How can OFDM spectrum efficiency be improved with advanced modulation and coding techniques?

In OFDM, each divided parallel low-sub streams are modulated onto a separate orthogonal subcarrier and these subcarriers are closely spaced in the frequency domain and are orthogonal to each other hence, they do not interfere with each other.

Higher modulation orders, such as 16-QAM or 64-QAM, allow more bits to be transmitted per symbol compared to lower modulation orders like QPSK or BPSK thus, using higher modulation orders, more information can be packed into each subcarrier, resulting into increased data rates and improved spectrum efficiency. However, higher modulation orders are more susceptible to noise and they do require higher SNR ratio for a reliable transmission.

This can be more easily and intuitively understood with the help of a constellation diagram. This shows how the number of final decision points increase as we go for higher order modulation schrmes [2^b baseband signal waveforms (constellation points) where "b" is the number of bits encoded onto the baseband signal]. This also however shows that for a decoder at the receiver end, the region where a distorted noisy signal can fall and still be decoded correctly shrinks accordingly, because the total spread area for the constellation is limited by the overall energy limitations for a signal transfer. This leads to a degradation in SNR for higher modulation schemes, as the limited transmit energy is spread across more signal waveforms.

1. Higher-Order Modulation. 2. Advanced Coding Techniques: Low-Density Parity-Check Codes and Turbo Codes. 3. Multiple Input Multiple Output: Spatial Multiplexing and Beamforming. 4. Channel State Information: Frequency-Selective Power Allocation and Precoding. 5. Interference Management: Coordinated Multipoint (CoMP) and Interference Cancellation. 6. Waveform Optimization: Filter Bank Multicarrier (FBMC) and Generalized Frequency Division Multiplexing (GFDM). 7. Coding and Modulation Combination (Link Adaptation).

Creo que por alguna razón comercial las empresas que tienen las patentes de los chipsets actuales con modulación QAM, no han permitido que una mejor técnica de modulación como al parecer es WAM no haya sustituído a la ya antigua QAM, de hecho cuando oí de tal modulación con demostración incluída en Barcelona, mejoraba por mucho la velocidad de la red y WAM es totalmente compatible con las redes actuales, sólo hay que cambiar los módulos QAM por los WAM lo cual es totalmente trasparente para la red, pero el impacto económico en las empresas que producen tales chipsets sería por no decir poco, desastroso y bueno por esas cuestiones perdemos mucho como usuarios.