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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Class 4 Microwave Antennas

What is a Class 4 Microwave Antenna?

Class 4 Antennas explained:

CableFree Class 4 Microwave Antenna 1Class 4 antennas provide the current best RF performance allowing mobile operators and Wireless Internet Service Providers (WISP) to increase the link capacity of a network by deploying new microwave links where high levels of interference are present. Class 4 antennas will allow customers to offer the highest performance in even the most congested environments. The higher side lobe suppression supports networks in ultra-dense areas and enables earlier reuse of frequencies. The lower interference increases the carrier-to-interference-ratio and allows smaller antennas with better link throughput, reducing tower leasing fees. The lower interference also enables higher modulation schemes, increasing the data capacity per antenna.

Benefits of a Class 4 Antenna

Increase the link capacity of the network
– Improved radiation patterns for ETSI Class 4 providing better performance
– Less interference and higher carrier-to-interference ratio
– Allows radios to operate at higher modulation levels
• Minimize the total cost of ownership
– Improved network efficiency
– Facilitates better re-use of a frequency channel
– Small antennas with better link throughput reduces tower leasing fees

Intended Use for Class 4 Antennas

CableFree Class 4 Microwave Antenna 1Class 4 antennas are intended for “extremely high interference potential” situations, according to ETSI. For a more detailed treatment of antenna classifications and radiation patterns, see the ETSI document “Fixed Radio Systems; Point to Point Antennas.”

Wider channels, larger capacity

For situations where the operator needs to increase capacity from a wireless backhaul site, the easiest way remains widening the channel size. But at sites that experience extremely high interference, the operator may not be able to coordinate radio frequency pairs in wide channels with Class 3 antennas. However, moving up to Class 4 antennas would allow the operator to optimize the signal-to-noise ratio and let higher modulations come into play, so wide channels could be coordinated with correspondingly higher data rates

Smaller is Better

In cases of high interference, larger antennas can be used to reduce it. For a subset, smaller Class 4 antennas can be used instead of their oversize Class 3 counterparts. Thus, operators who deploy Class 4 antennas gain the added benefit of dropping down a parabolic dish antenna size as compared to a Class 3 antenna in the same application. In general, smaller dishes advantage the operator due to their lighter weight and lower opex tower charges, albeit with an initially bigger upfront capex. Because Class 4 antennas represent an elevated level of precision tooling and more detailed manufacturing versus lower class antennas, capex of these passive, higher-performance infrastructure pieces always weighs in the balance.

 

According to Andy Sutton,  Principal Network Architect at EE:

Using Comsearch’s iQ.linkXG microwave planning software, CommScope analyzed the technical and commercial benefits of using Class 4 Sentinel antennas in the network. The results were most impressive. For the two frequency bands of the microwave backhaul network studied, which is comprised of over 6,200 links in total, the core findings were:

  • Potential savings of $5 million in total cost of ownership (TCO) over five years by enabling a greater link density and therefore reducing the need for third party Ethernet Leased Lines
  • Greater utilization of owned block allocated spectrum reduced the need for link by link licensing (from the national regulator) and therefore could save $44,000 in license fees over five years
  • $4.5 million could be saved per year based on optimizing capacity by freeing congested channels while still ensuring new links met the strict quality of service criteria
  • 96 percent and 31 percent of links which couldn’t be planned due to frequency congestion in 40 and 10 GHz could be assigned a channel, respectively
    • A strong opportunity to trade some of the above by reducing antenna size and thus reducing TCO on tower lease costs

(content from EE above reproduced with acknowledgement from Commscope. Other content from RFS).

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Mean Square Error (MSE) for Microwave Links

What is Mean Square Error ?

Mean Square Error (MSE) is similar to Signal-to-Noise Ratio (SNR) except that it accounts for distortion and interference in addition to noise power.

Mean Square Error MSE Microwave Link
CableFree Microwave ODU

Distortion may come from several sources such as bad Ethernet cables (poor shield, damaged, or low quality), path degradations such as multipath, or Fresnel zone encroachment.

Interference can come from other transmitters on the tower, as well as from sources inside an indoor shelter. High power transmitters inside a shelter can cause interference when near the PoE device or when located very close to the cabling.

There are maximum acceptable MSE values for each modulation which are useful in determining the quality of the link. The MSE value reported is only relevant to one tx-rx path, so the MSE of each tx-rx path must be evaluated to verify the link is operating as expected. The lower the number the better, so a -35dB is better than a -30dB.

Other possible causes for unacceptable MSE

These include

  • XPIC parameters are incorrect
  • Insufficient isolation between polarisations on an XPIC link
  • Insufficient performance to support high QAM modulation
  • Inbalance between paths on an XPIC dual polarity link

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