Refresher: LTE MAC Layer Protocol

This is following the RLC refresher post here. You can also view logs from real tests on a real LTE UE here.

3GPP LTE-MAC

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eICIC: Enhanced inter-cell interference coordination in 3GPP Release-10

Inter-cell interference coordination (ICIC) was introduced in Release-8/9 of the 3GPP LTE standards. The basic idea of ICIC is keeping the inter-cell interferences under control by radio resource management (RRM) methods. ICIC is inherently a multi-cell RRM function that needs to take into account information (e.g. the resource usage status and traffic load situation) from multiple cells.

Broadly speaking, the main target of any ICIC strategy is to determine the resources (bandwidth and power) available at each cell at any time. Then (and typically), an autonomous scheduler assigns those resources to users. Thus, from the Radio Resource Control perspective, there are two kind of decisions: (a) which resources will be allocated to each cell? and, (b) which resources will be allocated to each user?. Clearly, the temporality of such decisions is quite different. Whereas resources to users allocation is in the order of milliseconds, the allocation of resources to cells take much longer periods of time or can be fixed.

Static ICIC schemes are attractive for operators since the complexity of their deployment is very low and there is not need for new extra signaling out of the standard. Static ICIC mostly relies on the fractional reuse concept. This means that users are categorized according to their Signal-to-Noise-plus-Interference Ratio (SINR), that means basically according to their inter-cell interference, and different reuse factors are applied to them, being higher at regions with more interference, mostly outer regions of the cells. The total system bandwidth is divided into sub-bands which are used by the scheduler accordingly.

A simple way to explain ICIC is based on picture above. The users are divided into two categories, one is Cell Center User (CCU), and the other one is Cell Edge User (CEU). CCUs are the users distributed in the gray region of above figure, and CEUs are the users distributed in the above red, green and blue areas. CCU can use all the frequencypoints to communicate with the base station, while CEU must use corresponding specified frequency points to ensure orthogonality between different cells.

CEUs can be assigned a higher transmissionpower for the frequency reuse factor is greater than 1. The frequency points are not overlapped at the edges so the adjacent cell interference is small. CCU’s frequency reuse factor is 1; for the path loss is small and transmission power is low. Therefore the interference to the adjacent cells is not high either.

Dominant interference condition has been shown when Non-CSG/CSG users are in close proximity of Femto, in this case, Rel8/9 ICIC techniques are not fully effective in mitigating control channel interference, and hence, Enhanced interference management is needed At least the following issues should be addressed by any proposed solutions:

o Radio link monitoring (RLM)

o Radio Resource Management (including detection of PSS/SSS and PBCH)

o Interference from CRS

oo To PCFICH/PHICH/PDCCH

oo To PDSCH

o CSI measurement

o Interference from PDCCH masked with P-RNTI and SI-RNTI (for SIB-1 only) and associated PCFICH

As a result, from Release-10 onwards eICIC work was started. In Rel-10, two eICIC or Enhanced inter-cell interference coordination (also incorrectly referred to as Enhanced Inter-Cell Interference Cancellation) were being actively discussed. They are Time domain eICIC and autonomous HeNB power setting. More advanced ideas are being thought of beyong Rel-10 including Interference management techniques on carrier resolution ( optimally exploiting available Networks frequency assets (carriers in same or different bands) , combination with Carrier Aggregation; interference management schemes proposed both during LTE-Advanced Study Item phase, and during Rel-10 HetNet eICIC work.

From an earlier presentation in SON Conference:

eICIC:

- Effectively extends ICIC to DL control - time domain

- Requires synchronization at least between macro eNB and low power eNBs in its footprint

- No negative impact on legacy Rel 8 Use

Range Extension(RE)

- Refers to UE ability to connect and stay connected to a cell with low SINR

- Achieved with advanced UE receivers - DL interference cancellation (IC)

RE + eICIC technique:

– Eliminates coverage holes created by closed HeNBs

– Improves load balancing potential for macro network with low power eNBs and leads to significant network throughput increase

–Enables more UEs can be served by low power eNBs, which can lead to substantially higher network throughput

More details on eICIC is available in 3GPP CR's and TR's listed below:

• R1-105081: Summary of the description of candidate eICIC solutions, 3GPP TSG-WG1 #62, Madrid, Spain, August 23rd – 27th, 2010.
• R1-104942: Views on eICIC Schemes for Rel-10, 3GPP TSG RAN WG1 Meeting #62, Madrid, Spain, 23-27 August, 2010.
• R1-104238: eICIC Chairman’s note, 3GPP TSG RAN WG1 Meeting #61bis, Dresden, Germany, 28th June – 2nd July 2010.
• R1-103822: Enhanced ICIC considerations for HetNet scenarios, 3GPP TSG RAN WG1 #61bis Meeting, Dresden, Germany, June 28 – July 2, 2010.

You can also check out NTT Docomo's presentation on LTE Enhancements and Future Radio Access here.

Mobile Broadband Enablers in future

From a presentation by Huawei at the New Zealand Future Wireless Technologies Seminar. The presentation is available here.

Dilbert Humour: Face Recognition Apps

Source: http://www.dilbert.com/fast/2010-12-17/

Voice over LTE – a step towards future telephony (Ericsson Whitepaper)

The Fact boxes in the Appendix give a very good quick overview of how different technologies work with VoLTE.

Voice over LTE – a step towards future telephony

View more documents from Zahid Ghadialy

An Intellectual Property Rights Primer

Page 5-8 is a very good starting point to understand the IPR issues surrounding LTE.

The Essentials of Intellectual Property - Sep 2010

View more documents from Zahid Ghadialy.

An accompanying video and download information is available on Ericsson's website here.

HSPA and LTE carrier aggregation

Last week there were press releases about the Long Term HSPA Evolution. The only thing that got reported mostly is the 650Mbps peak rates. There are other interesting features in Release-11 that is covered in Nokia Siemens Networks presentation. Here is one of them:

The idea of aggregating multiple carriers to increase performance is included in both LTE and HSPA. A logical step to fully leverage existing HSPA deployments and future LTE deployments is to aggregate the capacity of both systems and tie them together into a single mobile system. The concept is illustrated in Figure 3.

The aggregation of LTE and HSPA systems enables the peak data rates of the two systems to be added together. It also allows for optimal dynamic load balancing between the two radios. A small number of active LTE and HSPA aggregation-capable devices is sufficient to exploit this load balancing gain, since the network can schedule these devices to carry more data on the radio that has lower instantaneous loading and less data on the radio with the higher load at any given moment.

Carrier aggregation is expected to have no impact on the core network.

Multicarrier and multiband HSPA aggregation

From NSN Whitepaper on HSPA Evolution:

HSPA Release 10 with 4-carrier HSDPA provides a peak downlink data rate of 168 Mbps using 2x2 MIMO (Multiple Input Multiple Output) over the 20 MHz bandwidth. This matches the LTE Release 8 data rates obtained using comparable antenna and bandwidth configurations. A natural next step for the HSPA Release 10 downlink is to further extend the supportable bandwidths to 40 MHz with 8-carrier HSDPA, doubling the Release 10 peak rate to 336 Mbps.

8-carrier HSDPA coupled with 4x4 MIMO doubles the peak rate again to reach 672 Mbps, see Figure 1. The evolution of HSPA beyond Release 10 will push the peak data rates to rival those provided by LTE Advanced.

In addition to increased peak rates, the aggregation of a larger number of carriers improves spectrum utilization and system capacity owing to inherent load balancing between carriers. Additional capacity gains from trunking and frequency domain scheduling will also be seen.

Typical spectrum allocations do not provide 40 MHz of contiguous spectrum. To overcome spectrum fragmentation, HSDPA carrier aggregation allows carriers from more than one frequency band to be combined. 3GPP Release 9 already makes it possible to achieve 10 MHz allocation by combining two 5 MHz carriers from different frequency bands, such as one carrier on 2100 MHz and another on 900 MHz.

The 4-carrier HSDPA of Release 10 extends this further, allowing the aggregation of up to four carriers from two separate frequency bands. Long Term HSPA Evolution allows eight carriers. Typical cases of HSDPA multiband aggregation are shown in Figure 2.

Packet Flow in 2.5G, 3G, 3.5G and 4G

The 'LTE Signaling' is a very interesting book just being released that is a must have for people who are involved in design, development and testing. A book that explains the basic concepts from beginning till advanced concepts and explains how different components and interfaces fit together.

Though I havent yet read this book, I have read the earlier one titled UMTS Signaling, from the same authors that is an excellent reference for understanding Signalling in UMTS. I have no doubt that this book will be the same high quality.

The Excerpt on Wiley's website provides complete chapter 1 which is quite detailed and the Packet flow pictures and details below is extracted from this book.

The first stage of the General Packet Radio Service (GPRS), that is often referred to as the 2.5G network, was deployed in live networks starting after the year 2000. It was basically a system that offered a model of how radio resources (in this case, GSM time slots) that had not been used by Circuit Switched (CS) voice calls could be used for data transmission and, hence, profitability of the network could be enhanced. At the beginning there was no pre-emption for PS (Packet Switched) services, which meant that the packet data needed to wait to be transmitted until CS calls had been finished.

In contrast to the GSM CS calls that had a Dedicated Traffic Channel (DTCH) assigned on the radio interface, the PS data had no access to dedicated radio resources and PS signaling, and the payload was transmitted in unidirectional Temporary Block Flows (TBFs) as shown in Figure 1.2.

In Release 99, when a PDP (Packet Data Protocol) context is activated the UE is ordered by the RNC (Radio Network Controller) to enter the Radio Resource Control (RRC) CELL_DCH state. Dedicated resources are assigned by the Serving Radio Network Controller (SRNC): these are the dedicated physical channels established on the radio interface. Those channels are used for transmission of both IP payload and RRC signaling – see Figure 1.7. RRC signaling includes the exchange of Non-Access Stratum (NAS) messages between the UE and SGSN.

The spreading factor of the radio bearer (as the combination of several physical transport resources on the Air and Iub interfaces is called) depends on the expected UL/DL IP throughput. The expected data transfer rate can be found in the RANAP (Radio Access Network Application Part) part of the Radio Access Bearer (RAB) assignment request message that is used to establish the Iu bearer, a GPRS Tunneling Protocol (GTP) tunnel for transmission of a IP payload on the IuPS interface between SRNC and SGSN. While the spreading factor controls the bandwidth of the radio connection, a sophisticated power control algorithm guarantees the necessary quality of the radio transmission. For instance, this power control ensures that the number of retransmitted frames does not exceed a certain critical threshold.

Activation of PDP context results also in the establishment of another GTP tunnel on the Gn interface between SGSN and GGSN. In contrast to IuPS, where tunnel management is a task of RANAP, on the Gn interface – as in (E)GPRS – the GPRS Tunneling Protocol – Control (GTP-C) is responsible for context (or tunnel) activation, modification, and deletion.

However, in Release 99 the maximum possible bit rate is still limited to 384 kbps for a single connection and, more dramatically, the number of users per cell that can be served by this highest possible bit rate is very limited (only four simultaneous 384 kbps connections per cell are possible on the DL due to the shortness of DL spreading codes).

To increase the maximum possible bit rate per cell as well as for the individual user, HSPA was defined in Releases 5 and 6 of 3GPP.

In High-Speed Downlink Packet Access (HSDPA) the High-Speed Downlink Shared Channel (HSDSCH) which bundles several High-Speed Physical Downlink Shared Channels (HS-PDSCHs) is used by several UEs simultaneously – that is why it is called a shared channel.

A single UE using HSDPA works in the RRC CELL_DCH state. For DL payload transport the HSDSCH is used, that is, mapped onto the HS-PDSCH. The UL IP payload is still transferred using a dedicated physical data channel (and appropriate Iub transport bearer); in addition, the RRC signaling is exchanged between the UE and RNC using the dedicated channels – see Figure 1.8.

All these channels have to be set up and (re)configured during the call. In all these cases both parties of the radio connection, cell and UE, have to be informed about the required changes. While communication between NodeB (cell) and CRNC (Controlling Radio NetworkController) uses NBAP (Node B Application Part), the connection between the UE and SRNC (physically the same RNC unit, but different protocol entity) uses the RRC protocol.

The big advantage of using a shared channel is higher efficiency in the usage of available radio resources. There is no limitation due to the availability of codes and the individual data rate assigned to a UE can be adjusted quicker to the real needs. The only limitation is the availability of processing resources (represented by channel card elements) and buffer memory in the base station.

From the user plane QoS perspective the two major targets of LTE are:

• a further increase in the available bandwidth and maximum data rate per cell as well as for the individual subscriber;

• reducing the delays and interruptions in user data transfer to a minimum.

These are the reasons why LTE has an always-on concept in which the radio bearer is set up immediately when a subscriber is attached to the network. And all radio resources provided to subscribers by the E-UTRAN are shared resources, as shown in Figure 1.9. Here it is illustrated that the IP payload as well as RRC and NAS signaling are transmitted on the radio interfaces using unidirectional shared channels, the UL-SCH and the Downlink Shared Channel (DL-SCH). The payload part of this radio connection is called the radio bearer. The radio bearer is the bidirectional point-to-point connection for the user plane between the UE and eNodeB (eNB). The RAB is the user plane connection between the UE and the Serving Gateway (S-GW) and the S5 bearer is the user plane connection between the S-GW and public data network gateway (PDN-GW).

The end-to-end connection between the UE and PDN-GW, that is, the gateway to the IP world outside the operator’s network, is called a PDN connection in the E-UTRAN standard documents and a session in the core network standards. Regardless, the main characteristic of this PDN connection is that the IP payload is transparently tunneled through the core and the radio access network.

To control the tunnels and radio resources a set of control plane connections runs in parallel with the payload transport. On the radio interface RRC and NAS signaling messages are transmitted using the same shared channels and the same RLC transport layer that is used to transport the IP payload.

RRC signaling terminates in the eNB (different from 3G UTRAN where RRC was transparently routed by NodeB to the RNC). The NAS signaling information is – as in 3G UTRAN – simply forwarded to the Mobility Management Entity (MME) and/or UE by the eNB.

You can read in detail about all these things and much more from the Wiley's website here.

The Innovation Driving Femto-powered Small Cells

Intelligence: the Innovation Driving Femto-powered 
Small Cells

View more presentations from Ubiquisys Femtocell.

Slideshow as presented by Richard Staveley from Ubiquisys at Femtocell Americas 2010.