NOTE : This note is about a tempary 5G specification that was implemented and tried before 5G specification is finalized. I keep this note for study purpose.

To me, 5G is a new interesting topics that I've been chasing for about 3 years now (as of Dec 2016), but 3GPP is still at too early stage as opposed to the expectations of industry (at least some major players in the industry). There might be some reasonable reasons with seeminly slow progress in 3GPP, but the demand in the industry (at least some part of the industry) got too high to wait until 3GPP is fully ready with the specification.

As a result, some major players in industry decided to deploy a small scale 5G system with their own specification that some people call 'Pre Trial'. As of now, there are two pretrial specifiction out there, one from Verizon (a US Carrier) and the other one from KT (a South Korea Carrier). You can get these specification in the following link :

• Verizon 5G TF : Verizon 5G Technical Forum
• KT 5G SIG : KT 5G SIG (Special Interest Group)

I haven't compared these two specification line by line , word by word.. but at least to me they seem to be almost same (at least at physical layer). This is good since I study only one of them and can understand both of them :)

The main purpose of this section ('PreTrial') is to give you brief introductions on these specification. I will start with physical layer and keep adding things towards higher layers later.

I cannot guarantee that all of these specification will be adopted by 3GPP official specification, but my guess that large portions of these specification will be absorbed by 3GPP specification (at least, early eMBB specification). So I would recommend you to look into these specifications and it will definitely help you understand the 3GPP specification when it is out.

• Summary of Key Parameters
• Comparison with 3GPP NR

Summary of Key Parameters

The three tables below describe the physical layer that Verizon and KT used in their 28 GHz trials. It helps to read them against LTE, because the PreTrial specification is built as a modified LTE. The channel names show this directly. xPDSCH, xPDCCH and ePBCH are LTE channels with a prefix, and the numerology keeps the LTE grid of 14 symbols and 12 subcarriers. Even the document number follows LTE, because the physical channel specification is numbered 211, like 3GPP TS 36.211.

Followings are some of the key parameters specified in Pre Trial specification (Based 211 in both Verizon and KT specification)

< Overall Specification >

Two rows of the table above need more explanation. The first is the ESS, the Extended Synchronization Signal. At 28 GHz, the base station sweeps its beams over the OFDM symbols of a synchronization subframe, and PSS and SSS repeat in each symbol. So PSS and SSS alone cannot tell the UE which symbol it received. The ESS carries the OFDM symbol index, and the UE uses it to find the subframe boundary.

The second is the group of beam reference signals. BRS lets the UE measure every transmit beam and report the best ones, and BRRS lets the network refine a beam after that. The phase noise reference signals appear in both directions for a different reason. Phase noise at 28 GHz rotates the constellation from one symbol to the next, and the receiver tracks that rotation with these signals.

< Numerology >

Every row of the numerology table follows from one decision: multiply the LTE subcarrier spacing by five. 75 kHz is five times 15 kHz, so each OFDM symbol is five times shorter, and a subframe of 14 symbols lasts 0.2 ms instead of 1 ms. The radio frame stays at 10 ms, so it now holds 50 subframes. The sampling rate grows by the same factor, from 30.72 Msps to 153.6 Msps, which is a 2048-point FFT at 75 kHz.

The reason for the wider spacing is phase noise. Oscillator phase noise grows with the carrier frequency, and a wider spacing makes each subcarrier less sensitive to it. The bandwidth numbers also scale in a simple way. 100 RB of 12 subcarriers occupy 90 MHz, which fits the 100 MHz carrier, just as 100 LTE RB occupy 18 MHz of a 20 MHz carrier.

< Squence Template >

The sequence table shows the same reuse of LTE design. Zadoff-Chu sequences have constant amplitude and good correlation properties. So they serve the signals that the receiver must detect without prior knowledge: PSS, the ESS, the SRS and the PRACH preamble. Gold sequences serve scrambling and most reference signals, because a cell specific or UE specific initial value gives each user of the sequence a different pattern. The ESS appears in both groups, because it is a Zadoff-Chu sequence that is then scrambled.

• PreTrial is LTE scaled by five : 75 kHz spacing, 0.2 ms subframes, 50 subframes per frame and 153.6 Msps.
• Beams need new signals : the ESS gives the symbol index inside a beam sweep, and BRS and BRRS measure and refine the beams.
• Phase noise needs its own reference signal : a phase noise reference signal exists in both the downlink and the uplink.
• The sequence families are the LTE ones : Zadoff-Chu for detection, and Gold for scrambling and reference signals.

Comparison with 3GPP NR

The introduction of this page ends with a guess made in December 2016: that 3GPP would absorb large portions of the PreTrial specification. NR Release 15 has been complete for years, so we can now check that guess. The table below puts each PreTrial parameter next to its closest NR counterpart in FR2, the frequency range that contains 28 GHz.

The comparison shows that the guess was right about the concepts and wrong about the values. The concepts moved into NR. NR sweeps beams across SSBs, measures them with SSB and CSI-RS, tracks phase noise with PT-RS, and scales the LTE numerology for high frequencies. But the values did not. NR allows only subcarrier spacings of 15 kHz times a power of two, so 75 kHz has no place in it. With power-of-two spacings, the slots of every numerology fit exactly inside one 1 ms subframe, and different numerologies can share a carrier.

The synchronization design changed as well. In NR, the UE learns the SSB index from the PBCH DMRS and the PBCH payload, so NR needs no separate ESS. As a result, a PreTrial device cannot work on an NR network. Verizon later moved its 28 GHz service to 3GPP NR.

• The concepts moved into NR : beam sweeping, beam reference signals and a phase noise reference signal all have NR counterparts.
• The values did not : 75 kHz is not an NR subcarrier spacing, because NR spacings are 15 kHz times a power of two.
• NR scales further : 400 MHz carriers, 16 carriers and 256QAM replace the PreTrial limits.
• PreTrial devices are not NR devices : the numerology and the signals differ, so the two cannot interoperate.