METHOD FOR DATA TRANSMISSION IN OFDM-BASED COMMUNICATION SYSTEMS

Abstract

A method for data transmission using OFDM-based communication systems comprising the steps of obtaining a binary vector to be transmitted; converting the binary vector in a b base vector, being b a base greater than 2. Encoding the b base vector by means of a modulation technique; Transmitting the encoded vector via multi-subcarriers, using an OFDM transmission scheme.

Claims

1. A method for data transmission using OFDM-based communication systems, which uses a new general digital modulation and comprises the steps of: obtaining a binary vector B of size N as an input for a given modulation; converting the binary vector B in a base b vector (T), being b a base greater than 2; encoding the b base vector (T) by a new modulation technique, which uses 3 modulation symbols; and transmitting the encoded vector via multi-subcarriers, using an OFDM transmission scheme.

2. The method according to claim 1, wherein the binary vector B of size N is the input for a standard BPSK modulation.

3. The method according to claim 1, wherein the 3 possible symbols comprise a symbol 0, which forces a corresponding subcarrier to be silent during the OFDM modulation.

4. The method according to claim 1, wherein the base b is 3, converting the binary vector B in a base 3 vector (T).

5. The method according to claim 2, wherein the step of encoding the vector T uses a modified BPSK modulation technique defined by the expression: $S\left[k\right]=\frac{1}{4}{\left(-2\right)}^{1+T\left[k\right]}\left(2-\left[k\right]\right).$

6. (canceled)

7. (canceled)

Description

DESCRIPTION OF THE DRAWINGS

[0051] FIG. 1. General OFDM-based communication scheme, using the introduced FD-IM technique as a modified digital modulation process.

[0052] FIG. 2A, 2B, 2C. shows a comparison between the method of the invention and two relevant methods in the literature.

[0053] FIG. 3A, 3B. Shows energy-spectral efficiency comparison between the method of the invention and the SIM-OFDM technique.

[0054] FIG. 4A, 4B. shows examples of received modulated symbols for standard and modified BPSK modulations (with a SNR=20 dB).

[0055] FIG. 5. shows the unencoded BPSK BERs variations vs. SNR for different schemes, with perfect known and estimated CSIs, and with energy saving.

[0056] FIG. 6. shows the unencoded BPSK BERs variations vs. SNR for different schemes, with perfect known and estimated CSIs, and with no energy saving.

[0057] FIG. 7. shows an exemplary embodiment of a testbed setup.

[0058] FIG. 8A, 8B. shows an example of a transmitted frame, and its received version, in time domain, with a Tx antenna's gain equals to 20 dBi, and a Rx antenna's gain equals to 30 dBi.

[0059] FIG. 9. shows practical unencoded BERs variations vs. antenna's gain for different schemes, with and without energy saving.

PREFERRED EMBODIMENTS OF THE INVENTION

[0060] The invention relates to a novel method for data transmission using OFDM-based communication systems. The new method includes a digital modulation that enhances the spectral efficiency, by increasing the total number of transmit bits via a given set of subcarriers, when compared to the equivalent standard modulation. In addition, it enhances the energy efficiency by forcing some subcarriers to be silent (with a null value), during the OFDM modulation, according to the corresponding transmit message.

[0061] To clearly explain the principle of the invention, a comparison between the proposed solution and the main relevant methods in the literature has been included in FIGS. 2A, 2B and 2C.

[0062] For instance, let us consider the standard binary phase-shift keying (BPSK) modulation. With this modulation, only two possible symbols can be sent via each subcarrier, [0 or 1], that will be encoded (before the OFDM modulation) to [1 and 1], respectively.

[0063] In this case, adapting our proposed solution to the BPSK modulation, we can send 3 possible symbols via each subcarrier: [0, 1, or 2], where the corresponding modulated symbol are, respectively, [1, 1, and 0].

[0064] To further explain the principle of the proposed method. Let us consider a binary vector B of size N as an input for a standard BPSK modulation. In this case, the output of this modulation is a vector S of size N that contains 1 s and 1 s, obtained by the expression:

[00003]$S\left[k\right]=2B\left[k\right]-1.$

[0065] With the method of the invention, comprising a modified BPSK modulation, 3 possible symbols instead of 2 can be modulated. For that, the binary vector B is converted from base 2 to base 3 to get a base 3 vector T of size L. Wherein L is defined as:

[00004]$L=.Math.\frac{\log \left(2\right)}{\log \left(3\right)}N.Math..$

[0066] Then, encoding the vector T elements to get the modified BPSK modulated vector S that contains L elements, with possible values: [1, 1, and 0], by using the expression:

[00005]$S\left[k\right]=\frac{1}{4}{\left(-2\right)}^{1+T\left[k\right]}\left(2-T\left[k\right]\right).$

[0067] Note that the symbol 0 forces the corresponding subcarrier to be silent during the OFDM modulation. As a result, while for the standard BPSK modulation, we can send N given bits via N given subcarriers, adopting the method of the invention the N bits can be sent via L (L<N) subcarriers, wherein the L subcarriers include the digits in base 3. Hence a gain of N/L (in terms of total number of transmit bits per subcarrier) can be obtained.

[0068] As an example, with N=1024, we have a gain of 1.5827 transmit bit per subcarrier, when using the proposed modulation, instead of the standard one.

[0069] Now, let us move to the general Mary multilevel modulation, to detail and derive the general bit gain and energy gain expressions of the proposed scheme, with respect to the standard one.

[0070] Let us consider an OFDM modulation, with N.sub.SC subcarriers, and energy per modulation symbol (or per subcarrier) Es. For the standard Mary multilevel modulation, N.sub.SC log.sub.2(M) bits per one OFDM symbol can be transmitted, using all the defined subcarriers. However, with the modified modulation, the number of subcarriers that are needed to transmit all the N.sub.SC log.sub.2(M) bits is given by:

[00006]$.Math.\frac{{\log }_2\left(2\right)N_{SC}{\log }_2\left(M\right)}{{\log }_2\left(M+1\right)}.Math.=.Math.\frac{N_{SC}{\log }_2\left(M\right)}{{\log }_2\left(M+1\right)}.Math.$

[0071] This number presents the length of the base M+1 vector that presents the binary vector of length N.sub.SC log.sub.2(M) bits. As a result, the number of transmit bits per subcarier, within the proposed scheme, is given by:

[00007]$\frac{N_{\mathrm{SC}}{\log }_2\left(M\right)}{.Math.\frac{N_{\mathrm{SC}}{\log }_2\left(M\right)}{{\log }_2\left(M+1\right)}.Math.}$

[0072] Accordingly, the bit transmit gain of the introduced FD-IM technique, with respect to the standard one is expressed by:

[00008]${\mathrm{bGain}}_{\mathrm{NM}}=\left[\frac{N_{\mathrm{SC}}{\log }_2\left(M\right)}{.Math.\frac{N_{\mathrm{SC}}{\log }_2\left(M\right)}{{\log }_2\left(M+1\right)}.Math.}-1\right]100\%.$

[0073] As shown in this derived expression, and with respect to the standard OFDM-based transmission scheme, the introduced technique is able to enhance the data transmission's spectral efficiency. For instance, and according to the derived bit gain expression in (11), the proposed technique offers a transmit bit gain of 58%, with respect to the standard BPSK modulation with M=2 and NSC=256. Hence, by using the proposed FD-IM technique, a significant gain in terms of total number of transmit bits can be observed in this case. In addition, by assuming that the base M+1 symbols have the same probability of existence in the transmit data, also a decrease of the transmission energy consumption can be obtained, quantified as:

[00009]${\mathrm{EGain}}_{\mathrm{NM}}=\left[1-\frac{M}{\left(M+1\right)\left(1+\frac{bGai{n}_{\mathrm{NM}}}{100}\right)}\right]100\%.$

[0074] In fact, with the introduced FD-IM technique, we only need

[00010]$\frac{M}{M+1}$

of the total used energy (N.sub.SCE.sub.S) per OFDM symbol, with respect to the standard one, as we transmit only M symbols from the M+1 digits in base M+1. This

[00011]$\frac{M}{M+1}$

of the total used energy can be used to carry

[00012]$N_{SC}{\log }_2\left(M\right)\left(1+\frac{{\mathrm{bGain}}_{\mathrm{NM}}}{100}\right)\mathrm{bits},$

so we have the corresponding bit energy equals to:

[00013]$E_b^{\left(2\right)}=\frac{M{N}_{SC}{E}_S}{\left(M+1\right)N_{SC}{\log }_2\left(M\right)\left(1+\frac{{\mathrm{bGain}}_{\mathrm{NM}}}{100}\right)}.$

[0075] Hence, the energy gain of the introduced technique, with respect to the standard one is expressed as follows:

[00014]${\mathrm{EGain}}_{NM}=\frac{E_b-E_b^{\left(2\right)}}{E_b}100\%=\left[1-\frac{M}{\left(M+1\right)\left(1+\frac{{\mathrm{bGain}}_{NM}}{100}\right)}\right]100\%.$

[0076] In FIGS. 3A and 3B, we present a performance comparison between the proposed solution and the SIM-OFDM technique, with respect to the conventional OFDM scheme. As depicted in these figures, despite the advantage of the SIM-OFDM scheme in terms of energy efficiency, It does not offer any spectral efficiency, with a null bGain value. However, the introduced FD-IM technique offers significant energy and bit gains, with respect to the conventional OFDM scheme.

[0077] Also, further numerical results have been generated using a generic OFDM-based communication simulator, which executed Monte Carlo simulations to evaluate the performances of the proposed scheme in terms of BER, with and without perfect known CSI.

[0078] Without loss of generality, the inputs of the simulator are presented in Table I, where the Typical Urban 6 path channel (TU6) has been used as a popular multipath channel model for different wireless telecommunications [40], and the same number of transmit bits has been considered for the different techniques for fair performance comparisons.

TABLE-US-00001 TABLE I The simulation parameter values Parameter Value Number of transmit bits 1024 Megabit Number of Subcarriers 1024 Number of FFT Points 1024 Number of Cyclic Prefix 32 Carrier Frequency 500 MHz Signal Sampling Frequency 1 GHz SNR 0:5:30 dB Modulation Type BPSK Multi-path Channel Type TU6 [40] Channel Estimation Method Spline Interpolation

[0079] In FIGS. 4A and 4B, samples of received modulated symbols for the standard and modified BPSK modulations, with SNR=20 dB, are represented. As shown in this figure, the CSI and the AWGN play a crucial role in the accuracy of the received/estimated modulated symbols for both schemes.

[0080] In the modified BPSK scheme of the proposed method, shown in FIG. 4B, 3 possible symbols form the constellation diagram: [1 and 1] that present the two original symbols of BPSK, and the [0] symbol that represents the null transmission.

[0081] At the reception, the transmitted symbols should be estimated/detected from the corresponding received samples that are affected by channel estimation error and additive noise, which results in a potential BER higher than that of the conventional BPSK, when the same SNR is used.

[0082] FIGS. 5 and 6 show the unencoded BERs variations vs. SNR, with and without energy saving, respectively. Thus, the BER variations of the considered schemes versus different SNR values are obtained.

[0083] For the energy saving case scenario, the saved energy due to the use of only fraction of the total OFDM subcarriers has been not used by or shared with the active subcarriers to increase the transmit power, and hence to improve the SNR.

[0084] In this case, and as shown in FIG. 5, the proposed method offers an acceptable BER, with respect to that of the standard one that has the lowest BER for the different SNR values. This is due to the fact that the standard BPSK demodulation is dealing with only two decisions, [1 or 1], as presented in FIG. 4A.

[0085] However, the new modulation has 3 possible decisions: [1, 0, and 1], which increases the decision errors, and hence increases the BER. The same results have been observed for the original SIM-OFDM, as it uses the same constellation diagram of the proposed scheme.

[0086] It is noteworthy that the proposed modulation is able to transmit more bits than the standard one, e.g., with a gain of 1.58 for the BPSK modulation, where some of the gained bits can be used within a given channel coding to significantly reduce the BER.

[0087] This is not the case for the original SIM-OFDM scheme, which does not offer extra transmit bits than the conventional OFDM scheme.

[0088] In FIG. 6, unencoded BERs variations vs. SNR are shown for the different considered schemes, with perfect known and estimated CSIs and without energy saving.

[0089] In this case, the saved energy was shared with the active subcarriers to increase the corresponding received SNRs. As a result, for low and medium SNR values, e.g., SNR20 dB, the proposed method and the SIM-OFDM scheme outperforms the standard OFDM scheme, with slightly better performance for the original SIM-OFDM scheme, when the same original SNR is used.

[0090] This is due to the fact that increasing the transmit power for an environment with high and medium noise levels can significantly enhances the received SNR and BER of the IM-schemes, when compared to that of the standard one without saved energy. In addition, the original SIM-OFDM offers a slightly higher saved energy than that of the proposed scheme.

[0091] For high SNR values, with low additive noise levels, the impact of increasing the transmit power on the BER is negligible, which results in better BER performances for the conventional OFDM scheme, when compared to the IM schemes.

[0092] In this case, the BER decision is directly related to the number of decisions, which is 2 for the original OFDM scheme, and 3 for the IM-based scheme. Although these results are for BPSK modulation, our solution is universal and applicable to any M-ary multilevel modulation.

[0093] Also, practical evaluations have been performed to assess the proposed method. FIG. 7 shows a preferred embodiment of a testbed setup configured according to the method of the invention. In this embodiment, a testbed is used, which consists of two nodes: a radio transmitter (Tx) and a radio receiver (Rx).

[0094] Each node has a host PC that is connected, by a Gigabit Ethernet (GbE) cable, to a USRPx310 device.

[0095] At the transmitter side, a generic OFDM-based transmitter has been implemented, using a flexible and adjustable Matlab software script, where the different considered modulations can be implemented to generate OFDM signals that convey random data/message.

[0096] This testbed has been implemented at a laboratory environment, to evaluate the considered schemes, with real radio channels, using the following hardware and software configuration parameters.

[0097] The used SW configuration parameters are listed in Table II. Except for the number of transmit bits per frame, the same presented parameters were used to test the performances of the different schemes. As shown in this table, there are two possible numbers of transmit bits per one frame. This is due to the fact that the proposed technique has the advantage of transmitting more bits than the other ones. In fact, with regard to the presented software configuration parameters, the proposed scheme is able to transmit 274.01 kbits per one frame. However, the two other schemes can transmit 174 kbits only per one frame.

TABLE-US-00002 TABLE II The software configuration parameters Parameter Value Number of transmit bits 1024 Megabit Number of OFDM Symbols per Frame 870 Number of Transmit bits per Frame 174/274.01 kbits Number of Subcarriers 200 Number of Zero Padding 56 Number of FFT Points 256 Number of Cyclic Prefix 16 Preamble Length 5 10.sup.4 Samples Frame Length 35 10.sup.4 Samples Modulation Type BPSK Pilot Subcarrier Spacing 4 Pilot Symbol Spacing 2 Channel Estimation Method Spline Interpolation

TABLE-US-00003 TABLE III The hardware configuration parameters Parameter Value Centre Frequency 2.4 Ghz Sampling Frequency 5 Ghz The Maximum Transmission Power 100 mW Output Data Type double Transmission Distance 15 m (Line of Sight) Transmitter Antenna Gain dBi 10:30 dBi Receiver Antenna Gain 30 dBi

[0098] In Table III, the considered HW configuration parameters are presented, where different values of the Tx antenna's gain were used to evaluate the BER of the different schemes, in various scenarios, with different transmit powers, and hence with different received SNRs.

[0099] FIGS. 8A and 8B show an example of a transmitted frame, and its received version, in time domain, with a Tx antenna's gain equals to 20 dBi, and a Rx antenna's gain equals to 30 dBi. These figures show the impact of the channel attenuation as well as the additive noise on the transmitted signal.

[0100] To evaluate the performance of the different considered schemes, Table IV and FIG. 9 present the average and the variance of uncoded-BER vs. the transmit antenna's gain for the different considered schemes, with and without energy saving.

TABLE-US-00004 TABLE IV Average uncoded-BER vs. the transmit antenna's gain for the different considered schemes Number of BER Transmit bit Energy Tx Antenna's Gain [dBi] bits/ Gain Gain 5 10 20 30 Frame [%] [%] .sup.2 .sup.2 .sup.2 .sup.2 Std-OFDM 174 kb 0 0 2. 1 10 8.49 10 9.64 10 7.3 10 7.19 10 2.30 10 2. 0 10 1.83 10 SIM-OFDM 174 kb 0 0 .54 10 1.87 10 6.05 10 3.16 10 4.76 10 1.54 10 2.59 10 1. 10 MIM-OFDM 274.01 kb 57.48 0 3 10 2.91 10 8.00 10 5.0 10 5.00 10 2.00 10 3.04 10 1.00 10 SIM-OFOM 174 kb 0 6.67 76 10 1.74 10 1.59 10 2.3 10 1.19 10 10 4. 6 10 8.37 10 NIM-OFDM 274.01 kb 57.48 57.66 2.76 10 1.59 10 1.60 10 1.84 10 1.14 10 6.97 10 4.13 10 1.15 10 indicates data missing or illegible when filed

[0101] Similar to the simulation results, for the transmission scenario without energy saving, the proposed method and the SIM-OFDM scheme outperform the standard OFDM scheme, with slightly better performance for the original SIM-OFDM scheme.

[0102] For the transmission scenario with energy saving, the proposed method and the SIM-OFDM scheme offer acceptable BERs, with respect to that of the standard one.

[0103] Thus, the proposed technique is able to enhance both the energy and the spectral efficiency, while offering a good BER, when compared to relevant techniques in the literature.

[0104] In particular, the proposed scheme has the power to increase the transmit data rate by up to 59%, with respect to the relevant presented schemes. For instance, with regard to the HW and SW configuration parameters presented in Tables II and III, the proposed scheme needs 27.63 ms to transmit a real data with a size of 2 MB. However, the standard and SIM-OFDM schemes need 43.52 ms to transmit the same data. In another scenario, the proposed scheme is able to transmit 72.35 MB, during 1 s, while the other scheme can transmit only 45.95 MB, during the same period.