1. What is QAM?

Quadrature Amplitude Modulation (QAM) is a modulation technique used in data transmission, particularly in IEEE 802.11ax (WiFi 6), to encode data onto a carrier signal. Modulation is a process that transmits a message signal inside another higher frequency carrier by altering the carrier to look more like the message. QAM combines two carriers, phase-shifted by 90 degrees, and varies the symbol rate (bits per symbol) to boost throughput. In 802.11ax, QAM enables efficient data transmission over multiple carriers using Orthogonal Frequency Division Multiple Access (OFDMA). The table below outlines common QAM levels, their bits per symbol, and capacity improvements.

2. Is higher-order QAM mandatory for microwave backhaul? 

Higher-order QAMs, such as 1024-QAM used in 802.11ax, are not mandatory for all operators using microwave backhaul. However, they are valuable for meeting the high-capacity demands of modern Wi-Fi networks, particularly for LTE and 5G backhaul, by enabling higher data throughput in the 2.4 GHz and 5 GHz bands.

3. What is the main advantage of using higher-order QAMs with microwave radios?

The primary advantage of using higher-order QAMs, like 1024-QAM, is significantly increased capacity and throughput, enabling data rates up to 10 Gb/s. However, the capacity gain diminishes with each step (e.g. from 256-QAM to 1024-QAM, the improvement is approximately 25%), requiring complementary techniques like OFDMA and MU-MIMO to maximise performance.

4. What are the trade-offs of higher-order QAMs on RF performance?

Higher-order QAMs introduce trade-offs in RF performance for 802.11ax systems, as shown in Figure 2:

• Increased Interference Sensitivity: Each QAM step increases the Carrier-to-Interference (C/I) ratio, e.g. a 5 dB increase from 256-QAM to 1024-QAM. This makes links more susceptible to interference and complicating link coordination.
• Reduced System Gain: System gain decreases (e.g. from ~90 dB at 256-QAM to ~80 dB at 1024-QAM), necessitating shorter links or larger antennas, which raises costs and complexity.
• Increased Design Complexity: Higher phase noise and intricate designs increase equipment costs.
• Diminishing Returns: Capacity gains follow a flattening curve, with each QAM step yielding smaller percentage increases compared to the added RF performance costs.

5. Do you need to use Adaptive Coding and Modulation (ACM) while using higher-order QAMs?

ACM is recommended when using higher-order QAMs to mitigate reduced system gain, especially in challenging propagation environments. ACM dynamically adjusts modulation and coding schemes to maintain link reliability. However, it cannot address the increased C/I ratio, requiring other interference management techniques.

6. What gives CableFree a “heads-up” here when other big name companies seem to be supporting the technology?

CableFree recognises that higher-order QAMs, while useful, are not a complete solution for capacity needs. Instead, they emphasise techniques offering substantial capacity increases, such as:

• Multichannel RF Bonding (N+0): Uses additional spectrum across different bands and channel sizes, achieving at least 200% capacity increase.
• Intelligent Network Dimensioning: Leverages Layer 2/3 QoS and best practices to optimise backhaul capacity, providing significant gains over the modest improvements from higher-order QAMs.

7. Will operators need to “retrofit” microwave radios to be capable of higher-order QAM operation in their existing microwave infrastructure? Or will completely new hardware be required?

Whether existing microwave radios can support higher-order QAMs like 1024-QAM depends on their age and model. Older systems may require retrofitting to handle 512-QAM or higher, while recently deployed 802.11ax-compatible radios typically support these modulations without new hardware, ensuring seamless integration.

8. How will QAM evolve in the future?

The evolution of QAM is limited by diminishing returns. As modulation order increases (e.g. beyond 1024-QAM), throughput improvements decrease (e.g. 2048-QAM offers only ~10% more capacity), while costs and complexity rise significantly. Beyond 1024-QAM, the benefits are unlikely to justify the trade-offs, making alternative techniques like OFDMA, MU-MIMO, and spectrum expansion more critical for future WiFi standards.