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Generalized Spatial Modulation (GSM)


Generalized Spatial Modulation (GSM) is an advanced technique for wireless communication systems that integrates spatial modulation with a dynamic antenna activation mechanism. This approach allows for significant improvements in both data rate and energy efficiency compared to traditional MIMO systems.

In GSM, instead of utilizing all available antennas for transmission, only a subset of antennas are activated at any given time. This selective activation reduces power consumption while still maintaining high throughput, which is a critical requirement for modern wireless communication.


Key Concepts of GSM

  1. Antenna Activation:
    • n_t = total number of transmit antennas.
    • n_rf = number of active RF chains (active antennas), where n_rf < n_t.
    • Inactive antennas are turned OFF to save power.
  2. Transmission with Active Antennas: Symbols are transmitted by activating specific antennas based on a predefined Antenna Activation Pattern (AAP). The AAP determines which antennas are ON or OFF during transmission.
  3. Combination of RF Chains and Transmit Antennas: The transmitter optimizes the combination of active antennas and RF chains to maximize the achievable rate.
  4. Rate Computation: The achievable rate depends on the number of activation patterns and the modulation scheme used on the active antennas.

Key Equations

Achievable Rate in GSM:
R = | log₂( C(n_t, n_rf) ) | + n_rf × log₂(M)
  • n_t → Number of transmit antennas
  • n_rf → Number of active RF chains
  • M → Modulation order (e.g., QPSK, 16-QAM)
  • C represents the number of possible antenna combinations

The first term represents the number of bits transmitted via antenna activation patterns, while the second term represents the bits transmitted through modulation on active antennas.


Mapping of Symbols to Antennas (Example)

When n_t = 4 and n_rf = 2, the symbols are mapped to antennas using specific Antenna Activation Patterns (AAP) as follows:

AAP Tx Antenna Status Ant 1 Ant 2 Ant 3 Ant 4
00 x₁, x₂ ON ON OFF OFF
01 x₁ ON OFF OFF OFF
10 x₂ OFF ON OFF OFF
11 x₁, x₂ OFF OFF ON ON

The OFF state indicates inactive antennas, which help save power and reduce interference.

Number of Activation Patterns

Number of possible patterns: L = C(n_t, n_rf)

Example: For n_t = 4 and n_rf = 2, L = C(4,2) = 6.
Thus, there are 6 unique antenna activation combinations.

Rate with M-ary Modulation

Bits transmitted on active antennas: n_rf × log₂(M)

The total achievable rate combines both the spatial activation bits and the modulation bits.


Example: Achievable Rate Comparison

  • For n_t = 32 and n_rf = 24, the optimal configuration gives R = 71 bps/Hz.
  • For V-BLAST with n_rf = n_t = 32, R = n_t × log₂(M) = 64 bps/Hz (for M = 2).

Hence, GSM achieves a higher rate even with fewer active antennas, demonstrating superior efficiency.


Conclusion

Generalized Spatial Modulation (GSM) offers a powerful trade-off between spectral efficiency and power savings. By activating only a subset of antennas based on Antenna Activation Patterns (AAP), GSM significantly reduces power consumption while maintaining high data rates.

The achievable rate in GSM depends on both the number of possible activation patterns (log₂(C(n_t, n_rf))) and the modulation scheme (n_rf log₂ M). The selective use of active and OFF antennas makes GSM an ideal solution for next-generation wireless networks requiring both speed and energy efficiency.

Further Reading


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