Hello.
I’m wondering why ARFCN of LTE since we have it already why we not using it in 5g nr sa?
why we need to use new technique called GSCN?
I asked myself 'why we need this kind of additional frequency specification like this when we already have the existing frequency spec(raster) called ARFCN ?
I tried to answer myself about scanning that would take too long time but didn’t understand why it would take much and long time if we complete to use ARFCN in 5g nr sa and not using GSCN?
Appreciated for any detailed explanation about GSCN and why it’s required in 5g nr sa and what would happen if we just continue with ARFCN technique in 5g nr sa that we already have in lte ?
When I first saw the frequency specification called GSCN(Global Synchronization Raster Channel). I asked myself ‘why we need this kind of additional frequency specification like this when we already have the existing frequency spec(raster) called ARFCN ?’.
I still don’t know of the offical answers in 3GPP TS or TDocs (I failed to find the answers in the specification). So I just came up with my personal guess as follows :
When NR is activated in NSA, UE does not need to do blind search for SSB since the frequency, subcarrier spacing etc are all configured by LTE RRC Connection Reconfiguration message.
But in SA, UE need to blindly detect SSB since UE need to detect this before it gets any RRC (MIB, SIB). If UE needs to search SSB based on ARFCN raster, it would take too long time since ARFCN raster is very narrow. So it would be good to define a SSB searching frequency in wider steps. This is the usage/purpose of GSCN.
And below is my note from a training (more than a year ago)
If UE needs to search SSB based on ARFCN raster, it takes king time since ARFCN is very narrow. So, it would be better to define a SSB searching frequencies in wider steps. This is called GSCN (Global Synchronization Raster Channel)
thanks for your reply , I almost understand you but still confused.
Why it would take more time if we work in 5g nr in ARFCN? you said because it’s narrow …what do you mean by narrow and how is that related to time of search? …appreciated if u can conceptually
example that by two examples , one that have ARFCN frequency so u claim it takes much time …
other example about GSCN when using GSCN frequency will take lesser time.
The difference in step size
For arfcn, the step is 5 kHz, 15kHz or 60 kHz for low band, mid band and high band respectively.
For GSCN, the corresponding step is 1.2 MHz, 1.44 MHz or 17.28 MHz
Think about maximum LTE BW is limited with 20 MHZ where NR could have 400 MHZ in FR2 range (mmwave) It is time demanding to find the sync signals in 5G compare to 4G for UE to latch on
LTE frequency raster is 100 KHZ where NR raster is wider based on O.B’s comment and below explanation from Sharetechnote
Some detail information here as well…
Please also see attached to clear more;
As you see in 38.104 Table 5.4.2.1-1, ARFCN step is 5Khz or 15kHz or 60Khz depending on frequency range. If UE has to scan the spectrum in this granularity for cell search, it will take very long time to find a cell. In case of GSCN, as you see in the following table, the granularity is 50 or 150 or 250 Khz under 3Ghz and it has the granularity of 1.44 Mhz in above 3 Ghz frequency range and below 24.25Ghz, and the granuilirity jumps to 17.28 Mhz if the frequency goes over 24.25 Ghz.
I still don’t know why in 5G NR using GSCN instead of ARFCN although power consumption in GSCN is higher than ARFCN.
Any idea/reason for the tradeoff why in 5G NR use GSCN intead of ARFCN?
And why we need this kind of frequency specification called GSCN when we already have the existing frequency spec called ARFCN?
Blockquote
The difference in step size
For arfcn, the step is 5 kHz, 15kHz or 60 kHz for low band, mid band and high band respectively.
For GSCN, the corresponding step is 1.2 MHz, 1.44 MHz or 17.28 MHz
Blockquote
Example band n78 spans from 3300-3800 (I.e. 500 MHz)
If UE scans on arfcn, there are 500000/15 options while scanning on GSCN there are 500/1.44 options
In 5G NR NSA, UE will never do a blind search for SSB at the time of UE initial access as UE gets the freq, Sub carrier spacing & SSB location configured by LTE in RRC connection Reconfiguration message.
Whereas
In Standalone Mode Blind search of SSB is required at the time of UE initial access as UE does not have any RRC message (MIB, SIB) decoded.
In blind search we have the channel raster so UE knows where to search for SSB.