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Wireless Communication Basics

Learn wireless communication with multiple-choice questions: wavelength and frequency, dBm and link budgets, free-space path loss and fading, ASK, PSK and QAM with symbol rates, Shannon's theorem, OFDM and OFDMA, MIMO and beamforming, the hidden node problem in wireless LANs, BLE, LPWA and NFC, LNAs, mixers, PAs and noise figure, and superheterodyne and direct-conversion receivers.

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Wireless Communication Basics: key points

Smartphones, wireless LANs and IoT devices put information onto radio waves and pick extremely weak signals out of noise. Their performance is set by the combination of communication schemes such as modulation and multiple access, RF circuits such as amplifiers, mixers and filters, and the semiconductor devices that make them possible.

This topic covers, with calculation questions, radio basics such as frequency and wavelength, link budgets in dB and dBm, free-space path loss, multipath fading and Doppler shift, and goes on to ASK, FSK, PSK and QAM, symbol rate and bit rate, Shannon's theorem, OFDM and the cyclic prefix, CDMA and OFDMA, and MIMO and beamforming. It also covers wireless LAN standards and the hidden node problem, WPA3, BLE, LPWA, NFC and GNSS, the concepts behind 4G LTE and 5G NR, the roles of LNAs, mixers, PAs and duplexers, noise figure and Friis's formula, receiver sensitivity, superheterodyne and direct-conversion receivers, and why compound semiconductors are used for PAs.

All Wireless Communication Basics questions (40)
  1. Taking the speed of light as 3×10⁸ m/s, which is closest to the wavelength in vacuum of a 2.4 GHz radio wave?
  2. A radio transmits 20 dBm. Which is this power expressed in mW?
  3. Which describes the antenna gain unit dBi?
  4. With transmit and receive antenna gains of 0 dBi, which is closest to the free-space path loss when a 2.4 GHz radio wave (wavelength 0.125 m) travels 100 m through free space?
  5. With a transmit power of 20 dBm, a transmit antenna gain of 2 dBi, a receive antenna gain of 2 dBi, a propagation loss of 100 dB and cable losses on the transmit and receive sides totaling 4 dB, which is the received power?
  6. In radio communication, which is the phenomenon in which waves reflected by buildings and other objects arrive late because of differing path lengths, interfere with the directly arriving wave, and cause the received power to drop sharply at certain locations or frequencies?
  7. A terminal receiving a 2 GHz radio wave is approaching the base station at 30 m/s (108 km/h). Taking the speed of light as 3×10⁸ m/s, which is closest to the magnitude of the frequency shift (Doppler shift) of the wave received by the terminal?
  8. Which is a general characteristic of radio waves in millimeter-wave bands such as the 28 GHz band used in 5G, compared with bands below 6 GHz?
  9. Which digital modulation scheme represents data by changing the phase while keeping the carrier's amplitude and frequency constant?
  10. In 64QAM, which is the number of bits that can be sent per symbol?
  11. With 16QAM at a symbol rate of 5 M symbols per second (5 Mbaud), which is the bit rate?
  12. Which is a general property when the modulation is changed from 16QAM to 256QAM at the same symbol rate?
  13. For a channel with a bandwidth of 20 MHz and an S/N (signal-to-noise ratio) of 30 dB, which is closest to the channel capacity (the theoretical upper limit on the error-free transmission rate) given by Shannon's theorem?
  14. Which describes OFDM (orthogonal frequency-division multiplexing)?
  15. In OFDM, which is the main purpose of the cyclic prefix (CP), formed by copying part of the end of each symbol to its beginning?
  16. In OFDM, the symbol length excluding the CP is the reciprocal of the subcarrier spacing. When the subcarrier spacing is widened from 15 kHz to 60 kHz, which is closest to the symbol length excluding the CP?
  17. Which describes direct-sequence spread spectrum (DSSS)?
  18. Which multiple-access scheme lets several users use the same frequency at the same time and distinguishes their signals by a different code for each user?
  19. Which describes OFDMA (orthogonal frequency-division multiple access)?
  20. In MIMO with 4 transmit antennas and 2 receive antennas, which is the upper limit on the number of independent data flows (streams) that spatial multiplexing can send at the same frequency and time?
  21. Which technique adjusts the phase (and amplitude) of the signal sent from each of several antenna elements so that the radio waves reinforce one another in a particular direction?
  22. Which is the most appropriate main reason that beamforming with array antennas of many elements is used in 5G millimeter-wave base stations and terminals?
  23. In cellular mobile phone systems, which is the main reason for dividing the service area into many small areas (cells), each with its own base station?
  24. Which is the most appropriate main reason that the 4G LTE uplink (transmission from terminal to base station) adopted SC-FDMA (DFT-spread OFDM) rather than the OFDMA used in the downlink?
  25. Which frequency band does an IEEE 802.11ac (Wi-Fi 5) wireless LAN use?
  26. Which describes the RTS/CTS mechanism used to mitigate the hidden node problem in wireless LANs?
  27. In the personal mode of the WPA3 wireless LAN security standard, which authentication and key exchange method replaced WPA2's pre-shared key (PSK) method and makes the password hard to guess by offline dictionary attack even if the traffic is intercepted?
  28. Which describes Bluetooth Low Energy (BLE)?
  29. Which is a characteristic of LPWA (low-power wide-area) wireless communication used in IoT?
  30. Which is a characteristic of NFC, used in transit IC cards and contactless smartphone payments?
  31. Which is the minimum number of satellites a GNSS (global navigation satellite system) receiver such as GPS needs to determine its three-dimensional position while also solving for its own clock error?
  32. In a radio receiver, which circuit is placed right after the antenna and amplifies the weak received signal while adding as little of its own noise as possible?
  33. In a receiver, which circuit multiplies the received signal by the signal of the local oscillator (LO) to produce a signal at the difference (or sum) of their frequencies, thereby converting frequency?
  34. A superheterodyne receiver receives a 900 MHz signal by setting its local oscillator (LO) to 800 MHz and converting to a 100 MHz intermediate frequency (IF). Which is the image frequency that would also be converted to the same 100 MHz IF?
  35. Which describes a direct-conversion receiver, which sets the local oscillator (LO) to the same frequency as the received signal and converts directly to baseband without an intermediate frequency, compared with a superheterodyne receiver?
  36. In a transmitter sending OFDM or multilevel QAM signals, which is the main reason for operating the power amplifier (PA) at a point below where its output saturates (with back-off)?
  37. An LNA with a noise figure of 1.5 dB and a gain of 15 dB is placed at the first stage of a receiver, followed by a circuit with a noise figure of 10 dB. Which is closest to the noise figure of the two stages together?
  38. Take the thermal noise power density at 290 K as −174 dBm/Hz. For a receiver with a bandwidth of 1 MHz and a noise figure of 6 dB that needs an S/N of 10 dB to demodulate, which is closest to the receiver sensitivity (the minimum signal power that can be received)?
  39. In an FDD mobile phone, which transmits and receives simultaneously on different frequencies, which component separates the transmit and receive signals using their frequency difference so that one antenna can be shared for transmitting and receiving?
  40. In smartphone radio circuits, the transceiver including the LNA, mixer and PLL is often integrated on a CMOS chip, while compound semiconductors such as GaAs HBTs are often used for the transmit PA. Which is the most appropriate main reason?

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