MOBILE COMMUNICATION (EC-503 (B)) - Unit 4 Short Notes

1.0 CELLULAR SYSTEM FUNDAMENTALS & CAPACITY ENHANCEMENT

1.1 Cellular Concept & Frequency Reuse

• Cellular System: Divides a service area into small geographic regions called cells. Each cell has a base station (BS). Frequency bands are reused in non-adjacent cells to increase system capacity.

  • Merits: Increased capacity, reduced transmitter power, seamless handoff.

• Frequency Reuse: Same set of frequencies (channel group) is used in cells separated by a sufficient distance to keep co-channel interference (CCI) below a threshold.

  • Reuse Factor: $$\displaystyle q = 1/N $$, where $N$ is the cluster size (total number of cells using distinct frequency groups).

  • Cluster Size ($N$): For a hexagonal grid, $$\displaystyle N = i^2 + ij + j^2 $$, where $i,j$ are integers.

    • Common clusters: $$\displaystyle N=7 $$ ($$\displaystyle i=2,j=1 $$), $$\displaystyle N=19 $$ ($$\displaystyle i=3,j=2 $$).

  • Co-channel Cells: Cells using the same channel. For cluster size $N$, the co-channel reuse distance $D$ is:

$$\boxed{D = R \sqrt{3N}}$$

    where $R$ is the cell radius.

> [!TIP] **Exam Focus:** You will be asked to **draw a 19-cell cluster** and **locate co-channel cells** for a given $i,j$. The cell at $(i,j)$ relative to a reference cell has the same channel set.

• Co-channel Interference (CCI): Interference from co-channel cells using the same frequency. It is the primary capacity-limiting factor.

  • Signal-to-Interference Ratio (SIR): For $n$ co-channel interferers, each at distance $$\displaystyle D_i $$:

$$\text{SIR} = \frac{S}{\sum_{i=1}^{n} I_i} = \frac{R^{-n}}{\sum_{i=1}^{n} D_i^{-n}}$$

    where $n$ is the **path loss exponent** (typically 3-4 for urban). **Higher SIR is required for digital systems** (e.g., >18 dB for GSM).

• Adjacent Channel Interference (ACI): Interference from adjacent frequency channels in the same or neighboring cells. Caused by imperfect receiver filtering and near-far effect. Reduced by guard bands and channel assignment strategies.

1.2 Capacity Expansion Techniques

1.3 Channel Assignment Strategies

• Fixed Channel Assignment (FCA): Each cell is permanently allocated a fixed set of channels.

  • Advantage: Simple, low control overhead.

  • Disadvantage: Low capacity during heavy traffic; channels in one cell cannot be borrowed by another.

• Dynamic Channel Assignment (DCA): Channels are allocated on demand from a central pool. A borrowing strategy (e.g., Borrowing with Channel Ordering) is used.

  • Advantage: Higher capacity, better traffic distribution.

  • Disadvantage: High complexity, signaling overhead, risk of increased CCI if borrowing is not controlled.

• Hybrid: Combines FCA (for most channels) and DCA (for a small pool for emergencies/borrowing).

1.4 Trunking & Grade of Service (GoS)

• Trunking: Multiple users share a limited pool of channels (like trunk lines in telephony). Allows a large number of users to be served by fewer channels than if each had a dedicated channel, based on statistical multiplexing.

• Grade of Service (GoS): Probability that a call is blocked (Erlang B) or delayed (Erlang C) due to unavailability of channels.

  • $$\displaystyle P_b $$ (Blocking Probability) = GoS.

  • Erlang B Formula (Blocked Calls Cleared):

$$P_b = \frac{\frac{A^c}{c!}}{\sum_{k=0}^{c} \frac{A^k}{k!}}$$

    where $A$ = offered traffic intensity (Erlangs), $c$ = number of channels.

*   **Erlang C Formula (Blocked Calls Delayed):**

$$P_{delay} = \frac{\frac{A^c}{c!} \frac{c}{c-A}}{\sum_{k=0}^{c-1} \frac{A^k}{k!} + \frac{A^c}{c!} \frac{c}{c-A}}$$

• System Capacity: Total number of users $M$ a system can support with a given GoS $$\displaystyle P_b $$ depends on:

  1. Number of cells.

  2. Channels per cell ($c$).

  3. Traffic per user ($$\displaystyle A_{user} $$).

$$M = \frac{\text{Total Channels} \times A}{A_{user}}$$

2.0 MOBILE RADIO PROPAGATION

2.1 Large-Scale Path Loss & Propagation Models

• Path Loss ($$\displaystyle L_p $$): Signal power attenuation over distance. Expressed in dB: $$\displaystyle L_p(dB) = 10n \log_{10}(d/d_0) + L_p(d_0) $$.

  • Path Loss Exponent ($n$): Varies with environment: Free space ($$\displaystyle n=2 $$), urban cellular ($$\displaystyle n=3-4 $$), indoor ($$\displaystyle n=4-6 $$).

• Models:

  1. Free Space Model: Valid for $$\displaystyle d \gg d_0 $$ in LOS.

$$\boxed{P_r(d) = P_t G_t G_r \left( \frac{\lambda}{4\pi d} \right)^2}$$

    or $$\displaystyle L_p(dB) = 32.44 + 20\log_{10}(f_{MHz}) + 20\log_{10}(d_{km}) $$.

2.  **Two-Ray (Ground Reflection) Model:** Includes direct and ground-reflected waves.

    *   **Critical Distance ($$\displaystyle d_c $$):** Beyond which two-ray model gives **lower path loss** than free space.

$$d_c = \frac{4\pi h_t h_r}{\lambda}$$

    *   Path loss exponent becomes **4** for $$\displaystyle d > d_c $$.

3.  **Log-Distance Model:** $$\displaystyle L_p(dB) = L_p(d_0) + 10n \log_{10}(d/d_0) + X_\sigma $$, where $$\displaystyle X_\sigma $$ is a **log-normal shadowing** variable (std dev $\sigma$ in dB).

4.  **Okumura-Hata Model (150-1500 MHz):** Empirical formula for urban/suburban areas.

$$L_{urban}(dB) = 69.55 + 26.16\log_{10}(f) - 13.82\log_{10}(h_t) - a(h_r) + [44.9 - 6.55\log_{10}(h_t)]\log_{10}(d)$$

    where $$\displaystyle a(h_r) $$ is mobile antenna height correction.

5.  **COST-231 Hata Model:** Extension of Okumura-Hata for 1500-2000 MHz (used for GSM-1800/1900, UMTS).

• Specific Losses:

  • Foliage Losses: Penetration loss through trees/vegetation. Frequency-dependent, increases with frequency. Can be modeled as additional attenuation (e.g., 0.1 dB/m at 1 GHz).

  • Near-in-Distance (Close-in) Propagation: For $$\displaystyle d < d_0 $$ (reference distance, e.g., 1m), path loss does not follow log-distance law. Measurements show rapid fluctuations due to reactive near-field effects. Requires close-in reference models.

2.2 Small-Scale Multipath Propagation & Fading

• Multipath Propagation: Signal arrives at receiver via multiple paths (reflection, diffraction, scattering). Causes constructive/destructive interference.

• Time Dispersion & Delay Spread ($$\displaystyle \tau_{max} $$): Difference in arrival times of first and last significant multipath components.

  • Impact: Causes Intersymbol Interference (ISI) if symbol duration $$\displaystyle T_s < \tau_{max} $$.

• Coherence Bandwidth ($$\displaystyle B_c $$): Frequency range over which channel impulse response is highly correlated (flat fading).

  • Approximate relation: $$\displaystyle B_c \approx \frac{1}{5\tau_{max}} $$ (for 50% correlation). If signal bandwidth $$\displaystyle B_s > B_c $$ → Frequency Selective Fading.

• Fading Classifications:

  | | Flat Fading | Frequency Selective Fading | | :--- | :--- | :--- | | Condition | $$\displaystyle B_s \ll B_c $$ | $$\displaystyle B_s > B_c $$ | | Effect | All freq components fade together. | Different freq components fade independently. | | ISI | No (symbol duration $$\displaystyle T_s \gg \tau_{max} $$). | Yes (symbol duration $$\displaystyle T_s \approx \tau_{max} $$). | | | Slow Fading | Fast Fading | | Condition | $$\displaystyle T_s \ll T_c $$ | $$\displaystyle T_s > T_c $$ | | Coherence Time ($$\displaystyle T_c $$): Time duration over which channel impulse response is correlated. Related to Doppler Spread ($$\displaystyle f_d $$). $$\displaystyle T_c \approx \frac{1}{f_d} $$.

  | Cause | Slow changes in path loss (e.g., shadowing). | Rapid changes due to mobile motion. $$\displaystyle f_d = \frac{v}{\lambda} $$ (max Doppler shift). |

• Clarke's Model (Rayleigh Fading):

  • Assumes isotropic scattering (no LOS), constant envelope modulation.

  • Received signal: $$\displaystyle r(t) = \text{Re}\left[ \sum_{n=1}^{N} a_n e^{j(2\pi f_c t + \theta_n)} \right] $$.

  • As $N \to \infty$, by Central Limit Theorem, in-phase (I) and quadrature (Q) components become independent Gaussian processes:

$$r(t) = r_I(t)\cos(2\pi f_c t) - r_Q(t)\sin(2\pi f_c t)$$

*   **Envelope** $$\displaystyle R(t) = \sqrt{r_I^2 + r_Q^2} $$ follows **Rayleigh distribution**. **Phase** $$\displaystyle \theta(t) = \tan^{-1}(r_Q/r_I) $$ is uniform $[0,2\pi)$.

*   **Level Crossing Rate (LCR)** & **Average Fade Duration (AFD)** are second-order statistics describing fading rate and duration.

2.3 Statistical Models for Multipath Fading

• Rayleigh Fading: No dominant LOS component. PDF of envelope $R$:

$$f_R(r) = \frac{r}{\sigma^2} e^{-r^2/(2\sigma^2)}, \quad r \ge 0$$

where $$\displaystyle \sigma^2 $$ is average power.

• Rician Fading: Presence of a strong LOS component plus scattered waves. Parameter $K$ = ratio of LOS power to scattered power.

$$f_R(r) = \frac{r}{\sigma^2} e^{-(r^2 + s^2)/(2\sigma^2)} I_0\left(\frac{rs}{\sigma^2}\right)$$

where $s$ is LOS amplitude, $$\displaystyle I_0 $$ is modified Bessel function. $$\displaystyle K=0 $$ → Rayleigh.

• Nakagami-m Distribution: Generalized model. PDF:

$$f_R(r) = \frac{2m^m}{\Gamma(m)\Omega^m} r^{2m-1} e^{-m r^2/\Omega}$$

$m$ is fading figure ($$\displaystyle m=1 $$ → Rayleigh). Fits experimental data well.

3.0 MULTIPLE ACCESS TECHNIQUES

3.1 Frequency Division Multiple Access (FDMA)

• Principle: Total bandwidth $$\displaystyle B_T $$ is divided into $C$ non-overlapping frequency channels of bandwidth $$\displaystyle B_c $$. Guard bands $$\displaystyle B_{guard} $$ separate channels to avoid ACI.

$$\boxed{B_T = C \cdot B_c + (C-1) \cdot B_{guard}}$$

> [!TIP] **Numerical:** Given $$\displaystyle B_T $$, $$\displaystyle B_c $$, $$\displaystyle B_{guard} $$, find $C$: $$\displaystyle C = \left\lfloor \frac{B_T + B_{guard}}{B_c + B_{guard}} \right\rfloor $$.

• Advantages: Simple, low latency, mature tech (e.g., analog cellular, satellite).

• Disadvantages: Rigid bandwidth allocation, inefficient for bursty traffic, vulnerable to narrowband interference/jamming.

3.2 Time Division Multiple Access (TDMA)

• Principle: Users share the same frequency channel in different time slots (TS). A frame consists of $N$ TS (one per user). Requires guard time between TS to prevent overlap due to timing errors.

  • Frame Efficiency: $$\displaystyle \eta = \frac{\text{User data time per TS}}{\text{Total TS duration}} $$.

• Advantages: Higher capacity than FDMA (same channel reused in time), flexible for bursty data.

• Disadvantages: High synchronization overhead, guard time reduces efficiency, vulnerable to synchronization errors.

3.3 Code Division Multiple Access (CDMA)

• Fundamental Principle: Spread Spectrum. All users transmit simultaneously over the entire bandwidth $W$. Users are separated by unique orthogonal or near-orthogonal spreading codes.

  • Processing Gain ($$\displaystyle G_p $$): Ratio of spread bandwidth to original data bandwidth.

$$\boxed{G_p = \frac{W}{R_b}}$$

    where $$\displaystyle R_b $$ = data rate (bps). Higher $$\displaystyle G_p $$ → better interference rejection & security.

• DS-CDMA Operation:

  1. Each user's data is multiplied by a high-rate pseudo-noise (PN) sequence (chip rate $$\displaystyle R_c \gg R_b $$).

  2. At receiver, correlate with the same PN sequence to despread desired signal; other users appear as wideband noise.

• Channels (IS-95 Forward Link - BS to MS):

  • Pilot Channel: Unmodulated PN sequence for coherent demodulation & handoff.

  • Sync Channel: Transmits system parameters & timing.

  • Paging Channel: Broadcasts control messages (paging, overhead).

  • Traffic Channels: Carry user voice/data (with power control bits).

• Power Control in CDMA:

  • Necessity: Near-far problem—a nearby user's signal can overwhelm a distant user's at the BS. All users must arrive at similar power at BS.

  • Open-Loop: MS estimates path loss from pilot and adjusts initial transmit power.

  • Closed-Loop: BS measures $$\displaystyle E_b/N_0 $$ from MS, sends power adjustment commands (up/down) at 800 Hz.

• Capacity in CDMA:

  • Interference-limited: Capacity = number of users that can be supported while keeping SIR above threshold.

  • Soft Capacity: Capacity degrades gracefully with more users (quality drops), unlike hard limit in FDMA/TDMA.

  • Uplink Capacity Estimate:

$$K \approx \frac{G_p}{\left( E_b/N_0 \right)_{req}}$$

    where $K$ = number of users.

• Advantages over FDMA/TDMA:

  • Soft capacity,抗干扰 (anti-jamming), security (spread spectrum), soft handoff (see 4.4), no frequency planning.

• Numerical Example (IS-95):

  • $$\displaystyle W = 1.25 $$ MHz, $$\displaystyle R_b = 13 $$ kbps → $$\displaystyle G_p = 1.25 \times 10^6 / 13 \times 10^3 \approx 96.15 $$ (19.8 dB).

  • Given $$\displaystyle E_b/N_0 = 7.8 $$ dB (linear = 6.03), max users $$\displaystyle K_{max} \approx G_p/(E_b/N_0) = 96.15/6.03 \approx 15.9 $$. But with perfect power control and orthogonal codes, theoretical limit is $$\displaystyle G_p $$ (96). Actual capacity lower due to non-orthogonality, imperfect power control.

3.4 Frequency Hopped Multiple Access (FHMA)

• Principle: Users change carrier frequency according to a pseudo-random hopping pattern over a wide band.

  • Slow FH: Several symbols per hop. Good for avoiding frequency-selective fades.

  • Fast FH: Hop per symbol. Good for avoiding jamming/interference.

• Comparison with FHSS: FHMA is a multiple access technique (different users have different hopping patterns). FHSS is a modulation/spread spectrum technique (one user hops to avoid interference).

3.5 Orthogonal Frequency Division Multiplexing (OFDM)

• Principle: High-rate data stream is split into many parallel low-rate streams, each modulating a subcarrier that is orthogonal to others (spectrally overlapping but mathematically separable via IFFT/FFT).

• Advantages:

  • Combats frequency-selective fading (each subcarrier sees flat fading).

  • High spectral efficiency (orthogonality eliminates need for guard bands).

  • Simple equalization (one-tap per subcarrier).

• Cyclic Prefix (CP): Copy of end of OFDM symbol prepended to combat ISI and maintain orthogonality. Duration > channel delay spread $$\displaystyle \tau_{max} $$.

• Implementation: Modulator uses IFFT, demodulator uses FFT.

3.6 Space Division Multiple Access (SDMA)

• Principle: Uses spatial separation of users via smart antennas (beamforming) to serve multiple users on the same frequency/time/code in different spatial directions.

• Key: Directional beams steer towards desired users, nulls towards interferers. Increases capacity by reusing resources in space.

4.0 WIRELESS SYSTEMS: GSM & IS-95 CDMA

4.1 GSM Architecture

• Block Diagram Components:

  1. Mobile Station (MS): Mobile phone + SIM card.

  2. Base Station Subsystem (BSS): Base Transceiver Station (BTS) + Base Station Controller (BSC). Manages radio resources, handoffs.

  3. Network Switching Subsystem (NSS): Mobile Switching Center (MSC) (core switch), Home Location Register (HLR), Visitor Location Register (VLR), Authentication Center (AuC), Equipment Identity Register (EIR). Handles call routing, mobility management, security.

  4. Operation & Support Subsystem (OSS): Network management.

• Key Interfaces:

  • Um (Air): MS ↔ BTS (radio).

  • A-bis: BTS ↔ BSC (standardized).

  • A: BSC ↔ MSC (SS7-based).

  • Iu (in UMTS): RNC ↔ MSC/SGSN.

4.2 GSM Radio Subsystem & Channels

• Traffic Channels (TCH):

  • TCH/F: Full-rate (13 kbps voice, 22.8 kbps channel).

  • TCH/H: Half-rate (two users per channel, 5.6 kbps each).

• Control Channels:

  • Broadcast Control Channel (BCCH): Broadcasts cell ID, neighbor list.

  • Frequency Correction Channel (FCCH): Allows MS to correct frequency drift.

  • Synchronization Channel (SCH): Provides frame timing & BSID.

  • Common Control Channel (CCCH): Shared by all MS in cell.

    • RACH: Random Access Channel (MS → BS, for call/signaling).

    • AGCH: Access Grant Channel (BS → MS, assigns SDCCH).

    • PCH: Paging Channel (BS → MS, for incoming calls).

  • Dedicated Control Channel (DCCH): Point-to-point.

    • SDCCH: Stand-alone Dedicated Control Channel (call setup, SMS, location update).

    • SACCH: Slow Associated Control Channel (link for measurement reports, SMS).

    • FACCH: Fast Associated Control Channel (steals TCH slots for urgent signaling like handoff).

4.3 GSM Frame & Multiframe Structure

• Hierarchy:

  • Bit Period: 3.69 µs.

  • Time Slot (TS): 156.25 bits ≈ 577 µs.

  • Frame: 8 TS (one from each user) ≈ 4.615 ms.

  • Multiframe:

    • 26-Frame Multiframe: For TCH & SDCCH/SACCH (26 frames = 120 ms).

    • 51-Frame Multiframe: For BCCH/CCCH (51 frames = 235.4 ms).

  • Superframe: 1326 frames (6.12 sec) or 2048 frames (9.48 sec).

  • Hyperframe: 2715648 frames (~3.28 hours). Repeats all encryption sequences.

• Channel Mapping:

  • TCH/F uses one TS per 26-frame multiframe.

  • SDCCH uses one TS per 51-frame multiframe (but mapped differently).

  • BCCH is on TS 0 of frame 51 in 51-frame multiframe.

4.4 Handoff in GSM & CDMA

• Handoff Necessity: Maintain call continuity when MS moves from one cell to another or cell quality degrades.

• Handoff Types (GSM):

  • Intra-cell: Change of channel within same cell (due to interference).

  • Inter-cell: Change of BS (most common).

  • Inter-BSC: Handoff controlled by different BSCs (MSC coordinates).

  • Inter-MSC: Handoff between MSCs (most complex).

• Mobile Assisted Handoff (MAHO):

  • MS continuously measures signal strength (RxLev) and quality (RxQual) of serving cell and up to 6 neighbor cells (from BCCH).

  • MS sends measurement reports to BSC via SACCH.

  • BSC makes handoff decision based on thresholds (e.g., serving cell power < threshold AND neighbor cell power > threshold + hysteresis).

• Queuing Concept in Handoff:

  • Prioritize handoff requests over new call attempts to prevent call drops.

  • If no channel available in target cell, handoff request may be queued briefly (if expected channel release soon) or dropped.

• CDMA Handoff:

  • Softer Handoff: MS communicates with multiple sectors of the same BTS (same frequency, same PN offset). Combines signals (maximal ratio combining). No new resources needed.

  • Soft Handoff: MS communicates with multiple BTSs (different PN offsets, same frequency). MSC combines signals. Resources allocated in all target cells until "drop" (traffic channel release). Zero drop probability if at least one leg succeeds.

  • Hard Handoff: Break-before-make (GSM style). MS releases old channel before acquiring new one. Used for inter-frequency or inter-system handoffs.

5.0 CAPACITY & INTERFERENCE MANAGEMENT

5.1 Co-channel Interference (CCI) Reduction

1. Increase $D/R$: Larger cluster size $N$ increases $$\displaystyle D = R\sqrt{3N} $$, but reduces capacity (fewer channels per cell).

2. Sectoring: Directional antennas reduce number of first-tier co-channel interferers (e.g., 120° sectors → only 2 first-tier interferers instead of 6).

3. Power Control: Reduces transmit power of mobiles/BSS to minimum required for acceptable SIR. Crucial in CDMA, helpful in FDMA/TDMA.

4. Cell Splitting/Microcells: Reduces $R$, but requires careful power control to maintain $D/R$ ratio.

5. Use of Diversity: At receiver (e.g., selection diversity) to combat fading, indirectly improving SIR.

5.2 Adjacent Channel Interference (ACI) Reduction

1. Guard Bands: Unused frequency bands between adjacent channels.

2. Efficient Modulation: Use constant envelope modulations (e.g., GMSK in GSM) to minimize out-of-band radiation.

3. Channel Assignment: Avoid assigning adjacent channels to the same cell or adjacent cells (frequency planning).

5.3 Trunking Efficiency & System Capacity

• Trunking efficiency $\eta$ = (carried traffic) / (total channels × traffic per channel). Decreases as number of cells increases (more channels total but same total traffic spread thinner).

• Impact of Techniques:

  • Cell Splitting: Increases number of cells $$\displaystyle N_{cell} $$. Total channels $$\displaystyle C_{total} = N_{cell} \times c $$ (channels/cell). If $c$ fixed, total capacity increases linearly with $$\displaystyle N_{cell} $$. But trunking efficiency may drop if $c$ becomes small.

  • Sectoring: Increases effective $N$ (reduces CCI), allowing smaller $N$ for same SIR → more channels per cell ($c$ increases). But each sector has its own channel pool, so trunking efficiency per sector may be lower than omni-cell. Overall system capacity increases.

6.0 ADVANCED TECHNIQUES & SHORT NOTE TOPICS

6.1 Multiple Input Multiple Output (MIMO) Systems

• Concept: Use multiple antennas at both transmitter ($$\displaystyle N_t $$) and receiver ($$\displaystyle N_r $$).

• Benefits:

  1. Spatial Multiplexing: Transmit independent data streams on different antennas → capacity increase proportional to $$\displaystyle \min(N_t, N_r) $$ in rich scattering.

  2. Diversity: Multiple copies of signal → improved reliability (lower BER).

  3. Beamforming: Focus energy in desired direction → increased range/capacity.

• Key Challenge: Channel estimation and signal processing complexity.

6.2 Diversity Techniques

• Purpose: Combat fading by providing independent fading replicas of the signal.

• Types:

  • Spatial Diversity: Multiple antennas separated by $\lambda/2$ or more.

  • Frequency Diversity: Transmit same signal on different frequencies (e.g., spread spectrum, OFDM subcarriers spaced > $$\displaystyle B_c $$).

  • Time Diversity: Transmit same signal at different times (e.g., interleaving, channel coding with interleaving). Requires $$\displaystyle T_s > T_c $$.

  • Polarization Diversity: Two orthogonally polarized antennas (vertical/horizontal).

  • Angle Diversity: Multiple directional antennas pointing in different directions.

6.3 Key Definitions & Parameters (Short Notes)

• Doppler Spread ($$\displaystyle f_d $$): Range of frequency shifts seen by a moving receiver due to Doppler effect. $$\displaystyle f_d = \frac{v}{\lambda} = \frac{v f_c}{c} $$, where $v$ is speed.

• Coherence Time ($$\displaystyle T_c $$): Time duration over which channel impulse response is correlated. $$\displaystyle T_c \approx \frac{1}{f_d} $$. Fast fading if symbol duration $$\displaystyle T_s < T_c $$.

• Coherence Bandwidth ($$\displaystyle B_c $$): Frequency separation over which channel frequency response is correlated. $$\displaystyle B_c \approx \frac{1}{5\tau_{max}} $$ (50% correlation). Frequency selective fading if signal bandwidth $$\displaystyle B_s > B_c $$.

• Delay Spread ($$\displaystyle \tau_{max} $$): Difference between arrival times of first and last significant multipath components. Causes ISI.

• Fading:

  • Slow vs. Fast: Based on $$\displaystyle T_s $$ vs. $$\displaystyle T_c $$.

  • Flat vs. Frequency Selective: Based on $$\displaystyle B_s $$ vs. $$\displaystyle B_c $$.

• Mobile-to-Mobile Propagation: Both ends moving. Doppler spread depends on relative velocity and angle of arrival. Path loss models similar but with two moving antennas.

• Cell-site vs. Mobile Antennas:

  • Cell-site: High gain, directional (sectoring), tall mast (height $$\displaystyle h_t $$ ≈ 30-100m), above most obstacles.

  • Mobile: Low gain, omnidirectional, low height ($$\displaystyle h_r $$ ≈ 1-3m), surrounded by local scatterers.