Comparing Microwave Links using 512-QAM, 1024-QAM, 2048-QAM, 4096-QAM

Microwave links using 512QAM, 1024QAM, 2048QAM & 4096QAM (Quadrature Amplitude Modulation)

What is QAM?

Quadrature amplitude modulation (QAM) including 16QAM, 32QAM, 64QAM, 128QAM, 256QAM, 512QAM, 1024QAM, 2048QAM and 4096QAM is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme.

Why are higher QAM levels used?

Modern wireless networks often demand and require higher capacities.  For a fixed channel size, increasing QAM modulation level increases the link capacity.  Note that incremental capacity gain at low-QAM levels is significant; but at high QAM, the capacity gain is much smaller.  For example, increasing
From 1024QAM to 2048QAM gives a 10.83% capacity gain.
From 2048QAM to 4096QAM gives a 9.77% capacity gain.

QAM Increase Capacity Table
QAM Increase Capacity Table

What are the penalties in higher QAM?

The receiver sensitivity is greatly reduced.  For every QAM increment (e.g. 512 to 1024QAM) there is a -3dB degradation in receiver sensitivity.  This reduces the range.  Due to increased linearity requirements at the transmitter, there is a reduction in transmit power also when QAM level is increased.  This may be around 1dB per QAM increment.

Comparing 512-QAM, 1024-QAM, 2048-QAM & 4096-QAM

This article compares 512-QAM vs 1024-QAM vs 2048-QAM vs 4096-QAM and mentions difference between 512-QAM, 1024-QAM, 2048-QAM and 4096-QAM modulation techniques. It mentions advantages and disadvantages of QAM over other modulation types. Links to 16-QAM, 64-QAM and 256-QAM is also mentioned.

Understanding QAM Modulation

Starting with the QAM modulation process at the transmitter to receiver in the wireless baseband (i.e. Physical Layer) chain. We will use the example of 64-QAM to illustrate the process. Each symbol in the QAM constellation represents a unique amplitude and phase. Hence they can be distinguished from the other points at the receiver.

64QAM Quadrature Amplitude Modulation
64QAM Quadrature Amplitude Modulation

Fig:1, 64-QAM Mapping and Demapping

• As shown in the figure-1, 64-QAM or any other modulation is applied on the input binary bits.
• The QAM modulation converts input bits into complex symbols which represent bits by variation in amplitude/phase of the time domain waveform. Using 64QAM converts 6 bits into one symbol at transmitter.
• The bits to symbols conversion take place at the transmitter while reverse (i.e. symbols to bits) take place at the receiver. At receiver, one symbol gives 6 bits as output of demapper.
• Figure depicts position of QAM mapper and QAM demapper in the baseband transmitter and receiver respectively. The demapping is done after front end synchronization i.e. after channel and other impairments are corrected from the received impaired baseband symbols.
• Data Mapping or modulation process is done before the RF upconversion (U/C) in the transmitter and PA. Due to this, higher order modulation necessitates use of highly linear PA (Power Amplifier) at the transmit end.

QAM Mapping Process

64QAM Mapping Modulation
64QAM Mapping Modulation

Fig:2, 64-QAM Mapping Process

In 64-QAM, the number 64 refers to 2^6.
Here 6 represents number of bits/symbol which is 6 in 64-QAM.
Similarly it can be applied to other modulation types such as 512-QAM, 1024-QAM, 2048-QAM and 4096-QAM as described below.

Following table mentions 64-QAM encoding rule. Check the encoding rule in the respective wireless standard. KMOD value for 64-QAM is 1/SQRT(42).

Input bits (b5, b4, b3) I-Out Input bits (b2, b1, b0) Q-Out
011 7 011 7
010 5 010 5
000 3 000 3
001 1 001 1
101 -1 101 -1
100 -3 100 -3
110 -5 110 -5
111 -7 111 -7

QAM mapper Input parameters :    Binary Bits
QAM mapper Output parameters : Complex data (I, Q)

The 64-QAM mapper takes binary input and generates complex data symbols as output. It uses above mentioned encoding table to do the conversion process. Before the coversion process, data is grouped into 6 bits pair. Here, (b5, b4, b3) determines the I value and (b2, b1, b0) determines the Q value.

Example: Binary Input: (b5,b4,b3,b2,b1,b0) = (011011)
Complex Output: (1/SQRT(42))* (7+j*7)

512-QAM modulation

512QAM Modulation
512QAM Modulation

Fig:3, 512-QAM Constellation Diagram

The above figure shows 512-QAM constellation diagram. Note that 16 points do not exist in each of the four quadrants to make total 512 points with 128 points in each quadrant in this modulation type. It is possible to have 9 bits per symbol in 512-QAM also. 512QAM increases capacity by 50% compare to 64-QAM modulation type.

1024-QAM modulation

1024QAM Modulation Constellation
1024QAM Modulation Constellation

The figure shows a 1024-QAM constellation diagram.
Number of bits per seymbol: 10
Symbol rate: 1/10 of bit rate
Increase in capacity compare to 64-QAM: About 66.66%

2048-QAM modulation

2048QAM Modulation Constellation
2048QAM Modulation Constellation

Following are the characteristics of 2048-QAM modulation.
Number of bits per seymbol: 11
Symbol rate: 1/11 of bit rate
Increase in capacity from 64-QAM to 1024QAM: 83.33% gain
Increase in capacity from 1024QAM to 2048QAM: 10.83% gain
Total constellation points in one quadrant: 512

4096-QAM modulation

4096QAM Modulation Constellation
4096QAM Modulation Constellation

Following are the characteristics of 4096-QAM modulation.
Number of bits per symbol: 12
Symbol rate: 1/12 of bit rate
Increase in capacity from 64-QAM to 409QAM: 100% gain
Increase in capacity from 2048QAM to 4096QAM 9.77% gain
Total constellation points in one quadrant: 1024

Advantages of QAM over other modulation types

Following are the advantages of QAM modulation:
• Helps achieve high data rate as more number of bits are carried by one carrier. Due to this it has become popular in modern wireless communication system such as LTE, LTE-Advanced etc. It is also used in latest WLAN technologies such as 802.11n 802.11 ac, 802.11 ad and others.

Disadvantages of QAM over other modulation types

Following are the disadvantages of QAM modulation:
• Though data rate has been increased by mapping more than 1 bits on single carrier, it requires high SNR in order to decode the bits at the receiver.
• Needs high linearity PA (Power Amplifier) in the Transmitter.
• In addition to high SNR, higher modulation techniques need very robust front end algorithms (time, frequency and channel) to decode the symbols without errors.

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QAM Modulation for Microwave Links

1. What is QAM?

Modulation is a data transmission technique that transmits a message signal inside another higher frequency carrier by altering the carrier to look more like the message. Quadrature Amplitude Modulation (QAM) is a form of modulation that uses two carriers—offset in phase by 90 degrees—and varying symbol rates (i.e., transmitted bits per symbol) to increase throughput. The table in this blog post (Figure 1) describes the various common modulation levels, associated bits/symbol and incremental capacity improvement above the next lower modulation step.

CableFree QAM Modulation Table
CableFree QAM Modulation Table

2. Must all operators who use microwave backhaul use higher-order QAMs?

Higher-order QAMs are not necessarily a must-have for all network operators. However, higher-order modulations do provide one method of obtaining higher data throughput and are a useful tool for meeting LTE backhaul capacity requirements.

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

The main advantage is increased capacity, or higher throughput. However, capacity improvement diminishes with every higher modulation step (i.e., moving from 1024QAM to 2048QAM the improvement is only about 10 percent!), so the real capability of higher-order modulations alone to address the objective of increasing capacity is very limited. Other techniques will be needed.

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

First, with each step increase in QAM the RF performance of the microwave radio is degraded as per the Carrier-to-Interference (C/I) ratio. For example, going from 1024QAM to 2048QAM will produce an increase of 5 dB in C/I (Figure 2). This results in the microwave link having much higher sensitivity to interference, making it more difficult to coordinate links and reducing link density. Along with this increase in phase noise there will be an increase in design complexity cost.

CableFree QAM Modulation Tradeoffs
CableFree QAM Modulation Tradeoffs

Also, by increasing from 1024QAM to 2048QAM, system gain will decrease from above 80 dB to just above 75 dB (Figure 2). With much lower system gain microwave links will have to be shorter and larger antennas will have to be employed—increasing total cost of ownership and introducing additional link design and path planning problems.

All of the above are the results of linear functions: they degrade in a one-to-one relationship with the move to higher-order QAMs. Meanwhile, the capacity increases derived from higher-order QAMs are the function of a flattening curve: Each step increase in QAM results in a reduced percentage increase in capacity compared to prior increases in QAM. The added capacity benefits are diminished when considering the added costs of higher C/I and lower system gain.

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

ACM should be implemented while employing high-order QAMs to offset lower system gain. However, while ACM does help mitigate the effects of more difficult propagation when using higher-order modulations, it cannot help offset increased C/I.

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

CableFree realizes higher-order modulations are not a panacea—a cure-all. While every minor technology improvement in throughput can help, a focus on technologies that grow capacity in hundreds of percentage points vs. tens of percentage points is most critical now. CableFree believes that these hundreds-of-percentage-points-of-improvement-in-capacity solutions will be the most important moving forward. It is in these technologies that CableFree has a “heads-up.” Such techniques include deploying more spectrum—particularly in the form of multichannel RF bonding (N+0) solutions—to achieve a minimum of 200 percent capacity increase. This technique is subject to frequency availability, but with flexible N+0 implementations (such as being able to use frequency channels in different bands and different channel sizes) many congestion issues can be avoided.

Second, intelligently dimensioning the backhaul network based on proven rules, best practices and L2/L3 quality of service (QoS) capabilities is another technique to provide potentially very large gains in backhaul capacity. Higher-order modulations can be one tool to achieve required capacity increases in the backhaul network. However, their inherent drawbacks should be well understood, while the most attention should be paid to other techniques that deliver more meaningful and quantifiable benefits.

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?

This depends on the age and model of the existing radios. Older microwave systems will likely need to be “retrofitted” to support 512QAM and higher modulations. Recently installed microwave systems should be able to support these technologies without new hardware.

8. How will QAM evolve in the future? Is the introduction of higher-order QAMs an indefinite process, with no end in sight?

The introduction of higher-order QAMs is not an endless process. As per Figure 1 above in this blog post, the law of diminishing returns applies: Throughput percentage improvement declines as modulation rates increase. The cost and complexity of implementing higher-order QAMs probably is not worth the capacity increase benefits derived—not past 1024QAM, in any event.

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Welcome to Microwave-Link.com

Find out information on technology, deployment and applications for modern Digital Microwave Links

Microwave Link
CableFree MW Link installed on a telecom tower

Microwave links are widely used for connectivity in modern digital IP networks. With capacities up to 6Gbps and beyond, a modern Microwave Link network can deliver bandwidth in a reliable, cost-effective and flexible manner – without need for disruption and delay caused by digging up streets and avoiding costly leased-line or leased fibre optic alternatives.

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CableFree Microwave Links used for Mobile Backhaul
CableFree MW Radio Links used for Mobile Backhaul

Microwave links are used extensively in 4G & 5G LTE backhaul networks, 2G (GSM) and 3G (UMTS) mobile operators, wireless metropolitan area networks (Wi-MAN) and corporate networks where high performance, flexibility, speed of deployment and low operating costs are required.  Key features of links include high spectral efficiency (256QAM, 1024QAM, 2048QAM and 4096QAM), Automatic Transmit Power Control (ATPC) and Adaptive Coding and Modulation (ACM).

Globally, MW radio links are used for around 60% of all mobile backhaul connections due to the compelling technical and commercial arguments in favour of MW radio compared to leased line and trenched fibre alternatives.  Speed of deployment and flexibility – the ability to move sites or provision rapidly – are greatly in favour of MW radio over fibre and cabled alternatives.

A Full Outdoor Microwave Link installed for ISP in Iraq with 880Mbps Full Duplex Capacity
A Full Outdoor Microwave Link installed for ISP in Iraq with 880Mbps Full Duplex Capacity

A  link typically features a radio unit and a parabolic antenna, which may vary in size from 30cm up to 4m diameter depending on required distance and capacity.  The radio unit is generally either a “Full Outdoor”, “Split Mount” or “Full Indoor” design depending on operator preference, deployment, features and available indoor space for specific sites and installation.

CableFree FOR3 Full Outdoor 1024QAM Microwave Link
Full Outdoor 1024QAM MW Radio Link

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