Technical Aspects of LTE Part I: OFDM

Technical Aspects of LTE Part I: OFDM By Mohammad Movahhedian, Ph.D., MIET, MIEEE m.movahhedian@mci.ir ITU regional workshop on Long-Term Evolution 9-11 Dec. 2013

Outline Motivation for LTE LTE Network architecture & LTE RAN elements LTE air-interface A review on the legacy networks air-interfaces (GSM & UMTS) Motivators for LTE air-interface What is OFDM? OFDM architecture Pros & cons of OFDM Use of OFDM in multiuser scenarios (OFDMA, SC-FDMA) Symbols, slots, radio blocks & frame structure in OFDMA 2

Motivation for LTE Despite evolving continuously, UMTS 1 has faced a number of limitations in terms of design Therefore the 3GPP 2 decided to make a redesign on the CN 3 as well as RN 4 The main redesign factors towards UMTS improvement: Reducing the multipath effect of communication channels via OFDM 5 air-interface Increasing the bandwidth and hence increasing the TX speed Making use of MIMO 6 transmission Making use of both FDD 7 and TDD 8 Making use of all-ip 9 approach 1 Universal mobile telecommunications system 5 Orthogonal frequency division multiplexing 9 Internet protocol 2 3 rd generation partnership project 6 Multiple-input multiple-output 3 Core network 7 Frequency-division duplexing 4 Radio network 8 Time-division duplexing 3

Progress Map Motivation for LTE LTE Network architecture & LTE RAN elements LTE air-interface A review on the legacy networks air-interfaces (GSM & UMTS) Motivators for LTE air-interface What is OFDM? OFDM architecture Pros & cons of OFDM Use of OFDM in multiuser scenarios (OFDMA, SC-FDMA) Symbols, slots, radio blocks & frame structure in OFDMA 4

LTE Network Architecture CP PDN-GW S5 UP Internet HSS S6 MME S11 Serving GW CP S1 UP enode-b X2 enode-b Mobile device 5

LTE Network Architecture Radio Access Network LTE UE 1 categories Category 1 2 3 4 5 Max. downlink data-rate in 20 MHz BW (Mbps) 10 50 100 150 300 Max. uplink data-rate (Mbps) 5 25 50 50 75 No. of receive antennas 2 2 2 2 4 No. of MIMO downlink streams (Mbps) 1 2 2 2 4 Support for 64 QAM in UL direction No No No No Yes 1 User equipment 6

LTE Network Architecture Radio Access Network enode-b The most complicated part of the LTE is the base-station that is referred to as e(evolved)node-b enode-b consists of three major parts: 1. The antennas 2. Radio modules, that modulate/demodulate the signals transmitted/received on the air-interface 3. Digital modules, as the processors of all signals on the air-interface and as an interface to the core network through high-speed backhaul connection Some of the other functionalities of enode-b includes: User management in general and scheduling air-interface resources Ensuring QoS, e.g., latency, minimum bandwidth requirements for real-time bearers, maximum throughput For load balancing between the different simultaneous radio bearers to different users Mobility management (MM) Interference management, i.e. to reduce the impact of DL transmissions on neighboring basestations particularly in cell edge scenarios 7

A review on GSM TX GSM is based on narrow 200 khz carriers that are split into 8 repeating timeslots for voice calls One timeslot carries the data of one voice call, thus limiting the number of simultaneous voice calls on one carrier to a maximum of 8 BSs use several carriers to increase the number of simultaneous calls Introduction of GPRS 1 facilitated packet-data transmission However the decision of 200KHz BW was still a limiting factor 1 General packet radio service 9

A review on UMTS TX In UMTS the GSM BW limitation is overcome by introduction of carriers BW of 5MHz Moreover, instead of having separated timeslots, UMTS makes use of uncorrelated codes for different users At the receiving end each codes is known and hence the original data for each user can be decoded HSPA combines the use of aforementioned codes with a timeslot structure for fast transporting of packet-switched data traffic 10

Motivators for LTE air-interface Today s hardware capabilities allows higher data-rates subject to provision of wider BW Due to non-ideality of CDMA for wider BW, UMTS air-interface is not capable of higher datarates When increasing the transmission speed, which results in a decrease in the time of each transmission step, the negative effect of the delayed signal paths increases As a consequence, CDMA is not suitable for carrier bandwidths beyond 5MHz Multicarrier operation has been defined for UMTS (i.e., MC-CDMA 1 ) to mitigate the problem to some degree at the expense of increased complexity What about Orthogonal Frequency Division Multiplexing (OFDM)? 1 Multi-carrier CDMA 11

OFDM Orthogonal frequency-division multiplexing is a digital multi-carrier modulation scheme Instead of sending multiuser data with high data-rates on a single-carrier, as in CDMA, OFDM splits the single high-rate stream to several parallel lower rate sub-streams Each sub-stream is assigned a number of overlapping but orthogonal subcarriers for simultaneous transmission In this way the transmission steps can be chosen to be sufficiently long to avoid the effects of multipath transmission for high transmission speeds To save bandwidth, the subcarriers (i.e., Sinc functions) are placed close to each other but with zero cross-correlation 12

FDD air-interface and radio network OFDM Subcarriers Orthogonality Source: M. Movahhedian, Frequency synchronization for OFDMA wireless cellular networks, PhD thesis, University of Surrey, UK, Aug. 2010. 13

OFDM Architecture Transmitter side OFDM has a particular architecture both at the transmitter and receiver At the transmitter, first the source data-stream is serial-to-parallel converted where the number of symbols in each batch is assumed to be equal to the number of subcarriers In this case, each batch is called an OFDM block Next, each block goes through an inverse discrete Fourier transform (IDFT) unit, used for modulating the data symbols over the allocated bandwidth A number of time-domain samples, which is referred to as CP (cyclic prefix), is copied from the tail of corresponding time-domain block and is added at the preamble of that block The length of CP is assumed to be large enough to mitigate the timing offset and also the channel image from one block to the adjacent one, i.e. ISI is fully mitigated 14

OFDM Architecture Receiver side At the receiver, CP that carries channel image is first discarded Then the received stream of symbols are passed through an FFT block with the purpose of demodulation Finally, channel equalization and symbols detection are performed on the received signal to recover the initially transmitted symbols 15

OFDM Architecture Schematic Representation Source: M. Movahhedian, Frequency synchronization for OFDMA wireless cellular networks, PhD thesis, University of Surrey, UK, Aug. 2010. s i i th data-stream (digitally modulated) Serial to parallel IFFT 1 Parallel to serial Add CP 2 CFO 3 Channel OFDM Transmitter Setup Medium (additive & multiplicative noise) AWGN 4 CP removal Serial to parallel FFT 5 CFO compensation, Channel equalization & Detection ŝ i Data out An estimation of i th data-stream OFDM Receiver Setup 1 Inverse fast Fourier transform 3 Carrier frequency offset 5 Fast Fourier transform 2 Cyclic prefix 4 Additive white Gaussian noise 16

Pros & Cons of OFDM + Robust against frequency-selective channels + Robust against inter-symbol interference (ISI) + Less sensitivity to timing-offsets + Ease of implementation in terms of architecture (i.e. FFT & IFFT blocks) - High sensitivity to carrier frequency offsets - High peak to average power ratio (PAPR) - Lower data-rate efficiency due to CP overhead 17

Use of OFDM in multiuser scenario OFDMA In multiuser scenarios, OFDM can be used with a number of multiple-access schemes such as TDMA, FDMA, etc. Alternatively if the total number of adjacent subcarriers are divided into a number of groups and each group is assigned to one user, this would form up a new multiple-access scheme called OFDMA In other words, OFDMA is a multiple-access scheme that divides the total bandwidth into a number of sub-channels and allocates them to a number of users for simultaneous data transmission LTE employs OFDMA In downlink (DL) direction 18

Use of OFDM in multiuser scenario OFDMA Architecture Schematic Representation Source: M. Movahhedian, Frequency synchronization for OFDMA wireless cellular networks, PhD thesis, University of Surrey, UK, Aug. 2010. s k th user data-stream ( k ) Serial to parallel Freq. mapping IFFT Parallel to serial Add CP CP removal Channel CFO Serial to parallel FFT Freq. de-mapping CFO compensation, Channel equalization & Detection Other users AWGN Data out ˆ ( k ) s 19

Use of OFDM in multiuser scenario Some physical parameters of OFDMA LTE uses a number of physical parameters related to the subcarriers Subcarrier spacing is set to 15 KHz regardless of overall channel BW OFDM symbol duration (length of each transmission step) is set to 66.667 microseconds CP length is either set to 4.7 microseconds or 16.67 microseconds depending on propagation channel conditions (i.e., the number of channel taps) 20

Use of OFDM in multiuser scenario SC-FDMA 1 For uplink transmissions, OFDMA is not ideal due to high PAPR 2 A high PAPR is affordable at the BS do to power abundance; therefore OFDMA does not cause PAPR problems in DL direction However, for UL transmissions, due to limited power of battery-driven UEs, OFDMA is not practical 3GPP has hence decided to use an alternative multiple-access scheme, i.e. SC-FDMA SC-FDMA consists of one additional function, i.e. FFT/IFFT, for each TX and RX side to spread out the information of each bit onto all subcarriers and hence reduce power differences 1 Single-carrier frequency-division multiple-access 2 peak to average power ratio 21

Use of OFDM in multiuser scenario SC-FDMA Architecture Schematic Representation s ( k ) Serial to parallel N-point FFT Freq. mapping M-point IFFT Parallel to serial Add CP CP removal Channel CFO Serial to parallel M-point FFT Freq. de-mapping N-point IFFT CFO Post-compensation, Channel equalization & Detection AWGN Other users Data out ˆ ( k ) s 22

Symbols, slots, radio blocks, frames structure in OFDMA The smallest transmission unit on each subcarrier is a single transmission step (a.k.a. symbol or RE 1 ) with a length of 66.667 microseconds To reduce the overhead of resource assignment, 7 consecutive symbols on 12 successive subcarriers are grouped into an RB 2. An RB occupies 1 slot of a duration of 0.5 milliseconds A subframe consists of 2 RBs (i.e., 2 slots) and hence is of a duration of 1 millisecond A subframe represents the LTE scheduling time which means at every 1 millisecond the enode-b decides which RBs are to be assigned to which user Each frame has a duration of 10 milliseconds consisting of 10 subframes There are two methods for the network to transmit a subframe: LVRBs 3 wherein the enode-b requires a narrowband channel feedback from UE to schedule the RBs on subcarriers that do not suffer from narrowband fading DVRBs 4 wherein the symbols that form a block are scattered over the whole carrier bandwidth. In this case the EU either transmits no feedback or a wideband channel feedback for the whole BW 1 Resource element 3 Localized virtual RBs 2 Resource block 4 Distributed virtual RBs 23

Symbols, slots, radio blocks, frames in OFDMA A schematic representation Previous frame 1 Frame (10ms)=10 sub-frames (1ms)= 20 slots (0.5ms) Next frame time 180 KHz = 1 symbol = 1 resource element (66.667 micro-sec, 15 khz) 1 sub-frame freq. 1 resource block = 1 slot (0.5 ms) = 12 subcarriers x 7 symbols 24

Conclusion OFDM is a digital multi-carrier modulation (multiplexing) scheme that splits the high-rate stream into several parallel lower rate sub-streams to counteract time-dispersive channels OFDMA is a multiple-access scheme that divides the total bandwidth into a number of subchannels and allocates them to a number of users for simultaneous data transmission The 2-dimensional time-frequency structure of OFDMA allows for flexible assignment of resources to different users depending on their channel condition Due to non-ideality of OFDMA in uplink direction of an LTE network, SC-FDMA is used wherein the symbols are first sent through a linear unitary precoder (i.e. Fourier) as a power spreader 25

Thank you for your attention 26