallel data streams by means of the serial to parallel (S/P) converter, that are then passed into an inverse fast Fourier transformation (IFFT) block to produce time sequence of the streams.Consequently, by totalling a cyclic prefix (CP), the OFDM symbol time sequences are extended.The CP is a copy of the latter portion of the symbol that is added in the start of the sequence and should be greater than the network deferral spread in order to diminish the inter symbol interference (ISI) produced by the influx of various OFDM symbols with distinct delay.The resultant digital signal is transformed into analog form and transmitted over the channel [5].
At the receiver end, the signal is reconstructed into digital form and the far Fourier transform (FFT) is achieved in the received streams after eradicating the CP [5].Finally, the parallel streams are collected into a single stream as the original transmitted one.Some of the disadvantages of OFDM are enumerated below: 1) Decreased spectral efficiency owing to the CP employed; 2) High spectral leakage owing to the rectangular windowing; 3) Interference amid the unsynchronized signal in the neighbouring bands.

Filter Bank Multi Carrier
The FBMC technique overcomes the limitations of OFDM by adding generalized pulse shaping filters which delivers a well localized subchannel in both time and frequency domain.Consequently, FBMC systems have more spectral containment signals and offer more effective use of the radio resources where no CP is required.In Figure 2 it can be seen that the filterbanks on the transmitter side and the receiver side consist of an array of N filters that processes N input signals to give N outputs.If the inputs of these N filters are associated together, the system in analogous manner can be measured as an analyzer to the input signal based on each filter characteristics.In Figure 2 the filter bank used at the transmitter side is called synthesis filter bank and the filter bank used in receiver side is called analysis filter bank.As depicted in Figure 2 the input signal is first converted from serial to parallel form and then passed through synthesis filter bank and then it is converted back to serial form after coming out of synthesis bank.After this it can be seen in Figure 2 that in the receiver side after the signal passes through the channel it is converted to parallel form by serial to parallel converter and passed through analysis filter bank.Finally when the output signal is obtained it is again converted to serial form by parallel to serial converter.[7].Hence the synthesis-analysis configuration depicted in Figure 2 is called transmulitplexer or TMUX and is applied in the MC communication systems [8].

Transmultiplexer Configuration of Filter Bank Multi Carrier
The core of the FBMC system is the TMUX configuration presented in Figure 3 the main processing blocks in this direct form depiction are OQAM pre-processing, synthesis filter bank, the analysis filter bank, and the OQAM post-processing.
The transmission channel is classically misplaced when analysing and planning TMUX systems as the channel equalization problem is controlled distinctly.The  a) The number of sub channels M is essentially a random even number, but characteristically it is a power of two acceptable to deliver effectual application.
b) So the greatest motivating prototype filter lengths are selected to be 1, , and

OQAM Pre/Post Processing
The TMUX system transmits OQAM symbols instead of QAM symbols.The pre-processing block, which utilizes the transformation between QAM and OQAM symbols, is shown in Figure 4.As can be seen, the first operation is a simple complex-to-real conversion, where the real and imaginary parts of a complex-valued symbol , k l c are separated to form two new symbols ,2 k l d and ,2 1 k l d + (this op- eration can also be called as staggering).So the order of these original symbols depends upon the sub channel number, i.e., the conversion is distinct for even and odd numbered sub channels.The complex-to-real conversion upsurges the sample rate by a factor of 2. After this the second operation is the multiplication by , k n θ sequence [9].
However, it should be noted that the signs of the , k n θ sequence can be cho- sen arbitrarily, but the pattern of real and imaginary samples has to follow the above definition.For example, an alternative sequence

Synthesis and Analysis Filter Banks
and p L is the filter length.In order to simplify nota- tions, it is assumed that the prototype filter is designed in such a manner that there is no need for the additional scaling factor.The k th analysis filter is simply a time-reversed and complex-conjugated version of the corresponding synthesis An interpretation to the above equations is that first zero-phase sub channel filters are generated from a linear-phase prototype filter and then their impulse responses are delayed by ( ) samples resulting in causal subchannel filters.Due to the nature of the modulation function, the m th sub channel filter is purely real and the M/2 th is purely imaginary.However, all the sub channel filters have linear phase.

Simulation Results
The analysis compares Filter Bank Multi-Carrier (FBMC) modulation with generic OFDM modulation.FBMC offers ways to overcome the known limitations of OFDM of reduced spectral efficiency and strict synchronization requirements.
The benefits have led it to being measured as one of the modulation techniques for 5G communication structures.

Power Spectral Densities of FBMC and OFDM
Power spectral density of the FBMC transmit signal is designed to give the low out-of-band leakage.In Figure 6 and Figure 7 the plots of the spectral densities for OFDM and FBMC schemes are compared.In Figure 6 it depicts that the FBMC has lesser side lobes.This lets an advanced utilization of the allotted spectrum, leading to advanced spectral efficiency.In Figure 7 the OFDM PSD curve has higher side lobes and out of band leakage.Hence the FBMC scheme is more advantageous in comparison to OFDM by providing higher spectral efficiency.
Owing to the per subcarrier filtering, it suffers a larger filter delay (in comparison to UFMC) and also needs OQAM processing.

Comparison of Computational Complexity of OFDM and FBMC in Terms of Magnitude Responses
The computational complexity of filter bank structures can be evaluated by calculating   the number of real multiplications and additions that are necessary to compute a new length-complex-valued output sequence.However, the analysis is just based on the number of multiplications because it is a known fact that adders are considerably cheaper to implement than multipliers.So the Figure 8 depicts that the FBMC is more complex than the conventional OFDM.Here the number of real multiplications as a function of number of sub channels in the case of OFDM and FBMC has been observed.Also when using the polyphase implementations of FBMC the complexity increases as depicted in Figure 8 but decreases when FBMC is implemented without the polyphase.

Comparison of Magnitude Responses of Prototype Filters of FBMC and OFDM
The main difference between FBMC and OFDM is the choice of the prototype filter.Such that OFDM uses a rectangular window filter and FBMC using a prototype filter designed with the Nyquist pulse shaping principle, which can reduce greatly the spectral leakage problem of OFDM.This results in negligible ICI and ISI.In Figure 9 the magnitude responses of prototype filters of FBMC and   OFDM have been compared against the normalized frequency taken on the x axis.

Conclusion
In this paper the performance comparison of OFDM and the FBMC as the most potential contenders of 5G has been carried out and simulated using MATLAB
where K is a positive integer called as overlapping factor and it is chosen to be 3 or higher.c) The prototype filter is intended in such a manner that only instantly adjacent sub channel filters are pointedly overlapping with each other in the frequency domain.The amount of sub channels is twice the up sampling and down sampling factors suggesting 2x oversampled filter banks if input and output signals are complex-valued.Hence, if input and output signals are purely real/imaginary-valued then the existing TMUX is comparable to a critically sampled TMUX.This happens because the sample rate (counted in terms of real-valued samples) of the SFB output and AFB input is equal to the sum of the sample rates of the sub channel signals.An extra delay D z − , with D depending on the length of the P. Kansal, A. K. Shankhwar DOI: 10.4236/wet.2017,has to be included either in AFB or SFB out- put [9].

A
direct form SFB consists of M up samplers and M synthesis filters.As shown in Figure 3, the input signals ( ) first up sampled by M/2 and then filtered with synthesis filters ( ) k G z The SFB output signal Y(z) is formed when all sub signals are added together.A direct form AFB is constructed using M analysis filters and M down samplers as shown in Figure 3.The input signal Y(z) is first filtered by analysis filters ( ) k F z and these signals are then down sampled by a factor of M/2 to form output signals ( ).
In the case of chosen class of complex modulated filter banks, all sub channel filters can be generated from a single real-valued linear-phase low pass prototype filter p[m] by using exponential modulation.Due to the modulation function, the resulting sub channel filters are complex-valued and therefore the useful fre-quency range is [ ] π, π − .Typically, two different types of channel stacking arrangements can be considered, namely, even-stacked filter banks and odd-stacked filter banks.The center frequencies of the k th sub channel filters are 2 π stacked and odd-stacked filter banks, respectively.The k th synthesis filter is given by P. Kansal, A. K. Shankhwar DOI: 10.4236/wet.2017.8400566 Wireless Engineering and Technology [ ] [ ]

Figure 8 .
Figure 8.Comparison of computational complexity of FBMC and OFDM in terms of magnitude responses.

Figure 9 .
Figure 9.Comparison of magnitude responses of prototype filters of FBMC and OFDM.
in terms of Power Spectral Density curves of OFDM and FBMC, sub channels of OFDM and FBMC, computational complexities of the two techniques and the prototype filter comparisons of FBMC and OFDM.All the simulations performed show that the FBMC technique is the most promising waveform contender for future wireless communications specially 5G telecommunications.The shortcomings of OFDM technique have been addressed and removed by the FBMC.FBMC is evolved from OFDM.P. Kansal, A. K. Shankhwar DOI: 10.4236/wet.2017.8400569 Wireless Engineering and Technology