TD-SCDMA Physical Layer Design Overview

Transcription

1 TD-SCDMA Layer Design Overview Author Information Coege: Information Security Engineering Coege Cass: F Name: SHI Junxiao Instructor: WU Yue Instructor's coege: Information Security Engineering Coege ABSTRACT TD-SCDMA is the first 3G mobie network aunch in China. This paper takes a ook at the specification of TD-SCDMA, and focuses at the physica ayer design. KEYWORDS TD-SCDMA, 3G, mobie, physica ayer 1 Overview 1.1 Current Status Time Division-Synchronous Code Division Mutipe Access, or TD-SCDMA, is a 3G mobie teecommunication standard, being persu in the Peope's Repubic of China, in an attempt not to be "dependant on Western technoogy". "Commercia trias" across eight cities was aunch on Apri 1, 2008 and wi eventuay incude 60,000 users. As I read in cnbeta.com, the resuts were unsatisfactory. Many users report that the signa is not as good as the 2G networks, GSM or CDMA. There are ony a sma number of TD-SCDMA users, so users don't know who they can make Video-Ca to. More Internet cards were sod than ce phones. 1.2 Vaue of my Topic TD-SCDMA standard has been adopt by 3GPP since Re-4, and offer as air interfaces for the UMTS-TDD system. It's interesting and usefu to know something deep of this

2 booming 3G network. 1.3 Purpose and Scope I am writing this paper to share my findings when I'm reading a part of TD-SCDMA Technica Specification. I wi focus on the overa architecture and the physica ayer, ony a itte of other parts wi be mention. I wi try to find out why they design the physica ayer ike that. 2 Radio Interface Protoco Architecture TD-SCDMA is assum to use an architecture of User Equipment (UE), RAN, and Core Network. Radio interface acts as the Access Stratum of TD-SCDMA. The radio interface is divid into three protoco ayers: L3, network ayer (RRC) L2, data ink ayer RLC, Radio Link Contro subayer MAC, Mium Access Contro subayer L1, physica ayer

3 It's obvious that these three ayers are taken from the famous OSI 7-ayer mode. Define the protoco in each ayer, define the interface on each SAP, then a change in any ayer won't affect other ayers. The future of communication is A-IP (everything wi be pack in IPv6 packets). As a 3G standard, using the OSI 7-ayer mode is appropriate. Radio interface protoco incudes ony the owest three ayers of the OSI mode, because this standard defines ony the access stratum. It's enough to use ony those three ayers to provide a QoS-aware end-to-end data transfering and necessary controing & charging functions. 2.1 Layer The physica ayer offers information transfer services to MAC ayer. L1 shoud know how to transfer those data over the radio interface. It provides two types of transport channes:

4 common transport channes: there is a ne for in-band identification of the UEs, by using addresses dicat transport channes: UEs are identifi by the physica channe Major functions: error detection, FEC encoding/decoding rate matching mapping transport channes to physica channes (code, time sot, frequency) power contro synchronization contro beamforming for upink & downink user positioning 3. More about the physica ayer wi be discuss in chapter 2.2 MAC Layer The MAC subayer provides: unacknowg data transfer: send SDUs to another MAC entity. But no ACK is provid, so there is no guaranteeing. This service does not provide any data segmentation, either. reaocation of radio resources and MAC parameters reporting of measurements: This can be us for charging. Major MAC functions: mapping ogica channes to transport channes seection of "Transport Format" or "Transport Format Set" depending on instantaneous source rate. This is one of the technica highights in TD-SCDMA: supporting dynamic bit-rate, and even different downink / upink rates. priority handing between data fows of one UE, or between UEs identification of UEs on common transport channes This MAC subayer is simiar to the one in TCP/IP. Unike the wir computer network, its bit-rate is changing instantaneousy, and it shoud use precious transport resources efficienty. 2.3 RLC Layer

5 The RLC subayer provides: RLC connection data transfer transparent data transfer: without adding RLC header unacknowg data transfer: no guaranteeing, out-ofsequence; assures error-free, unique, immiate acknowg data transfer: guaranteeing (maybe by ARQ); assures error-free, unique; can be in-sequence or outof-sequence QoS setting It seems that RLC can provide some services ike UDP and TCP, but this is in the data ink ayer! I' expain this ater. Major RLC functions: connection contro segmentation, compression, concatenation, padding transfer of user data, error correction, in-sequence deivery, dupicate detection fow contro 2.4 RRC Layer The RRC ayer provides: genera contro notification dicat contro RRC is focusing on "contro". In fact, RRC & RLC are divid into Contro-pane and User-pane. 2.5 Why is "UDP & TCP" appearing in RLC? As I mention above, three types of data transfer services is provid in the RLC subayer. Unacknowg data transfer ooks ike UDP (however UDP does not ensure unique), and acknowg data transfer ooks ike TCP. In computer network, UDP and TCP are on transport ayer (ayer 4). But they are here on the data ink ayer, why? Let take a ook at ayer 2-4 in TCP/IP. LAYER MAJOR PROTOCOLS MAJOR SERVICES & FUNCTIONS TD-SCDMA equivaent L2 data ink Ethernet, unacknowg MAC subayer

6 wireess peer-to-peer data transfer, identification of stations with MAC addresses L3 network IP routing with IP addresses none L4 transport TCP UDP unacknowg / acknowg end-to-end data transfer, fow contro, (TCP)Automatic Repeat request RLC subayer Now it's cear that "UDP & TCP" are mov into the data ink ayer because of the absence of "IP" routing requirements. TD- SCDMA's MAC ayer do have routing function, but that's the routing of signaing. In mobie communication systems, radio interface is between Mobie Station (UE in TD-SCDMA) and Base Station, and controing is mosty done by Mobie Switching Center rather than data stations themseves. Routing of user data is not necessary in radio interface, so "UDP & TCP" can be mov down to RLC subayer on L2. 3 Detais about the Layer 3.1 Transport s There are two types of transport channes: common channes, where there is a ne for in-band identification; dicat channes, where UEs are identifi by physica channes. Transport channes and mapping to physica channes: (optiona channes are omitt) PHY PRACH PHYSIC AL CHANN EL Random Access TIME SLOT / CODE Tu0, code 0/1/2/3 L1 RACH TRANS PORT CHANN EL Random Access TYPE UP/DO WN CONTE NT common upink initia access, nonreatim e

7 dicat contro / traffic CCPCH Common Contro Td1/Td0, code C/D/E/F FACH Forward Access common downin k sma amount of data DPCH Dicat DSCH Downin k Shar common downin k (severa UEs) dicat contro / traffic DPCH Dicat USCH Upink Shar common upink (severa UEs) dicat contro / traffic CCPCH Common Contro Td0, code 0/1 BCH Boardca st common downin k system informa tion (into entire ce) PSCH Synchro nizatio n SCH Synchro nizatio n common downin k synchro nizatio n informa tion (into entire ce) CCPCH Common Contro Td0, code 0/1 PCH Paging common downin k paging & notific ation (for UE ide

8 mode procu res) DPCH Dicat one or more (TS,cod e) DCH Dicat dicat up & down TD-SCDMA is design to use two types of transport channes. boardcast on common channes, because they shoud be receiv by a UEs initia access on common channes, because there is no way for a "new" UE to know its dicat channe arge amount of (reatime) data (contro information or user data) on dicat channes, because doing CSMA/CD on common channes may deay the transmit and can't ensure reatime sma amount of (non-reatime) data on common channes, because CSMA/CD won't deay this itte piece so much Mobie communication systems shoud be design to save bandwidth / channes, so TD-SCDMA is design ike that. 3.2 s Every signa is eventuay transmitt on a physica channe. channes are identifi by code, time sot, frequency. TD-SCDMA's physica channes take a four-ayer structure: superframe, 720ms; contains 72 radio frames radio frame, 10ms; contains 2 subframes subframe, 5ms, 1.28Mchip/s; contains 7 main time sots and 3

9 specia time sots 7 main time sots can be us as downink or upink. 1 downink & 6 upink, or 1 upink & 1 downink are both aow, it's not requir to have down/upinks in pair. This design make TD-SCDMA fit the ne where "down rate!= up rate" downink sots come first, foow by DwPTS & G & UpPTS (us for synchronization), upink sots appear ast time sots main time sot (TS), 675μs Downink Piot Time Sot (DwPTS), 75μs Upink Piot Time Sot (UpPTS), 125μs Guard Period (G), 75μs; indicates the switching point from downink to upink Dicat physica channes are ocat in main time sots. With spreading codes, up to 16 users' bursts can be transmitt within one TS. Each burst contains 704 chips for transmitting data. If there are 16 users in this TS, these 704 chips can transmit 22 symbos(bits) of each user; if there are ony 4, 88 symbos are transmitt. "Data symbos" are us to transmit user data from the data ink ayer. But when there is ne to transmit L1 contro data (eg. CRC), the physica ayer may "stea" severa symbos. 3.3 Mutipexing and Coding Data stream from/to MAC is encod/decod to offer transport services. coding scheme incudes error detection, error correcting, intereaving and mapping transport channes onto physica channes (one data stream mapp onto one or severa physica channes).

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11 1. error detection: provid through CRC. Higher ayers shoud te L1 to use 24,16,8 or 0 bits of CRC 2. transport bock concatenatation / segmentation 3. channe coding: convoutiona code or turbo code may be appi, chosen by higher ayers. 4. radio frame size equaisation: pad the input to ensure the output can be segment in severa segments of same size 5. 1st intereaving: to avoid interfere 6. radio frame segmentation 7. rate matching: bits are repeat for sow rate 8. TrCH mutipexing 9. physica channe segmentation 10. 2nd intereaving: to avoid interfere 11. physica channe mapping 12. mutipexing & mapping to CCTrCH 13. transport format detection 3.4 Spreading and Moduation 1G: ony FDMA is us, ony a very sma number of users are support. GSM: FDMA & TDMA are us, supports severa thousand users. When there is more, "CMCC" disappears from UEs' screen. W-CDMA: FDMA & CDMA is us, supports "unimit" users. When an impementation wants to support so many users, mutiuser detection and beamforming schemes are too compex. TD-SCDMA: using a combination of FDMA, TDMA & CDMA, supports "unimit" users. Usage of TDMA ruces the number of users in each time sot (and improves the orthogonaity between the codes), so mutiuser detection and beamforming become easier, but the non-continuous transmission may ruce coverage & mobiity.

12 4 Concusion TD-SCDMA is one of 3G radio interface standards. Its highights are: combination of FDMA, TDMA, CDMA: mutiuser detection becomes simpe TDD: downinks & upinks do not ne to be in pair, accommodating asymmetric traffic with dynamic rates REFERENCE 1. TD-SCDMA Technica Specification, China Wireess Teecommunication Standard (CWTS) Working Group 1 (WG1), 2. TD-SCDMA - Wikipia,