The IoT will make heavy use of fifth-generation mobile networks that
use a yet-to-be-determined modulation scheme. Here are the major contenders.
Fifth-generation mobile networks, abbreviated 5G, will
form the telecommunications standards for the internet of things. Planners say
5G will have a higher capacity than the current 4G equipment partly to support
the device-to-device, ultra reliable, and massive machine communications
expected to help define the IoT of the future. Among the goals of 5G: lower
latency than 4G equipment and lower battery consumption, data rates of tens of
megabits per second for tens of thousands of users, several hundreds of
thousands of simultaneous connections available for wireless sensors, along
with better spectral signaling efficiency.
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The better spectral efficiency will partly be
a function of the modulation schemes used in 5G. However, those modulation
schemes have yet to be standardized. There are several contenders, and
derivatives of the same quadrature-style schemes in use by mobile networks
today haven’t been ruled out for 5G. So it is interesting to review the major
modulation techniques now up for consideration as part of 5G.
Techniques discussed for 5G tend to use
multiple carriers as a means of obtaining spectral efficiency. At present, 4G
LTE uses QAM (quadrature amplitude modulation) with OFDM (orthogonal frequency
division multiplexing) as modulation and OFDMA (OFDM multiple access) as access
scheme. 5G will provide a high bit rate so it will need to make efficient use
of the spectrum. Several of the ideas proposed for 5G are hybrids of QAM and
OFDM principles.
Firstly, Quadrature
techniques represent a transmitted symbol as a complex number and modulate a
cosine and sine carrier signal with the real and imaginary parts. This lets the
symbol be sent with two carriers. The two carriers are generally referred to as
quadrature carriers. A coherent detector can independently demodulate these
carriers. This principle of using two independently modulated carriers is the
foundation of quadrature modulation.
Quadrature amplitude modulation 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 o+r
amplitude modulation (AM) analog modulation scheme. The two carrier waves of
the same frequency are out of phase with each other by 90° and are thus called
quadrature carriers. The modulated waves are summed, and the final waveform is
a combination of both phase-shift keying (PSK) and amplitude-shift keying
(ASK), or, in the analog case, of phase modulation (PM) and amplitude
modulation.
QAM conveys information by modulating the amplitudes of
the two carrier waves, using either amplitude-shift keying (ASK) for digital
data or straight amplitude modulation for analog. The two carrier waves of the
same frequency, usually sinusoids, are out of phase with each other by 90°. The
modulated waves are summed, and the final waveform is a combination of both
phase-shift keying (PSK) and amplitude-shift keying (ASK).
QAM is said to be spectrally efficient, and the reason becomes
clear by comparing a QAM signal with that of an ordinary AM’ed carrier. A
straight amplitude-modulated signal has two sidebands. The carrier plus the
sidebands occupy twice the bandwidth of the modulating signal. In contrast, QAM
places two independent double-sideband suppressed-carrier signals in the same
spectrum as one ordinary double-sideband suppressed-carrier signal.
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QAM can give arbitrarily high spectral
efficiencies by setting a suitable constellation size. As a quick review, a
constellation diagram represents the signal as a scatter diagram in the Q and I
axes and represents the possible symbols as points on the plane. The more
symbols defined in the modulation scheme, the more points on the constellation
diagram. The number of points at which the signal can rest, i.e. the number of
symbols, is indicated in the modulation format description: 16QAM uses a
16-point constellation, and so forth.
Constellation points are normally arranged in
a square grid with equal vertical and horizontal spacing. Use of higher-order
modulation formats, i.e. more points on the constellation, makes it possible to
transmit more bits per symbol. However, use of higher-order symbols positions
constellation points closer together, making the link more susceptible to noise.
Specifically, it takes less noise to move the signal to a different decision
point on the constellation diagram.
A point to note about QAM is that it is
considered a single-carrier system. The two digital bit streams come from one
source that is split into two independent signals.
QAM signals are often sent via multi-carrier modulation
schemes that transmit one QAM signal over one of several subcarriers. The point
of doing this is to simplify the task of compensating for distortions arising
in the communication channel. Each of the subcarriers has a small bandwidth.
The communication channel has a relatively flat frequency response over each of
these small bands. So it is relatively easy to compensate for distortions over
each of the small subcarrier bands.
In OFDM, many closely spaced orthogonal
sub-carriers carry data on several parallel data streams or channels. Each
sub-carrier is modulated with a conventional modulation scheme such as QAM at a
low symbol rate, maintaining total data rates similar to conventional
single-carrier modulation schemes in the same bandwidth.
The primary advantage of
OFDM over using a single carrier is its ability to cope with severe
interference as caused by RF sources at nearly the same frequency or
frequency-selective fading from multipath. OFDM may be viewed as using many
slowly modulated narrowband signals rather than one rapidly modulated wideband
signal. The low symbol rate makes the use of a guard interval between symbols
affordable, making it possible to eliminate inter-symbol interference (ISI) and
use echoes and time-spreading to improve signal-to-noise.
The orthogonality of OFDM comes from the selection of the sub-carrier
frequencies so they are orthogonal to each other. This basically means the
spectrum space between sub-carriers obeys a mathematical relationship where it
is inversely proportional to the symbol duration. Sub-carriers spaced this way
don’t experience any cross-talk and thus eliminate the need for inter-carrier
guard bands, simplifying the design of both the transmitter and the receiver.
There are a few inherent difficulties with OFDM. One is that an OFDM
signal can have a high instantaneous peak compared to its average level. There
can also be a large signal amplitude swing when the signal traverses from a low
to a high instantaneous power. The power amp used must be linear over a wide
bandwidth to prevent a high out-of-band harmonic distortion. This phenomenon
can potentially interfere with adjacent channels.
Other difficulties arise with the time and frequency synchronization
between the OFDM transmitter and receiver. Numerous techniques have been
proposed for estimating and correcting both timing and carrier frequency
offsets at the OFDM receiver. For example, one idea is to embed pilot tones
into OFDM symbols, then use timing and frequency acquisition algorithms to sync
on them.
HYBRID SCHEMES FOR 5G
Several of the modulation schemes under review for 5G are hybrids
employing elements found in QAM and OFDM. One is called F-QAM or FSK-QAM. F-QAM
is a combination of QAM and frequency shift keying (FSK). It has been proposed
in conjunction with OFDMA, the multi-user version of OFDM where individual
users are assigned subsets of subcarriers.
F-QAM combines MF-FSK
(multiple frequency FSK) and MQ-QAM (multiple QAM modulation levels). F-QAM has
many similarities with OFDM-IM (OFDM with index modulation). In both cases the
information is not only conveyed through the modulated symbols but also via the
indices of the active subcarriers. At the receiver side, the detection process
is similar to that of the OFDM-IM. The receiver employs what’s called a
log-likelihood-ratio (LLR) detector to determine the active subcarrier in each
sub-block and, afterwards, estimates the received symbols using a maximum
likelihood (ML) detector.
One drawback of current OFDMA schemes is that they
require accurate synchronization of the user signals at the base station. Such
synchronization is not straightforward and demands a lot of resources. So a lot
of the work on 5G aims at a way around this base station syncing. One idea from
AlcatelLucent Bell Labs is a modified OFDM waveform dubbed universal filtered
multicarrier (UFMC). UFMC passes each bundle of adjacent subcarriers that
belong to a user through a filter to minimize multi-user interference.
Bandwidth efficiency is kept at the same level as OFDM, but UFMC uses no cyclic
prefix (CP). The interval the CP normally occupies instead absorbs the
transient of the underlying filters, making the filtering more effective.
Generalized frequency division multiplexing
(GFDM) is another candidate waveform. GFDM may be thought of as a modified
OFDM, where each subcarrier is shaped by a high-quality filter. To allow the
addition of the CP, the subcarrier filtering operation in GFDM is based on a
circular convolution.
Another 5G contender is based on filter bank multicarrier with offset QAM
(FBMC-OQAM). FBMCs employ two sets of band pass filters called analysis and
synthesis filters, one at the transmitter and the other at the receiver, to
filter the collection of subcarriers being transmitted simultaneously in
parallel frequencies. FBMC filter bandwidth, and therefore selectivity, is a
parameter that can be varied during design. FBMC also offers better bandwidth
efficiency when compared to OFDM. FBMC eliminates the need for CP processing
while efficiently attenuating interferences within and close to the frequency
band. FBMC systems are also comparatively more resistant to narrowband noise.
OTHER IDEAS
Though multi-carrier systems seem to be getting most of the attention for
5G, experts say single-carrier modulation could still be part of the spec.
There is also what might be termed odd-ball techniques still in the mix. One is
called faster than Nyquist (FTN) modulation. It is a non-orthogonal subcarrier
system that actually makes use of intersymbol interference to pack more data into
a communication channel.
Another non-orthogonal idea is called time-frequency packing. The carriers
are close together, and a super-sophisticated detector in the receiver decodes
the closely packed signals. TFS is implemented either with QAM or OQAM.
Finally, a couple of ideas from independent companies have been floated as
5G specs. One is called wave modulation (WAM) which comes from MagnaCom, an
Israeli startup acquired by Broadcom. Here a set of algorithms implement a form
of spectral compression. Details about WAM are sparse, but the spectral
compression is said to enable a higher signaling rate thereby affording the use
of lower-order symbol alphabet, which reduces complexity. It is also said to
give an overall 10% system gain advantage, up to 4x increase in range, a 50%
spectrum savings, improved noise tolerance, and increase in data speed.
Another company called Cohere Technologies patented a modulation
technology called Orthogonal Time Frequency and Space (OTFS). Again, details
about OTFS are sparse, but press releases put out by Cohere speak highly of it.
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Source: microcontrollertips
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