This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Multipath induced by reflection of radio signal off of obstacles• Buildings, vehicles, even people
Signal arrives at receiver staggered in time due to different path lengths• Amount of added relay referred to as “delay spread”• Results in distortion of signal at receiver
Problem becomes more difficult to deal with as data rates increase• LTE data rates represent a major increase of earlier technology• New approaches required to provide high system reliability WHILE minimizing complexity
Signal arrives at receiver staggered in time due to different path lengths• Amount of added relay referred to as “delay spread”• Results in distortion of signal at receiver
Multipath results in ISI• Higher data rates mean sorter symbols periods in single carrier systems• Severe ISI can span several symbol periods in high rate systems (e.g. LTE)
Received Signal Distorted byFrequency Selective Fading
frequency
Signal Bandwidth
Transmitted Single Carrier Signal
MultipathDistortion
Frequency Selective Fading
Different Signal Paths Have Different Physical Lengths• Some frequencies combine constructively• Other frequencies combine destructively
Frequency Selective Fading• FSS becomes more severe at higher data rates and/or longer delay spreads• Combination of constructive and destructive interference within signal passband
Channel Equalization• Channel equalization can compensate for frequency selective fading• Basic limitations with single carrier modulation
Single Carrier Systems Employ Channel Equalization to Compensate for Freq Selective Fading• a.k.a time-domain equalization
FIR Transversal Filter is a Common Equalizer Topology• Utilizes delay taps, variable coefficients, and summing amp
But at higher data rates• Sample clock is higher --- more delay taps required • Adaptive algorithm becomes increasingly complex
Single Carrier Modulation / Time Domain Equalization has fundamental limitations• System complexity drastically increased at high data rates• Fundamentally unsuitable for LTE, so….
LTE PHY Employs OFDM for Downlink• Fundamentally different approach to high data rates and channel compensation
OFDM uses many closely spaced subcarriers rather than a single carrierData transmitted in parallel streams using VERY LONG symbolsOFDM symbols preceded by CP
At the Receiver…• Delay spread is less than CP duration
CP is removed by receiver --- eliminating ISIResulting signal CAN by amplitude and phase distorted, but…The distortion is CONSTANT over the FFT Period (TFFT) within each subcarrierThe result is a RECT function in the time domain
OFDM Symbol is a linear summation of individual subcarriers• Individual subcarriers are constant within TFFT, but composite symbol amplitude is NOT
FFT Converts OFDM symbol into Frequency Domain• RECT functions in time domain result in SINC functions in frequency• Resulting subcarriers are very closely spaced (15 kHz) but are non-interfering (i.e. orthogonal)
Demodulated Signal w/o Freq Offset (Zero ICI) Demodulated Signal w/Freq Offset Causing ICI
OFDM Drawbacks
OFDM Has Two Major Drawbacks• Sensitivity to Carrier Offset and High Peak-to-Average Power Ratio (PAPR)
Sensitivity to Carrier Offset• Subcarrier orthogonality depends on sampling precisely at SINC function zero-crossings• Carrier offset shifts same points to locations exhibiting Inter Carrier Interference (ICI)
High PAPR• Individual subcarriers have constant amplitude in FFT window, but composite symbol does not• Composite OFDM symbol amplitude varies about mean value much more than a single carrier signal
at roughly similar data rates• High PAPR reduces RFPA efficiency AND drives A/D D/A converter requirements.
Orthogonal Frequency Division Multiple Access Differs from Packet Orient Networks• OFDMA is more efficient, and generally has lower latency• OFDMA is more complex to implement
Packet Oriented Networks• Transmit data packets to users one at a time
Channel estimates performed at start of each headerPHY preambles can become significant contributor to overhead in some circumstances
• Each user consumes all network resources during transmission / receptionLatency can be problematic in high user density situations
Generic Frame Format• Basic building block for Uplink and Downlink Multiplexing Scheme• Generic frame format used for FDD. Alternative frame formats defined for TDD schemes
Generic Frame Format Parameters• Frame duration: 10 msec• Frame consists of 10 subframes, 1 msec each• Each subframe contains two slots, 0.5 msec each
Slot Parameters• Short or Long CP used depending on propagating conditions within cell• Short CP: Each slot contains 7 symbols• Long CP: Each slot contains 6 symbols
Two Dimensional Resource Grid• Downlink resources can be visualized via resource grid• Two dimensions
Time (symbols, slots, etc.)Frequency (subcarriers)
• In MIMO apps, there is one resource grid per antenna
Physical Resource Block (PRB)• Basic unit of resource allocated by eNodeB• Consists of 12 contiguous subcarriers• Duration in time is 1 slot• # available PRBs determined by system bandwidth
Bandwidth (MHz) 1.25 2.5 5.0 10.0 15.0 20.0
Subcarrier Bandwidth (kHz) 15
PRB Bandwidth (kHz) 180
#Available PRBs 6 12 25 50 75 100
Number of Available PRBs Depends on System Bandwidth
Reference Signals Interspersed with data-bearing resource elements• Channel Impulse Response (CIR) and network timing updated continuously by UE
Reference Signals transmitted on specified subcarriers on specified symbols• 1st reference signal in slot
Transmitted on every sixth subcarrier– Subcarrier #1, #7, #13….
Transmitted during first OFDM symbol• 2nd reference signal in slot
transmitter on every sixth subcarrier, but offset from 1st reference symbol– Subcarrier #4, #10, #16…
Transmitted on third-to-last OFDM symbol• CIR for intervening subcarriers estimated via interpolation• Frequency hopping of Reference Signals under consideration
Conventional Receivers Often have Antenna Diversity• Not to be confused with Receiver Diversity
MRC Exploits Receiver Diversity to Increase System RELIABILITY by:• Capturing multiple copies of the signal with different multipath characteristics and noise signatures• Uses baseband combining to eliminate frequency selective fading and improve SNR
MIMO Exploits Receiver Diversity to Increase System DATA RATE by:• Capturing multiple copies of multiple transmitted signals
Each with different multipath characteristics and noise signatures• Using techniques in baseband to resolve transmitted signals into separate channels
Each resolvable channel capable of carrying high data rate traffic
Signal from Transceiver A Signal from Transceiver B
SNR for All Subcarriers Enhancedin Composite MRC Signal
MRCCombining
MRC
MRC Exploits Receiver Diversity to Increase System RELIABILITY by:• Capturing multiple copies of the signal with different multipath characteristics and noise signatures• Uses baseband combining to eliminate frequency selective fading and improve SNR
MRC Improves SNR• Coherent combination of signals with uncorrelated noise improves SNR by 3 dB
MRC Enhances System Robustness in Presence of Severe Frequency Selective Fading• Coherent combination improves SNR for all subcarriers• Statistically unlikely to get deep fades on same subcarriers if antennas are adequately separated
LTE Reference Signals Transmitted Sequentially in Multi-Antenna Applications• When Reference Signal is sent on Antenna 0, Antenna 1 is idle• When Reference Signal is sent on Antenna 1, Antenna 0 is idle
Data is transmitted simultaneously• A priori CIR knowledge enables receiver to resolve separate data steams
Functions Common to OFDMA and SC-FDMA SC-FDMA Only
Symbol
Block
P/SConvert
M-PointIDFT
Symbol
Block
Const.De-map
SC-FDMA
Single Carrier – Frequency Division Multiple Access• Basic transmission scheme for LTE Uplink• Extension of Single Carrier – Freq Domain Equalization to accommodate multiple users• Main advantage is lower PAPR, thus improving RFPA efficiency at UE• Also has less sensitivity to carrier offset
High degree of Commonality with OFDM Signal Chain• SC-FDMA sometimes refered to as “DFT-Spread OFDMA”
Basic Operation• Sequence of single carrier symbols S/P converted and fed to DFT• Discrete freq domain representation mapped to subcarriers & transmitted in same manner as OFDM
Physical Signals, Physical Channels & Transport Channels
Physical Signals• Resource elements used to support PHY functions only• Physical Signals do not transport information originating in higher layers
Physical Channels• Resource elements used to transport info originating in higher layers• Physical Channels have unique characteristics depending on function• Physical Channels map to Transport Channels
Transport Channels• Service Access Points for higher layers• Unique characteristics depending on function
Two types of DL Physical Signals• Reference Signal: used by UE for CIR estimation• Synchronization Signal: facilitates Cell Search procedure
Reference Signal• Selected from a set of 510 unique cell ID sequences
Product of 1 of 3 orthogonal sequences and 1 of 170 PRN sequences• Mapped to resource elements as shown on following slide
Synchronization Signal• utilize the same type of cell ID sequences as reference signals • primary and secondary synchronization signals
– Used for different purposes within cell search algorithm– transmitted 72 subcarriers centered around the DC subcarrier– Primary: transmitted during the 0th slot of a frame– Primary: transmitted during the 10th slot of a frame
Downlink Physical Channels• Physical Downlink Shared Channel (PDSCH)• Physical Downlink Control Channel (PDCCH)• Common Control Physical Channel (CCPCH)
Physical Downlink Shared Channel (PDSCH)• Supports multiple functions including transport of high rate data and multimedia content• Capable of supporting highest LTE data rates• Subcarriers modulated with QPSK, 16QAM, or 64 QAM depending on channel conditions
PDSCH Supports Spatial Multiplexing• Up to two code words sent per subframe• Term “layer” in this context corresponds to transmitting antenna
Up to four Tx antennas used for LTE spatial multiplexingCode word-to-antenna mapping shown in table below
Transmission rank Number of code words Codeword-to-layer mapping
1 1 x(0)(i) = d(0)(i)x(0)(i) = d(0)(i)x(1)(i) = d(1)(i)d(0)(i) is mapped to layer 0d(1)(i) is mapped to layers 1 and 2d(0)(i) is mapped to layers 0 and 1d(1)(i) is mapped to layers 2 and 3
Physical Downlink Control Channel• Conveys UE-specific Control information• Robustness is main consideration• QPSK is only modulation format• Mapped onto first three symbols off the 1st slot in a subframe
Common Control Physical Channel• Conveys cell-wide control information• Robustness is main consideration• QPSK is only modulation format• Transmitted as close to center frequency as possible
transmitted exclusively on the 72 active subcarriers centered around the DC subcarrier
Broadcast ChannelFixed formatMust be broadcast over entire coverage area of cell
Downlink Shared ChannelSupports Hybrid ARQ (HARQ)Supports dynamic link adaption by varying modulation, coding and transmit powerSuitable for transmission over entire cell coverage areaSuitable for use with beamformigSupport for dynamic and semi-static resource allocationSupport for discontinuous receive (DRX) for power save
– Demodulation signal facilitates coherent demod– Channel sounding signal
Both based on Zadoff-Chu sequences
Random Access PreambleTransmission by UE initiates cell search procedureDerived from Zadoff-Chu sequencesTransmitted on contiguous blocks of 72 subcarriersRandom Access Preamble format shown below
UL Slot StructureUplink Slot structure also same as DL
– 7 SC-FDMA symbols/slot with normal CP– 6 SC-FDMA symbols/slot with extended CP
Physical Uplink Shared Channel (PUSCH)Resource allocation controlled on sub-frame basis by base station schedulerResources allocated in PRB’s
– Same BW (12 subcarriers) as Downlink– May be frequency hopped from sub-frame to sub-frame
QPSK, 16 QAM, 64 QAM modulation used
Physical Uplink Control Channel (PUCCH)Carries Uplink control information
– CQI, ACK/NACK, HARQ, and uplink scheduling requests Never transmitted simultaneously with PUSCH data Freq hopped at slot boundary for added reliability (see below)
Uplink Shared Channel (UL-SCH)Support possible use of beam formingSupport dynamic link adaption (varying modulation, coding, and/or Tx power)Support for HARQSupport for dynamic and semi-static resource allocation
Random Access Channel (RACH)Supports transmission of limited control informationPossible risk of collision