Wireless Communication Fundamentals
Q. Examples of non-wireless (guided) media
A. Co-axial cables (used in cable TV), Fiber Optics (high-speed data), and Twisted Pair cables (Ethernet/Telephone lines).
Q. Who pioneered millimeter-wave communication?
A. Sir Jagadish Chandra Bose conducted pioneering research into millimeter-length electromagnetic waves in the 1890s.
Q. Which device provides wireless signals from towers to home routers?
A. Base Stations (eNodeB/gNodeB) use high-gain antennas to transmit data. A typical macro base station transmits at roughly 40 Watts per sector.
Q. Which wireless technology provides Metropolitan Area coverage (WMAN)?
A. WiMAX (802.16) and LTE/5G are designed for Metropolitan Area Networks, whereas WLAN (Wi-Fi) is for short-range local areas.
Antenna, dB, and MIMO Systems
Q. What are dB and dBm?
A. dB (Decibel) is a relative ratio: $dB = 10 \times \log_{10}(P_{out} / P_{in})$.
dBm is absolute power relative to 1 milliwatt: $dBm = 10 \times \log_{10}(Power / 1mW)$.
Note: 30 dBm is equal to 1 Watt.
Q. What is Spatial Division Multiplexing (SDM)?
A. SDM is a MIMO technique that transmits multiple independent data streams simultaneously over the same frequency channel by using the spatial separation of antennas.
Q. How many antennas are used in the Alamouti scheme?
A. It typically uses 2 transmitter antennas and 1 or more receiver antennas (2x1 or 2x2 MIMO) to achieve transmit diversity.
Q. What is an outage in wireless communication?
A. An outage occurs when the required data rate (R) exceeds the instantaneous channel capacity (C). In this state, the Signal-to-Noise Ratio is too low to decode the data reliably.
Q. What are S11 and S21 in MIMO antennas?
A. S11 is the Reflection Coefficient (Return Loss); it measures how much power is reflected back from the antenna. S21 is the Transmission Coefficient; in MIMO, it measures the "mutual coupling" or interference between two adjacent antenna elements. Ideally, S21 should be less than -20 dB.
Q. What is Beamforming?
A. Beamforming is a signal processing technique used in antenna arrays to focus the transmitted power in a specific direction toward a user, increasing gain and reducing interference to other users.
Q. What is the primary difference between Rayleigh and Rician fading?
A. **Rayleigh fading** occurs when there is no direct Line-of-Sight (LoS) path between the transmitter and receiver; the signal arrives via multiple reflected paths. **Rician fading** is used when a dominant LoS component is present along with the multipath components.
Q. What causes "deep fades" in a Rayleigh environment?
A. Deep fades are caused by the **destructive interference** of multipath components. Since the In-phase and Quadrature components of the signal are modeled as zero-mean Gaussian random variables, they can occasionally sum to zero, causing the signal amplitude to drop to near-zero.
Q. How can deep fades be mitigated without increasing transmit power?
A. The most effective way is through **Diversity techniques**. This includes Time Diversity (interleaving and coding), Frequency Diversity (OFDM), or Spatial Diversity (Multiple antennas/MIMO). These ensure that if one version of the signal is in a deep fade, another version likely isn't.
Q. How does Doppler Spread affect the coherence time of a channel?
A. Doppler Spread ($B_D$) is inversely proportional to the Coherence Time ($T_C$). High mobility (high Doppler) results in a low coherence time, meaning the channel characteristics change very rapidly, leading to "Fast Fading."
Formula: $T_C \approx \frac{1}{B_D}$
Q. Why is the BER in a Rayleigh fading channel much higher than in an AWGN-only channel?
A. In an AWGN-only channel, the signal power is constant. In Rayleigh fading, the instantaneous SNR fluctuates. The **deep fades** (low SNR events) dominate the average Bit Error Rate, causing the BER curve to decay linearly (1/SNR) rather than exponentially as it does in AWGN.
Q. What is the role of an Adaptive Equalizer in a fading channel?
A. An adaptive equalizer tracks the time-varying impulse response of the channel in real-time. It attempts to "invert" the channel's distortion to mitigate Intersymbol Interference (ISI) caused by multipath delay spread.
Q. What is "Outage Probability" and why is it used?
A. Outage Probability is the probability that the instantaneous SNR falls below a specific threshold required for reliable communication. In fading channels, we cannot guarantee 100% connectivity, so we design systems for a specific "Reliability" (e.g., 99.9%).
Q. Why is BPSK more robust than 64-QAM in a fading environment?
A. BPSK has the largest **Euclidean distance** between its constellation points. In a deep fade, the signal amplitude shrinks; because 64-QAM points are packed very tightly, even a small amount of noise or a shallow fade can cause the receiver to pick the wrong symbol.
Q. How do we demodulate a signal if the instantaneous channel phase is unknown?
A. By using Differential Modulation (e.g., DPSK). Instead of absolute phase, information is encoded in the phase *difference* between consecutive symbols. This allows the receiver to decode the data without needing an explicit estimate of the channel's phase.
Q. What is Coherence Bandwidth ($B_C$)?
A. It is the range of frequencies over which the channel is considered "flat" (all frequencies experience similar fading). If the signal bandwidth is much larger than $B_C$, the signal experiences **Frequency Selective Fading**, which causes ISI.