About Multiplexing Technologies for Wireless Technologies

    Since the advent of the first networks, there has been a sharp question about the simultaneous operation of several devices. And for decades now, there has been a struggle against this problem and the main task is to keep up with the needs of society.
    In the article I propose to quickly go over the main technologies used for multiplexing. If you lied where, correct.

    Actually, what are the opportunities for organizing comfortable simultaneous work of several users? There are only two for wired networks: diversity of data from different devices in space and time.

    It all started with 10Base2 standard wired networks, where a common copper bus, a coaxial cable, was used as a data transmission medium. We briefly touch on this topic.
    At a time when one of the devices is broadcasting, the other cannot start, because it will just noise instead of data, because the electrical signal propagates along the entire length of the conductor. In such conditions, spatial data is not particularly explode. It takes time: the network card is waiting for silence on the network. If the channel is busy, then it tries to repeat after a random period of time. As soon as silence appears, the device starts broadcasting. This is the first multiplexing mechanism - CSMA / CD - Carrier Sense Multiple Access with Collision Detection .
    This situation continued with the advent of hubs, since they were essentially the same tires. But when the switches came to the network market, everything changed. They divided the network into several collision domains- in fact, each device is separate, which meant the spatial diversity of data from different devices.

    The situation is different with wireless technology. They are much more complex in terms of developing standards and in terms of implementation. Here, intersectoral and interchannel interference, signal attenuation, and more complex QoS support, and most importantly, the simultaneous operation of dozens of subscribers, are added.
    How can it be provided for wireless devices? There are two approaches here: time diversity and frequency diversity.

    TDMA - Time Division Multiple Access. This is one of the technologies that we resort to using every day - GSM is built on it. Here, the data is posted in time. For each subscriber station, a certain number of time slots is allocated with full use of the allocated radio channel width. These time slots are assigned to the device until the end of the session. Remember the dawn of GSM, when you tried to dial a number for 5-10 minutes, and in response you saw a message that “the network is busy” and that it is impossible to make a call? At this moment, there were no free time slots for you at the base station. Subsequently, TDMA received support in GPRS, EDGE. But it can hardly be called optimal and, hardly, there is a place for it in our wireless future. FDMA

    Technology - Frequency Division Multiple Access- in its pure form it is practically not used (it was implemented in 1G standards, for example, AMPS) For each subscriber station a certain band is allocated from the used frequency range and is released only after the end of the session.
    As well as in TDMA, such use of a radio channel is not optimal and requires a balance between the number of online users and their speed.

    CDMA - Code Division Multiple Access . At the moment, this is one of the most promising multiplexing mechanisms. Used in CDMA (2000, EVDO) and UMTS standards.
    Its essence is that each device registered on the network is provided with its own unique code. On one side, the signal is modulated using such a unique code, on the other, the reverse process occurs - demodulation. Thus, user data is not distributed either in time or frequency, but is mixed using the entire channel width.
    CDMA can be synchronous — using the properties of orthogonal vectors to select a coding method — and asynchronous — using pseudo-random sequences. Each approach has its pros and cons - this is the topic of an independent article and I will not dwell on this in detail.

    DSSS - Direct Sequence Spread Spectrum. The technology has much in common with CDMA (a pseudo-random sequence is used for modulation), it is quite noise-resistant due to elevenfold redundancy, but at the same time it has low efficiency. Used in Wi-Fi at speeds up to 2 Mb / s.

    Now let's move on to one of, in my opinion, the most interesting multiplexing technologies, which has a huge variety of applications, including Wi-Fi, WiMAX and LTE - OFDMA ( Orthogonal frequency-division multiple access ).
    OFDM itself, on which OFDMA is based, is also used in a number of standards and not only wireless: DVB-T, ADSL, VDSL.
    Why is he so interesting? It's all about the high efficiency of using the radio channel. The secret is revealed already in the title: Orthogonal Frequency. For transmission, not one carrier with the entire allocated channel width is used, but several subcarriers with intersecting subchannels.
    OFDMA implies allocating time slots to each user, the size of which is nxm, where n is the number of subchannels, m is the number of OFDMA symbols !!!! .. Thus, this type of multiplexing can be called a symbiosis of TDMA and FDMA, each of which individually is not optimal.

    The advantages of OFDMA include:
    • Insensitivity to multipath interference
    • Support for MIMO and AAS
    • Narrow Band Insensitivity

    By cons:
    • Frequency bias sensitivity
    • The FFT (Fast Fourier Transform) and FEC (Forward Error Correction) mechanisms operate continuously, regardless of the connection speed, which leads to high power consumption.

    I will be glad for your additions and notes.

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