Showing posts with label NTT DoCoMo. Show all posts
Showing posts with label NTT DoCoMo. Show all posts

Tuesday, 24 January 2012

LTE Base station equipment


If interested, more details available in the NTT Docomo whitepaper here.

Monday, 9 January 2012

Overview of LTE Handovers


From the NTT Docomo Technical journal:


The LTE handover is broadly divided into a backward handover (PS handover) and forward handover. In the former, the network performs cell switching and notifies the mobile terminal of the destination cell, and in the latter, the mobile terminal performs autonomous switching to pick up the destination cell.


To control packet loss due to a momentary cutoff at the time of radio switching, PS handover supports a data forwarding process that transfers undelivered data from the switching-source eNodeB to the switching-destination eNodeB and a reordering process that corrects sequencing mistakes between forwarded data and new data.


The forward handover can be classified into Release with Redirection triggered by a cutoff signal from the network and Non Access Stratum (NAS) Recovery in which the mobile terminal autonomously performs a NAS recovery, either of which is accompanied by data loss due to a momentary cutoff. From a different perspective, handover can be classified in the following two ways according to whether it is accompanied by Radio Access Technology (RAT) or frequency switching or by eNodeB or EPC switching (Figure 7).


1) Intra-RAT handover: This is a handover that occurs within the LTE system in which node transition occurs between sectors within an eNodeB, between eNodeBs within an EPC switch, or between EPC switches. 


A handover between eNodeBs within an EPC switch may be an X2 or S1 handover. In an X2 handover, signal processing is performed by the X2 logical interface between eNodeBs, while in an S1 handover, signal processing is performed by the S1 logical interface between an eNodeB and the EPC switch. There is a tradeoff between the cost of maintaining an X2 link and the cost incurred by an S1 handover, and operations are configured accordingly.


Handover can also be classified by whether the center frequency is the same before and after handover, that is, whether the handover occurs within the same frequency or between frequencies.


2) Inter-RAT handover: This is a handover that occurs between RATs either as a transition from LTE to 3G or from 3G to LTE.

A detailed post on LTE to 3G Inter-RAT handover is available here.

Saturday, 1 October 2011

Future Mobile devices: Winners & Losers in technology

NTT DOCOMO announced a range of futuristic ideas and products that they are going to demo at CEATEC this month. Some of the products/ideas as follows:

Extra-high-speed, next-generation LTE service

  • Experience the first Xi-compatible tablets “docomo Tablet GALAXY Tab 10.1 LTE SC-01D” and “docomo Tablet ARROWS Tab LTE F-01D,” set for October release.
  • Try out new broadband services for the high-performance Xi network, including internationally popular services and games such as Hulu and Qik Video.

Smartphone-ready device to measure acetone in breath for diet support

  • The compact device measuring acetone for diet support that can easily be used anywhere and anytime.
  • Visitors blow into the smartphone-connected device to measure acetone in their breath-the higher the concentration, the greater the level of hunger.
  • The device also identifies when people are burning fat, based on the concentration of acetone in their breath, which rises when body fat is being burnt.

Smartphone jacket for ultra-high-speed battery charging

  • This special battery jacket for smartphones achieves a full charge in just 10 minutes. A recharging indicator will show how the jacket will charge a smartphone 10~15 times faster than conventional charging devices.

Smartphone jackets for various purposes

  • Three types of special jackets for smartphones that are equipped with sensors that can be customized to measure ultraviolet light and bad breath, gamma radiation and body fat.

Environmental sensor network

  • Presentation of real-time atmospheric data (temperature, humidity, wind direction/speed, precipitation, ultraviolet intensity) collected by DOCOMO’s nationwide network of approximately 2,500 environmental sensors, and a demonstration of visualizing the data using augmented reality.

The DOCOMO booth will also offer a mobile handset recycling service, where visitors can drop off old and unwanted mobile phones, rechargers, battery packs and stands. This service is open to all mobile phone users regardless of their carrier, and mobile phones will be destroyed using specialized tools to ensure the protection of personal information.

You can watch these in action here:








I also recently attended a Cambridge Wireless Handset SIG event and David Wood gave an interesting presentation that is embedded below:

Other presentations from that event available here.

Monday, 8 August 2011

Radio-over-Fiber (RoF): The existing alternative to Femtocells

Recently while going through NTT Docomo Technical Journal, I came across an article on Radio over Fibre. This is the first time I have come across RoF but apparently this is a common way to provide indoor coverage before Femtocells.
My intention here is not to compare this with Femtocells as I can think of advantages and disadvantages of both of them.


I found the following extract in the book Femtocells: Technologies and Deployment:

Active Fibre DAS (Radio over Fibre)

Active fibre DAS is the most efficient in term of performance. Optical fibres are used to make the link between the MU and the RU. They can cover very long distances (up to 6 km) and support multiple radio services. With such a system the RU directly converts the optical signal into radio signal and vice versa. The other advantage is that optical fibre is very cheap and easy to install. Radio over fibre is now the most common technique used for indoor radio coverage. As detailed in [16], radio over fibre is today the optimal solution to extending indoor coverage, because it provides scalability, flexibility, easy expandability, and also because the signal degradation is very low compared with DAS using standard connections.


The following is from Wikipedia:

Radio over Fiber (RoF) refers to a technology whereby light is modulated by a radio signal and transmitted over an optical fiber link to facilitate wireless access. Although radio transmission over fiber is used for multiple purposes, such as in cable television (CATV) networks and in satellite base stations, the term RoF is usually applied when this is done for wireless access.

In RoF systems, wireless signals are transported in optical form between a central station and a set of base stations before being radiated through the air. Each base station is adapted to communicate over a radio link with at least one user's mobile station located within the radio range of said base station.

RoF transmission systems are usually classified into two main categories (RF-over-Fiber ; IF-over-Fiber) depending on the frequency range of the radio signal to be transported.

a) In RF-over-Fiber architecture, a data-carrying RF (Radio Frequency) signal with a high frequency (usually greater than 10 GHz) is imposed on a lightwave signal before being transported over the optical link. Therefore, wireless signals are optically distributed to base stations directly at high frequencies and converted to from optical to electrical domain at the base stations before being amplified and radiated by an antenna. As a result, no frequency up/down conversion is required at the various base station, thereby resulting in simple and rather cost-effective implementation is enabled at the base stations.

b) In IF-over-Fiber architecture, an IF (Intermediate Frequency) radio signal with a lower frequency (less than 10 GHz) is used for modulating light before being transported over the optical link. Therefore, wireless signals are transported at intermediate frequency over the optical.


Access to dead zones

An important application of RoF is its use to provide wireless coverage in the area where wireless backhaul link is not possible. These zones can be areas inside a structure such as a tunnel, areas behind buildings, Mountainous places or secluded areas such a jungle.


FTTA (Fiber to the Antenna)

By using an optical connection directly to the antenna, the equipment vendor can gain several advantages like low line losses, immunity to lightening strikes/electric discharges and reduced complexity of base station by attaching light weight Optical-to-Electrical (O/E) converter directly to antenna.


Sunday, 22 May 2011

LTE World Summit 2011 - Pics and Notes from Day 1

Here are few pics and discussions from the day 1 of LTE world summit 2011. They are quite brief and I will try and add some info from the tweets as well.

Adrian Scrase from 3GPP said that as there are already over 200 operators committed to LTE, its the fastest growing mobile technology ever.

Bart Weijermars from TMobile Netherlands said future growth will be data centric fuelled by appealing terminals, new usages, broadband and ubiquity.

Future challenges include Network sharing, required to keep the cost under control; everyone allowed to roam freely everywhere; all the content is stored in the cloud and voip is the only option for voice.

New services will be possible with the advent of '4G' but care has to be taken because background apps are already using up a lot of capacity.

There is still work that needs to be done on Spectrum, Smartphone challenges and Network of networks.

Huawei has been one of the main sponsors of the event and the award and Ying Weimin spoke on how LTE is the way to more competitive Mobile Broadband.

According to him, wireless solution is a personalized solution and will go everywhere you go.

Spectrum is the main concern though as a combination of low and high frequency will be needed. Hetnets are coming and they will be the future of the networks.

The way forward is to start the LTE with data only and build on top of that. The network should be simple evolution and will contain of cloud baseband, wideband RRU and AAS.

Pocket Wifi is definitely going to be a killer device and Innovative LTE business and Apps will be needed in the long term like Instant LTE broadcast, Wireless Video surveillance, etc.

LTE is faster than expected and this is the reason there are so many operator commitments. Huawei has 40+ LTE contracts and 10 have already been launched. This is just the beginning.


Seizo Onoe from NTT Docomo spoke about Crossy. In fact during Christmas the employees were wishing each other 'Merry Crossy'. Docomo believes that the users dont care about HSPA or LTE so the Crossy is a service they are selling to the users.

Docomo are getting 75Mbps max DL speeds (using 10MHz band). The phones are capable of 100Mbps though (category 3).

Docomo has recently announced 24 new devices. 2 are Wifi routers. Unlimited data plans on the LTE network cost 5000 yens.

On the spectrum side they are expecting the LTE network to co-exist with UMTS and will be using the 2.1GHz band. In fact Docomo thinks that 2.1GHz should be the universal band that all devices should support so In future when the networks are deployed all these existing devices start working without problems.

The RRE equipment that Docomo has been deploying works with both HSPA and LTE.

Japan has already shutdown its 2G PDC networks but other cannot do the same for GSM. Onoe-san believes that we should stop the evolution of 2G as EDGE has still been evolving and we should focus all the energy into LTE evolution.

Onoe-san did emphasise that LTE is 3.9G and not 4G.

I did check with Onoe-san later that as FOMA was not compliant to the '3G' standards completely, is Crossy compliant to the LTE Specs completely and he said it is.

Onoe-san also said that ETWS was very helpful in the recent tsunami in Japan and services like these should be standardised quickly as they will be useful for someone or the other.


Thomas Wehelier from Informa presented the survey results of LTE ecosystem.

In 2011, the LTE deployments will quadruple but 2012 will have most deployments. Spectrum fragmentation is still prevalent but the core bands for LTE are 800MHz, 1800MHz and 2.6GHz. Capacity still cited as a big driver for the deployments.

TD-LTE represents a new market and new opportunity. In fact Ericsson bills this a year of TD-LTE.


Chris Kimm from Verizon spoke on their 2020 vision.

In Dec. 10 Verizon were covering 110 million people in the US by 2013 the plan is that 290 million will have LTE coverage. In fact LTE was launched in new cities on the day. At the moment though only 250K users are using LTE.

The rate of change is breathtaking and as a result CIO has changed from Chief Information officer to chief innovation officer.

In the Q&A, Chris said that they will deploy voice in 2012 using VoLTE. OTT voice will also be ok.
Tommy Ljunggren from Teliasonera spoke of their LTE deployments.

Last year they had 2 'kids' (as he called their networks) but now they have 4 more. Norway and Sweden got their LTE network in 2009. Network in Finland was launched 30th Nov. 2010. Then on 9th Dec 2010 network in Denmark was launched followed by Estonia on 17th Dec. 2010 and finally Lithuania on 28th April 2011. In fact in Estonia the network was launched 6 minutes after the auction.

Their deployments are in 800/1800/2600 MHz band. This will give them capacity and coverage.

In Sweden the downlink speed is over 20Mbps. In Nordics and Baltics the end users can roam without borders.

Once consumers are using 4G they dont want to go back to 3G. During the royal wedding on Stockholm last year, LTE was used by a TV station to transmit from 6 movable cameras without the need of satellite or any other connection. The transmit was without jitters and a revolution. Nippon TV used LTE to transmit the Noble Peace prize live from Sweden to Tokyo. In fact a bank uses 4G connection as a backup.

TeliaSonera plans to make money by having data caps in place, monthly fees, etc. VoIP would be charged. Right now the charges are €60 for 30GB allowance on LTE.


Cameron Rejali from BT Wholesale spoke on whether the future of mobile was fixed.

According to him WiFi offload is just the start as whenever the speed of the network increases the data usage increases as well.

The network has to do a balancing act. Greater user experience versus network complexity and seamless mobility.

Finally with LTE we will have true convergence at last. The future of mobile is fixed and that of fixed is mobile.

Panel Discussion:

Adrian Scrase brought up the topic of Global Roaming. As there are already 30 bands specified for LTE, do we need a roaming band that should be standardised. Should this band be an Industry initiative or will it be market driven?

The consensus was that this will be market driven.

Question was asked if LTE will be more profitable than previous technology.

NTT Docomo believes that LTE as a technology cannot generate new revenues but the services around it can.

Adrian Scrase asked the question that a lot of Services are defined by the standards but most of them do not get deployed. Does NTT DoComo think ETWS has been defined correctly as per the standards.

Onoe-san from NTT Docomo said that this service has been of utmost importance in the recent tsunami disaster. Even though the service was implemented and available on the phones, it was not used so people were not aware of it. So when the disaster struck everyone was surprised to receive this message. Now everyone knows about this service. Docomo has been using meteorological data since 2009 for this service.

In response to another question Tommy from TeliaSonera said that they will have CSFB next year for voice and VoLTE later. I asked similar question to Onoe-san about the voice support in crossy devices and he said that they will support CSFB. Someone did mention in the panel discussion that VoLTE is not needed and CSFB is enough.

That was my summary of the first day of #LTEWS. You can read the twitter conversations that have much more information.

Wednesday, 30 March 2011

Quick Recap of MIMO in LTE and LTE-Advanced

I had earlier put up some MIMO presentations that were too technical heavy so this one is less heavy and more figures.

The following is from NTT Docomo Technical journal (with my edits):

MIMO: A signal transmission technology that uses multiple antennas at both the transmitter and receiver to perform spatial multiplexing and improve communication quality and spectral efficiency.

Spectral efficiency: The number of data bits that can be transmitted per unit time and unit frequency band.

In this blog we will first look at MIMO in LTE (Release 8/9) and then in LTE-Advanced (Release-10)

MIMO IN LTE

Downlink MIMO Technology

Single-User MIMO (SU-MIMO) was used for the downlink for LTE Rel. 8 to increase the peak data rate. The target data rates of over 100 Mbit/s were achieved by using a 20 MHz transmission bandwidth, 2 × 2 MIMO, and 64 Quadrature Amplitude Modulation (64QAM), and peak data rates of over 300 Mbit/s can be achieved using 4×4 SU-MIMO. The multi-antenna technology used for the downlink in LTE Rel. 8 is classified into the following three types.

1) Closed-loop SU-MIMO and Transmit Diversity: For closed-loop SU-MIMO transmission on the downlink, precoding is applied to the data carried on the Physical Downlink Shared Channel (PDSCH) in order to increase the received Signal to Interference plus Noise power Ratio (SINR). This is done by setting different transmit antenna weights for each transmission layer (stream) using channel information fed back from the UE. The ideal transmit antenna weights for precoding are generated from eigenvector(s) of the covariance matrix of the channel matrix, H, given by HHH, where H denotes the Hermitian transpose.

However, methods which directly feed back estimated channel state information or precoding weights without quantization are not practical in terms of the required control signaling overhead. Thus, LTE Rel. 8 uses codebook-based precoding, in which the best precoding weights among a set of predetermined precoding matrix candidates (a codebook) is selected to maximize the total throughput on all layers after precoding, and the index of this matrix (the Precoding Matrix Indicator (PMI)) is fed back to the base station (eNode B) (Figure 1).


LTE Rel. 8 adopts frequency-selective precoding, in which precoding weights are selected independently for each sub-band of bandwidth from 360 kHz to 1.44 MHz, as well as wideband precoding, with single precoding weights that are applied to the whole transmission band. The channel estimation used for demodulation and selection of the precoding weight matrix on the UE is done using a cell specific Reference Signal (RS) transmitted from each antenna. Accordingly, the specifications require the eNode B to notify the UE of the precoding weight information used for PDSCH transmission through the Physical Downlink Control Channel (PDCCH), and the UE to use this information for demodulation.

LTE Rel. 8 also adopts rank adaptation, which adaptively controls the number of transmission layers (the rank) according to channel conditions, such as the received SINR and fading correlation between antennas (Figure 2). Each UE feeds back a Channel Quality Indicator (CQI), a Rank Indicator (RI) specifying the optimal rank, and the PMI described earlier, and the eNode B adaptively controls the number of layers transmitted to each UE based on this information.

2) Open-loop SU-MIMO and Transmit Diversity: Precoding with closed-loop control is effective in low mobility environments, but control delay results in less accurate channel tracking ability in high mobility environments. The use of open-loop MIMO transmission for the PDSCH, without requiring feedback of channel information, is effective in such cases. Rank adaptation is used, as in the case of closed-loop MIMO, but rank-one transmission corresponds to open-loop transmit diversity. Specifically, Space-Frequency Block Code (SFBC) is used with two transmit antennas, and a combination of SFBC and Frequency Switched Transmit Diversity (FSTD) (hereinafter referred to as “SFBC+FSTD”) is used with four transmit antennas. This is because, compared to other transmit diversity schemes such as Cyclic Delay Diversity (CDD), SFBC and SFBC+FSTD achieve higher diversity gain, irrespective of fading correlation between antennas, and achieve the lowest required received SINR. On the other hand, for PDSCH transmission with rank of two or higher, fixed precoding is used regardless of channel variations. In this case, cyclic shift is performed before applying the precoding weights, which effectively switches precoding weights in the frequency domain, thereby averaging the received SINR is over layers.

3) Adaptive Beamforming: Adaptive beamforming uses antenna elements with a narrow antenna spacing of about half the carrier wavelength and it has been studied for use with base stations with the antennas mounted in a high location. In this case beamforming is performed by exploiting the UE Direction of Arrival (DoA) or the channel covariance matrix estimated from the uplink, and the resulting transmit weights are not selected from a codebook. In LTE Rel. 8, a UE-specific RS is defined for channel estimation in order to support adaptive beamforming. Unlike the cell-specific RS, the UE specific RS is weighted with the same weights as the data signals sent to each UE, and hence there is no need to notify the UE of the precoding weights applied at the eNode B for demodulation at the UE. However, its effectiveness is limited in LTE Rel. 8 because only one layer per cell is supported, and it is an optional UE feature for Frequency Division Duplex (FDD).

Uplink MIMO Technology

On the uplink in LTE Rel. 8, only one-layer transmission was adopted in order to simplify the transmitter circuit configuration and reduce power consumption on the UE. This was done because the LTE Rel. 8 target peak data rate of 50 Mbit/s or more could be achieved by using a 20 MHz transmission bandwidth and 64QAM and without using SU-MIMO. However, Multi-User MIMO (MU-MIMO) can be used to increase system capacity on the LTE Rel. 8 uplink, using multiple receiver antennas on the eNode B. Specifically, the specification requires orthogonalization of the demodulation RSs from multiple UEs by assigning different cyclic shifts of a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence to the demodulation RSs, so that user signals can be reliably separated at the eNode B. Demodulation RSs are used for channel estimation for the user-signal separation process.


MIMO TECHNOLOGY IN LTE-ADVANCED

Downlink 8-Layer SU-MIMO Technology

The target peak spectral efficiency in LTE-Advanced is 30 bit/s/Hz. To achieve this, high-order SU-MIMO with more antennas is necessary. Accordingly, it was agreed to extend the number of layers of SU-MIMO transmission in the LTE-Advanced downlink to a maximum of 8 layers. The number of transmission layers is selected by rank adaptation. The most significant issue with the radio interface in supporting up to 8 layers is the RS structure used for CQI measurements and PDSCH demodulation.

1) Channel State Information (CSI)-RS: For CQI measurements with up-to-8 antennas, new CSI-RSs are specified in addition to cell-specific RS defined in LTE Rel. 8 for up-to-four antennas. However, in order to maintain backward compatibility with LTE Rel. 8 in LTE-Advanced, LTE Rel. 8 UE must be supported in the same band as in that for LTE-Advanced. Therefore, in LTE Advanced, interference to the PDSCH of LTE Rel. 8 UE caused by supporting CSI-RS must be minimized. To achieve this, the CSI-RS are multiplexed over a longer period compared to the cell-specific RS, once every several subframes (Figure 3). This is because the channel estimation accuracy for CQI measurement is low compared to that for demodulation, and the required accuracy can be obtained as long as the CSIRS is sent about once per feedback cycle. A further reason for this is that LTE-Advanced, which offers higher data-rate services, will be developed to complement LTE Rel. 8, and is expected to be adopted mainly in low-mobility environments.


2) UE-specific RS: To allow demodulation of eight-layer SU-MIMO, the UE-specific RS were extended for SU-MIMO transmission, using a hybrid of Code Division Multiplexing (CDM) and Frequency Division Multiplexing (FDM) (Figure 4). The UE-specific RS pattern for each rank (number of layers) is shown in Figure 5. The configuration of the UE-specific RS in LTE-Advanced has also been optimized differently from those of LTE Rel.8, extending it for SU-MIMO as well as adaptive beamforming, such as by applying twodimensional time-frequency orthogonal CDM to the multiplexing between transmission layers.


Downlink MU-MIMO Technology

In addition to the peak data rate, the system capacity and cell-edge user throughput must also be increased in LTE-Advanced compared to LTE Rel. 8. MU-MIMO is an important technology for satisfying these requirements. With MU-MIMO and CoMP transmission (described earlier), various sophisticated signal processing techniques are applied at the eNode B to reduce the interference between transmission layers, including adaptive beam transmission (zero-forcing, block diagonalization, etc.), adaptive transmission power control and simultaneous multi-cell transmission. When these sophisticated transmission techniques are applied, the eNode B multiplexes the UE-specific RS described above with the PDSCH, allowing the UE to demodulate the PDSCH without using information about transmission technology applied by the eNode B. This increases flexibility in applying sophisticated transmission techniques on the downlink. On the other hand, PMI/CQI/RI feedback extensions are needed to apply these sophisticated transmission techniques, and this is currently being discussed actively at the 3GPP.

Uplink SU-MIMO Technology

To reduce the difference in peak data rates achievable on the uplink and downlink for LTE Rel. 8, a high target peak spectral efficiency of 15 bit/s/Hz was specified for the LTE-Advanced uplink. To achieve this, support for SU-MIMO with up to four transmission antennas was agreed upon. In particular, the two-transmission-antenna SU-MIMO function is required to satisfy the peak spectral efficiency requirements of IMT-Advanced.

For the Physical Uplink Shared Channel (PUSCH), it was agreed to apply SU-MIMO with closed-loop control using multiple antennas on the UE, as well as codebook-based precoding and rank adaptation, as used on the downlink. The eNode B selects the precoding weight from a codebook to maximize achievable performance (e.g., received SINR or user throughput after precoding) based on the sounding RS, which is used for measuring the quality of the channel transmitted by the UE. The eNode B notifies the UE of the selected precoding weight together with the resource allocation information used by the PDCCH. The precoding for rank one contributes to antenna gain, which is effective in increasing cell edge user throughput. However, considering control-information overhead and increases in Peak-to-Average Power Ratio (PAPR), frequency-selective precoding is not very effective in increasing system throughput, so only wideband precoding has been adopted.

Also, for rank two or higher, when four transmission antennas are used, the codebook has been designed not to increase the PAPR. The demodulation RS, which is used for channel estimation, is weighted with the same precoding weight as is used for the user data signal transmission. Basically, orthogonalization is achieved by applying a different cyclic shift to each layer, but orthogonalizing the code region using block spread together with this method is adopted.


Uplink Transmit Diversity Technology

Closed-loop transmit diversity is applied to PUSCH as described above for SU-MIMO. Application of transmit diversity to the Physical Uplink Control Channel (PUCCH) is also being studied. For sending retransmission request Acknowledgment (ACK) and Negative ACK (NAK) signals as well as scheduling request signals, application of Spatial Orthogonal-Resource Transmit Diversity (SORTD) using differing resource blocks per antenna or an orthogonalizing code sequence (cyclic shift, block spread sequence) has been agreed upon (Figure 6). However, with LTE-Advanced, the cell design must be done so that LTE Rel. 8 UE get the required quality at cell-edges, so applying transmit diversity to the control channels cannot contribute to increasing the coverage area, but only to reducing the transmission power required.

Monday, 21 March 2011

A quick primer on Coordinated Multi-point (CoMP) Technology

From NTT Docomo Technical Journal:

CoMP is a technology which sends and receives signals from multiple sectors or cells to a given UE. By coordinating transmission among multiple cells, interference from other cells can be reduced and the power of the desired signal can be increased.

Coordinated Multi-point Transmission/Reception:

The implementation of intracell/inter-cell orthogonalization on the uplink and downlink in LTE Rel. 8 contributed to meeting the requirements of capacity and cell-edge user throughput. On the downlink, simultaneously connected UE are orthogonalized in the frequency domain. On the other hand, they are orthogonalized on the uplink, in the frequency domain as well as the code domain, using cyclic shift and block spreading. It is possible to apply fractional frequency reuse (A control method which assigns different frequency ranges for cell-edge UE) to control interference between cells semi-statically, but this is done based on randomization in LTE Rel. 8. Because of this, we are planning to study CoMP technology, which performs signal processing for coordinated transmission and reception by multiple cells to one or more UE, as a technology for Rel. 11 and later in order to extend the intracell/ inter-cell orthogonalization in LTE Rel. 8 to operate between cells.


Independent eNode B and Remote Base Station Configurations:

There are two ways to implement CoMP technology: autonomous distributed control based on an independent eNode B configuration, or centralized control based on Remote Radio Equipment (RRE) (Figure 7). With an independent eNode B configuration, signaling over wired transmission paths is used between eNode B to coordinate among cells. Signaling over wired transmission paths can be done with a regular cell configuration, but signaling delay and overhead become issues, and ways to increase signaling speed or perform high-speed signaling via UE need study. With RRE configurations, multiple RREs are connected via an optical fiber carrying a baseband signal between cells and the central eNode B, which performs the baseband signal processing and control, so the radio resources between the cells can be controlled at the central eNode B. In other words, signaling delay and overhead between eNode B, which are issues in independent eNode B configurations, are small in this case, and control of high speed radio resources between cells is relatively easy. However, high capacity optical fiber is required, and as the number of RRE increases, the processing load on the central eNode B increases, so there are limits on how this can be applied. For these reasons, it is important to use both distributed control based on independent eNode B configurations and centralized control based on RRE configurations as appropriate, and both are being studied in preparation for LTE-Advanced.

Downlink Coordinated Multi-point Transmission:

Downlink coordinated multi-point transmission can be divided into two categories: Coordinated Scheduling/ Coordinated Beamforming (CS/CB), and joint processing (Figure 8). With CS/CB, a given subframe is transmitted from one cell to a given UE, as shown in Fig. 8 (a), and coordinated beamforming and scheduling is done between cells to reduce the interference caused to other cells. On the other hand, for joint processing, as shown in Fig. 8 (b-1) and (b-2), joint transmission by multiple cells to a given UE, in which they transmit at the same time using the same time and frequency radio resources, and dynamic cell selection, in which cells can be selected at any time in consideration of interference, are being studied. For joint transmission, two methods are being studied: non-coherent transmission, which uses soft-combining reception of the OFDM signal; and coherent transmission, which does precoding between cells and uses in-phase combining at the receiver.

Uplink Multi-cell Reception:

With uplink multi-cell reception, the signal from a UE is received by multiple cells and combined. In contrast to the downlink, the UE does not need to be aware of whether multi-cell reception is occurring, so it should have little impact on the radio interface specifications.

Wednesday, 9 March 2011

ETWS detailed in LTE and UMTS

Its been couple of years since the introductory post on 3GPP Earthquake and Tsunami Warning service (ETWS). The following is more detailed post on ETWS from the NTT Docomo technical journal.

3GPP Release 8 accepted the standard technical specification for warning message distribution platform such as Area Mail, which adopts pioneering technology for faster distribution, in order to fulfil the requirements concerning the distribution of emergency information e.g. earthquakes, tsunamis and so on in LTE/EPC. The standard specifies the delivery of emergency information in two levels. The Primary Notification contains the minimum, most urgently required information such as “An earthquake occurred”; the Secondary Notification includes supplementary information not contained in the Primary Notification, such as seismic intensity, epicentre, and so on. This separation allows implementation of excellent information distribution platforms that can achieve the theoretically fastest speed of the warning distribution.

The purpose of the ETWS is to broadcast emergency information such as earthquake warnings provided by a local or national governments to many mobile terminals as quickly as possible by making use of the characteristic of the widespread mobile communication networks.

The ETWS, in the same way as Area Mail, detects the initial slight tremor of an earthquake, the Primary Wave (P wave - The first tremor of an earthquake to arrive at a location), and sends a warning message that an earthquake is about to happen to the mobile terminals in the affected area. ETWS can deliver the first notification to mobile terminals in the shortest theoretical time possible in a mobile communication system (about four seconds after receiving the emergency information from the local or national government), which is specified as a requirement by 3GPP.

The biggest difference between Area Mail and the ETWS is the disaster notification method (Figure 1). Earthquake warnings in Area Mail have a fixed-length message configuration that notifies of an earthquake. ETWS, on the other hand, achieves distribution of the highest priority information in the shortest time by separating out the minimum information that is needed with the most urgency, such as “Earthquake about to happen,” for the fastest possible distribution as a Primary Notification; other supplementary information (seismic intensity, epicentre, etc.) is then distributed in a Secondary Notification. This distinction thus implements a flexible information distribution platform that prioritizes information distribution according to urgency.

The Primary Notification contains only simple patterned disaster information, such as “Earthquake.” When a mobile terminal receives a Primary Notification, it produces a pre-set alert sound and displays pre-determined text on the screen according to the message content to notify users of the danger. The types of disaster that a Primary Notification can inform about are specified as “Earthquake,” “Tsunami,” “Tsunami + Earthquake,” “Test” and “Other,” regardless of the type of radio access.

The Secondary Notification contains the same kind of message as does the existing Area Mail service, which is, for example, textual information distributed from the network to the mobile terminal to inform of the epicentre, seismic intensity and other such information. That message also contains, in addition to text, a Message Identifier and Serial Number that identifies the type of disaster.

A major feature of the ETWS is compatibility with international roaming. Through standardization, mobile terminals that can receive ETWS can receive local emergency information when in other countries if the local network provides the ETWS service. These services are provided in a manner that is common to all types of radio access (3G, LTE, etc.).

Network Architecture

The ETWS platform is designed based on the Cell Broadcast Service (CBS). The ETWS network architecture is shown in Figure 2. Fig. 2 also shows the architecture for 3G network to highlight the features differences between LTE and 3G.

In the ETWS architecture for 3G, a Cell Broadcast Centre (CBC), which is the information distribution server, is directly connected to the 3G Radio Network Controller (RNC). The CBC is also connected to the Cell Broadcast Entity (CBE), which distributes information from the Meteorological Agency and other such sources.

In an LTE radio access network, however, the eNodeB (eNB) is directly connected to the core network, and eNB does not have a centralized radio control function as the one provided by the RNC of 3G. Accordingly, if the same network configuration as used for 3G were to be adopted, the number of eNB connected to the CBC would increase and add to the load on the CBC. To overcome that issue, ETWS for LTE adopts a hierarchical architecture in which the CBC is connected to a Mobility Management Entity (MME).

The MME, which acts as a concentrator node, is connected to a number of eNBs. This architecture gives advantages to the network, such as reducing the load in the CBC and reducing the processing time, and, thus preventing delay in distribution.

Message Distribution Area

In the 3G ETWS and Area Mail systems, the distribution area can be specified only in cell units, which creates the issue of huge distribution area database in CBC. In LTE ETWS, however, the distribution area is specified in three different granularities (Figure 3). This allows the operator to perform area planning according to the characteristic of the warning/emergency occasions, e.g. notice of an earthquake with a certain magnitude needs to be distributed in a certain width of area, thus allowing efficient and more flexible broadcast of the warning message.

1) Cell Level Distribution Area: The CBC designates the cell-level distribution areas by sending a list of cell IDs. The emergency information is broadcasted only to the designated cells. Although this area designation has the advantage of being able to pinpoint broadcast distribution to particular areas, it necessitates a large processing load in the network node (CBC, MME and eNB) especially when the list is long.

2) TA Level Distribution Area: In this case, the distribution area is designated as a list of Tracking Area Identities (TAIs). TAI is an identifier of a Tracking Area (TA), which is an LTE mobility management area. The warning message broadcast goes out to all of the cells in the TAIs. This area designation has the advantage of less processing load when the warning message has to be broadcast to relatively wide areas.

3) EA Level Distribution Area: The Emergency Area (EA) can be freely defined by the operator. An EA ID can be assigned to each cell, and the warning message can be broadcasted to the relevant EA only. The EA can be larger than a cell and is independent of the TA. EA is a unit of mobility management. EA thus allows flexible design for optimization of the distribution area for the affected area according to the type of disaster.




Message Distribution

The method of distributing emergency information to LTE radio networks is shown in Figure 4. When the CBC receives a request for emergency information distribution from CBE, it creates the text to be sent to the terminals and specifies the distribution area from the information in the request message (Fig. 4 (1) (2)).

Next, the CBC sends a Write-Replace Warning Request message to the MME of the specified area. This message contains information such as disaster type, warning message text, message distribution area, Primary Notification information, etc. (Fig. 4 (3)). When the MME receives this message, it sends a response message to the CBC to notify that the message was correctly received. The CBC then notifies the CBE that the distribution request was received and the processing has begun (Fig. 4 (4) (5)). At the same time, the MME checks the distribution area information in the received message (Fig. 4 (6)) and, if a TAI list is included, it sends the Write-Replace Warning Request message only to the eNB that belong to the TAI in the list (Fig. 4 (7)). If the TAI list is not included, the message is sent to all of the eNB to which the MME is connected.

When the eNB receives the Write-Replace Warning Request message from the MME, it determines the message distribution area based on the information included in the Write-Replace Warning Request message (Fig. 4 (8)) and starts the broadcast (Fig. 4 (9) (10)). The following describes how the eNB processes each of the specified information elements.

1) Disaster Type Information (Message Identifier/Serial Number): If an on-going broadcast of a warning message exists, this information is used by the eNB to decide whether it shall discard the newly received message or overwrite the ongoing warning message broadcast with the newly received one. Specifically, if the received request message has the same type as the message currently being broadcasted, the received request message is discarded. If the type is different from the message currently being broadcast, the received request message shall overwrite the ongoing broadcast message and the new warning message is immediately broadcasted.

2) Message Distribution Area (Warning Area List): When a list of cells has been specified as the distribution area, the eNB scans the list for cells that it serves and starts warning message broadcast to those cells. If the message distribution area is a list of TAIs, the eNB scans the list for TAIs that it serves and starts the broadcast to the cells included in those TAIs. In the same way, if the distribution area is specified as an EA (or list of EAs), the eNB scans the EA ID list for EA IDs that it serves and starts the broadcast to the cells included in the EA ID.

If the received Write-Replace Warning Request message does not contain distribution area information, the eNB broadcasts the warning message to all of the cells it serves.

3) Primary Notification Information: If Primary Notification information indication exists, that information is mapped to a radio channel that is defined for the broadcast of Primary Notification.

4) Message Text: The eNB determines whether or not there is message text and thus whether or not a Secondary Notification needs to be broadcasted. If message text exists, that text is mapped to a radio channel that is defined for the broadcast of Secondary Notification. The Secondary Notification is broadcast according to the transmission intervals and number of transmissions specified by the CBC. Upon the completion of a broadcast, the eNB returns the result to the MME (Fig. 4 (11)).


Radio Function Specifications

Overview : In the previous Area Mail service, only mobile terminals in the standby state (RRC_IDLE) could receive emergency information, but in ETWS, emergency information can be received also by mobile terminals in the connected state (RRC_CONNECTED), and hence the information can be delivered to a broader range of users. In LTE, when delivering emergency information to mobile terminals, the eNB sends a bit in the paging message to notify that emergency information is to be sent (ETWS indication), and sends the emergency information itself as system information broadcast. In 3G, on the other hand, the emergency information is sent through the paging message and CBS messages.

Message Distribution method for LTE: When the eNB begins transmission of the emergency information, a paging message in which the ETWS indication is set is sent to the mobile terminal. ETWS-compatible terminals, whether in standby or connected, try to receive a paging message at least once per default paging cycle, whose value is specified by the system information broadcast and can be set to 320 ms, 640 ms, 1.28 s or 2.56 s according to the 3GPP specifications. If a paging message that contains an ETWS indication is received, the terminal begins receiving the system information broadcast that contains the emergency information. The paging message that has the ETWS indication set is sent out repeatedly at every paging opportunity, thus increasing the reception probability at the mobile terminal.

The ETWS message itself is sent as system information broadcast. Specifically, the Primary Notification is sent as the Warning Type in System Information Block Type 10 (SIB10) and the Secondary Notification is sent as a Warning Message in SIB11. By repeated sending of SIB10 and SIB11 (at an interval that can be set to 80 ms, 160 ms, 320 ms, 640 ms, 1.28 s, 2.56 s, or 5.12 s according to the 3GPP specifications), the probability of the information being received at the residing mobile terminal can be increased. In addition, the SIB10 and SIB11 scheduling information is included in SIB1 issued at 80-ms intervals, so mobile terminals that receive the ETWS indication try to receive SIB10 and SIB11 after first having received the SIB1. By checking the disaster type information (Message Identifier and Serial Number) contained in SIB10 and SIB11, the mobile terminal can prevent the receiving of multiple messages that contain the same emergency information.

3G Message Distribution Method: For faster information delivery and increased range of target uers in 3G also, the CBS message distribution control used in Area Mail was enhanced. An overview of the 3G radio system is shown in Figure 5.

In the Area Mail system, a Common Traffic Channel (CTCH) logical channel is set up in the radio link, and emergency information distribution is implemented by sending CBS messages over that channel. To inform the mobile terminals that the CTCH logical channel has been set up, the RNC orders the base station (BTS) to set the CTCH Indicator information element in the system information broadcast to TRUE, and transmits the paging message indicating a change in the system information broadcast to the mobile terminals. When the mobile terminal receives the CTCH Indicator, it begins monitoring the CTCH logical channel and can receive CBS messages.

In ETWS, by including the Warning Type in the paging message indicating a change in the system information broadcast, processing for a pop-up display and alert sound processing (Primary Notification) at the mobile terminals according to the Warning Type can be executed in parallel to the processing at the mobile terminals to start receiving the CBS messages. This enhancement allows users whose terminals are in the connected state (RRC_CONNECTED) to also receive emergency information. In the previous system, it was not possible for these users to receive emergency information. Also including disaster type information (Message Identifier and Serial Number) in this paging message makes it possible to prevent receiving multiple messages containing the same emergency information at the mobile terminal.

More detailed information (Secondary Notification) is provided in CBS messages in the same way as in the conventional Area Mail system, thus achieving an architecture that is common to ETWS users and Area Mail users.

Thursday, 3 March 2011

LTE to 3G Handover Procedure and Signalling

It may be worthwhile brushing up the LTE/SAE Interfaces and Architecture before proceeding.

1) Overview of Handover Operation

With EPC, continuous communication is possible, even while the terminal switches from one type of radio access system to another.

Specifically, in order to achieve the internal network path switching required to change radio access systems, the S-GW provides a mobility management anchor function for handover between 3GPP radio access systems, and the P-GW provides the function for handover between 3GPP and non-3GPP radio access systems. In this way, the IP address does not change when the terminal switches radio access systems, and communications can continue after handover.



In handover between the 3GPP radio access systems, LTE and 3G, handover preparation is done before changing systems, including tasks such as securing resources on the target radio access system, through cooperation between the radio access systems (Figure 3 (a)(A)). Then, when the actual switch occurs, only the network path needs to be switched, reducing handover processing time (Fig.3 (a)(B)). Also, loss of data packets that arrive at the pre-switch access point during handover can be avoided using a data forwarding function (Fig.3 (b)).

In this way, through interaction between radio access systems, fast handover without packet loss is possible, even between radio access systems such as LTE and 3G which cannot be used simultaneously.

2) Handover Preparation Procedure (Fig.3 (a)(A))

The handover preparation procedure for switching radio access from LTE to 3G is shown in Figure 4.


Step (1):The terminal sends a radio quality report containing the handover candidate base-stations and other information to the eNodeB. The eNodeB decides whether handover shall be performed based on the information in the report, identifies the base station and RNC to switch to, and begins handover preparation.

Steps (2) to (3): The eNodeB sends a handover required to the MME, sending the RNC identifier and transmission control information for the target radio access system. The MME identifies the SGSN connected to the target RNC based on the received RNC identifier and sends the communication control and other information it received from the eNodeB to the SGSN in a forward relocation request signal. The information required to configure the communications path between the S-GW and SGSN, which is used for data transmission after the MME has completed the handover, is sent at the same time.

Steps (4) to (5): The SGSN forwards the relocation request to the RNC, notifying it of the communications control information transmitted from the eNodeB. The RNC performs the required radio configuration processing based on the received information and sends a relocation response to the SGSN. Note that through this process, a 3G radio access bearer is prepared between the SGSN and RNC.

Step (6): The SGSN sends a forward relocation response to the MME in order to notify it that relocation procedure has completed. This signal also includes data issued by the SSGN and required to configure a communications path from the S-GW to the SGSN, to be used for data forwarding.

Steps (7) to (8): The MME sends a create indirect data forwarding tunnel request to the S-GW, informing it of the information issued by the SSGN that it just received. From the information that the S-GW receives, it establishes a communications path from the S-GW to the SGSN for data forwarding and sends a create indirect data forwarding tunnel response to the MME.

Through this handover preparation, target 3G radio-access resources are readied, the radio access bearer between the SGSN and RNC is configured, and the data forwarding path from the
S-GW to the SGSN configuration is completed.


3) Handover Procedure for Radio Access System Switching (Fig. 3(a)(B)):

The handover process after switching radio access system is shown in Figure 5.



Steps (1) to (2): When the handover preparation described in Fig.4 is completed, the MME sends a handover command to the eNodeB. When it receives this signal, the eNodeB sends a handover from LTE command for the terminal to switch radio systems. Note that when the eNodeB receives the handover command from the MME, it begins forwarding data packets received from the S-GW. Thereafter, packets for the terminal that arrive at the S-GW are forwarded to the terminal by the path: S-GW, eNodeB, S-GW, SGSN, RNC.

Steps (3) to (6): The terminal switches to 3G and when the radio link configuration is completed, notification that it has connected to the 3G radio access system is sent over each of the links through to the MME: from terminal to RNC, from RNC to SGSN, and from SGSN to MME. This way, the MME can perform Step (10) described below to release the eNodeB resources after a set period of time has elapsed.

Step (7): The MME sends a forward relocation complete acknowledgement to the SGSN. A set period of time after receiving this signal, the SGSN releases the resources related to data forwarding.

Step (8): The SGSN sends a modify bearer request to the S-GW to change from the communications path before the handover, between the S-GW and eNodeB, to one between the S-GW and SGSN. This signal contains information elements required to configure the path from S-GW to SGSN, including those issued by the SGSN. When the S-GW receives this signal, it configures a communications path from the S-GW to the SGSN. In this way, the communications path becomes: S-GW, SGSN, RNC, terminal; and data transmission to the target 3G radio access system begins.

Note that after this point, data forwarding is no longer needed, so the S-GW sends a packet to the eNodeB with an “End Marker” attached, and when the eNodeB receives this packet, it releases its resources related to data forwarding.

Steps (9) to (10): The S-GW sends a modify bearer response to the SGSN, indicating that handover procedure has completed. The MME also releases eNodeB resources that are no longer needed.

Through this handover procedure, data is forwarded during the handover, the switch of radio access bearer is completed, and the communications path from the P-GW to the terminal is updated.

In the examples above, we described the handover procedure between 3GPP radio access systems in which the S-GW did not change, but handovers with S-GW relocation are also possible. In these cases, the P-GW provides the anchor function for path switching, as with switches to non-3GPP access systems.

TERMS

Anchor function: A function which switches the communications path according to the area where the terminal is located, and forwards packets for the terminal to that area.

Relocation: Switching communications equipment such as area switches during communication.


Friday, 25 February 2011

Attach Sequence for LTE Radio

I have in past posted a complete Attach Sequence on the 3G4G website for LTE Radio Signaling but included the signalling on a few nodes. Recently I came across a signalling example in NTT Docomo technical journal which was less detailed but at a higher level and detailed signalling on these other nodes. It may be worthwhile brushing up the LTE Architecture diagram before diving into this.

With EPC, when a terminal connects to the LTE radio access system it is automatically connected to the PDN and this connected state is maintained continuously. In other words, as the terminal is registered on the network (attached) through the LTE radio access system, a communications path to the PDN (IP connectivity) is established.

The PDN to which a connection is established can be preconfigured on a per-subscriber basis, or the terminal can specify it during the attach procedure. This PDN is called the default PDN. With the always-on connection function, the radio link of the connection only is released after a set amount of time has elapsed without the terminal performing any communication, and the IP connectivity between the terminal and the network is maintained. By doing this, only the radio link needs to be reconfigured when the terminal begins actual communication, allowing the connection-delay time to be reduced. Also, the IP address obtained when the terminal attaches can be used until it detaches, so it is always possible to receive packets using that IP address.

The information flow for the terminal attaching to the network up until the connection to the PDN is established is shown in Figure 2 below.

Steps (1) to (3): When the terminal establishes a radio control link for sending and receiving control signals with the eNodeB, it sends an attach request to the MME. The terminal and MME perform the required security procedures, including authentication, encryption and integrity.

Steps (4) to (5): The MME sends an update location request message to the Home Subscriber Server (HSS), and the HSS records that the terminal is connected under the MME.

Step (6): To begin establishing a transmission path to the default PDN, the MME sends a create session request to the S-GW.

Steps (7) to (8): When the S-GW receives the create session request from the MME, it requests proxy binding update to the P-GW. The P-GW allocates an IP address to the terminal and notifies the S-GW of this information in a proxy binding acknowledgement message. This process establishes a continuous core-network communications path between the P-GW and the S-GW for the allocated IP address.

Step (9): The S-GW prepares a radio access bearer from itself to the eNodeB, and sends a create session response signal to the MME. The create session response signal contains information required to configure the radio access bearer from the eNodeB to the S-GW, including information elements issued by the S-GW and the IP address allocated to the terminal.

Steps (10) to (11) and (13): The MME sends the information in the create session response signal to the eNodeB in an initial context setup request signal. Note that this signaling also contains other notifications such as the attach accept, which is the response to the attach request. When the terminal receives the attach accept in Step (11), it sends an attach complete response to the MME, notifying that processing has completed.

Step (12): The eNodeB establishes the radio data link and sends the attach accept to the terminal. It also configures the radio access bearer from the eNodeB to the S-GW and sends an initial context setup response to the MME. The initial context setup response contains information elements issued by the eNodeB required to establish the radio access bearer from the S-GW to the eNodeB.

Steps (14) to (15): The MME sends the information in the initial context setup response to the S-GW in a modify bearer request signal. The S-GW completes configuration of the previously prepared radio bearer from the S-GW to the eNodeB and sends a modify bearer response to the MME.

Through these steps, a communications path from the terminal to the P-GW is established, enabling communication with the default PDN.

If the terminal performs no communication for a set period of time, the always-on connection function described above releases the radio control link, the LTE radio data link, and the LTE radio access bearer, while maintaining the core network communications path.

After the terminal has established a connection to the default PDN, it is possible to initiate another connection to a different PDN. In this way it is possible to manage PDNs according to service.

For example the IMS PDN, which provides voice services by packet network, could be used as the default PDN, and a different PDN could be used for internet access.

To establish a connection to a PDN other than the default PDN, the procedure is the same as the attach procedure shown in Fig.2, excluding Steps (4) and (5).

TERMS:

Attach: Procedure to register a terminal on the network when, for example, its power is switched on.

Detach: Procedure to remove registration of a terminal from the network when, for example, its power is switched off.

Integrity: Whether the transmitted data is complete and has not been falsified. Here we refer to pre-processing required to ensure integrity of the data.

Bearer: A logical transmission path established, as between the S-GW and eNodeB.

Context setup: Configuration of information required for the communications path and communications management.