What is 64 QAM?

Rohde and Schwarz recently achieved what it claims is the first Global Certification Forum validation of 64 quadrature amplitude modulation in the uplink, for LTE-Advanced Pro.

Carrier aggregation in the uplink takes the maximum uplink rate for an LTE user from 50 megabits per second to 100 Mbps with traditional 16 QAM, while LTE-Advanced Pro with 64 QAM supports up to an additional 50% increase in uplink data speed – so the uplink could, at least in theory, be as fast as 150 Mbps. Most LTE features so far have focused on downlink speeds, which don’t address upload speeds for applications such as social networking and cloud services.

RCR Wireless News asked Bryan Helmick, product manager for Rohde & Schwarz, to outline the basics of 64 QAM and the necessary testing to ensure its functionality.

RCR Wireless News: What is 64 QAM? 

Helmick: Information can be converted to a digital format in the form of bits (0s & 1s). These bits can be sent over-the-air using electromagnetic waves (radio waves) by changing one or more of the properties (i.e. amplitude, frequency, phase) of each wave. Modulation techniques define which of the properties are being manipulated. Higher order modulation schemes allow more information to fit into a single radio wave. In other words, higher order modulation equals more bits per wave. This is a powerful way of improving spectral efficiency.

The simplest modulation techniques allow one wave to represent just one single bit of information (0 or 1). Sixty-four QAM is a higher order modulation technique, which allows one single radio wave to represent six bits of data by manipulating the amplitude and phase of the radio wave into one of 64 different discrete and measurable states.

The advantage of higher order modulation is the possibility to transmit more bits per radio wave. The disadvantage is that the data becomes more susceptible to noise and interferers since the receiver must accurately detect more discrete phases and amplitudes of a signal. Advancements in electronics have made it possible to use higher and higher order modulation techniques. And the small cell LTE deployment revolution is helping to create more and more environments with signal-to-noise ratios appropriate for 64 QAM on the LTE uplink.

RCRWN: How is 64 QAM used in LTE-Advanced Pro?

Helmick: Sixty-four QAM was defined for use in LTE even before LTE-A Pro. Sixty-four QAM was included as part of the initial LTE Release 8 specification to be used on both the downlink (mandatory) and on the uplink (optional). It is only very recently that there has been a requirement by operators (and now devices) to send data modulated at 64 QAM on the uplink.  

RCRWN: How does it improve upon other modulation schemes? 

Helmick: LTE allows QPSK, 16 QAM and 64 QAM modulation schemes on the uplink and the downlink. Higher order modulation schemes can typically only be used when the RF conditions are ideal. In fact, UL 64 QAM provides the most benefit in small cell environments. When the conditions are right, such as in a small cell serving a building with slow-moving or fixed wireless devices, modulation schemes of higher order make it possible to send much more data (up to 50% more) with the same amount of frequency resource (spectrum). When using a 20-megahertz bandwidth, the maximum LTE data rate on the uplink increases from 50 Mbps with UL 16 QAM to 75 Mbps with UL 64 QAM. Combining UL 64 QAM with UL carrier aggregation with 20 megahertz x two makes it possible to achieve a maximum data rate of 150 Mbps.  

That said, the evolution is not complete. LTE-A Release 12 added 256 QAM as an additional modulation scheme for the downlink and we are beginning to see devices supporting this. And with LTE-A Pro Release 14 is adding 256 QAM for the uplink. We will probably see devices support UL 256 QAM in 2018.

RCRWN: When is testing for 64 QAM in the uplink required? 

Helmick: There are three main concerns with the roll out of UL 64 QAM on commercial devices. These concerns are multiplied when also enabling uplink carrier aggregation.
1. RF performance: UE transmitter performance. These measurements include:
        A. Error vector magnitude measures how accurately a device can transmit symbols within the 64 QAM constellation.
        B. Spectrum emission mask measures the total amount of excess unwanted power transmitted outside the carrier bandwidth that could interfere with other channels.
        C. Adjacent channel leakage power ratio measures the average power relative to the transmitter power that leaks from a transmitted signal into adjacent channels that could interfere with other channels.
        D. Spurious emissions measures unwanted emissions “far out” from the transmitted signal. Spurious emissions are emissions caused by unwanted transmitter effects such as harmonics emissions and intermodulation products, but they exclude out-of-band emissions measured by SEM and ACLR.

2. Data performance: UE uplink and bidrectional data performance with and without uplink carrier aggregation enabled. Testing data performance in ideal scenarios (i.e. small cell with no mobility) and nonideal (with AWGN and fading profiles applied) is very important.

3. Battery life: Higher order modulation schemes and the transmission of more data could have an adverse effect on power consumption and ultimately a shorter battery life.

RCRWN: Can you give a summary of the testing process of ensuring 64 QAM functionality in the uplink? 

Helmick: After the initial R&D phase is complete by the chipset manufacturers and the features are integrated by the device manufacturer, there are two main areas of certification/acceptance that a UL 64 QAM capable device must pass to be accepted onto an operator’s LTE network.  
1. Devices need to pass all PTCRB or GCF tests related to UL 64 QAM. There are LTE RF conformance tests (in 3GPP 36.521-1 Chapter 6) specific to UL 64 QAM that must be passed at an approved PTRCB/GCF lab.
2. Devices need to pass all supplemental network operator acceptance tests related to UL 64 QAM. These tests go above and beyond what is defined in conformance – either with tighter limits than what is defined in the 3GPP tests, or covering additional areas deemed important by a specific network operator. These tests must be passed at a lab approved by the specific network operator.

RCRWN: What network and/or device changes are required to support 64 QAM in the uplink? 

Helmick: UL 64 QAM will mainly be used in a small cell environment. The roll out of LTE small cells by network operators around the world is creating the perfect environment for higher order modulation schemes to be used commercially. The exact changes being made by the infrastructure and device manufacturers depend on the vendor. But the demand for UL 64 QAM by network operators is growing which has quickly lead to solutions from several infrastructure, device chipset and device manufacturers.

RCRWN: How would you categorize the development stage for 64 QAM in the uplink? When would you expect to see broad adoption/availability? 

Helmick: UL 64 QAM is very close to becoming commercial. We will see it rolling out beginning this year and it will become a common feature by the middle of 2018.

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