The second option, called d istributed SC-FDMA , distributes the subcarriers over the entire bandwidth and again uses zeros for the unused bins. Interleaved SC-FDMA is a special case of the distributed version where the subcarriers are evenly distributed over the whole bandwidth. In that case, the separation between each subcarrier is typically equal to the number users. However, by using an intelligent resource allocation that assigns the subcarriers with the best propagation conditions for each terminal, it is possible to achieve better performance using localized SC-FDMA.
By more efficiently using the power amplifier, LTE terminals are able to increase coverage and reduce their power consumption, which is extremely important in battery powered devices.
Myung, Hyung G. Each subcarrier is able to carry data at a maximum rate of 15 ksps kilosymbols per second. This gives a 20 MHz bandwidth system a raw symbol rate of 18 Msps. In turn this is able to provide a raw data rate of Mbps as each symbol using 64QAM is able to represent six bits. It may appear that these rates do not align with the headline figures given in the LTE specifications.
The reason for this is that actual peak data rates are derived by first subtracting the coding and control overheads. Then there are gains arising from elements such as the spatial multiplexing, etc. One of the primary reasons for using OFDM as a modulation format within LTE and many other wireless systems for that matter is its resilience to multipath delays and spread.
However it is still necessary to implement methods of adding resilience to the system. This helps overcome the inter-symbol interference ISI that results from this. In areas where inter-symbol interference is expected, it can be avoided by inserting a guard period into the timing at the beginning of each data symbol.
It is then possible to copy a section from the end of the symbol to the beginning. This is known as the cyclic prefix, CP. The receiver can then sample the waveform at the optimum time and avoid any inter-symbol interference caused by reflections that are delayed by times up to the length of the cyclic prefix, CP.
The length of the cyclic prefix, CP is important. If it is not long enough then it will not counteract the multipath reflection delay spread. If it is too long, then it will reduce the data throughput capacity. For LTE, the standard length of the cyclic prefix has been chosen to be 4. This enables the system to accommodate path variations of up to 1. With the symbol length in LTE set to The symbol length is defined by the fact that for OFDM systems the symbol length is equal to the reciprocal of the carrier spacing so that orthogonality is achieved.
With a carrier spacing of 15 kHz, this gives the symbol length of An attractive technique for spatial diversity is the conventional space-frequency block code C-SFBC strategy [ 6 ]. In the presence of CFO and high Doppler, severe ICI from adjacent carriers occurs which destroys the Alamouti structure and results in performance degradation. The received signal for m th user, after demapping, is expressed as. Now 12 can be written as.
At the receiver side, the reordering is done after demapping by using a matrix O T. We can approximate these matrices as banded tridiagonal matrices. Assuming the feedforward taps matrices have similar structure as the channel matrices, the equalized signal can be written as. At the k th instant, the output of the equalizer is. Now, exploiting the special structure of SFBC, it can be seen that there is no matrix inversion involved altogether, and hence, complexity is significantly reduced.
This structure allows us to easily find the inverse in 16 using block matrix inversion [ 21 ] where all the sub-matrices are diagonal. Therefore, the inverse in 18 can be found easily similar to In summary, the RLS algorithm is updated according to 5 whereas the steps describing the avoidance of the matrix inversion are detailed in the respective complexity reduction section of the SISO and SFBC scenarios.
The modulation scheme used is 16 QAM and the channel is frequency selective with 12 paths and each path fades independently according to the Rayleigh distribution. Our work is similar to [ 16 ], when to comes to stopping criteria; therefore, here convergence analysis is not presented. Practical correct decision feedback and known channel and impractical error decision feedback with known channel MMSE-DFE are also shown. To further investigate the robustness of the proposed technique, a five-tap filter is used and compared with the three-tap scenario.
Using a five-tap filter also avoids matrix inversion if the approach mentioned in the paper is followed. However, similar performance is attained by both filters as shown in Fig. In this regard, our design is optimal. In addition, Fig. Furthermore, a five-tap filter will increase the computational complexity as compared to the three-tap filter. Hence, using a three-tap filter is a better compromise in terms of performance and complexity as compared to a one-tap filter.
To complete the discussion on the computational complexity of the algorithm with matrix inversion and without matrix inversions, these two scenarios are compared. For matrix inversion, the approach given in [ 22 ] is followed. First, the LU decomposition of Q is performed as follows. Eventually, Q contains the upper triangular matrix U and the lower triangular matrix L when the diagonal part is replaced by ones [ 22 ]. The equalizer operates without channel estimation at the receiver.
The proposed algorithm delivers superior performance at low complexity due to the special structure of the matrices involved in computing the weights of the feedforward and feedback filters in the frequency domain. Simulation results demonstrate the significant performance gain and robustness of a three-tap frequency-domain adaptive equalizer, compared to a one-tap equalizer when dealing with ICI due to CFO.
In [ 16 ], a one-tap strategy was used without concentrating on the mitigation of CFO. As shown in the simulation results and from the channel matrix structure shown in Fig. Google Scholar. IEEE Trans. Inf Theroy. Global Telecommun Conference Globecom.
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