Updates from the 3GPP RAN 5G Workshop - Part 2

I have finally got round to having a look at some more presentations on 5G from the recently concluded 3GPP RAN 5G Workshop. Part 1 of the series is here.

Panasonic introduced this concept of Sub-RAT's and Cradle-RAT's. I think it should be obvious from the picture above what they mean but you can refer to their presentation here for more details.

Ericsson has provided a very detailed presentation (but I assume a lot of slides are backup slides, only for reference). They have introduced what they call as "NX" (No compatibility constraints). This is in line to what other vendors have referred to as well that above 6GHz, for efficiency, new frame structures and waveforms would serve best. Their slides are here.



Nokia's proposal is that in the phase 1 of 5G, the 5G Access point (or 5G NodeB) would connect to the 4G Evolved Packet Core (EPC). In phase 2, both the LTE and the 5G (e)NodeB's would connect to the 5G core. Their presentation is available here.

Before we move on to the next one, I should mention that I am aware of some research that is underway, mostly by universities where they are exploring an architecture without a centralised core. The core network functionality would be distributed and some of the important data would be cached on the edge. There will be challenges to solve regarding handovers and roaming; also privacy and security issues in the latter case.

I quite like the presentation by GM research about 5G in connected cars. They make a very valid point that "Smartphones and Vehicles are similar but not the same. The presentation is embedded below.

Qualcomm presented a very technical presentation as always, highlighting that they are thinking about various future scenarios. The picture above, about phasing is in a way similar to the Ericsson picture. It also highlights what we saw in part 1, that mmW will arrive after WRC-19, in R16. Full presentation here.

The final presentation we are looking is by Mitsubishi. Their focus is on Massive MIMO which may become a necessity at higher frequencies. As the frequency goes higher, the coverage goes down. To increase the coverage area, beamforming can be used. The more the antennas, the more focused the beam could be. They have also proposed the use of SC-FDMA in DL. Their presentation is here and also embedded below.

Enhanced Uplink SC-FDMA: PUSCH+PUCCH Simultaneous Transmission

OFDM and SC-FDMA

OFDM has been around since the mid 1960s and is now used in a number of non-cellular wireless systems such as Digital Video Broadcast (DVB), Digital Audio Broadcast (DAB), Asymmetric Digital Subscriber Line (ADSL) and some of the 802.11 family of Wi-Fi standards. OFDM’s adoption into mobile wireless has been delayed for two main reasons. The first is the sheer processing power which is required to perform the necessary FFT operations. However, the continuing advance of signal processing technology means that this is no longer a reason to avoid OFDM, and it now forms the basis of the LTE downlink. The other reason OFDM has been avoided in mobile systems is the very high peak to average ratio (PAR) signals it creates due to the parallel transmission of many hundreds of closely-spaced subcarriers. For mobile devices this high PAR is problematic for both power amplifier design and battery consumption, and it is this concern which led 3GPP to develop the new SC-FDMA transmission scheme.

The LTE downlink transmission scheme is based on OFDM. OFDM is an attractive downlink transmission scheme for several reasons. Due to the relatively long OFDM symbol time in combination with a cyclic prefix, OFDM provides a high degree of robustness against channel frequency selectivity. Although signal corruption due to a frequency-selective channel can, in principle, be handled by equalization at the receiver side, the complexity of the equalization starts to become unattractively high for implementation in a mobile terminal at bandwidths above 5 MHz. Therefore, OFDM with its inherent robustness to frequency-selective fading is attractive for the downlink, especially when combined with spatial multiplexing.

Additional benefits with OFDM include:
• OFDM provides access to the frequency domain, thereby enabling an additional degree of freedom to the channel-dependent scheduler compared to HSPA.
• Flexible bandwidth allocations are easily supported by OFDM, at least from a baseband perspective, by varying the number of OFDM subcarriers used for transmission. Note, however, that support of multiple spectrum allocations also require flexible RF filtering, an operation to which the exact transmission scheme is irrelevant. Nevertheless, maintaining the same baseband-processing structure, regardless of the bandwidth, eases the terminal implementation.
• Broadcast/multicast transmission, where the same information is transmitted from multiple base stations, is straightforward with OFDM.

For the LTE uplink, single-carrier transmission based on DFT-spread OFDM (DFTS-OFDM) is used. The use of single-carrier modulation in the uplink is motivated by the lower peak-to-average ratio of the transmitted signal compared to multi-carrier transmission such as OFDM. The smaller the peak-to-average ratio of the transmitted signal, the higher the average transmission power can be for a given power amplifier. Single-carrier transmission therefore allows for more efficient usage of the power amplifier, which translates into an increased coverage. This is especially important for the power-limited terminal. At the same time, the equalization required to handle corruption of the single-carrier signal due to frequency-selective fading is less of an issue in the uplink due to fewer restrictions in signal-processing resources at the base station compared to the mobile terminal.

In contrast to the non-orthogonal WCDMA/HSPA uplink, which also is based on single-carrier transmission, the uplink in LTE is based on orthogonal separation of users in time and frequency. Orthogonal user separation is in many cases beneficial as it avoids intra-cell interference. However allocating a very large instantaneous bandwidth resource to a single user is not an efficient strategy in situations where the data rate mainly is limited by the transmission power rather than the bandwidth. In such situations, a terminal is typically allocated only a part of the total transmission bandwidth and other terminals can transmit in parallel on the remaining part of the spectrum. Thus, as the LTE uplink contains a frequency-domain multiple-access component, the LTE uplink transmission scheme is sometimes also referred to as Single-Carrier FDMA (SC-FDMA).

Via: 'Agilent Whitepaper' and '3G evolution'.

MIMO schemes in LTE

SU-MIMO (Single User MIMO)

•This is an example of downlink 2x2 single user MIMO with precoding.

•Two data streams are mixed (precoded) to best match the channel conditions.

•The receiver reconstructs the original streams resulting in increased single-user data rates and corresponding increase in cell capacity.

•2x2 SU-MIMO is mandatory for the downlink and optional for the uplink

MU-MIMO (Multiuser MIMO)

•Example of uplink 2x2 MU-MIMO.

•In multiple user MIMO the data streams come from different UE.

•There is no possibility to do precoding since the UE are not connected but the wider TX antenna spacing gives better de-correlation in the channel.

•Cell capacity increases but not the single user data rate.

•The key advantage of MU-MIMO over SU-MIMO is that the cell capacity increase can be had without the increased cost and battery drain of two UE transmitters.

•MU-MIMO is more complicated to schedule than SU-MIMO

Everything you want to know on Single Carrier FDMA

While working on our LTE training, I came across this very interesting website that contains probably everything you want to know on SC-FDMA. Bookmark it if this is an area of interest.

Single Carrier FDMA Discussion Forum