不知道小伙伴們有沒有聽說過汽車的可變懸掛原理？簡單來說，一些高檔汽車為了應(yīng)付不同的路況，可以通過調(diào)整本身系統(tǒng)參數(shù)達(dá)到升高或者降低汽車底盤高度的目的，以獲取更好的通過性或者駕駛體驗(yàn)。

這看似和5G的物理資源與幀結(jié)構(gòu)配置是風(fēng)馬牛不相及，不過小編還是建議閱讀下文之前先在wifi環(huán)境下戳戳看下面的視頻（土豪請隨意），也許對5G的設(shè)計(jì)初衷能有更深入的理解。

LTE中對于OFDM調(diào)制符號(hào)的子載波間隔只有兩種規(guī)定，一種是基于標(biāo)準(zhǔn)循環(huán)前綴（Normal cyclic prefix）模式下的15kHz，另外一種是基于擴(kuò)展循環(huán)前綴（Extended cyclic prefix）模式下的7.5kHz。5G NR中對于OFDM調(diào)制符號(hào)的不同子載波間隔有了新的命名，稱作為參數(shù)集（Numerology），5G NR系統(tǒng)所支持的參數(shù)集如表1所示：

             表1 5G NR系統(tǒng)所支持的傳輸參數(shù)集配置

其中上行參數(shù)以及與載波帶寬對應(yīng)的循環(huán)前綴可由高層參數(shù)UL-BWP-mu和UL-BWP-cp分別進(jìn)行配置，下行參數(shù)以及與載波帶寬對應(yīng)的循環(huán)前綴可由高層參數(shù)DL-BWP-mu和 DL-BWP-cp分別進(jìn)行配置。這些小區(qū)級的公共參數(shù)配置可通過RRC重配信令攜帶下發(fā)。

        BandwidthPart.Config information element

-- ASN1START

-- TAG-BANDWIDTH-PART-START

BandwidthPart-Config ::=         SEQUENCE {

--FFS: Conditions! What to do when certain fields or the entire bandwidth part isomitted? Assume parameters of the carrier instead?

--     Or use the initialBWP derived fromSIB1 or ServingCellConfigCommon? Or make it mandatory to provide at least oneBWP.

--FFS: How can a BandiwdthPart be changed? Only via synchronousReconfiguration oralso without?

--NOTE: The changes in this section are based on RAN1 agreements (not from theofficial L1 parameter list):

--The bandwidth parts for downlink. (see 38.211, 38.213, section 12)

    downlinkBandwidthPartsToReleaseList         SEQUENCE(SIZE (1..maxNrofBandwidthParts)) OFBandiwdthPartId       OPTIONAL,

    downlinkBandwidthPartsToAddModList          SEQUENCE(SIZE (1..maxNrofBandwidthParts)) OF BandwidthPart        OPTIONAL,

--ID of the downlink bandwidth part to be used upon MAC-activation of an  SCell. If not provided, the UE uses thedefault BWP

    firstActiveDownlinkBwp-Id               BandiwdthPartId                                                 OPTIONAL, -- CondSCellOnly

--Corresponds to L1 parameter 'default-DL-BWP'.

--ID of the downlink bandwidth part to be used upon expiry of txxx.

--This field is UE specific. When the field is absent the UE the initial BWP asdefault BWP.

--(see 38.211, 38.213, section 12)

--FFS: May the NW change the default BWP with a regular RRC reconfiguration oronly with Reconfiguration with sync?

--FFS: Whether to add a default uplink BWP

    defaultDownlinkBwp-Id                   BandiwdthPartId                                                 OPTIONAL,

--The bandwidth parts for uplink. In case of TDD uplink- and downlink BWP withthe same bandwidthPartId are considered

--as a BWP pair and must have the same center frequency.

    uplinkBandwidthPartsToReleaseList           SEQUENCE(SIZE (1..maxNrofBandwidthParts)) OF BandiwdthPartId       OPTIONAL,

    uplinkBandwidthPartsToAddModList        SEQUENCE(SIZE (1..maxNrofBandwidthParts)) OF BandwidthPart        OPTIONAL,

--ID of the uplink bandwidth part to be used upon MAC-activation of an  SCell. If not provided, the UE uses the FFS:default BWP

    firstActiveUplinkBwp-Id                     BandiwdthPartId                                                 OPTIONAL,-- Cond SCellOnly

--The duration in ms after which the UE falls back to the default Bandwidth Part.(see 38.321, section FFS_Section)

--The UE starts the timer when it switches its active downlink BWP to a downlinkBWP other than the default downlink BWP.

--The UE restarts the timer to the initial value when it successfully decodes aDCI to schedule PDSCH(s) in its active downlink BWP.

--When the timer expires, the UE switches its active downlink BWP to the defaultdownlink (FFS: and uplink?) BWP.

--FFS: For TDD the UE switches also the paired uplink BWP to the one with thedefaultDownlinkBwp-Id.

--FFS: For FDD the UE switches the uplink BWP?????

--When the network releases the timer configuration, the UE stops the timerwithout swithching to the default (FFS: and uplink?) BWP.

    bandwidthPartInactivityTimer             SetupRelease{ ENUMERATED {

                                               FFS:Value range }}                                          OPTIONAL,  --  Need M

}

BandwidthPart ::=                SEQUENCE {

--An identifier for this bandwidth part.

--Corresponds to L1 parameter 'UL-BWP-index'. (see 38.211, 38.213, section 12)

       bandwidthPartId                  BandiwdthPartId,

--Frequency domain location of this bandwidth part as a distance in number ofPRBs in relation to the reference PRB (PRB 0)

--of the associated carrier. Corresponds to L1 parameter 'DL-BWP-loc'. (see38.211, section FFS_Section).      

       -- Incase of TDD, a BWP-pair (UL BWP and DL BWP with the same bandwidthPartId) musthave the same location (see 38.211, section REF)

--FFS_Value: RAN1 seems to discuss the final range. 

       location                     INTEGER(0.. maxNrofPhysicalResourceBlocksTimes4)                      OPTIONAL,

--Bandwidth of this bandwidth part (see 38.211, section REF)

       bandwidth                    INTEGER(1.. maxNrofPhysicalResourceBlocks)                            OPTIONAL,

--Subcarrier spacing to be used in this BWP. It is applied to at least PDCCH,PDSCH and corresponding DMRS.

--The values provided here are converted into a subcarrier spacing as indicatedin 38.211, Table 4.1-2.

       subcarrierSpacing            ENUMERATED {n0, n1, n2, n3, n4}                                     OPTIONAL,

--Indicates whether to use the extended cyclic prefix for this bandwidth part. Ifnot set, the UE uses the normal cyclic prefix.

--Normal CP is supported for all numerologies and slot formats. Extended CP issupported only for 60 kHz subcarrier spacing.

--(see 38.211, section 4.2.2)

       cyclicPrefix                 ENUMERATED { extended }                                                OPTIONAL,

-- Frequency location of the uplink"direct current" frequency.

       -- Correspondsto L1 parameter 'UL-BWP-DC'. (see 38.211, section FFS_Section)

       directCurrentLocation        INTEGER (0..3299)                                                         OPTIONAL,-- Cond UplinkOnly

}

BandwidthPartId   ::=                   INTEGER(0..maxNrofBandwidthParts-1)

-- TAG-BANDWIDTH-PART-STOP

-- ASN1STOP

圖1 RRC重配消息攜帶的子載波間隔和循環(huán)前綴參數(shù)配置

UE也可以通過讀取MIB獲取SIB1，Msg2/Msg4的子載波間隔，對于主服務(wù)小區(qū)（初始接入小區(qū)），子載波間隔可選配置分別為載波頻率{15kHz，30kHz，60kHz，120kHz}，其中前兩個(gè)參數(shù)配置{15kHz，30kHz}適用于系統(tǒng)載波頻率小于6GHz，后兩個(gè)參數(shù)配置{60kHz，120kHz}適用于系統(tǒng)載波頻率大于6GHz。

對于副載波或者第二小區(qū)組主載波的同步信號(hào)子載波間隔可以通過RRC重配消息攜帶，其中參數(shù)配置{15kHz，30kHz}適用于系統(tǒng)載波頻率小于6GHz，而參數(shù)配置{120kHz，240kHz}則適用于系統(tǒng)載波頻率大于6GHz。

OFDM調(diào)制符號(hào)的頻域子載波間隔設(shè)置與載波頻率是息息相關(guān)的。在固定帶寬下，頻域子載波間隔越小，意味著在子載波間隔內(nèi)并發(fā)傳輸?shù)臄?shù)據(jù)速率越低，對于符號(hào)間干擾(Inter-symbol interference，ISI)的抑制效果越好，而當(dāng)小區(qū)載波頻率越高（>6GHz），帶寬越大時(shí)，可以采取更大子載波間隔以減少FFT器件處理復(fù)雜度。

另外，載波頻率越高，對于一些特定場景，例如室內(nèi)等靜態(tài)/低速移動(dòng)性場所等，由于多徑反射帶來的ISI情況相對較輕，更大的子載波間隔的參數(shù)配置也可以滿足適配這樣場景下的數(shù)據(jù)傳輸。

為了支持多種部署模式下的不同信道寬度，5G NR必須適應(yīng)同一部署下不同的參數(shù)配置（如圖2所示），在統(tǒng)一的框架下提高多路傳輸效率，同時(shí)，5G NR也能跨參數(shù)實(shí)現(xiàn)載波聚合，比如聚合毫米波和6GHz以下頻段的載波，因而5G NR也具有更強(qiáng)的連接性能。

       圖2 5G NR不同子載波間隔參數(shù)配置適配傳輸場景

伴隨著頻域子載波間隔的差異化，采樣頻率也隨之進(jìn)行調(diào)整，相應(yīng)時(shí)域采樣的時(shí)間單位也進(jìn)行了重新定義

其中，，，

定義常量，LTE中的時(shí)域采樣單位，，其中

，。

5G NR中定義的無線幀時(shí)域長度依然為10ms，包含了10個(gè)1ms時(shí)域長度的子幀。每個(gè)10ms無線幀依然可劃分為兩個(gè)5ms半幀。

5G NR根據(jù)實(shí)際的應(yīng)用需求以及場景提出了靈活多變的“微時(shí)隙”的概念，每個(gè)子幀中可以包含多個(gè)時(shí)隙。根據(jù)子載波間隔，定義一個(gè)子幀中的時(shí)隙數(shù)為，而一個(gè)無線幀的時(shí)隙個(gè)數(shù)為，每個(gè)子幀中包含的OFDM符號(hào)為，不同循環(huán)前綴格式下每個(gè)時(shí)隙中所包含的OFDM符號(hào)個(gè)數(shù)

分別由表2和表3進(jìn)行定義：

表2 標(biāo)準(zhǔn)循環(huán)前綴格式下系統(tǒng)幀相關(guān)參數(shù)配置（一個(gè)時(shí)隙包含OFDM符號(hào)數(shù) /一個(gè)無線幀包含的時(shí)隙個(gè)數(shù)/一個(gè)子幀包含時(shí)隙數(shù)）

表3 擴(kuò)展循環(huán)前綴格式下系統(tǒng)幀相關(guān)參數(shù)配置（一個(gè)時(shí)隙包含OFDM符號(hào)數(shù)/一個(gè)無線幀包含的時(shí)隙個(gè)數(shù)/一個(gè)子幀包含時(shí)隙數(shù)）

5G NR中沒有專門針對幀結(jié)構(gòu)按照FDD或者TDD進(jìn)行劃分，而是按照更小的顆粒度OFDM符號(hào)級別進(jìn)行上下行傳輸?shù)膭澐?，一個(gè)時(shí)隙內(nèi)的OFDM符號(hào)類型可以被定義為下行符號(hào)（D），靈活符號(hào)（X）或者上行符號(hào)（U）。在下行傳輸時(shí)隙內(nèi)，UE假定所包含符號(hào)類型只能是D或者X。而在上行傳輸時(shí)隙內(nèi)，UE假定所包含的符號(hào)類型只能是U或者X。

                             表4 時(shí)隙格式類型

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