DFTS-OFDM, fast facts

DFTS-OFDM, fast facts

DFTS-OFDM is used in the LTE uplink

Discrete Fourier Transform Spread OFDM (DFTS-OFDM) is used for the LTE uplink.

Basic properties

• Also known as Single Carrier Frequency Division Multiple Access (SC-FDMA)
• Small variations in transmitted power (and low PAR).
• Possible to use low-complexity, highly efficient equalization in a frequency selective channel.
• Possibility to use FDMA with flexible bandwidth assignment.

·         Same as for the downlink, the sub-carrier spacing is 15 khz and a resource block consists of 12 sub-carriers

Basic principles

• DFTS-OFDM can be interpreted as normal OFDM with DFT-based pre-coding.
• Main benefit of DFTS-OFDM is the reduced variations in transmitted power (and low PAR).
• An equalizer is needed to compensate for the radio channel frequency selectivity (MMSE).

·         Similar to OFDM, the physical resource is a time-frequency grid of resource elements/blocks.

·         Un-like conventional OFDM. The sub-carriers must be adjacent in frequency.

Uplink reference signals

·         Demodulation reference signals (DRS) is needed for channel estimation and coherent demodulation.

·         DRS are always transmitted together with the corresponding physical channel to be demodulated (PUSCH or PUCCH)

·         Sounding Reference Signals (SRS) are transmitted for estimation of the channel quality at different frequencies.

·         SRS are used as basis for assigning resource blocks to a user.

Uplink L1/L2 control signaling

·         Main content is: HARQ ack/nack's, reports on downlink channel conditions and scheduling requests.

·         Unlike the downlink, there is no need to indicate the transport format since the terminal always uses the transport format details that were sent in the scheduling grants by the NW.

·         The basis for channel status reports is aperiodic reports where the enodeB requests a report by setting the CQI request bit in a scheduling grant

Uplink scheduling

• In EUL, the scheduling grant consists of a Tx power limit that the terminal is not allowed to exceed. If a terminal does not exploit all if it’s granted Tx power, it is available for use by another terminal.
• LTE UL scheduling grants consists of resource blocks (time, frequency) which the terminal will always use for data or fill up with padding bits.
• The e nodeB decides which transport format (TBS, modulation, code rate) the terminal must use.
• Since the e nodeB knows the transport format and resource blocks, there is no need for outband signaling of this information.

Channel status reporting

The channel status reports consists of one or several pieces of information

• Rank Indication (RI)
• Pre-coder Matrix Indication (PMI)
• Channel Quality Indication (CQI)

Channel status reports can be

• Wideband (one report) or sub-band (several reports)
• Periodic on a fixed interval and a fixed UL scheduling grant
• A-periodic or trigger-based, following a request from the NW or a fulfilled event in the terminal.
• If padding is applied and the number of padding bits exceeds the number required for a channel status report. A report will then be sent instead.

OFDM fast facts

OFDM fast facts

OFDM is used for the LTE downlink

Basic principles

• OFDM is highly robust to channel frequency selectivity due to the relatively long symbol period in combination with a cyclic prefix
• OFDM enables scheduling in the frequency domain as well as the time domain.
• Large number of frequency adjacent sub-carriers which are used for flexible transmission bandwidth by simply changing the number of subcarrier that are allocated at each scheduling moment (per TTI).
• Rectangular pulse shaping corresponding to sinc-square shaped per-sub-carrier spectrum.
• Tight sub-carrier spacing, in LTE 15khz.
• During each OFDM symbol interval, multiple modulation symbols are transmitted in parallel on the sub-carriers.
• QPSK, 16QAM, 64QAM can be used.
• Sub-carrier orthogonality is made possible due to the sinc-square shaped sub-carrier spectrum combined with a sub-carrier spacing equal to the per sub-carrier symbol rate
• Fast Fourier Transform (FFT) processing

Frequency diversity with OFDM

• Channel coding is used to spread the information bits over many code bits. These code bits are then mapped, via the OFDM symbols, over a set of sub-carriers that are spread over the overall OFDM transmission bandwidth spectrum. This distribution of the code bits in the frequency domain is known as frequency interleaving which is similar to time domain interleaving.
• LTE makes use of interleaving in both the frequency domain and the time domain as compared to WCDMA/HSPA which only uses the time domain for interleaving.
• The benefits of interleaving are equal in the two domains.

Downlink L1/L2 control signaling

• Downlink L1/L2 control signaling is needed for data transfer in both UL&DL.

• For the downlink it consists of scheduling information necessary for receiving, demodulating and decoding the DL-SCH.
• For the up-link it consists of scheduling grants with information about the resources and transport format to use for UL-SCH. Also, it contains HARQ ACK/NACK’s.

• Downlink L1/L2 control signaling is carried on three different physical channels:

1. Physical Control Format Indicator Channel (PCFICH) – carries information about the size of the control region which is the part of the sub-frame in which the control signaling is transmitted.
2. Physical Downlink Control Channel (PDCCH) – carries downlink scheduling assignments and uplink scheduling grants.

      3    Physical HARQ Indicator Channel (PHICH) carries HARQ ACK/NACK’s

Overall Time Domain Structure

• A radio frame of length 10ms consists of ten equally sized sub-frames of length 1ms.
• Each frame is identified by its System Frame Number (SFN).
• A sub-frame of length 1ms consists of two slots of length 0,5 ms.
• A slot then consists of six or seven symbols including cyclic prefix.
• A resources block consists of 12 sub-carriers during one 0,5ms slot.
• A resources element consists of one sub-carrier during one symbol interval.

Downlink traffic distribution

Eeeh, I guess I can stay awake that late for testing max throughput in Live NW's.........   :o

Multi Antenna Systems

Multi Antenna Systems – fast facts

Basic characteristic

• The relation of the distance between the antenna elements in a multi antenna system and the mutual correlation between the radio-channel fading that is experienced by the signals at the different antennas defines many of the properties and capabilities of the system.

Benefits of multi antenna techniques

• Multiple antennas at the receiver and/or the transmitter can be used to provide additional diversity against fading on the radio channel.
• Multiple antennas at the receiver and/or the transmitter can be used to shape and direct the antenna beam in a certain way.
• Spatial Multiplexing (MIMO): Multiple antennas at both the transmitter and the receiver can be used to create what can be seen as multiple, parallel channels.

Maximum Ratio Combining (MRC) weights:

• Phase rotation of the signals received at different antennas to make them phase aligned when adding them together (coherent combining).
• Weight the signals in proportion to their corresponding channel gains, that is apply higher weights for signals that are received stronger (= more reliable).

Interference Rejection Combining (IRC)

• In situations with a single or a small number of interferers, improved performance can be achieved if selecting the weights so that the interferer is suppressed maximally.

The antenna weights can also be selected to achieve Minimum Mean Square Error (MMSE) combining.

In case of OFDM, due to the narrow bandwidth of the sub-carriers, there is little or no frequency selectivity on the channel. This means that low-complexity combining schemes such as MRC and IRC can be used.

Transmit antenna diversity

• Multi-path propagation may actually be beneficial if it can be used for delay diversity (RAKE)
• Space time coding is a general concept used to describe multi antenna transmission schemes where the symbols are mapped both in time and the spatial domain that the multiple antennas can provide.

Transmitter side beam forming

• If some knowledge of the downlink channel (phase) from the different antennas exist at the transmitter, the antennas can be used to provide beam-forming in a certain direction.
• The overall transmission beam can be steered in a certain direction by applying different phase shifts to the signals to be transmitted by the antennas.
• Beam-forming can increase the received signal strength with a factor that is in relation to the number of antennas

Spatial multiplexing (MIMO)

• Multiple antennas at both sides (transmitter and receiver)
• Possible to utilize high signal to noise ratio for higher data rates. F ex near the base station and/or in very good channel conditions.
• Data rate can be increased by a factor relative to the number of simultaneous receiver and transmitter antennas (rank, layers)
• In low signal to noise conditions, spatial multiplexing is less efficient as compared to beam-forming and/or diversity.

twinratman

HSUPA/EUL

HSUPA/Enhanced Uplink

Basics:

• EUL is based on some of the same basic procedures as for HSDPA: fast scheduling, fast Hybrid ARQ and short (2ms) TTI
• The shared resource for the users in the uplink can be seen as the maximum tolerable interference at the nodeB which is generated by the transmission powers of all active UE’s in the cell.
• Contrary to HSDPA, the data buffers and the scheduler are located in different nodes (buffers are in the UE’s) which means that the buffer status must be signaled to the scheduler in the nodeB.
• The scheduler can adjust the data rate of the E-DCH by a power offset which is relative to the associated power-controlled uplink control-channel. TPC’s (Transmit Power Control) is transmitted on the F-DPCH (Fractional Dedicated Physical Channel).
• Unlike HSDPA, EUL supports soft handover.
• SF2 is introduced in EUL since it leads to a lower PAR (Peak to Average Ratio).
• The reason why it's difficult to use higher modulation in HSUPA is the fact that it requires more energy per bit to be transmitted. Since the transmitter in the UL is the UE, the available UL Tx power is limited and less than in the DL transmitter, which is the nodeB.

Scheduling

• The scheduling in EUL is based on scheduling grants from the nodeB and scheduling requests from the UE’s.
• Scheduling grants can be sent with every TTI or slower.
• Due to the limited transmission power of a single UE compared to the nodeB, in most cases multiple users can transmit simultaneously.
• Inter cell interference is controlled by a down-scheduling grant (overload indicator) which is transmitted by the non-serving cells while the UE is in soft handover.
• Absolute grants are sent from the serving cell only on the E-AGCH ( E-Absolute Grant Channel) and may increase or decrease the grant in arbitrary steps.
• Absolute grant value ranges from 0-31 and are directly mapped to E-DPDCH/DPCCH power ratio.
• Absolute grants are valid for one UE, a group of UE’s or all UE’s using a primary identity (E-RNTI) which is UE specific or a secondary identity (E-RNTI) which is assigned to multiple UE’s. In this way the scheduler has more flexibility in controlling the overall cell load.
• Relative grants are sent from all involved nodeB’s and typically only adjusts the grant by one step up/down at a time. It is sent on the E-RGCH (E – Relative Grant Channel)
• Relative grants from the non-serving cells can only have to values: DTX or “Down” and is transmitted to all UE’s (aka the overload indicator)
• The UE maintains a Serving grant which is used in the E-TFC selection algorithm unless the HARQ process is in retransmission. In that case, the E-TFC should be unchanged.
• In order to adapt to the fast changing variations of the channel quality, the scheduling grant is only seen as an upper limit of the used resources (total interference) and the UE is free to do its own instantaneous adjustments to the transmission parameters.
• Out-band signaling consisting of scheduling information is used in the form of a “happy bit” which should be set if the UE has more data in its buffer to transmit and has more available transmission power.
• The basic information that the happy bit transmits is whether or not the UE can empty its buffers in N TTI's where the value of N is signaled by the NW.
• The terminal may only increase the bit rate if there is an UP from the serving HSUPA cell and no DOWN from any other cell in the E-DCH active set.

Scheduling information

• The scheduling information in the MAC-e pdu consists of:
  -total E-DCH buffer status
  -highest priority logical channel ID
  -highest priority logical channel buffer status
  -UPH
• UE transmission power headroom (UPH) is defined as being the ratio of the maximum UE transmission power and the corresponding DPCCH code power.
• UPH indicates the available power resources, or if and how much the UE can increase the bit rate.
• The nodeB scheduler may use this scheduling information in addition to the happy bit in the E-DPCCH

HARQ

• In soft handover, the HARQ protocol is terminated at multiple (UE’s) locations.
• As long as one ACK is received from any of the involved nodeB’s. The UE will consider the transmission as successful.
• For continuous transmission to a single UE, multiple HARQ processes can operate in parallel.
• The number of parallel processes depends on the TTI: 8 for 2ms TTI and 4 for 10ms TTI
• Individual HARQ processes can be de/activated in order to adjust the bit rate.
• To make the most efficient use of the HARQ features and to achieve maximum bit rates, it has been shown that a typical rate of retransmissions up to 10% should be targeted.
• Same reordering mechanism as for HSDPA is also necessary in EUL however, due to the multiple nodeB’s involved in soft handover, reordering takes place in the RNC.
• The E – HARQ Indicator Channel (E-HICH) is a dedicated physical channel that is transmitted from each cell in the active set and is used for sending the HARQ ACK/NACK’s.
• E-HICH from multiple users are transmitted on the same channelization code where each user has it’s own orthogonal signature sequence for identification. This ensures that channelization code space is not wasted at the same time as each ACK/NACK is transmitted with sufficient energy.
• E-HICH from non serving cells only contains ACK's
• UL-HARQ is fully synchronous which means that even transmitted redundancy versions can be predetermined.
• One HARQ profile consists of a power offset attribute and a maximum number of retransmissions.
• Incremental redundancy is supported with Chase combining as a subcase
• The node B transmits the responding ACK/NACK a well-defined time after receiving each transport block. In this way the UE knows to which HARQ-process the ACK/NACK belongs.
• In HSUPA/EUL, HARQ retransmissions are operated in synchronous and non-adaptive mode which means that retransmissions can only be made at pre-defined times after the initial transmission and that the same transport format is used.
• A 2 bit Retransmission Sequence Number  (RSN) is used to distinguish retransmissions from new data.

MAC

UE

• A new MAC entity, MAC-es/MAC-e is introduced in the UE which handles: HARQ protocol, scheduling and E_DCH Transport Format Combination (TFC )selection
• The UE can autonomously select the data rate by choosing the E-TFC( E-DCH Transport Format Combination)  as long as the resulting data rate does’nt exceed the scheduling grant.

NodeB

• A new Mac entity, MAC-e is added in the node B to handle HARQ, Scheduling and MAC-e demultiplexing.
• There is one MAC-e entity in each node B for a UE in soft handover.
• There is one E-DCH scheduler function in the node B

RNC

• For each UE there is one MAC-es entity in the SRNC which handles: Reordering queue distribution, Reordering and combining of data from different node B’s in case of soft handover.
• The MAC-es header includes the Transmission Sequence Number (TSN) necessary for reordering (in-sequence delivery) in the RNC.
• Since the RNC does not know when a PDU has been transmitted the maximum number of times, a stall avoidance timer is used. On its expiry, the packet delivery from the reordering entity to RLC is resumed and a RLC retransmission is triggered for the missing PDU.

Mobility

• E-DCH keeps its own active set of nodeB’s. However, most of the time it’s the same as the DCH active set.
• Serving cell is the same for both E-DCH and HS-DSCH and is changed in the same way as for HSDPA

Physical channels

• E-DCH maps to one or multiple E-DPDCH
• A separate and parallel code channel E-DPCCH is carrying all the necessary information to receive the E-DPDCH.
• The 10 information bits on E-DPCCH is carrying the following information:
  E-TFCI, RSN and happy bit
• From a performance perspective, coding is always better than spreading implying that the number of channelization codes (E-DPDCH’s) should be high and the spreading factor low.

RRM for HSUPA

• RNC: admission control, handover control, resource management, packet reordering, congestion control
• NodeB: resource management, packet scheduling, congestion control, HARQ
• UE: packet scheduling, power control, HARQ

• the HSUPA scheduler has instant information of uplink interference since it is located in the nodeB.
• the RNC can send a UE-specific congestion indicator to the nodeB

HSPA Evolution

HSPA Evolution

Some of the new features that builds on and enhances the performance of Rel5 HSDPA and Rel 6 HSUPA include:

MIMO

Multiple Input Multiple Output (MIMO) may in a most generalized way be viewed as the use of multiple antennas at both the transmitter and the receiver arranged to operate in one of the 2 following ways:

• As diversity gain for increasing the carrier-to-interference-ratio which is typically used in bad channel conditions or at the cell edge borders for increased coverage. Pre-coding is used so that the data streams from the two antennas can be combined coherently.
• Transmission in multiple layers or streams for increasing the maximum achievable data rate(spatial multiplexing) which requires high carrier-to-interference-ratio and is therefore typically used near the nodeB and in good channel conditions. Pre-coding is used in this case to create two orthogonal data streams that can carry separate flows of information.  

Higher Order Modulation

• Release 7 supports 64QAM in the downlink and 16QAM in the uplink
• Higher Order Modulation can in some cases be combined with MIMO for maximum bit rates.

Continuous Packet Connectivity (CPC)

In order to reduce delay after a period of no data transfer, it is advantageous to let the terminal keep the HS-DSCH and the E-DCH that were assigned when the UE first entered state CELL-DCH. However this comes with a cost in the form of increased interference in the uplink and reduced battery life for the UE. In earlier releases this situation has been targeted by different power-saving states (FACH: CELL_PCH: URA_PCH: IDLE). With Rel7 CPC, it is possible to keep the UE in state CELL_DCH efficiently during periods of low activity. CPC consist of three main features:

• Discontinous transmission (DTX)
• Discontionous reception (DRX)
• HS-SCCH less operation

After a certain period of inactivity the UE goes into a preconfigured cycle where it is only allowed to transmit/receive during certain intervals. DTX/DRX should be used in combination and corresponding cycles should be matched to each other.

• HARQ is not part of CPC which means that the UE will transmit/receive the ACK/NACK’s regardless of the DTX/DRX cycles.
• DTX/DRX cycles are configured and activated by RRC signalling

HS-SCCH less operation

• The serving HS-DSCH cell can activate or deactivate HS-SCCH less mode for a UE by sending a HS-SCCH less order.

• It is possible to transmit user data on HS-DSCH without the accompanying HS-SCCH control channel however this will require blind decoding of the HS-DSCH.
• The complexity of the blind decoding is reduced by using one out of four predefined formats for the HS-DSCH.
• All formats are limited to QPSK modulation and at most two channelization codes.
• The HS-PDSCH CRC is 24-bits long and is UE specific. Its generation follows the same procedure as the CRC for the second part of the HS-SCCH.
• HARQ is limited to two retransmissions with a predefined redundancy version.
• HS-SCCH less operation is mainly targeted at relative low bitrate services (such as VOIP) where the signaling overhead becomes proportionally large.

Enhanced CELL_FACH operation

To further reduce latency due to state changes by the UE, the following additions has been made to state CELL_FACH

• In release 7 it is possible to use the HS-DSCH in state CELL_FACH during the transition to state CELL_DCH
• In release 8 it is also possible to use E-DCH in state CELL_FACH
• The HS-DSCH in state CELL_FACH is used to enable reception of BCCH, CCCH and DCCH logical channels.
• Using HS-DSCH’s higher transmission capabilities decreases the transmission times for signaling and system information broadcast.
• In release 10, the HS-DSCH reception is also supported in CELL_PCH and URA_PCH state in a similar way as for CELL_FACH.

Layer 2 protocol enhancements

Flexible RLC PDU size:

• Larger PDU sizes for keeping the relative overhead of header information small.
• Smaller PDU sizes for keeping the padding overhead small when user date content is small.

DUAL Cell HSDPA operation

• Dual Cell operation is characterized as simultaneous reception of up to two HS-DSCH transport channels.
• Certain categories of UE’s may be configured for DUAL Cell operation with or without MIMO simultaneously.
• Each of these HS-DSCH transport channels has its own associated UL and DL signaling and own HARQ entity.
• The maximum number of HS-SCCH’s for a UE in Dual Cell operation is 6.
• Certain categories of UE’s may be configured for simultaneous reception of two transport channels  carried over separate frequency bands.
• In Rel10, Four Carrier HSDPA operation with or without simultaneous MIMO has been defined.

HSDPA fast facts 2

HSDPA fast facts 2

Mobility

• The UE measurements reports are initiated and managed by the RNC.
• The RNC can order the UE to change it’s serving cell based on the measurements reports.

Change of serving HS-DSCH cell

• When change of HS-DSCH serving cell takes place between different node B's. The source node B will flush it's buffers and it's up to RLC retransmissions to recover the lost PDU's. However, with perfect timing of when to stop forwarding PDU's to the source cell retransmission can be completetely avoided. If both source and target cell belong to the same nodeB and it supports HARQ preservation, the buffer content at the time of the serving cell change will be transfered from the source cell to the target cell.
• Change in the serving HS-DSCH cell may be triggered by measurement event 1D

UE categories

The UE HSDPA categories describe the UE capabilities in terms of:

• Maximum number of HS-DSCH codes received
• Minimum inter-TTI interval
• Maximum transport block size
• Maximum number of schemes
• Supported modulation

Constellation rearrangement

• Due to the outline of the symbol constellation diagram for higher modulation degrees, certain symbols (and transmitted bits) have a shorter distance to some of the neighbors in the diagram which makes them more likely to be received in error.
• For turbo codes, systematic bits are of greater importance than parity bits.
• For these reasons, there is a gain in rearranging the symbol constellation between retransmissions with regards to both parity bits and bits that were previously received in error.

Channel Quality Indicator (CQI)

• Each of the 30 available, five-bit CQI values are directly mapped to a given transport block size, modulation scheme and number of channelization codes.
• For highest efficiency and utilization of the retransmission and error correcting coding schemes, the CQI value chosen should result in a block error rate (BLER) not exceeding 10%.
• Each step in the CQI value corresponds to an increase/decrease in the carrier-to-interference ratio by one dB.
• CQI values is not only based on measurements of the common pilot channel SIR and EcN0. Other factors include: multipath environment, terminal receiver type, ratio of interference of the own base station compared with others.

Downlink Control Signalling (HS-SCCH)

The following information is carried on the HS-SCCH:

• HS-DSCH transport format: channelization code set, modulation scheme and transport block size.
• HARQ information: HARQ process number, redundancy version and new data indicator.

The HS-SCCH information is split in two parts depending on how urgently the receiver needs the information:

• Part 1: channelization code set and modulation scheme for the HS-DSCH.
• Part 2: transport block size and HARQ parameters.

For identification needs, part 1 and 2 use different methods:

• part two contains a CRC which is also used for identification of the receiving UE
• part one uses a terminal specific masking operation which enables identification of the receiving UE

• HS-SCCH power control is recommended at every TTI and can be based on associated DPCCH power control commands and CQI's
• The highest number of different HS-SCCH’s that the UE has to monitor is 4. However, if data was received in the previous TTI and the current, there can be no change of HS-SCCH between TTI’s.

F-DPCH

• In essence, the F-DPCH is a slot format DPCH for Transmission Power Control (TPC) bits only which allows up to ten different users to share a single channelization code.
• The SRB can be mapped to the HS-DSCH using the F-DPCH

RRM for HSDPA

• At the RNC, the RRM algorithms include: resource allocation, admission control and mobility management.
• At the nodeB its link adaption, HS-SCCH power control and MAC-hs packet scheduler.