If a phone supports Band 28 but doesn't support Band 20, Vodafone Ghana may still be detected. This is because the downlink (site to phone) frequencies for the center part of its Band 20 are a part of Band 28.
There's a difference in the frequencies phones transmit around.
On Band 20, phones transmit around 798.5 + 41 = 839.5
On Band 28, phones transmit around 798.5 - 55 = 743.5
Logs:
Cur Time : 62.512sec
drop count = 0, drop time = 0.0sec, high water mark = 0
TIME(Diag) STRING
0.001 init
16:02:16.180 ST=1 LTE_SUSPEND
16:02:16.195 ST=1 3G_CellSelection
16:02:16.398 ST=2 LTE_CellSelection
16:02:16.401 ST=2 LTE_CellSelection
16:02:34.127 ST=2 LTE_CC0_rx mode=3, LTE_BW=4
16:02:34.146 ST=2 LTE_CellSelection
------------------------------------------------------
Cur Time : 62.512sec
Used msg ram = 0.0 %
Elapsed time for making this log = 0.0 sec
------------------------------------------------------
PROTOCOL.
Cur Time : 62.621sec
drop count = 0, drop time = 0.0sec, high water mark = 0
TIME(Diag) STRING
0.001 init
16:02:16.308 [LTE NAS][UE-->NW] DetachRequest [Type: COMBINED_POWER_OFF][Reason: NONE]
16:02:16.308 [LTE NAS] SAEMM_NsDetReq_DeregiLimited
16:02:16.308 [LTE NAS] To send NOT_REGI status in NET_REGI_NTF in case of power-off detach
16:02:34.115 [LTE RRC] MasterInformationBlock
16:02:34.138 [LTE RRC] SystemInformationBlockType1
16:02:34.235 [LTE RRC] MasterInformationBlock
16:02:34.258 [LTE RRC] SystemInformationBlockType1
16:02:34.258 [LTE RRC] Pmax = 23
16:02:34.258 [LTE RRC][F] Add RRC barring List Earfcn(9615) Tbarred(300000) FreqBarr(1)
------------------------------------------------------
Cur Time : 62.621sec
Used msg ram = 0.0 %
Elapsed time for making this log = 0.0 sec
------------------------------------------------------
The Band 28 phone selected the frequency but did not attempt to connect. The channel number is now barred by it, so the phone doesn't select it again. Center frequencies overlap, but the radio frequency channel numbers are not the same. It's 6225 on Band 20, 9615 on Band 28.
Here's the SystemInformationBlockType1:
message: c1 (0)
c1: systemInformationBlockType1 (1)
cellAccessRelatedInfo
plmn-IdentityList: 1 item
Item 0
PLMN-IdentityInfo
plmn-Identity
mcc: 3 items
Item 0
MCC-MNC-Digit: 6
Item 1
MCC-MNC-Digit: 2
Item 2
MCC-MNC-Digit: 0
mnc: 2 items
Item 0
MCC-MNC-Digit: 0
Item 1
MCC-MNC-Digit: 2
cellReservedForOperatorUse: notReserved (1)
trackingAreaCode: 9c54 [bit length 16, 1001 1100 0101 0100 decimal value 40020]
cellIdentity: 0023e020 [bit length 28, 4 LSB pad bits, 0000 0000 0010 0011 1110 0000 0010 .... decimal value 146946]
cellBarred: notBarred (1)
intraFreqReselection: allowed (0)
.... ..0. csg-Indication: False
cellSelectionInfo
q-RxLevMin: -128dBm (-64)
freqBandIndicator: 20
schedulingInfoList: 2 items
Item 0
SchedulingInfo
si-Periodicity: rf16 (1)
sib-MappingInfo: 1 item
Item 0
SIB-Type: sibType3 (0)
Item 1
SchedulingInfo
si-Periodicity: rf64 (3)
sib-MappingInfo: 2 items
Item 0
SIB-Type: sibType6 (3)
Item 1
SIB-Type: sibType7 (4)
si-WindowLength: ms40 (6)
systemInfoValueTag: 20
nonCriticalExtension
nonCriticalExtension
cellSelectionInfo-v920
q-QualMin-r9: -18 dB
How it knows not to connect by transmitting on the wrong frequency is likely thanks to the freqBandIndicator.
Having both 20 and 28 indicated does not make sense, because...
1. the uplink (phone to site) frequency is still different, and authorization is required before a frequency can be used for a network.
2. some of the downlink frequencies are out of the bounds of Band 28.
If there are two bands where the only difference is the minimum and/or maximum frequency, and the frequencies fully qualify for both bands, Multi Freq Band Indicator (MFBI) can then be used, so that phones that support either band can connect.
Phones must have Band 20 to use its 4G, even if it appears while manually searching for networks in phones that can only use Band 28.
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