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Interference free multiuser modulation for mmWave radio links

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Abstract

Radiocommunications in the mmWave band present strong free-space attenuation, which requires power maximization at the receiver. A code division multiple access (CDMA) modulation system increases the receiver’s ability to detect the transmitted signal by means of spectrum spreading. The main benefit of direct sequence spread spectrum for mmWaves is the increase in coverage due to the process gain. The main drawback of its use in high-speed communications is bandwidth reduction. Fortunately, this can be compensated for by the huge bandwidth available at mmWave frequencies. However, most of the codes available for CDMA have mutual interference. The zero correlation zone (ZCZ) codes are orthogonal to each other, but only under the condition that no multipath far echoes are received. This condition is fulfilled in mmWave radio communications, owing to strong propagation attenuation and also because of the beamforming that radio links usually do at this frequency. In this study, a field experiment is carried out consisting of data reception at 28 GHz from many different users who are simultaneously transmitting using CDMA on BPSK modulation and then comparing the performance of conventional maximal pseudorandom (PR) codes versus ZCZ codes. It is verified that the latter are received with the same process gain but no mutual interference, approximately resulting in a 12 dB improvement on the modulation error ratio (MER).

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The author confirms that all data generated or analyzed during this study are included in this published article.

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Funding

This research was funded by the Spanish Government, Ministry of Science and Innovation, under grant number PID2020-112545RB-C52, Xunta de Galicia, grants ED431C 2019/26 and ED481A-2020/049 and the European Regional Development Fund (ERDF).

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Authors and Affiliations

  1. atlanTTic Research Center of Universidade de Vigo, Escuela de Ingeniería de Telecomunicación, Campus de Vigo, As Lagoas, Rúa Maxwell, Estrada de Marcosende, s/n, Vigo, Pontevedra, 36310, Spain

    Pablo Torío, Manuel G. Sánchez & Luis A. López-Valcárcel

Authors
  1. Pablo Torío
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  2. Manuel G. Sánchez
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  3. Luis A. López-Valcárcel
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Contributions

All of the authors contributed to the study’s conception and design. The manuscript was mainly written by Pablo Torío who also came to the fundamental conclusions. Analysis of the state of the art, analysis of the results, and partial redaction of the manuscript were carried out by Manuel G. Sánchez. Experiment set-up and measurements were made by Luis A. López-Valcárcel. All of the authors read and approved the final manuscript.

Corresponding author

Correspondence to Pablo Torío.

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Supplementary Material 1

Appendix: experiment details

Appendix: experiment details

Basically, the experiment consists of the following stages:

  1. 1.

    Synthesis of the transmitted signal with Matlab.

  2. 2.

    Translation of the Matlab into a format that is readable by the vector signal generator. This is made by the proprietary Rhode ARB Toolbox application.

  3. 3.

    Cyclic transmission of the synthesized signal from the vector signal generator.

  4. 4.

    Reception of a snapshot, containing many signal cycles, at the IQ analyzer.

  5. 5.

    Import of the snapshot to Matlab for offline processing. Including synchronization, normalization, and error assessment.

  • The data.

The data sent in the experiment consist of a maximal PR [7 1] code sequence of length 127, which is cyclically generated. A sequence like this has two advantages: first, it can be easily replicated at reception in order to compare with the incoming data and assess the error; second, the good autocorrelation properties of maximal PR codes make it easy to align and synchronize the sampling times in the received signal. In order to introduce some variation, each user is supposed to transmit the same data but with a different shift delay.

  • Synthesis and transmission of the signal.

The data from each user are modulated with their corresponding code by code sequence modulation. Hence, we have M different sequences from independent users. Now, we simply add them to make a single signal that will be sampled at 1 Gsps and transmitted by direct carrier modulation at the frequency of 28 GHz.

  • Offline processing.

Let us assume that we are modulating with the ZCZ codes. The reasoning for the maximal PN codes would be the same.

Every record received at the IQ analyzer consists of many cycles of mixed signals from the M users. The first step is signal interpolation. Following, we choose one random user to get data from and carry out a periodic correlation with its corresponding ZCZ code (which is also upsampled to the same degree as the signal).

  • Time synchronization.

Now, it is time to detect the starting data bit. Thanks to the fact that the data within the signal is a [7 1] maximal PR sequence, we can easily do it by carrying out a periodic correlation between the data and another [7 1] maximal PR sequence.

This way, we locate the starting bits where the correlation maxima are. At this point, the sequence timing has been synchronized.

  • Phase and frequency synchronization.

Now it is time for phase and frequency synchronization. The original data sequence was BPSK; therefore, we will use the known technique of signal squaring in order to remove the phase shift.

Despite the accurate frequency stability of the oscillators at the transmitter and receiver, we took care of any possible frequency offset, calculating this frequency by applying to the phase shift the finite difference scheme.

At the end, the phase ambiguity caused by the squaring method is removed and the amplitude normalized.

  • MER.

The MER is calculated as follows:

$$MER=10{log}_{10}\left(\frac{{P}_{simb}}{\frac{1}{N}\sum _{i=1}^{N}{e}_{i}}\right)$$

Psimb is the true symbol power. It is calculated as the power of averaging all the received symbols on each side of the real part of the complex plane (1 and − 1).

ei are the error powers, that is to say, the squared distance of the received symbols to the true symbol.

We have to calculate a MER for symbol 1 and another one for symbol − 1. The final value is the average of both.

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Torío, P., Sánchez, M.G. & López-Valcárcel, L.A. Interference free multiuser modulation for mmWave radio links. Wireless Netw 30, 1271–1276 (2024). https://doi.org/10.1007/s11276-023-03590-4

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