Spectral Efficiency: FDMA, TDMA, and CDMA

In wireless communication, radio frequency resources are precious. Maximizing their utilization is crucial. This article explores the concept of spectral efficiency and how it applies to different multiple access techniques like FDMA, TDMA, and CDMA.

Methods to Improve Spectrum Utilization

Several methods help optimize spectrum usage:

• Cell planning: Designing cell layouts to minimize interference.
• Frequency reuse: Reusing the same frequencies in different cells to increase capacity.
• Channel assignment: Allocating channels efficiently to users.
• Multiple access techniques: Allowing multiple users to share the same frequency band.

Factors Affecting Spectral Efficiency

Spectral efficiency hinges on these key parameters:

• Channel spacing (KHz)
• Frequency reuse factor
• Cell area (Km²)
• Modulation techniques employed
• Multiple Access techniques used

Spectral efficiency can be expressed in two primary ways:

• Data channel per unit bandwidth per unit coverage area: Measured in channels/MHz/Km².
• Total data traffic per unit bandwidth per unit area: Measured in Erlangs/MHz/Km².

Spectral Efficiency Equation

Here’s an image visually representing the spectral efficiency equation:

Spectral Efficiency of FDMA (Frequency Division Multiple Access)

The spectral efficiency (η) of an FDMA system is calculated as:

Where:

• $A_{\text{cell}}$ is the area of each cell.
• $N_{\text{data/cell}} = [ { (BW_t - 2*B_g)/B_{\text{ch}} } - N_{\text{cntl}} ]/N$
• $BW_t$ is the total bandwidth.
• $B_{\text{ch}}$ is the channel bandwidth.
• $B_g$ is the guard band bandwidth.
• $N$ is the number of cells per cluster.

Spectral Efficiency of TDMA (Time Division Multiple Access)

Spectral efficiency varies for wideband and narrowband TDMA:

Wideband TDMA (WTDMA)

Narrowband TDMA (NTDMA)

Where:

• $T_p$ = preamble period
• $T_t$ = trailer time period
• $T_f$ = frame duration
• $N_s$ = number of symbols in a time slot
• $N_i$ = number of information bits
• $N_{\text{sub}}$ = Number of sub bands
• $N_{\text{slot}}$ = Number of slots

Spectral Efficiency of CDMA (Code Division Multiple Access)

The spectral efficiency (η) of a DS-CDMA (Direct-Sequence CDMA) system is:

Where:

• $R_b$ is the information bit rate.
• $W$ is the system bandwidth for one direction.
• $U_s$ is the system utilization.