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5G新在哪儿(5)-灵活多变的物理资源与帧结构配置

不知道小伙伴们有没有听说过汽车的可变悬挂原理?简单来说,一些高档汽车为了应付不同的路况,可以通过调整本身系统参数达到升高或者降低汽车底盘高度的目的,以获取更好的通过性或者驾驶体验。


这看似和5G的物理资源与帧结构配置是风马牛不相及,不过小编还是建议阅读下文之前先在wifi环境下戳戳看下面的视频(土豪请随意),也许对5G的设计初衷能有更深入的理解。



LTE中对于OFDM调制符号的子载波间隔只有两种规定,一种是基于标准循环前缀(Normal cyclic prefix)模式下的15kHz,另外一种是基于扩展循环前缀(Extended cyclic prefix)模式下的7.5kHz。5G NR中对于OFDM调制符号的不同子载波间隔有了新的命名,称作为参数集(Numerology),5G NR系统所支持的参数集如表1所示:

             表1 5G NR系统所支持的传输参数集配置



Cyclic  prefix

0

15

Normal

1

30

Normal

2

60

Normal, Extended

3

120

Normal

4

240

Normal


其中上行参数以及与载波带宽对应的循环前缀可由高层参数UL-BWP-mu和UL-BWP-cp分别进行配置,下行参数以及与载波带宽对应的循环前缀可由高层参数DL-BWP-mu和 DL-BWP-cp分别进行配置。这些小区级的公共参数配置可通过RRC重配信令携带下发。

        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

图1 RRC重配消息携带的子载波间隔和循环前缀参数配置


UE也可以通过读取MIB获取SIB1,Msg2/Msg4的子载波间隔,对于主服务小区(初始接入小区),子载波间隔可选配置分别为载波频率{15kHz,30kHz,60kHz,120kHz},其中前两个参数配置{15kHz,30kHz}适用于系统载波频率小于6GHz,后两个参数配置{60kHz,120kHz}适用于系统载波频率大于6GHz。


对于副载波或者第二小区组主载波的同步信号子载波间隔可以通过RRC重配消息携带,其中参数配置{15kHz,30kHz}适用于系统载波频率小于6GHz,而参数配置{120kHz,240kHz}则适用于系统载波频率大于6GHz。


OFDM调制符号的频域子载波间隔设置与载波频率是息息相关的。在固定带宽下,频域子载波间隔越小,意味着在子载波间隔内并发传输的数据速率越低,对于符号间干扰(Inter-symbol interference,ISI)的抑制效果越好,而当小区载波频率越高(>6GHz),带宽越大时,可以采取更大子载波间隔以减少FFT器件处理复杂度。

另外,载波频率越高,对于一些特定场景,例如室内等静态/低速移动性场所等,由于多径反射带来的ISI情况相对较轻,更大的子载波间隔的参数配置也可以满足适配这样场景下的数据传输。


为了支持多种部署模式下的不同信道宽度,5G NR必须适应同一部署下不同的参数配置(如图2所示),在统一的框架下提高多路传输效率,同时,5G NR也能跨参数实现载波聚合,比如聚合毫米波和6GHz以下频段的载波,因而5G NR也具有更强的连接性能。


       图2 5G NR不同子载波间隔参数配置适配传输场景


伴随着频域子载波间隔的差异化,采样频率也随之进行调整,相应时域采样的时间单位也进行了重新定义

                                             

其中,,,

定义常量,LTE中的时域采样单位,,其中

,。


5G NR中定义的无线帧时域长度依然为10ms,包含了10个1ms时域长度的子帧。每个10ms无线帧依然可划分为两个5ms半帧。


5G NR根据实际的应用需求以及场景提出了灵活多变的“微时隙”的概念,每个子帧中可以包含多个时隙。根据子载波间隔,定义一个子帧中的时隙数为,而一个无线帧的时隙个数为,每个子帧中包含的OFDM符号为,不同循环前缀格式下每个时隙中所包含的OFDM符号个数

分别由表2和表3进行定义:


表2 标准循环前缀格式下系统帧相关参数配置(一个时隙包含OFDM符号数 /一个无线帧包含的时隙个数/一个子帧包含时隙数)





0

14

10

1

1

14

20

2

2

14

40

4

3

14

80

8

4

14

160

16

5

14

320

32


表3 扩展循环前缀格式下系统帧相关参数配置(一个时隙包含OFDM符号数/一个无线帧包含的时隙个数/一个子帧包含时隙数)





2

12

40

4


5G NR中没有专门针对帧结构按照FDD或者TDD进行划分,而是按照更小的颗粒度OFDM符号级别进行上下行传输的划分,一个时隙内的OFDM符号类型可以被定义为下行符号(D),灵活符号(X)或者上行符号(U)。在下行传输时隙内,UE假定所包含符号类型只能是D或者X。而在上行传输时隙内,UE假定所包含的符号类型只能是U或者X。

                             表4 时隙格式类型

<td width="38" valign="top" style="margin: 0px; padding: 0px 7px; word-break: break-all; border-top-style: none; border-left-style: none; border-rig

Format

                   Symbol  number in a slot

0

1

2

3

4

5

6

7

8

9

10

11

12

13

0

D

D

D

D

D

D

D

D

D

D

D

D

D

D

1

U

U

U

U

U

U

U

U

U

U

U

U

U

U

2

X

X

X

X

X

X

X

X

X

X

X

X

X

X

3

D

D

D

D

D

D

D

D

D

D

D

D

D

X

4

D

D

D

D

D

D

D

D

D

D


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