MobTech

OFDMA vs SC-FDMA

2009-12-11T10:49:00.012Z

The multiple access of the radio channel in LTE is based on OFDMA (Orthogonal Frequency Division Multiple Access) in the DL and SC-FDMA (Single-Carrier Frequency Division Multiple Access) in the UL. These two techniques have a number of similarities but also some major differences as the UL and DL have different issues to cope with.

Both multiple access techniques aim at spreading the narrowband data signal over a higher bandwidth to provide high data rates and cope with the frequency selective radio channels. In order to achieve that the wideband channel is split into a number of narrowband subcarriers and each user is allocated subcarriers which are not fading for the user’s particular radio channel. The data for this user is then transmitted using these subcarriers only. The main advantage of OFDMA and SC-FDMA over other multicarrier techniques is that the subcarriers are orthogonal to each other allowing higher spectrum efficiency.

Let's first analyse the OFDMA technique which is a bit more standard than the SC-FDMA. The signal processing chain is shown in the next figure:

Each data symbol (QAM symbol) modulates a single narrowband subcarrier (15kHz). All these subcarriers are orthogonal to each other in order to avoid interference. All the modulated symbols are transmitted simultaneously over the air.

Now the superposition of all the modulated subcarriers is a noise-like signal according to the central limit theorem. Thus the variation of the signal amplitude is quite high leading to a high Peak-to-Average-Ratio (PAPR). This requires expensive power amplifiers and high power transmission which is acceptable for the eNB but not desirable for the UE.

Solution to this problem is the introduction of SC-FDMA which is a similar technique offering the same advantages as OFDMA without the high PAPR issue. In SC-FDMA each data symbol modulates the whole used wideband carrier instead of a narrowband one and the modulated symbols are transmitted sequentially over air. Thus the final transmitted signal is a single carrier one unlike OFDMA where the final signal is the superposition of a great amount of carriers.

It is obvious from the figures that the two techniques transmit the same amount of data symbols in the same time period and using the same bandwidth. However in SC-FDMA the UE needs to transmit only one carrier (wideband) at a time containing the information of one data symbol, while in OFDMA a number of subcarriers (narrowband) need to be transmitted at each time period. Thus the SC-FDMA technique is more power efficient.

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Mobility Management in LTE/EPS

2009-11-24T18:26:00.018Z

Mobility Management is the function of controlling the mobility (and security) of the UE when using the RAN while the Session Management is the function of providing IP connectivity (and resources) to the UE. Connection Management is a subfunction of Mobility Management controlling the connection of the UE to the network. These functions are implemented by protocols belonging to the NAS (Non-Access Stratum).

Mobility Management in LTE/EPS known as EPS Mobility Management (EMM) is similar to the UMTS one with some changes stemming mainly from the LTE requirement for “Always On” IP connectivity. This requirement is achieved by providing an EPS Bearer (Default EPS Bearer) to the UE once it attaches to the network and by keeping it until it switches off. Thus the Mobility and Session Management mechanisms in LTE are interconnected. For more info on the EPS Bearer look here.

Mobility Management (EMM)

The main EMM procedures are as follows:

Attach
Detach
Tracking Area update
Paging
Identification
Security Mode Control

The EMM defines two main states for the UE, while other intermediate states are also defined:

EMM-Deregistered
When the UE is not attached to the network and thus the network doesn’t know its location and cannot reach it.

EMM-Registered
When the UE is attached to the network and an EPS Bearer is established for it.

Independently of the EMM states there are two EPS Connection Management (ECM) states the UE can be in:

ECM-Idle
When there is no NAS connection established for the UE and no physical resources are allocated to it (i.e. Radio and S1 Bearers). It can still exchange data as its EPS Bearer still exists but a NAS connection needs to be established first in order to get physical resources allocated.

ECM-Connected
When there is a NAS connection established for the UE and physical resources are allocated to it. In order to get to the EMM-Registered state the UE needs to get to the ECM-Connected mode first.

The NAS connection which changes the ECM-Idle to the ECM-Connected mode is established when the UE needs to send an initial NAS message such as Attach/Detach Request, Tracking Area Update, etc. Then an RRC connection needs to be established between the UE and the eNodeB and in sequence an S1_MME connection. After that the NAS connection between the UE and the MME is established. The initial NAS message can then be sent by the UE. In the same manner the NAS connection release which brings the UE back to ECM-Idle mode takes place after the RRC connection and the S1_MME connections are released.

Session Management (ESM)

The main EPS Session Management (ESM) procedures which realize the bearer handling and IP connectivity are as follows:

Default EPS Bearer Context Activation
EPS Bearer Context Deactivation
Dedicated EPS Bearer Context Activation
EPS Bearer Context Modification
UE Requested PDN Connectivity
UE Requested Disconnect
UE Requested Bearer Resource Allocation
UE Requested Bearer Resource Modification

All these procedures can be performed only after a NAS connection is established (ECM-Connected). The Default EPS Bearer setup is performed during the EMM Attach procedure.

ESM defines two main states for the UE:

Bearer Context Inactive
When there is no EPS Bearer for the UE.

Bearer Context Active
When there is at least EPS Bearer for the UE. An EPS Bearer Context can exist even if no physical resources are allocated to the UE (i.e. Radio and S1 Bearers).

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Detailed Description of the QoS Mechanism in EPS

2009-11-18T14:15:00.015Z

A Packet Filter has to be set up at the PDN GW (and signaled to the UE) for each SDF or SDF Aggregate in order to allow the correct mapping of data to the EPS Bearer and effectively the correct routing. The EPS Bearer is associated with one TFT (one in UL and one in DL) and thus an EPS Bearer can carry only one SDF or only one SDF Aggregate while all data of the same EPS Bearer will experience the same QoS treatment meaning. The SDF can be mapped to an EPS Bearer only if they have the same QCI and ARP.

The Packet Filters are sequentially applied to the incoming data (on the UE in UL and PDN GW in DL) according to the Evaluation-Precedence-Index values of the Packet Filters and eventually the SDF (if one Packet Filter exists for the TFT) or the SDF Aggregate (If more Packet Filters exist for the TFT) is mapped to a TFT and thus to an EPS Bearer.

In UL the UE creates a binding between the SDF or SDF Aggregate and the Radio Bearer (RB) carrying it, the eNB creates a binding between the RB and S1 Bearer, the SGW creates a binding between the S1 Bearer and the S5/S8 Bearer. In the DL the PDN GW creates a binding between the SDF or SDF Aggregate and the S5/S8 Bearer carrying it, while the rest mappings are as in UL.

Any non-matching data should be sent to the Bearer which has no Packet Filters associated. If no such Bearer exists, data shall be discarded.

Traffic Flow Templates
A TFT (Traffic Flow Template) consists of one or multiple Packet Filters (1-8) and is used to discriminate data packets from different applications, with different QoS requirements, etc. in order to route them appropriately. Each Packet Filter is aimed at isolating the data from one application protocol (e.g. FTP), so effectively a TFT can carry data from more than one application protocol, which have to share the same QoS characteristics though. The TFT’s are pre-configured at the PDN GW.

Packet Filters
The Packet Filter has a unique packet identifier (1-8) within the TFT and consists of one or more of the following attributes depending on its configuration with regards to the application to be carried:

Source/Destination address with subnet mask
IP address with subnet mask; source is valid on DL and destination on UL
Protocol number
Number of higher protocol (e.g. TCP/UDP)
Destination port range
Port range of the application (e.g. HTTP)
Source port range
Port range of the application (e.g. HTTP)
IPsec security parameter index
Arbitrary number between 256 -16639 to identify the secure connection between two entities
Type of service
Identifies the QoS
Flow level
IPv6 flow level. Not used in IPv4.

Each Packet Filter has an Evaluation-Precedence-Index which is unique across all TFT’s of the same APN which indicates the priority in which the Packet Filters will be applied to the packets. Highest priority is 0 and lowest is 255. Packet filters are signaled to the UE during NAS procedures.

TFT Example

A PDN GW has 2 different TFT’s stored as follows. The first one is aimed at transferring VoIP, with two different options (over TCP or UDP) and the second one is aimed for FTP services. The PDN GW checks first the two packet filters of the VoIP TFT and then the FTP one.

When data from an FTP service reaches the PDN GW, the PDN GW applies Packet Filter 1 first, then Packet Filter 2 and finally Packet Filter 3 as per their Evaluation-Precedence-Index values. Packet Filter 3 matches its attributes (protocol number and ports) and the PDN GW creates a binding between the FTP SDF and the TFT of Packet Filter 3 (i.e. FTP TFT).Then the PDN GW forwards the data to the EPS Bearer which is associated with that TFT. The data will then follow the appropriate S5/S8, S1 and finally Radio Bearer which constitute that EPS Bearer.

For a description of the EPS Bearer and the QoS Model in EPS click here.

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QoS in LTE/EPS

2009-11-17T21:50:00.014Z

EPS Bearer Architecture

EPS introduces an E2E QoS model based on the EPS Bearer, which replaces the PDP Context of GPRS. The EPS Bearer is a logical link between the UE and the PDN GW providing a specific QoS along the whole path. The EPS Bearer Architecture consists of multiple layers as in UMTS and the service of each layer is provided through the service of the layer immediately below it. Thus the required QoS to the end user is provided by each Bearer across all layers. The E2E Service is completed by an external Bearer from the edge of PLMN to the final external destination and not controlled by the PLMN.

Bearer Level QoS Paremeters

The QoS of an EPS Bearer is defined by the following QoS parameters:

QCI (Quality Class Identifier)
QCI is an index referring to a number of different sets of minimum QoS characteristics, such as priority, delay, etc. required by a service. The different QCI’s can be achieved by different packet forwarding treatment (e.g. scheduling, queue management, RLC config, etc.) at the network nodes. The network has to be pre-configured to provide the supported QCI’s. There are 9 standardised QCI’s and associated with example services. The characteristics of the QCI are:

ARP (Allocation and Retention Priority)
It is a priority indicator in order to allow the network to reject the establishment or modification of new Bearers or discard existing ones in cases of limited resources. After the Bearer establishment, it does not affect routing.

GBR (Guaranteed Bit Rate)
Applies to Bearers with Resource Type = GBR and indicates the minimum bitrate to be provided for this service.

MBR (Max Bit Rate)
Applies to Bearers with Resource Type = GBR and indicates the maximum bitrate to be provided for this service. GBR=MBR for now.

APN - AMBR (APN – Aggregated MBR)
Applies to Bearers with Resource Type = non-GBR. It is the maximum bitrate allowed across all Bearers of the same UE for each UE-APN connection and it is stored in HSS. One non-GBR Bearer can have the whole capacity if all other non-GBR are zero.

UE - AMBR (UE – Aggregated MBR)
Applies to Bearers with Resource Type = non-GBR. It is the maximum bitrate allowed of the same UE and it is stored in HSS. One non-GBR Bearer can have the whole capacity if all other non-GBR are zero. UE-AMBR = sum(APN-AMBR) for all APN connections of the UE.

QoS Mechanism
When an UE is attaching to the network, an EPS Bearer is always set up and lasts until the UE detaches in order to provide an ‘Always On’ IP connectivity, reducing set up latency and excessive signaling. This Bearer is the Default Bearer and its QoS parameters are set by the network based on the UE subscription profile stored in the HSS. The Default Bearer is always non-GBR. More EPS Bearers can be established for one UE, known as Dedicated Bearers. The Dedicated Bearers are modified or created always after network trigger. The GBR Bearers have fixed allocated dedicated network resources related to the GBR, while non-GBR not.

In order to map the incoming data packets to the correct EPS Bearer, the packets need to have a QoS requirement which will then be satisfied by the Bearer. All the data packets from one application protocol (e.g. FTP) within the same IP-CAN session constitute an SDF (Service Data Flow) and the SDF has a QoS requirement as defined by its QoS parameters, described before: QCI, ARP, MBR, and GBR. A set of SDF’s within the same IP-CAN session with the same QCI and ARP constitutes an SDF Aggregate which can be treated in the same way as a single SDF. One EPS Bearer can then carry all the SDF’s which have exactly the same QoS parameters as the Bearer. Thus an EPS Bearer carries the data from one application or from multiple applications which share exactly the same QoS requirements.

The exact mechanism for the mapping of SDF’s to EPS Bearers is using the TFT concept and is described in detail here.

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Do We Actually Need LTE??

2009-11-17T19:25:00.016Z

Everyone involved somehow in the mobile technologies and market knows very well that the hot topic of the industry is LTE (Long Term Evolution of UMTS) or EPS (Evolved Packet System) as is the correct term as per 3GPP standards.

From a technology point of view it is easy to understand that EPS is by far better than 2G and 3G in order to support the current and future packet-centric services. However a lot of the industry thought leaders remain sceptical towards EPS as they claim that the return on investment of such a dramatic change is low for the operators. The cost of deploying EPS is quite high as new spectrum licence has to be acquired, new equipment in the RAN and Core need to be purchased and new IP-based transmission network needs to be deployed. While on the other hand the data services haven't really taken off yet in most of the countries and the voice and SMS services remain the main source of income for most mobile operators.

I think the answer to this question came to me after I got my iPhone. The use of data services really EXPLODES once someone gets one of the new iPhone-like smartphone consuming on average 30 times more bandwidth than a voice user! Even people who never used to do packet services before, become regular data users with the smartphones. And the experience from UK and US has shown that once these devices become available people start getting them in bulk exploding the bandwidth usage from one day to another.

One can now claim that HSPA is fast enough to provide the data rates required by most of the current applications which are not that demanding (email, browsing, IM, facebook, etc.). However the main killer is not the air interface, but the backhaul (see also The Future of Mobile Backhaul for details). Operators need to increase the backhaul bandwidth significantly and the cost for that is not trivial as they need to purchase a huge amount of leased lines in the current TDM/ATM based deployments. Moving to IP is the way forward but this what EPS is all about! Another killer is signaling. Smartphones generate so much signaling in the current voice-centric networks that make it really inefficient for the capacity but also for the mobile's battery life.

Moving a bit away from the immediate problems that operators are facing now and thinking longer term, we will see that EPS will allow the support of completely new services such as online gaming, mobile TV, etc. which are now very difficult to get with current networks. These services may seem very distant for most of the people, but we shouldn't underestimate the fact that young people and kids or the "internet generation" as we call them adapt very fast to the new fancy services and change their consuming behaviours very quickly. Facebook is a good example, which has almost become the killer application that 3G operators were looking for a long time.

Finally if EPS can provide the data rates that is promising, then mobile operators can drastically enter the broadband market and compete with the fixed line operators making significant amount of business in this field before WiMAX operators do so. Why should I have a fixed broadband service if I can get all my telecoms services with one contract? Furthermore they could also provide mobile broadband to countries with poor infrastracture and become the main broadband providers as their implementation costs will be lower than the fixed ones.

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HSPA Evolution

2009-11-15T23:33:00.028Z

HSPA Evolution (HSPA+) is the latest enhancement of the existing WCDMA/HSPA technologies as defined by 3GPP standards Releases 7&8. HSPA+ is aimed at improving the performance of current 3G networks while making another step closer to LTE/EPS implementations. It is an easy and cost effective solution as it can be realised by simply upgrading the software of RAN equipment.

In summary the main drivers of HSPA+ are:

Higher data rates (up to 42Mbps DL and 11Mbps UL) and lower latencies for the support of new data services (e.g. VoIP, PTT, interactive gaming)
Better spectrum utilisation will provide higher throughput and also lower cost for the operators
Allow a packet-only operation for both voice and data (better support for VoIP)
Strengthen the role of mobile technologies to the competition with fixed broadband networks and keep up with WiMAX technologies
Exploit the full CDMA capabilities and provide a smooth migration towards LTE/EPS

The major technical characteristics introduced by HSPA+ are:

Higher Order Modulation Schemes
=> higher data rates

64QAM is the highest allowed modulation scheme for HSPDA (from 16QAM in Release 6)
16QAM is the highest allowed modulation scheme for HSUPA (from QPSK in Release 6)

MIMO for DL
=> higher data rates

2 Tx antennas at NodeB transmitting different streams with up to 64QAM modulation scheme and at least 2 Rx antennas at UE

L2 enhancements in both UL and DL
=> higher data rates, higher throughput

Larger RLC PDU sizes can be used to support the higher rates achieved
Flexible RLC size selection
RLC PDU segmentation
RLC PDU's of different RAB's can be multiplexed in one MAC PDU to reduce overheads

Continuous connectivity of data users (CPC)
=> lower latencies, improved battery life, lower interference, increased capacity

Remain in the CELL_DCH even when inactive to reduce latencies
UE DTX/DRX of the Dedicated Physical Control CHannel (DPCCH) when there is no data to send and receive
No transmission of the High Speed - Shared Control CHannel (HS-SCCH) to reduce signaling and improve capacity; the decoding of the HS-DSCH is achieved blindly

Enhanced CELL_FACH
=> higher data rates, lower latencies

HSDPA traffic can be carried on FACH
HSUPA traffic can be carried on FACH

Improved Receivers
=> higher data rates, better efficiency

Interference cancellation of ‘other cell’ power for HSDPA terminals
Better equalisers

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How Does HSUPA Work?

2009-11-13T12:42:00.008Z

HSUPA is an improved UL for HSDPA in order to support higher data rates than the ones achieved by R’99 whilst introducing reduced UL latency as well. It uses most of the R’99 features, while employing some new methods of delivering user data from the terminal to Node B. HSUPA is standardised in 3GPP Release 6 and its main characteristics are summarised below:

Multi-code transmission
Node B scheduling
Option for shorter TTI (TTI = 2ms or TTI = 10ms)
Node B retransmissions (H-ARQ)
Variable spreading factor of minimum SF = 2
Power control
Soft handover

Channel Structure
A new transport channel dedicated to each HSUPA user carrying the user data with HSUPA operation is introduced and denoted as Enhanced Dedicated Channel (E-DCH). Only one E-DCH is dedicated to one HSUPA user and it is mapped onto a pool of newly introduced physical channels (i.e. 1, 2 or 4 E-DCH Dedicated Physical Data Channels - E-DPDCH's and 1 E-DCH Dedicated Physical Control Channel - E-DPCCH). The channel slot structure is shown below:

E-DPDCH carries the HSUPA data and has a variable spreading factor of minimum SF2. E-DPCCH is used for the transmission of the essential information for decoding E-DPDCH and has a fixed SF = 256. It also carries information about retransmissions and power allocation.

E-DPDCH transmission requires simultaneous transmission of the R’99 DPCCH and the E-DPCCH. The first one contains important information about channel estimation and power control, while the other one about decoding.

Three DL physical control channels without carrying any higher layer information are also introduced. These are the E-DCH HARQ Acknowledgement Indicator Channel (E-HICH), the E-DCH Relative Grant Channel (E-RGCH) and the E-DCH Absolute Grant Channel (E-AGCH).

The E-HICH is a dedicated channel with a fixed SF = 128 carrying DL information about retransmissions (i.e. ACK/NACK from the Node B). E-RGCH is also dedicated with SF = 128 carrying the power step up/down scheduling commands from the Node B. Both E-HICH and E-RGCH are transmitted in a single code channel using different signatures. Finally E-AGCH is a common channel with SF = 256 carrying information about the exact amount of power each terminal can use for the E-DPDCH transmission. Each terminal checks whether an E-AGCH is meant for it by recognizing its ID contained in the frame.

Multi-Code Transmission
Adaptive modulation and coding (AMC) is not used in HSUPA, as higher modulation schemes require more energy per bit and the terminals have limited power transmission capability. The solution for achieving higher data rates is found in the use of multi-code transmission, which is more efficient in the UL than AMC.

Multiple (i.e. up to 4) E-DPDCH's can be transmitted for one E-DCH (i.e. one HSUPA user) during one TTI, using code multiplexing. The multiplexed E-DPDCH's can use the same or even different SF's with the allowed combinations being described explicitly in the standards. In order of descending data rates, the allowed combinations for multi-code transmission are 2xSF2 + 2xSF4, 2xSF2, 2xSF4.

For each E-DCH transmission exactly 1 E-DPCCH has to be sent in the same TTI. E-DPCCH and the E-DPDCH'(s) carrying data of the same E-DCH are transmitted using different codes in different branches of QPSK modulation (I-Q multiplexing) leading to transmission of BPSK symbols. The E-DPCCH is always transmitted using I branch.

Node B Based Scheduling
HSUPA scheduling is also shifted to Node B as in HSDPA. However, the scheduling procedure is very similar to the R’99 rather than the HSDPA one. The scheduling algorithm simply tries to keep the cell throughput high by adjusting the E-DPDCH transmit powers and thus the TFC selections. The terminal provides information about its capability of emptying its buffers and the Node B decides whether it can transmit more power or not according to the noise rise limit.

Node B Based Re-Transmissions (H-ARQ)
H-ARQ functionality in HSUPA is very similar to the HSDPA one. The terminal keeps in memory unacknowledged packets and retransmissions take place in case of NACK reception from the Node B. Both IR and CC strategies are supported in HSUPA and the only main difference is that HSUPA HARQ procedure is synchronous. This means that the system knows which HARQ process is used without need for including such information in the data stream.

Option for Shorter TTI
HSUPA allows for the support of two TTI lengths to be chosen; TTI = 2ms and TTI = 10ms. Same as in HSDPA, shorter TTI length (i.e. 2ms) can provide for reduced delays between retransmissions and better utilisation of the resources by reacting faster to the channel conditions. When the channel conditions for a user are changed, the TFC used by the user can be easily changed as well in order to adapt to the new conditions. Furthermore TTI = 2ms supports higher data rates, as above 2Mbps the block size for TTI = 10ms becomes too large and is limited.

However for great number of users, short TTI length requires the transmission of very high number of DL control signals and thus can lead to very high consumption of the Node B power especially, for areas at the cell edge. In HSDPA, the users are time-multiplexed and such problem does not exist. Therefore, the shorter TTI is expected to be used for distances closer to the Node B and as the distance grows, TTI = 10ms will be used.

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How Does HSDPA Work?

2009-11-13T12:16:00.025Z

HSDPA represents an evolution of the WCDMA radio interface, which uses very similar methods to those employed by EDGE (Enhanced Data Rates for GSM Evolution) technology for the GSM radio interface. The fundamental characteristics which enable the increase in the data throughput and capacity with reduced latency are summarised below:

Time and code multiplexing of the users
Multi-Code transmission
Fixed Spreading Factor (SF = 16)
Shorter TTI = 2ms
No DTX (Discontinuous transmission)for the data channel
Adaptive modulation and coding (AMC) supporting higher order modulation
Node B scheduling and link adaptation
Node B retransmissions (H-ARQ - Hybrid Auotmatic Repeat-Request)
No power control
No soft handover

Channel Structure
A new transport channel carrying the user data with HSDPA operation is introduced and denoted as High Speed Downlink Shared Channel (HS-DSCH). This is mapped onto a pool of newly introduced physical channels (i.e. High Speed Physical Downlink Shared channel - HS-PDSCH) to be shared among HSDPA users in a time multiplexed manner. One user only transmits during a single TTI, which has a fixed shorter duration of 3 slots (i.e. 2ms). If code multiplexing is used as well, then multiple users can transmit during the same TTI, using different codes. The HS-PDSCH slot structure is shown below.

Two new physical control channels are also introduced for the handling of associated signalling without carrying information from higher layers. Namely, they are the High Speed Shared Control Channel (HS-SCCH) for the DL and the High Speed Dedicated Physical Control Channel (HS-DPCCH) for the UL.

HS-SCCH has a fixed spreading factor (SF = 128) and carries mainly information essential for demodulation and retransmissions. One HS-SCCH per TTI is required for each active HSDPA user. HS-DPCCH has a fixed spreading factor (SF = 256) and carries mainly channel quality information and ACK/NACK for retransmissions.

An HS-PDSCH corresponds to one channelling code of fixed spreading factor SF=16, and thus there are maximum 15 such codes available for HS-DSCH transmission under one code tree, as at least 1 such code is needed for UMTS and HSDPA control channels. It is noted here that the same code tree is used for both UMTS and HSDPA traffic when a single carrier deployment is used, to do otherwise the use of multiple scrambling codes per cell would be required. The use of a separate carrier for HSDPA would add another code tree and thus increase the capacity of the system.

Multi-code transmissions are also supported in HSDPA, which means that the user equipment can be assigned multiple codes during one TTI, depending on its capability. This allows users to transmit large amounts of data in one TTI leading to a better resource utilisation which increases system throughput. Throughput is also increased by not supporting DTX for HS-PDSCH and users are scheduled only if they have data to transmit.

AMC (Adaptive Modulation and Coding)
An HS-PDSCH may use QPSK or 16-QAM modulation symbols, and several channel coding rates depending on the channel conditions, as reported by the terminal in the HS-DPCCH frames. Thus the system adapts to the channel conditions and can achieve higher data rates when the conditions are favourable. This procedure is denoted as link adaptation and has similar effects as power control without need for power control overhead. Each modulation scheme and coding rate combination is denoted as Transport Format Combination (TFC) or Modulation and Coding Scheme (MCS).

Node B Based Fast Scheduling and Link Adaptation
The scheduling and link adaptation in HSPDA are performed in the Node B unlike R’99, where scheduling is done in the RNC. Node B estimates the channel quality for each active user based on the feedback provided by them in the HS-DPCCH frame. Afterwards the user to be served first depends on the scheduling algorithm and the user prioritisation.

In general, there are three principal scheduling algorithms depending on the equipment (the algorithms used to implement these can take many different forms) and these are:

C/I based (Max C/I) – serves in every TTI the user with the largest C/I
Round Robin (RR) – users are served in a cyclic order ignoring the channel quality conditions
Proportional Fair (PF) – serves users with the largest relative channel quality (i.e. the channel quality conditions are weighted by the user throughput)

Node B scheduling in combination with the shorter TTI provides with faster scheduling and better resource utilisation, as the network reacts faster to the channel conditions and higher rates can be achieved for users with good channel conditions.

Node B Based Retransmissions (H-ARQ)
Apart from scheduling, retransmission handling is also shifted from the RNC to Node B in HSDPA and thus from RLC to the physical layer. The packets sent to a user are kept in the Node B buffer until an ACK is received from the terminal carried in the HS-DPCCH frame. The RNC will retransmit only in case that the physical operation fails. This makes retransmissions faster, reducing the latency but also provides with combining gain, as the terminal keeps the energy of both transmissions.

The retransmission protocol selected in HSDPA is the Stop and Wait (SAW) due to the simplicity of this form of ARQ. The Hybrid ARQ (H-ARQ) technique is fundamentally different from WCDMA retransmissions because the UE decoder combines the soft information of multiple transmissions of transport blocks at bit level imposing significant memory requirements on the UE. Two different Hybrid ARQ strategies are generally used:

Chase Combining (CC) – the basic idea of the CC strategy is to transmit identical versions of the erroneously detected data packet and then use maximal ratio combining effectively providing with time diversity and soft combining gain.
Incremental Redundancy (IR) – additional redundant information is incrementally transmitted if the decoding fails on the first attempt.

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Quick HSPA Overview

2009-11-13T12:07:00.013Z

WCDMA Release '99 enables peak rates of 384kbps with latency of 100-200ms and can adequately support various voice and data services. However new services such as mobile video-streaming, voice-over-IP, real time gaming, etc. require even higher data rates and especially reduced latency making the need for a new radio interface essential.

A straightforward and cost effective upgrade of the existing UMTS networks was made possible with the 3GPP Releases 5 (High Speed DL Packet Access – HSDPA) and 6 (High Speed UL Packet Access – HSUPA), together known as High Speed Packet Access (HSPA). The two technologies aim at increasing the achieved peak data rates beyond 10Mbps in the DL and up to 3-4 Mbps in the UL whilst providing reduced latency. Apart from the performance improvement, HSPA provides an increase in the capacity and spectral efficiency allowing support of both high rate symmetric and asymmetric services.

The HSPA deployment is performed on top of the WCDMA network through the introduction of additional software and hardware to the existing network elements (i.e. Node B’s), potentially keeping the required cost relatively low. The same carrier for both WCDMA and HSPA can be used, although a separate carrier provides a greater increase in capacity. HSPA support has already been introduced for laptop computers in the form of data cards and there are also several HSDPA enabled 3G mobile phones in the market place.

Technical details about HSDPA and HSUPA can be found in How Does HSDPA Work? and How Does HSUPA Work?.

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The Future of Mobile Backhaul

2009-11-13T11:11:00.005Z

The high availability of the new smartphones, like iPhone and the introduction of the new 3G technologies – like HSPA (High Speed Packet Access) – already deployed in many countries have led to an increasing demand for new bandwidth-hungry services such as web browsing and mobile video streaming. This demand is also fuelled by the emerging 4G technologies – such as EPS (Evolved Packet System) and WiMAX (Worldwide interoperability for Microwave Access) – which are now beginning to be implemented and are aimed at realising mobile broadband. The end-users can now receive peak rates up to 45Mbps with HSPA+ which are expected to reach up to 100Mbps DL in 4G.

These high data rates can be achieved by applying a number of changes to the radio interface, such as introducing MIMO, adaptive modulation and coding along with higher modulation schemes, new multiple access techniques (OFDMA), etc.

However the high rates achieved over air can be limited by the backhaul network and lead to severely degraded service provided to the end users or even signalling failures and dropped calls. This is due to the fact that most of the current backhaul networks are deployed using TDM/ATM techniques which are efficient for voice services but not optimal for the new data services. Thus a re-design of the backhaul needs to be considered by the operators in order to cope with the forthcoming backhaul bottleneck.

The straightforward solution is to increase the number of leased lines and microwave links in order to provide more bandwidth in the backhaul. However this approach is neither cost effective nor bandwidth efficient for such a dramatic increase. Furthermore the 4G technologies require an all-IP transport logic.
Thus operators have to start introducing packet switched technologies such as MPLS/IP in the current backhaul deployments in order to be able to support the new services advertised but also to make a step towards 4G upgrade.

This upgrade will probably happen in phases. As a first step towards IP backhaul the HSPA traffic can be offloaded over Ethernet transmission network, while the voice services can still be transmitted over traditional TDM/ATM networks and equipment already support this hybrid technology. Pseudowires (PWE3) can then be introduced to emulate circuit switched networks and allow the transmission of TDM/ATM over Ethernet.

Summarising, the new technologies and the supported services require a significant increase in the supported data rates, which is realised by improvements in the radio interface. This data rate increase is causing a bottleneck in the current backhaul deployments which are mainly designed for voice services and a subsequent re-design of the backhaul is also required in order to provide an efficient and cost effective solution for the operators. IP/Ethernet technologies are the way forward for the next generation of the backhaul networks being in the form of hybrid TDM/IP solution, circuit emulation, or native IP.

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