Voice over HSPA (VoHSPA) and CS over HSPA (CSoHS)

4G Americas has recently released a whitepaper entitled, "Delivering voice over HSPA". This paper describes the technological features that are being developed to make Voice over HSPA (VoHSPA) a reality. It describes the two potential options for VoHSPA. The first option leverages IP Multimedia Subsystem (IMS) technology developed in conjunction with Long Term Evolution (LTE), and is referred to as IMS Voice over HSPA or simply IMS Voice. The other option delivers voice by modifying existing circuit-switch based techniques so that those communications can be transmitted over an HSPA infrastructure, and is referred to as CS Voice over HSPA (CSoHS). Both the options are shown in the picture above. Note that there is no discussion about Over the top (OTT) type voice services like Skype, etc. 

The chief among benefits anticipated from VoHSPA are increases in the spectral efficiency of mobile networks. With these new techniques, voice calls can be delivered more efficiently from a spectral standpoint over Packet Switched (PS) rather than Circuit Switched (CS) networks freeing up radio resources for additional data traffic.

The 4G Americas report defines work completed by the GSMA for a minimum mandatory set of features defined in existing 3GPP Release 8 specifications (IR 58: IMS Profile for VoHSPA) that should be implemented in order to insure an interoperable, high quality, IMS-based telephony service over an HSPA radio access layer. In the white paper, 4G Americas recommends additional features, above the minimum mandatory features in IR 58, for VoHSPA either under an IMS or a CS approach, in order to minimize packet losses and variations in packet arrival times that can impair the quality of voice communications.

The whitepaper is available to download from here.

HSDPA multiflow data transmission

From RP-111375:

HSPA based mobile internet offerings are becoming very popular and data usage is increasing rapidly. As a result, HSPA has begun to be deployed on more than one transmit antenna and/or on more than one carrier. As an example, the single cell downlink MIMO (MIMO-Physical layer) feature was introduced in Release 7. This feature allowed a NodeB to transmit two transport blocks to a single UE from the same cell on a pair of transmit antennas thus improving data rates at high geometries and providing a beamforming advantage to the UE in low geometry conditions. Subsequently, in Release-8 and Release-9, the dual cell HSDPA (DC-HSDPA) and dual band DC-HSDPA features were introduced. Both these features allow the NodeB to serve one or more users by simultaneous operation of HSDPA on two different carrier frequencies in two geographically overlapping cells, thus improving the user experience across the entire cell coverage area.

When a UE falls into the softer or soft handover coverage region of two cells on the same carrier frequency, the link from the serving HS-DSCH cell is capacity or coverage limited and the non-serving cell in its active set has available resources, it would be beneficial to schedule packets to this UE also from the non-serving cell and thereby improve this particular user’s experience.

From 4G Americas whitepaper:

One family of such schemes parses the incoming data for the user into multiple (restricted to two cells in the study) data streams or flows, each of which is transmitted from a different cell [8] Concurrent transmission of data from the two cells may either be permitted or the UE may be restricted to receiving data from only one cell during a given TTI. The former type of scheme is designated an aggregation scheme while the latter is termed a switching scheme. The aggregation scheme can be seen as subsuming the switching scheme at the network when scheduling to the user is restricted to the cell with better channel quality.

Figure 14 illustrates the basic multi-flow concept with both cells operating on the same carrier frequency F1.

3GPP studied different multipoint transmission options for HSDPA and documented the findings and performance gains in TR25.872 providing feasibility and performance justification for the specification work.

For more details also see:
http://www.3gpp.org/ftp/Specs/html-info/FeatureOrStudyItemFile-530034.htm

See also old blog post on Multipoint HSDPA/HSPA here.

New 4G Americas whitepaper on HSPA evolution in 3GPP standards

Some forecasts put HSPA at over 3.5 billion subscribers by the end of 2016. Operators with HSPA and LTE infrastructure and users with HSPA and LTE multi-mode devices will be commonplace. There are 412 commercial deployments of HSPA in 157 countries, including 165 HSPA+ networks. Thus, with the continued deployment of LTE throughout the world, and the existing ubiquitous coverage of HSPA in the world, HSPA+ will continue to be enhanced through the 3GPP standards process to provide a seamless solution for operators as they upgrade their networks. While LTE, with 33 commercial deployments to date and over 250 commitments worldwide, will be the mobile broadband next generation technology of choice for HSPA, EV-DO, WiMAX and new wireless operators, HSPA will continue to be a pivotal technology in providing mobile broadband to subscribers.

The white paper explains that as 3GPP specifications evolve, their advanced features help to further the capabilities of today’s modern mobile broadband networks. With each release there have been improvements such as better cell edge performance, increased system efficiencies, higher peak data rates and an overall improved end-user experience. 3GPP feature evolution from Rel-7 to Rel-10 has pushed possible HSPA peak data rates from 14 Mbps to 168 Mbps. Continued enhancements in 3GPP Rel-11 will again double this capability to a possible peak data rate of 336 Mbps:

• Rel-7: 64QAM or 2X2 MIMO => 21 or 28 Mbps
• Rel-8: DC + 64QAM or 2X2 MIMO + 64QAM => 42 Mbps
• Rel-9: DC + 2X2 MIMO + 64QAM => 84 Mbps
• Rel-10: 4C + 2X2 MIMO + 64QAM => 168 Mbps
• Rel-11: (8C or 4X4 MIMO) + 64QAM => 336 Mbps

“If operators are able to gain new additional harmonized spectrum from governments, they will no doubt deploy LTE, However, it is clear that HSPA+ technology is still exceptionally strong and will continue to provide operators with the capability to meet the exploding data usage demands of their customers in existing spectrum holdings,” Pearson said.

The paper is embedded as follows:

The Evolution of HSPA

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This paper and other similar papers are available to download from the 3G4G website here.

LTE Rollouts and Deployment Scenarios

According to GSA report, as of August 2011, 26 commercial LTE networks in 18 countries are already rolled out as below:

As of Aug. 2011, 237 operators in 85 countries are investing in LTE:

* 174 LTE network commitments in 64 countries

* 63 pre-commitment trials in 21 more countries

* 26 commercial LTE networks launched

* At least 93 LTE networks are expected to be in commercial service by end 2012

The following is from the 4G Americas whitepaper:

There are many different scenarios that operators will use to migrate from their current networks to future technologies such as LTE. Figure 10 presents various scenarios including operators who today are using CDMA2000, UMTS, GSM and WiMAX. For example, as shown in the first bar, a CMDA2000 operator in scenario A could defer LTE deployment to the longer term. In scenario B, in the medium term, the operator could deploy a combination of 1xRTT, EV-DO Rev A/B and LTE and, in the long term, could migrate EV-DO data traffic to LTE. In scenario C, a CDMA2000 operator with just 1xRTT could introduce LTE as a broadband service and, in the long term, could migrate 1xRTT users to LTE including voice service.

3GPP and 3GPP2 both have specified detailed migration options for current 3G systems (UMTS-HSPA and EV-DO) to LTE. Due to economies of scale for infrastructure and devices, 3GPP operators are likely to have a competitive cost advantage over Third Generation Partnership Project 2 (3GPP2) operators. One option for GSM operators that have not yet committed to UMTS, and do not have an immediate pressing need to do so, is to migrate directly from GSM/EDGE or Evolved EDGE to LTE with networks and devices supporting dual-mode GSM-EDGE/LTE operation.

New '4G Americas' Whitepaper on 'Mobile Broadband Explosion'

4G Americas has published new whitepaper and presentation on Mobile Broadband explosion.

Mobile Broadband Explosion: 3GPP Broadband Evolution to IMT-Advanced

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A copy of these whitepaper, presentation and all other presentation on MBB is available on the 3G4G website here.

Non-Voice Emergency Services (NOVES)

Its been a while we talked about SMS for Emergency purposes and eCall. A new study item in 3GPP has looked at non-voice alternatives for Emergency purposes.

Picture Source: Dailymail

The following is from the recent 4G Americas report entitled: 4G Mobile Broadband Evolution: 3GPP Release-10 and Beyond:

Non-verbal communications such as text messaging and instant messaging via wireless devices has been very successful and continues to expand. Many of the consumers assume that they can utilize these types of non-verbal communications as mechanisms to communicate with emergency services whenever emergency assistance is required. Such mechanisms currently do not exist. The Emergency Services community has a desire to have multimedia emergency services supported with the same general characteristics as emergency voice calls.

Currently, service requirements for emergency calls (with or without the IP Multimedia Core Network) are limited to voice media. The Non-Voice Emergency Services (NOVES) is intended to be an end-to-end citizen to authority communications. NOVES could support the following examples of non-verbal communications to an emergency services network:

1. Text messages from citizen to emergency services

2. Session-based and session-less instant messaging type sessions with emergency services

3. Multimedia (e.g., pictures, video clips) transfer to emergency services either during or after other communications with emergency services.

4. Real-time video session with emergency services

In addition, to support the general public, this capability would facilitate emergency communications to emergency services by individuals with special needs (e.g., hearing impaired citizens).

The objectives of this study include the following questions for NOVES with media other than or in addition to voice:

1. What are the requirements for NOVES?

2. What are the security, reliability, and priority handling requirements for NOVES?

3. How is the appropriate recipient emergency services system (e.g., PSAP) determined?

4. What are the implications due to roaming?

5. Are there any implications to hand over between access networks?

6. Are there any implications due to the subscriber crossing a PSAP boundary during NOVES communications (e.g., subsequent text messages should go to the same PSAP)?

7. Do multiple communication streams (e.g., voice, text, video emergency services) need to be associated together?

8. What types of “call-back” capabilities are required?

9. What are the load impacts of NOVES in the case of a large scale emergency event or malicious use?

NOVES will be applicable to GPRS (GERAN, UTRAN) and to EPS (GERAN, UTRAN, E-UTRAN and non-3GPP). The content may be transmitted between the subscribers and the emergency services which might bring new security issues. Therefore, the security impacts need to be studied.

You can spend your weekend reading the 3GPP Study Item TR 22.871: Study on Non-Voice Emergency Services (Release 11).

A word of caution, the name NOVES may be changed in future as Emergency agencies in Europe have an objection to the name. See here and here.

VoLTE: Semi-Persistent Scheduling (SPS) and TTI Bundling

The following is from the recently released 4G Americas paper '4G Mobile Broadband Evolution: 3GPP Release-10 and beyond:

With the support of emergency and location services in Rel-9, interest in Voice over LTE (VoLTE) has increased. This is because the Rel-9 enhancements to support e911 were the last step to enable VoLTE (at least in countries that mandate e911) since the Rel-8 specifications already included the key LTE features required to support good coverage, high capacity/quality VoLTE. There are two main features in Rel-8 that focus on the coverage, capacity and quality of VoLTE: Semi-Persistent Scheduling (SPS) and TTI Bundling.

SPS is a feature that significantly reduces control channel overhead for applications that require persistent radio resource allocations such as VoIP. In LTE, both the DL and UL are fully scheduled since the DL and UL traffic channels are dynamically shared channels. This means that the physical DL control channel (PDCCH) must provide access grant information to indicate which users should decode the physical DL shared channel (PDSCH) in each subframe and which users are allowed to transmit on the physical UL shared channel (PUSCH) in each subframe. Without SPS, every DL or UL physical resource block (PRB) allocation must be granted via an access grant message on the PDCCH. This is sufficient for most bursty best effort types of applications which generally have large packet sizes and thus typically only a few users must be scheduled each subframe. However, for applications that require persistent allocations of small packets (i.e. VoIP), the access grant control channel overhead can be greatly reduced with SPS.

SPS therefore introduces a persistent PRB allocation that a user should expect on the DL or can transmit on the UL. There are many different ways in which SPS can setup persistent allocations, and Figure below shows one way appropriate for VoLTE. Note that speech codecs typically generate a speech packet every 20 ms. In LTE, the HARQ interlace time is 8 ms which means retransmissions of PRBs that have failed to be decoded can occur every 8 ms. Figure below shows an example where a maximum of five total transmissions (initial transmission plus four retransmissions) is assumed for each 20 ms speech packet with two parallel HARQ processes. This figure clearly shows that every 20 ms a new “first transmission” of a new speech packet is sent. This example does require an additional 20 ms of buffering in the receiver to allow for four retransmissions, but this is generally viewed as a good tradeoff to maximize capacity/coverage (compared to only sending a maximum of two retransmissions).

The example in Figure above can be applied to both the DL and UL and note that as long as there are speech packets arriving (i.e. a talk spurt) at the transmitter, the SPS PRBs would be dedicated to the user. Once speech packets stop arriving (i.e. silence period), these PRB resources can be re-assigned to other users. When the user begins talking again, a new SPS set of PRBs would be assigned for the duration of the new talkspurt. Note that dynamic scheduling of best effort data can occur on top of SPS, but the SPS allocations would take precedent over any scheduling conflicts.

TTI bundling is another feature in Rel-8 that optimizes the UL coverage for VoLTE. LTE defined 1 ms subframes as the Transmission Time Interval (TTI) which means scheduling occurs every 1 ms. Small TTIs are good for reducing round trip latency, but do introduce challenges for UL VoIP coverage. This is because on the UL, the maximum coverage is realized when a user sends a single PRB spanning 180 kHz of tones. By using a single 180 kHz wide PRB on the UL, the user transmit power/Hz is maximized. This is critical on the UL since the user transmit power is limited, so maximizing the power/Hz improves coverage. The issue is that since the HARQ interlace time is 8 ms, the subframe utilization is very low (1/8). In other words, 7/8 of the time the user is not transmitting. Therefore, users in poor coverage areas could be transmitting more power when a concept termed TTI bundling (explained in the next paragraph) is deployed.

While it’s true that one fix to the problem is to just initiate several parallel HARQ processes to fill in more of the 7/8 idle time, this approach adds significant IP overhead since each HARQ process requires its own IP header. Therefore, TTI bundling was introduced in Rel-8 which combined four subframes spanning 4 ms. This allowed for a single IP header over a bundled 4 ms TTI that greatly improved the subframe utilization (from 1/8 to 1/2) and thus the coverage (by more than 3 dB).

Martin Sauter puts it in a simpler way in his blog as follows: The purpose of TTI Bundling is to improve cell edge coverage and in-house reception for voice. When the base station detects that the mobile can't increase it's transmission power and reception is getting worse it can instruct the device to activate TTI bundling and send the same packet but with different error detection and correction bits in 2, 3 or even 4 consecutive transmit time intervals. The advantage over sending the packet in a single TTI and then detecting that it wasn't received correctly which in turn would lead to one or more retransmissions is that it saves a lot of signaling overhead. Latency is also reduced as no waiting time is required between the retransmissions. In case the bundle is not received correctly, it is repeated in the same way as an ordinary transmission of a packet. Holma and Toskala anticipate a 4dB cell edge gain for VoIP with this feature which is quite a lot. For details how the feature is implemented have a look at 3GPP TS 36.321.

A whitepaper explaining the concepts of TTI Bundling is available on Slideshare here.

LTE Self Optimizing Networks (SON) enhancements for Release-10

Capacity and Coverage Optimisation (CCO) was already nominally part of the Release-9 WI, but could not be completed due to amount of work related to other use cases.

Energy Savings are a very important topic, especially for operators, as solutions derived for this use case can significantly limit their expenses. According to TR 36.902 this solution should concern switching off cells or whole base stations. This may require additional standardised methods, once there is need identified for.

Basic functionality of Mobility Load Balancing (MLB) and Mobility Robustness Optimisation (MRO), also listed in TR 36.902, were defined in Rel.9. However, successful roll-out of the LTE network requires analysing possible enhancements to the Rel.9 solutions for MLB and MRO. In particular, enhancements that address inter-RAT scenarios and inter-RAT information exchange must be considered. These enhancements should be addressed in Rel.10.

There may also be other use cases for LTE for which SON functionality would bring optimisations.

Although, it is of primary interest to provide coverage to users during a roll-out, it is equally important to enhance the capacity of the network during operation. As such, both coverage and capacity are considered in the use case and supported by the SON function. The CCO SON function should be configured through appropriate objectives and targets in order to meet the operator’s requirement on coverage and capacity, and the prioritization between them.

The following use cases and scenarios are planned for Release-10:

Coverage and Capacity Optimisation (CCO)

The use case is to enable detection of following problems:

• Priority 1: coverage problems, e.g. coverage holes

• Priority 2: capacity problems

Mobility Robustness Optimisation (MRO) enhancements

The use case is to enable detection and to provide tools for possible correction of following problems:

• Connection failures in inter-RAT environment:

o Priority 1: at HOs from LTE to UMTS/GSM

o Priority 2: at HOs from UMTS/GSM to LTE

• Obtaining UE measurements in case of unsuccessful re-establishment after connection

failure

• Ping-pongs in idle mode (inter-RAT and intra-LTE environment)

• Ping-pongs in active mode (inter-RAT)

• HO to wrong cell (in intra-LTE environment) that does not cause connection failure (e.g. short stay problem)

Mobility Load Balancing (MLB) enhancements

The use case is to fulfil following objectives:

• Improving reliability of MLB in intra-LTE scenarios

• Improving functionality of the MLB in inter-RAT scenarios (the transport method agreed for R9 should be used for R10).

For more info see 3GPP TS 32.521: Self-Organizing Networks (SON) Policy Network Resource Model (NRM) Integration Reference Point (IRP); Requirements; Release-10

There is also a Self-Organising Networks Conference that I am attending next month and I plan to give SON lots of coverage before and after the event.

If you havent read the 3G Americas whitepaper on SON, it is definitely worth a read. I have embedded it below.

Benefits of SON in LTE

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