1. Modulation

Modulation is the process of varying the properties of a carrier signal such as frequency, phase and amplitude with a modulating signal that contains the actual information to be transmitted. A modulator does the modulation of a signal and a demodulator does exactly the reverse of modulation. The device which does both modulation and demodulation is called as the MODEM.

The most fundamental digital modulation techniques are Phase-shift keying (PSK), Frequency-shift keying (FSK), Amplitude-shift keying (ASK) and Quadrature Amplitude Modulation (QAM). Lets discuss only the QAM which is widely used in radio communication especially in LTE. Quadrature amplitude modulation is used in radio communication as it is able to carry higher data rates when compared to other modulation techniques. The most common forms of QAM are 16-QAM, 64-QAM and 256-QAM. By using higher order modulation formats, it is possible to transmit more bits per symbol. The advantage of moving to the higher order formats is that it is possible to transmit more bits per symbol. However the disadvantage is that it is more susceptible to noise. As a result, higher versions of QAM are used only when there is a sufficiently high signal to noise ratio(S/N).

LTE Modulation Techniques

1.1 Orthogonal Frequency Division Multiplexing (OFDM)

LTE is based on Orthogonal Frequency Division Multiple Access (OFDM), and achieves high data rates by combining large bandwidths, higher order modulation and spatial multiplexing. There are multi path fading problems in UMTS so LTE uses OFDM in the downlink to overcome such problems.

 

Orthogonal frequency-division multiplexing (OFDM) is a method for encoding digital data transmission which uses a large number of closely spaced carriers that are modulated with low rate data stream. By making the signal orthogonal to each other, the signals would not interfere with other signals and thus mutual interference is avoided. By carrying the data at a lower rate across all carriers, the effects of reflections and inter-symbol interference are also overcome. If some of the carriers are lost due to multi-path effects, then the data can be reconstructed by using error correction techniques.

The OFDM signal used in LTE comprises a maximum of 2048 different sub-carriers having a spacing of 15 kHz. Although it is mandatory for the mobiles to have capability to be able to receive all 2048 sub-carriers, not all need to be transmitted by the base station which only needs to be able to support the transmission of 72 sub-carriers. In this way all mobiles will be able to talk to any base station.

 

Within the OFDM signal it is possible to choose between four types of modulation for the LTE signal:

  1. QPSK 

  2. 16QAM  

  3. 64QAM 

  4. 256QAM

 

The exact LTE modulation format is chosen depending upon the actual conditions. When there is a sufficient signal to noise ratio the higher order modulation format can be used and more data can be transferred. The lower forms of modulation, do not require a large signal to noise ratio but are also not able to send the data as fast as possible.

Features of OFDM:

  • The sub-carriers are orthogonal to each other.

  • Multiple sub-carriers carry the information stream

  • A guard interval is added to each symbol to minimize the channel delay spread and intersymbol interference (ISI).

 

Advantages of OFDM:

  • High spectral efficiency 

  • Robust against narrow-band co-channel interference. Hence not all the data is lost.

  • Resilient to intersymbol interference (ISI) and fading caused by multipath propagation

  • Low sensitivity to time synchronization errors

 

Disadvantages of OFDM:

  • Sensitive to carrier frequency offset and drift

  • High peak-to-average ratio which affects the efficiency of RF amplifier as they cannot operate at high efficiency level

In the downlink, the subcarriers are split into resource blocks. This enables the system to be able to compartmentalise the data across standard numbers of subcarriers. Resource blocks comprise 12 subcarriers, regardless of the overall LTE signal bandwidth. They also cover one slot in the time frame. This means that different LTE signal bandwidths will have different numbers of resource blocks.

 

1.2 Single Carrier Frequency Division Multiple Access (SC-FDMA)

In the uplink, LTE uses a different access technique concept called as Single Carrier Frequency Division Multiple Access (SC-FDMA). SC-FDMA is a pre-coded form of OFDM technology which is used to compensate for high Peak to Average Power Ratio. This combines the low peak to average ratio offered by single-carrier systems with the multipath interference resilience and flexible subcarrier frequency allocation that OFDM provides.

One of the key parameters that affects all mobiles is the battery life. Even though battery performance is improving from time to time, it is still necessary to make sure that the mobiles use little battery power. Therefore it is necessary that the RF amplifiers operate in as efficient mode as possible. However this is not a problem for the base stations but only for the mobiles.

 

In OFDM there is high Peak to Average Power Ratio so we will require expensive and efficient RF power amplifiers that transmits the radio frequency signal via the antenna to the base station being the highest power item within the mobile. This increases the cost of the mobile terminal and drains the battery faster. As a result it is necessary to employ a transmission mode that has as almost a constant power level when operating. SC-FDMA solves this problem by grouping together the resource blocks in such a way that reduces the need for linearity, and also the power consumption, in the RF power amplifier.

2. 256QAM in Downlink

One of the LTE features introduced in Rel-12 is the support for higher order modulation, that is 256QAM for DL data transmission to enhance the spectral efficiency and the system throughput. UE categories 11-15 have been introduced to accommodate support for 256QAM with increased peak data rates. 256QAM means sending more (8) bits with each transmission. In terms of peak data rates, the largest transport block size with 256QAM is about 30% larger than that of 64QAM.

But there are some challenges associated with 256QAM deployments.

  • Packing that many bits per symbol means that only a small percentage of devices with relatively high Signal to Interference + Noise Ratio (SINR) will be able to successfully process 256QAM.

  • As the device moves away from the cell center, SINR decreases and it becomes increasingly more difficult for devices to extract the usable signal and demodulate 256 QAM, therefore the less efficient modulation scheme is used. 

To support 256QAM, a couple of new IEs are added to "UE Capability Information" message as shown below. In order to activate 26QAM in DL both the UE and network has to support 256QAM or else 256QAM cannot be used in downlink. If the UE supports 256QAM the network will send “RRCConnectionReconfiguration” message which will contain element “altCQI-Table-r12” IE. This means that 256QAM in downlink is activated and the UE should switch to it.

When 256QAM in DL is  activated for PCell, the "RRC Connection Reconfiguration" message shall contain the IE "cqi-ReportConfigPCell-v1250"(See below image).

When 256QAM in DL is  activated for SCell, the "RRC Connection Reconfiguration" message shall contain the IE "cqi-ReportConfigSCell-v1250" along with the SCell index to which it has to be applied(See below image),

The difference between the frames before and after applying 256QAM in DL is the modulation technique. Before the network activates 256QAM in DL, the UE uses one of the modulation out of QPSK, 16QAM and 64QAM. After the network activates 256QAM via RRC Connection Reconfiguration, the UE switches to 256QAM as it is the best one to be used that will increase the performance. Higher order modulation schemes can typically only be used when the RF conditions are ideal. Even after activating 256QAM in DL the UE might switch to QPSK, 16QAM or 64QAM:

  • When the PCell or SCell cell conditions are poor

  • Data is more susceptible to noise and has interference issues 

If we look at the frames that are downloaded via the PCell/SCell before and after 256QAM activation, we can see some difference in the modulation technique that is used under good radio conditions.

3. 64QAM in Uplink

Uplink 64 QAM provides the most benefit in small cell environments. When the conditions are right, modulation schemes of higher order make it possible to send much more data with the same amount of frequency resource (spectrum).

64QAM uplink in LTE was introduced in 3GPP release 13 as a new modulation technique. It boosts the uplink data throughput by transmitting 6 bits per transmitted LTE uplink symbol instead of the current 4 bits per LTE uplink symbol with 16QAM. This increases the uplink throughput by 50 percent.

In LTE if the SIB2 contains IE "pusch-ConfigCommon-v1270" then it will tell us that if 64QAM in UL is supported by the network.

 

If we look at the frames that are uploaded via the PCell/SCell before and after the 256QAM activation we can see the difference in the modulation technique that is used under good radio conditions.