To Question or Not to Question? That Is the Question !

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René Descartes was a French philosopher and his best known philosophical statement is “I think, therefore I am”. Statement itself can be argued. However, whole idea is that — if you can question about your own existence, it is enough proof of your existence. If you didn’t exist, you wouldn’t be able to question it.

Questions are important. It may sound trivial but asking right scientific questions is not an easy task. objectives of asking questions may differ, one asks questions to discover something new, one asks questions to understand already well established facts. Questions and subsequently answers give clarity of thought and you get clarity about something you did not understand.

However, most obvious question really is: Can I understand something new in science or technology without asking a question. Almost always answer to this question is ‘NO’. And, then another question is: What really is a good question specially in scientific context. Some would say that a good scientific question is something that can be answered by direct observations, experiments and results of those experiments. These questions are different from questions that are based on people’s opinion or their belief.

In this blog post (and upcoming blog posts) I want to list some of the question that can be asked to understand 5G-NR physical layer.

Question-1: In LTE, Cell Specific Reference Signal (CRS) is an Always-On signal and transmitted in 4 or 6 OFDM non-continuous symbols of each sub-frame. LTE and 5G-NR can share the same DL frequency. So, what are design choices made in design of 5G-NR DL signal so that collision with LTE-CRS can be avoided.

Question-2: In 5G-NR, PSS/SSS/PBCH signals are structured in time and frequency in a very specific way and this structure is known as SS-PBCH block (or SSB in short).

  • What is the reason that this structure uses 240 sub-carries in frequency domain and is 4 OFDM symbol long.
  • Why SSS is sandwiched between two PBCHs ?
  • Were there other alternatives arrangements discussed, what were the disadvantages of other alternatives ?

Question-3: In LTE, Synchronization signals are always located at the center of the RF carrier. There is no decoupling between channel raster and synchronization raster. When UE detects PSS/SSS, it also knows center of the RF carrier.

  • What is the reason that 5G-NR decouples channel raster and synchronization raster ?
  • How does UE know exact SS block frequency-domain location on the carrier ?

Question-4: Multiple PUCCH formats are specified in NR to carry various UCI payload sizes on different formats.

  • Why sequence based design was adopted for PF0 in 5G-NR ?
  • Why DMRS based design with evenly distributed pattern and 1/3 overhead was adopted for PF2 in 5G-NR ?
  • Why specific DMRS based design was adopted for PF1 and PF3 in 5G-NR ?

Question-5: Why specific design choices are made for synchronization signal design?

  • Why does 5G-NR use m-sequence for PSS instead of ZC-sequence based as in 4G-LTE ? 
  • Why does 5G-NR use gold-sequence for SSS instead of interleaved concatenation of two m-sequences with different cyclic shifts as in 4G-LTE ? 

.. To be Continued

~~Peace

Dheeraj

The Saga of Sync-up!

If I need to pick two things that are fascinating about nature, I would pick symmetry and synchronization. Mathematician Steven Strogatz in his famous TED talk explored how flocks of creatures (like birds, fireflies and fish) manage to synchronize and act as a unit — when no one’s giving orders. Similarly, The book “The Equation That Couldn’t Be Solved” explores symmetry in everything from biology and physics to music and the visual arts.

A lot of scientific and mathematical discoveries are based on concepts associated with symmetry and synchronization . Albert Einstein used symmetry as a guiding principle when he devised his General Theory of Relativity. James Clerk Maxwell, mathematical physicist, demonstrated symmetry between electric and magnetic fields. 

Initial synchronization is a very important process in communication systems. It comes into picture when a user equipment (UE) tries to connect to network very first time or it tries to handover to new cell. These procedures allow UE to synchronize with the network both in terms of time and frequency, identify itself with the network and acquire knowledge about the network to which it is trying to connect. To aid UE in this process, Synchronization signals are defined. These synchronization signals are transmitted by network and UE detects them to synchronize itself with the network. Keeping in mind importance of these signals, good amount of effort is given to design these signals. One of the design requirement for these signal is that synchronization signals should be based on mathematical sequences that have sharp (time/frequency offset detection) ambiguity function so that mis-detection at UE end does not happen due to time and frequency impairments.

This blog post tries to explore how these synchronization signals are changed with each generation of cellular communication standard starting from 2G-GSM to 5G-NR.

GSM uses TDMA technique for transmitting information and it uses
Frequency Correction Burst (FCB) signal as the frame synchronization information.  The FCB signal consists of a single Continuous-Wave (CW) tone (Pure Sine Wave) transmitted at a frequency 67.708 Hz above the nominal carrier frequency of the downlink signal. The tone is transmitted for 148 symbol intervals in the first time slot of every tenth frame, equivalent to 148/270833=546.4 microseconds. So, the task of the receiver is to detect this pure sine-wave for detecting the frame structure of down-link frame structure. This may look like a trivial signal processing task. However, detecting a sine wave that is deep buried under the noise and impaired by frequency offset and other impairments can be a daunting task.

In 4G-LTE, There are two types of synchronization signals: Primary Synchronization Signal (PSS) and Secondary Synchronization Signals (SSS). PSS is based on class of complex exponential sequences known as Zad-off Chu (ZC) sequence. Its waveform is defined by class of waveforms known as Constant Amplitude Zero-Autocorrelation (CAZAC). SSS is based on maximum length sequences (m-sequences). The sequence used for the SSS is an interleaved concatenation of two length-31 m-  sequences s0(m0) (n) and s1(m1) (n) (also known as “short codes”), where mand mare the indices for the short codes/sequences sand s1, respectively. An m-sequence is a pseudo random binary sequence which can be created by cycling through every possible state of a shift register of length resulting in a sequence of length.

In 5G-NR also there are same two synchronization signals as 4G-LTE. However, type of the signals used is different. PSS is based on m-sequence and SSS is based on Gold sequence. NR-PSS sequence is constructed based on a length-127 BPSK modulated m-sequence. In freq. domain 3 cyclic shifts (0, 43, 86) to get the 3 PSS signals carrying the index of cell ID in a cell ID group. NR-SSS sequence is generated by the multiplication of two BPSK modulated m-sequences with cyclic shifts determined by the cell ID.

Why does 5G-NR use m-sequence for PSS instead of ZC-sequence based as in 4G-LTE ? Given below is great article that explains the reason. In nutshell, The LTE PSS based on Zadoff-Chu (ZC) sequences has a time-frequency ambiguity problem, i.e., the time-frequency auto-correlation of each LTE PSS contains several significant side lobes besides the main lobe at zero time delay and zero carrier frequency offset (CFO). This may lead to false PSS/SSS detection. http://www.dpi-proceedings.com/index.php/dtcse/article/viewFile/26293/25707

Why does 5G-NR use gold-sequence for SSS instead of interleaved concatenation of two m-sequences with different cyclic shifts as in 4G-LTE ?
LTE cell searches can result in false alarms (FAs), where non-existent cells (also known as false cells or ghost cells) are detected. One cause of this is when two nearby cells have SSSs which share a short code. e.g., the cell ID NID(1)=1 has an m0=0, but so does NID(1)=30, NID(1)=59, NID(1)=87, NID(1)=114, NID(1)=160, and NID(1)=165. Accordingly, if a cell has NID(1)=30, and a nearby cell has NID(1)=114, they would have the same s0(m0) (n). This overlap in constituent short codes results in what is known as a “short code collision”, which, in turn, can result in a ghost cell being detected. Other causes of ghost cells include imperfect PSS/SSS cross-correlations between two cell IDs and random noise/interference. [Ref: System and method for cell search enhancement in an LTE system United States Patent 9432922]. In LTE, this issue can to a certain extent be alleviated by performing cell ID verification via the always-on common reference signal (CRS), which is scrambled by the cell ID. For NR, there is no always-on reference signal and the problem is solved inherently by proper SSS sequence selection.

~Peace ~~Dheeraj

3GPP Release-17, What is cooking ?

3GPP is still giving final touches to Release-15 and started working on functionalities of Release-16. But this does not prevent 3GPP to defined new Study Items for Release-17. These preliminary study Items are listed below:

SP-180340: Feasibility Study on Audio-Visual Service Production

  • The 3GPP system already plays an important role in the distribution of audio-visual (AV) media content and services. Release 14 contains substantial enhancements to deliver TV services of various kinds, from linear TV programmes for mass audiences to custom-tailored on-demand services for mobile consumption. However, it is expected that also in the domain of AV content and service production, 3GPP systems will become an important tool for a market sector with steadily growing global revenues.

SP-180341: Study on Network Controlled Interactive Service in 5GS

  • With the expectation that 5G consumer UEs, existing or some new form devices (e.g. VR/AR devices, robot, etc.) being used for different use cases in a number of different environments, e.g. entertainment in home party or bar, or education in office, becomes interested for supporting data sharing and data exchanging between users, thus, it is necessary to investigate new use cases and requirements, like lower latency, higher throughput, higher reliability, higher resource/power efficiency etc, for such interactive services.
    3GPP has completed various studies and works to provide efficient communication for mobile broadband service and proximity service, but how to support interactive service between users, in different use cases has not been studied.

SP-180666: Study on Typical Traffic Characteristics of Media Services

  • For the optimized design of 3GPP radio and core network protocols, especially in the context of 5G, understanding typical traffic characteristics of services running on top of 3GPP networks is of utmost importance. In particular, media services (audio-visual services including TV type of services, Virtual and Augmented Reality, user generated content, etc.) are expected to dominate mobile traffic.
    In addition, with the emergence of new media services such as FLUS for live broadcast and VR/AR/MR, service requirements will vary greatly. For instance, in the former, QoS allocation cannot be reflective and more bandwidth and delay requirements are needed for the Uplink. For the latter, latency requirements are essential.
    Whereas still some traffic will be distributed through operator services, also a significant amount of traffic will result from Third-Party Services.
    Based on this and recent communication between SA4 and SA1 in S4-AHI793, it was identified that collecting and providing typical media traffic characteristics (including bandwidth and latency requirements) is of importance for SA1 and other groups in 3GPP. This includes demands based on current services, but also expectations for new services or emerging services, taking into account developments in the industry in terms of efficiency improvements. While this initial communication proved to be useful, it is preferred to maintain a central and permanent document for collecting such information.
    In addition, it should be useful identify suitable standardized Quality Indicators for such services and applications, but as the services and applications evolve, the necessity of additional Quality Indicators or other QoS parameters may be identified.

SP-180667: Study on eXtended Reality (XR) in 5G

  • Market studies indicate that Augmented Reality (AR) and Extended Reality (XR, an envelope that includes AR and Virtual Reality) are expected to grow significantly over the next few years. Many use cases and applications are expected to be wireless on mobile and portable devices (including new form factors such as AR glasses), requiring many different enablers that play together to create immersive services and experiences.
    AR is expected to share certain requirements with Virtual Reality, enabling seamless integration of different worlds. Studying the relevancy of interoperability enablers for such new services requires detailed understanding of use cases and requirements to address them It also requires understanding how emerging 5G core network and radio technologies can successfully contribute to XR services, for example to support latency and/or bitrate requirements. The relevancy of 3GPP codecs in such services is of importance.
    Also the migration from not only three degrees of freedom (3DoF, enabling movement along 3 axes), but enabling movement in 6DoF (including rotation about the axes) enables new opportunities, but also provides new challenges, such as higher data rate requirements, etc.
    3GPP initiated a study for VR360 services in the context of FS_VR and the process in this study proved useful to draw relevant conclusions on the potential needs for standardization in 3GPP in Rel-14. This study resulted in several normative work items in Rel-15, including VRStream. In the spirit of FS_VR, a new study on XR seems well justified.
    Therefore, a study is proposed to identify needs for XR (AR and VR) to which 3GPP can successfully contribute.

SP-180783: Study on Communication Services for Critical Medical Applications

  • 3GPP and ETSI specifications have already tackled a number of e-health related use cases that are mostly related to remote patient monitoring. As a matter of fact, as part of FS-CAV study item, a use case called “Telecare data traffic between home and remote monitoring centre” has been described and in ETSI TR 102.732 one can find the following use cases:
    – “Remote Patient Monitoring”
    – “Patient – Provider Secure Messaging”
    – “Measurement of Very Low Voltage Body Signals”
    – “Telecare data traffic between home and remote monitoring centre”
    They all relate to the description of communication services needed to ensure patient/healthcare equipment monitoring outside of hospitals and/or care centres.
    Additionally, although the adoption of wireless technologies has increased across most hospital functions (patient monitoring, nurse call systems, etc.) critical medical service, such as surgical operations, remote diagnosis are lagging behind. 5G system offering wireless communication services targeting radiologists or surgeons at work shall help addressing reliability, efficiency and flexibility for critical medical applications will improved operational efficiency through increased medical throughput. Therefore, it is proposed to specifically address use cases happening in this environment and define specific constraints related to e.g. the following areas:
    – Real time medical imaging in operating rooms
    – Augmented Reality Assisted Surgery
    – Robotic Aided Surgery
    – Remote Robotic Diagnosis
    – Remote Robotic Operation

SP-180784: Study on Asset Tracking Use Cases

  • As every organisation owns assets (machines, medical devices, containers, pallets, trolleys …) and since assets can be (extremely) valuable, asset tracking represents a huge market (billions of units) that so far is mostly untapped by 3GPP technology.
    These assets are often not stationary: they are transported all over the world by different kinds of vehicles; and the assets are also moved inside various kinds of buildings.
    The ownership of assets may change many times during the life-cycle of the asset as different stakeholders take possession of the assets and pass them on to other stakeholders along the supply chain and value chain.
    The value of an asset is not fixed as it typically changes all along the supply chain and value chain.
    The emergence of the sharing economy also implies that one asset can be used by different people, which further amplifies the need of asset tracking.
    So, many stakeholders want to track their assets anytime and anywhere (indoor & outdoor) in a global and multi-modal context (sea, air, road, rail…).
    Asset tracking encompasses distinct use cases such as pallets, trolley, crates, containers, parcels and security asset tracking but also luggage, vehicles and even animals (pets / farm livestock) tracking.
    The asset tracking topic implies more than just knowing the location of an asset. Asset tracking includes real time monitoring of several asset-related properties depending on the asset and its content (condition of the asset, environmental factors – temperature, mechanical shock…). An asset is pre-conditioned on a low-cost approach, for which the two main requirements are coverage (need to support full coverage: indoor / urban / rural / harsh environments / metallic obstructions…) and energy efficiency (15 years’ lifetime of an asset tracking device without changing the battery or the UE).
    The majority of current solutions are based on active RFID tags coupled with IoT sensors and GPS. These solutions are limited as they require readers to be disseminated almost everywhere. RFID tags can also turn out to be very expensive.
    Proprietary solutions are emerging, addressing for instance the container market and the associated requirements for battery efficiency, multi-hop device-to-device communication and security issues.
    Other proprietary solutions that are based on LPWAN (Low Power Wireless Access Network) type networks are rather limited (message size, coverage, scalability, duty cycle management, etc.).
    Concerning asset tracking requirements, 3GPP has already addressed asset tracking aspects through:
    • Feasibility Study on New Services and Markets Technology Enablers (TR.22.891) in the context of Release 14;
    • Communication for Automation in Vertical Domains (TR 22.804) in the context of Release 16.
    • Feasibility Study on Business Role Models for Network Slicing in the context of Release 16
    However, these requirements are covering only a few asset tracking use cases, are too generic, or need to be updated to take into account the particular characteristics of a larger variety of asset tracking use cases.
    Therefore, it’s in the interest of 3GPP to further investigate the asset tracking topic and to identify missing features and requirements for fulfilling as many asset tracking use cases as possible.

SP-180785: Study on enhanced Relays for Energy eFficiency and Extensive Coverage

  • 5G contemplates many different scenarios and verticals (inHome, SmartFarming, SmartFactories, Public Safety and others). Many of them are new while others has been already covered in earlier generations of mobile networks. What all of them has in common is that we can find use cases where better energy efficiency and more extensive coverage are needed in comparison to what earlier generations (3G, 4G) could offer.
    Nevertheless, all these use cases present an heterogenous set of performance requirements. While for IoT use cases will address small data transmissions this is not the case for inHome scenarios where high bandwidth is expected. SmartFactories use cases will need latency requirements that other use cases will not need.
    Release 16 service requirements already include the possibility of having direct 3GPP communication or indirect 3GPP communication with the use of relays. Nevertheless, this may is not enough for the needs of the possible use cases from the area listed. Incorporating multihop relays into 5G will help to improve the energy efficiency and the coverage of the 5G system.

SP-180786: Study on enhancement for Unmanned Aerial Vehicles (UAVs)

  •   Human’s natural vision of flying makes UAVs more and more widely used in the world. The cost of UAVs is gradually reduced. The cost of the UAV is the same as the price of the mobile phone. UAVs can integrate advanced technologies in many fields, such as data mining, machine learning, image recognition, AR/VR, high definition real beat, and so on, to open up the new market blue sea.
    After market analysis, the vertical industry needs more bandwidth for upstream data traffic. And the current technology implementation have not guaranteed the flexible time delay requirements in vertical industry. Example use cases could be Electric power inspection, agricultural insurance, environmental protection, film and television drama shooting, biological field, and oil monitoring. Even for 4G technology, there is still a mismatch of technical implementation and market requirements. This study is intended to correct the deviations of these technologies.

SP-180681: Application layer support for V2X services

  • 3GPP system provides transport services to the V2X applications. To enable V2X applications on 3GPP networks, an application layer support and application architecture to support V2X applications (e.g. vehicles platooning, intersection safety) as defined in Stage 1 specifications (TS 22.185, TS 22.186), is necessary and will ensure efficient use and deployment of V2X applications on 3GPP networks.
    A detailed study has been conducted in SA6 to identify key issues, architecture requirements, functional architecture model, and corresponding solutions that are relevant to the definition of the application layer support for V2X services. The study includes the analysis of stage 1 requirements specified in TS 22.185 and TS 22.186, EPS architecture for V2X communications specified in TS 23.285 and ongoing V2X application layer standards e.g. SAE, ETSI ITS. The application layer support capabilities illustrated by the solutions can be utilized by the V2X applications to enhance the interaction with the 3GPP system(s).
    The results of the study are captured in 3GPP TR 23.795.

~Dheeraj

Downlink Control Region: 4G-LTE Vs 5G-NR

NR introduced a new concept of CORESETs (COntrol REsource SETs). This concept is related to Downlink Control signaling. To understand this, let’s first revisit “LTE CORESET”. You may argue that there is no such concept in LTE. So, Let’s first define what could be LTE CORESET (or concept closest to CORESETs).
As it can be seen from figure below, In LTE, up-to 4 OFDM symbols are used for Downlink control signaling and it uses the full carrier bandwidth. This region is known as “Control Region”. Control region is situated a the start of the subframe and length can change dynamically and indicated by PCFICH channel. This region can be known as LTE CORESET.

Figure: LTE Control Region

Below given are some of the drawbacks of LTE CORESET

  1. There do not exist mechanisms to perform neither frequency domain scheduling nor Intercell Interference Coordination (ICIC) over the PDCCH and hence, low Signal to Interference plus Noise Ratio (SINR) levels at cell edges.
  2. Release-12 eMTC devices does not use full system bandwidth. This leads to the complexities.
  3. UE first needs to decode PCFICH channel and then only it knows the length of control region. This dependency makes pipelined processing complex.
  4. Resources assigned for control region can not be used for data channels.

Although LTE-A provides some mechanisms like introduction of ePDCCH to overcome some of the issues related to LTE PDCCH yet those are not enough. Therefore, a more flexible structure was introduced in NR.

  • In NR, CORESET is defined as time-domain and frequency-domain resources. In time domain, a CORESET is semi-statically configured with one or a set of contiguous OFDM symbols (up to 3 OFDM symbols) and it can be located anywhere in the slot. The configuration indicates the starting OFDM symbol and time duration. In frequency domain, a CORESET can be located anywhere in the frequency range of the carrier (However, not outside the active Bandwidth Part). And, is made up of multiple of resource bocks (i.e. multiples of 12 REs). This flexibility to blank-out certain CORESET or to configure overlapping CORESET provides greater flexibility and helps in avoiding intercell interference.
  • There are two types of CORESETs : Common CORESET and UE-Specific CORESET. After UE has detected the serving cell and decoded PBCH in SS- block to acquire the essential system information such as bandwidth, SFN and etc. Then the UE needs to decode the remaining system information to get the initial access related parameters. The remaining system information is scheduled by the NR-PDCCH, however, UE cannot expect any configuration information for the CORESET except for the information carried in the NR-PBCH at this stage. In this sense, the configuration for CORESET 0 (common CORESET) is indicated by MIB (Master Information Block). In the connected mode, UE can be configured with UE-specific CORESETS. There can be up-to 12 CORESETs configured (up-to 3 CORESETs in each BWP).
  • when the control resource set spans multiple OFDM symbols, NR support a control channel candidate to be mapped to multiple OFDM symbols or to a single OFDM symbol. (As shown in figure below). In this case, The gNB informs UE which control channel candidates are mapped to each subset of OFDM symbols in the control resource set.

 

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~Dheeraj

5G-New Radio (NR) NoMA and Reciever challenges

In cellular communication systems, Multiple Access Techniques define ways by which multiple users access a single communication channel.

For example:

  • 2G uses TDMA (Time division multiple access) where multiple users are assigned a time-slot to access the channel.
  • 3G uses CDMA (Code division multiple access) where multiple users are assigned a code to access the channel.
  • LTE uses OFDMA (Orthogonal frequency division multiple access) where users are assigned orthogonal frequencies to access the channel.
  • LTE-A also uses Multi User Superposition coding Transmission (MUST) where eNB serves two DL users at same OFDMA subcarrier. Users are separated by different power levels. Users with better channel conditions are assigned less power and users with bad channel conditions are assigned more power. eNB uses supposition coding and sends information for two users at the same time. This is a class of Non orthogonal Multiple Access (NoMA) scheme and only applicable in DL direction.  [Ref: https://www.youtube.com/watch?v=JXkBSb-mhjQ]

NoMA Study Item for 5G-New Radio (NR) was approved and revised recently. Clear industry interest in NoMA has been shown, given the support of nearly 40 companies. It is expected that NoMA will benefit various use cases and deployments. This will be supported in both DL and UL directions.

NoMA is expected to serve many scenarios and use cases, for example, connection density, UE power consumption and signaling overhead reduction, Coverage extension, Reliability and resource utilization, Latency reduction and Number of Users and capacity enhancements.

One of the challenge in supporting NoMA is complex receiver design. For example,

In orthogonal multiple access, the DMRS of different UEs are not overlapping and there is no pilot contamination. In fact, one has to estimate the channel only for the RE positions where a particular UE is active. Therefore, we only need to place pilots over the indices where each UE is active. In NOMA, though, multiple UEs are transmitting over the same set of resources hence we need to alleviate the impact of pilot contamination.” [Ref: R1-1805005]

“When multiple UEs are allocated on the same time/frequency resources then the gNB does not have a priori knowledge of the UEs’ ID; when the gNB detects a transmission over the shared resources, it does not know which of the assigned UE transmitted. Therefore, it is important to establish mechanisms for acquiring the UE ID, which is especially relevant for NoMA where resources are shared. Moreover, the identification mechanism should be at least as reliable as the payload transmission. The reason is that a passed CRC-check without a UE-ID, tied to the payload, will be lost effort since the gNB cannot acknowledge the packet reception; it will instead have to wait for a new transmission. In the opposite scenario, where the UE-ID is detected but the payload decoding fails, at least we can exploit the acquired UE-ID in order to take remedy actions, such as retransmissions on orthogonal resources.” [Ref: R1-1805005]

“One key issue with any multiuser detection is the power imbalance between users. More specifically, in most designs it is assumed that the received power from different users at the receiver is either the same or can be ideally controlled. However, in reality the power control process is not ideal and there can be a difference of ∆P between the target power and the realistic power at the receiver. It is important that the power control imperfection is considered in the evaluation of different NoMA schemes.”  [Ref: R1-1805005] 

“One of the most pronounced features of NR NOMA is the UE and gNB capabilities to operate in RRC inactive/idle states without a UL grant. The support for grant-free transmission can significantly reduce the power consumption and latency, which is a desirable property for NR mMTC and URLLC.” [Ref: R1-1804826]

“For mMTC use case, managing timing advance and maintaining synchronization across all NOMA UEs requires large power consumption and incur large latency. Therefore, the ability to operate asynchronously without any TA is an important feature for NOMA schemes. For long codes based RSMA with DFT-s-OFDM waveform, time domain processing, e.g. Rake receiver, can be immediately applicable for NOMA multi-user detection, even for asynchronous scenario. However, for other NOMA schemes especially the ones based on short NOMA codes, the chip-level alignment across the UEs are crucial. It is unclear yet how to apply the short-codes based NOMA schemes for asynchronous transmission.” [Ref: R1-1804826]

~Dheeraj

 

 

 

 

 

5G-NR channel raster and sync raster

Channel raster is the distance between neighboring channels in a frequency band.

During Cell search procedure and Initial synchronization process user equipment (UE) tries to synchronize with the network both in terms of time and frequency, identify itself with the network and acquire knowledge about the network to which it is trying to connect.

A naïve cell search procedure will look like as follows:

1) Do a frequency scan of all the configured frequency bands using the range defined as channel raster. For example if channel raster is 200 KHz, UE will perform search in every 200 KHz in all the bands UE supports. This actually means UE calculates DL received signal power and detects a set of frequency candidates.
2) Search for synchronization signals over all the candidates it has detected in step-1.

In practical systems above naïve procedure is not followed. UE implements more efficient search algorithm. For example, performing spectrum analysis on the frequencies before engaging search operation. Also, it is to be noted that this kind of sophisticated frequency search procedure only needs to done for the very first time. Once the UE successfully acquires the synchronization signal, the later initial access procedure can be based on apriori information from the previous search cell results.

In any case, Channel raster defines step-size for this search procedure and consequently it influences time UE will take to complete initial search procedure. The Channel raster for GSM is 200 kHz for all bands. WCDMA and TD-SCDMA also use a channel raster of 200 kHz for all bands. The selected center frequency must therefore be a whole number multiple of 200 kHz. However, there are exceptions.

The center frequency of an LTE carrier is assumed to be on a 100 kHz channel raster (known also as carrier grid) in order to allow the LTE UEs to search a limited number of carrier frequencies in the frequency bands they support and synchronize within a reasonable amount of time after being activated. Synchronization signals are centered around DC subcarrier. So, there is no decoupling between channel raster and synchronization raster.

5G-NR is expected to be deployed in the wide frequency bands, frequency scanning using the channel raster is not efficient. Further, There may be confusion between NR/LTE netowrks, as both NR and LTE employ
OFDM based waveforms in the downlink. furthermore “alien” waveform confusion may occur when some massive MTC waveforms or some on-demand signals exist in the frequency bands. [Ref: US patent application 2018/0131487]

5G-NR decouples channel raster and synchronization raster. Therefore, [Ref: R1-1608967]

  • Frequency raster (used for synchronization and NR cell search) is sparser than carrier/channel raster.
  • Frequency raster (used for synchronization and NR cell search) depends on frequency bands.
  • NR UE does not assume a fixed frequency location of the synchronization signal(s) relative to the center of the carrier.
  • Sparser frequency raster (than channel raster) means that the center frequency locations of the synchronization signal and physical carrier are different.

To come-up to this there were extensive discussions in 3GPP about following questions.

Question 1: Is there any benefit for UE power consumption in allowing that the candidate frequency locations of synchronization signal(s) for NR be sparser than for LTE ?
Question 2: If there is benefit, what are possible synchronization signal raster per frequency ranges and/or bands?
Question 3: what should be relation between the candidate frequency locations of synchronization signal(s) and the frequency locations of the center of NR carrier bandwidth.
Question 4: Are there drawbacks in allowing that the candidate frequency locations of synchronization signal(s) are sparser than the possible frequency locations of the center of NR carrier bandwidth?
Question 5: What about the scenarios where synchronization signal(s) of neighbor cells may not be on the same center frequency, and the impact on intra/inter frequency RRM measurements.

~ Dheeraj

Condition number and MIMO systems

Wireless communication system often requires solution of linear algebraic equation like y = H*x + w. Where H is channel matrix, x is transmitted signal, y is received signal and w is noise.

  • If small change in H results in small change in y, we say system of equation is well-conditioned.
  • If small change in H results in large change in y, we say system of equation is ill-conditioned.

It is to note that sometimes we say that Matrix “H” is ill-conditioned or well-conditioned (Instead of system of equation). In this case, we are only referring to the fact that how sensitive matrix is to its inversion.

In the presence of round-off errors, ill-conditioned systems are inherently difficult to handle.In this regard, Channel matrix condition number is an important indicator to tell how sensitive a Matrix or solution of a system of equation is to  round-off errors or input to the system. . If the condition number of a matrix is large, the matrix is said to be ill-conditioned.

Higher order MIMO detector systems require to solve equation with large matrix dimensions. And. things become even worse when these detectors are implemented on finite precision machines. So, It becomes very important to know about condition numbers.

Probably following answer by “Mario Carneiro” explains it best.  [ref: https://math.stackexchange.com/q/261300]

Mathematically, if the condition number is less than , the matrix is invertible. Numerically, there are round-off errors which occur. A high condition number means that the matrix is almost  non-invertible. For a computer, this can be just as bad. But there is no hard bound; the higher the condition number, the greater the error in the calculation. For very high condition number, you may have a number round to 0 and then be inverted, causing an error. This is the same thing that would happen if you tried to invert a truly non-invertible matrix, which is why I say that a high condition number may as well be infinite in some cases.

In order to find out if the matrix is really too  ill-conditioned, you should invert the matrix, and then check that AA1=I, to an acceptable precision. There is simply no hard cap on the condition number, just heuristics, which is why your references differ.

So, below are some of the questions that attached video that I created long ago tries to answer.

  • What is Geometric interpretation of  condition number.
  • How to calculate condition number of a matrix.

~Dheeraj

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