Cell Size Configuration in RACH (II) Cyclic Shift We have discussed the preamble format configuration vs. the maximum cell radius during the random access procedure. For example, Preamble Format 0 supports a maximum cell radius of 14.5 km. Another random access parameter that affects the cell size is the cyclic shift. Let's look at it now.The random access preamble is generated using Zadoff-Chu sequences; there are multiple root Z-C sequences used in LTE. From each root sequence, multiple preambles can be obtained by applying different cyclic shifts. This cyclic shift also determines the maximum radius of the cell.The cyclic shift, Ncs, is defined in 3GPP TS 36.211, section 5.7. (Note that the unrestricted set is for normal speed cells, and the restricted set is for high mobility cells.)

Table 5.7.2-2: for preamble generation (preamble formats 0-3).How is the cyclic shift related to cell radius? As shown in Figure 1, assume that there are 2 UEs, UE1 at the cell edge and UE2 close to the eNB. The cyclic shift used by UE1 is 0 and the cyclic shift used by UE2 is Ncs. At the eNB, the observed cyclic shift of UE1 will not be 0 but some value x because of the transmission delay. As long as x is less than Ncs, the auto-correlation between the shifted x and shifted Ncs (as perceived by the eNB) will be zero, and the eNB will be able to distinguish between the accesses from UE1 and UE2 (This is one of the nice properties of Z-C sequences). So, the maximum cell radius is limited by the cyclic shift.

Figure 1: Cyclic shift vs. Cell RadiusNow, let's calculate the maximum supported cell radius by a cyclic shift, Ncs. Based on 3GPP, the preamble sequence length is 839 and spans 800 milliseconds.

[Exercise time: Please fill the blanks.]

Getting back to the basics, why is a cyclic shift needed? The cyclic shift can be used to expand the preamble capacity. There are a total of 838 Zadoff-Chu sequences defined in LTE, and the default setting for the number of preambles in each sector is 64. For areas covered with a large number of small cells, if the preamble capacity is limited, preamble interference may cause more collisions and longer random access delay. A small Ncs value generates more preambles, which extend the preamble reuse distance and mitigate the interference. However, the cyclic shift cannot be configured smaller than expected cell radius, since that will block random accesses from the cell edge and may cause drops during handovers.PBCH: Does the MIB tell the UE how many antennas are used in the cell? Before answering this question let's review the contents of the Master Information Block (MIB). You will find the definition of the MIB in the RRC specification, 36.331. It contains just three parameters: DL Bandwidth, PHICH Configuration and System Frame Number as well as ten spare bits for future expansion. In a previous blog, "PBCH: How quickly can a UE read the MIB?" I discussed at length the SFN. The presence of the system bandwidth in the MIB is a reflection of the PBCH being in the center 1.4MHz regardless of the actual bandwidth of the channel. Why does the UE need the PHICH Configuration information to be in the MIB? Not because it will be receiving ACKs or NAKs immediately, but it must know WHERE this channel is so that it can read the PDCCH.You have probably noticed that there is no parameter in the MIB for the number of antennas in the cell. The MIB has a CRC however, which is scrambled with one of three sequences which maps to the number of antennas used in the cell. Perhaps then, when the UE calculates the CRC from the decoded MIB it can compare against each of the three descrambled CRCs looking for a match and hence discover the number of antennas - perhaps not. It is tempting to think of this scrambling sequence as a parameter. But it is not a parameter in the same way that a C-RNTI used to scramble the CRC of the PDCCH is a parameter. Ask yourself the question "How does the UE decode the MIB in each of the three possible scenarios, namely, one antenna, two antennas or four antennas?" In the one antenna case there is nothing special the UE has to do, but for both the two and four antenna cases the UE has to be in sync with the base station. LTE uses a specific type of transmit diversity in which both the transmitter and receiver are aware of the method and participate in its application. For the two antenna case LTE uses Space-Frequency Block Coding (SFBC) and for the four antenna case a combination of SFBC and Frequency Switched Transmit Diversity (SFBC+FSTD) is used. The LTE UE will need to blindly detect the number of antennas by trying each possible antenna configuration in turn, decoding the MIB and descrambling the MIB's CRC with the corresponding antenna mask in order to compare the CRC.So the next time you are told that the MIB tells the UE how many antennas are used in the cell you'll know exactly what this means.LTE System Information: Part 1 LTE system information is one of the key aspects of the air interface. It consists of the Master Information Block (MIB) and a number of System Information Blocks (SIBs). The MIB is broadcast on the Physical Broadcast Channel (PBCH), while SIBs are sent on the Physical Downlink Shared Channel (PDSCH) through Radio Resource Control (RRC) messages. SIB1 is carried by "SystemInformationBlockType 1" message. SIB2 and other SIBs are carried by "SystemInformation (SI)" message. An SI message can contain one or several SIBs.1. The MIB is the first thing a UE looks for after it achieves downlink synchronization. The MIB carries the most essential information that is needed for the UE to acquire other information from the cell. It includes: The downlink channel bandwidth The PHICH configuration. The Physical Hybrid ARQ Indicator Channel carries the HARQ ACKs and NACKs for uplink transmissions The SFN (System Frame Number) which helps with synchronization and acts as a timing reference The eNB transmit antenna configuration specifying the number of transmit antennas at eNB such as 1, 2, or 4, which is carried by CRC mask for PBCH 2. SIB1 is carried in a SystemInformationBlockType1 message. It includes information related to UE cell access and defines the schedules of other SIBs, such as: The PLMN Identities of the network The tracking area code (TAC) and cell ID The cell barring status, to indicate if a UE may camp on the cell or not q-RxLevMin, which indicates the minimum required Rx Level in the cell to fulfill the cell selection criteria The transmissions times and periodicities of other SIBs 3. SIB2 contains radio resource configuration information common for all UEs, including: The uplink carrier frequency and the uplink channel bandwidth (in terms of the number of Resource Blocks, for example n25, n50) The Random Access Channel (RACH) configuration, which helps a UE start the random access procedure, such as preamble information, transmit time in terms of frame and subframe number (prach-ConfigInfo), and powerRampingParameters which indicates the initial Tx power and ramping step. The paging configuration, such as the paging cycle The uplink power control configuration, such as P0-NominalPUSCH/PUCCH The Sounding Reference Signal configuration The Physical Uplink Control Channel (PUCCH) configuration to support the transmission of ACK/NACK, scheduling requests, and CQI reports The Physical Uplink Shared Channel (PUSCH) configuration, such as hopping 4. SIB3 contains information common for intra-frequency, inter-frequency, and/or inter-RAT cell reselection. This information does not necessarily apply to all scenarios; please refer to 3GPP TS 36.304 for the details. The basic parameters include: s-IntraSearch: the threshold for starting intra-frequency measurement. When s-ServingCell (i.e., cell selection criterion for serving cell) is higher than s-IntraSearch, the UE may choose not to perform measurement in order to save battery life. s-NonIntraSearch: the threshold for starting inter-frequency and IRAT measurements q-RxLevMin: the minimum required Rx level in the cell Cell reselection priority: the absolute frequency priority for E-UTRAN or UTRAN or GERAN or CDMA2000 HRPD or CDMA2000 1xRTT q-Hyst: the hysteresis value used for calculating the cell-ranking criteria for the serving cell, based on RSRP. t-ReselectionEUTRA: the cell reselection timer value for EUTRA. t-ReselectionEUTRA and q-Hyst can be configured to trigger cell reselection sooner or later. 5. SIB4 contains the intra-frequency neighboring cell information for Intra-LTE intra-frequency cell reselection, such as neighbor cell list, black cell list, and Physical Cell Identities (PCIs) for Closed Subscriber Group (CSG). CSG can be used to support Home eNBs.6. SIB5 contains the neighbor cell related information for Intra-LTE inter-frequency cell-reselection, such as neighbor cell list, carrier frequency, cell reselection priority, threshold used by the UE when reselecting a higher/lower priority frequency than the current serving frequency, etc.(Note that 3GPP states that LTE neighbor cell search is feasible without providing an explicit neighbor list. Since the UE can do blind detection of neighbor cells in LTE, the broadcast of LTE neighbor cells is optional.)LTE TDD (TD-LTE): How much different from LTE FDD? While initial commercial deployments have focused on FDD (Frequency Division Duplex) version of LTE (Long Term Evolution), interest in the TDD (Time Division Duplex) version of LTE has been rising. The TDD-based LTE is also known as TD-LTE (Time Division- LTE). We will discuss TD-LTE from the eyes of LTE FDD; the reader is assumed to be familiar with LTE FDD.First and foremost, TD-LTE shares the same channel bandwidth between the uplink (UL) and the downlink (DL). As an example, the LTE FDD uses paired spectrum such as 10 MHz in the UL and separate 10 MHz in the DL. In contrast, the TD-LTE would use the same 10 MHz for both UL and DL. While FDD allows simultaneous transmission and reception at an entity such as the eNodeB or the UE, TDD involves either transmission or reception at a given time instant. The main implications of using TDD instead of FDD are that the service operator does not need large amount of spectrum to deploy TDD and that the average throughput is slightly lower in TDD due to relatively higher overhead. We summarize below key differences between TD-LTE and LTE FDD related to the frame structure, radio channels and signals, data transmission in UL and DL, and deployment aspects. LTE FDD uses Type 1 Frame structure, whereas TD-LTE uses a Type 2 Frame Structure. While traditional FDD systems use symmetric spectrum in UL and DL (LTE FDD does allow asymmetric bandwidth), TDD has inherent support for an asymmetric use of UL and DL. The Type 2 frame structure defines various configurations that basically specify how much time is dedicated to the DL and to the UL. The UL:DL ratio varies from 3:2 ("uplink-heavy") to 1:9 ("downlink-heavy"). Within a 10 ms frame, subframe 0 and subframe 5 are always used for the DL in TD-LTE. TD-LTE defines one or two special subframes in a 10 ms frame. The special subframe has three parts- DwPTS (Downlink Pilot Time Slot), GP (Guard Period), and UpPTS (Downlink Pilot Time Slot). DwPTS and UpPTS are legacy terms from TDD version of UMTS (Universal Mobile Telecommunication System); formally, there is no "pilot" channel in LTE. The traditional "pilot" channel is called Reference Signal in LTE. DwPTS facilitates downlink synchronization, and UpPTS facilitates uplink synchronization. GP helps avoid interference between the uplink and the downlink and provides the transceiver adequate time to switch from transmit function to receive function and vice versa. Roles of radio channels and signals remain the same in TD-LTE. However, structures of certain signals and channels are different in TD-LTE. The main reason for structure differences is to support different UL:DL ratios. The primary synchronization signal is sent in the third OFDM symbol in slot 2 (subframe 1) and slot 12 (subframe 6), and the secondary synchronization signal is sent in the last OFDM symbol of slot 1 (subframe 0) and slot 11 (subframe 5). Recall that the primary synchronization signal is sent in the last OFDM symbol of slot 0 (subframe 0) and slot 10 (subframe 5) and the secondary synchronization signal is sent in the second last OFDM symbol of these slots/subframes in LTE FDD. Multiple PRACHs (Physical Random Access Channels) (up to six) may be present in a given UL subframe in TD-LTE, whereas LTE FDD supports zero or one PRACH in a given subframe. While four random access preamble formats are available to TD-LTE and LTE FDD, an additional fifth format is also available for use in TD-LTE for small cells. The PDCCHs (Physical Downlink Control Channels) can use up to 2 OFDM symbols in subframes 1 and 6. In other downlink subframes, up to 3 or 4 OFDM symbols can be occupied by the PDCCHs in LTE TDD just like LTE FDD. Number of PHICH (Physical HARQ Indicator Channel) groups differs as a function of UL:DL ratio. HARQ is Hybrid Automatic Repeat reQuest. Sounding reference signal in the UL has different configurations in TD-LTE.The overall DL/UL data transmission approach remains the same for TD-LTE and LTE FDD. There are additional parameters in the DCI (Downlink Control Information) messages carried over the PDCCHs to support resource allocation in TD-LTE. The UL resource allocation (and UL power control command) specified by the PDCCH in subframe "n" is valid for the UL subframe "(n+4)" in LTE FDD and subframe "(n+k)" in TD-LTE, where k ranges from 4 to 7. HARQ and semi-persistent scheduling are also affected due to different UL:DL ratios. While DL HARQ supports up to 8 HARQ processes in LTE FDD, TD-LTE supports up to 15 HARQ processes in the DL. To support DL transmission, the TD-LTE UE could use ACK/NACK bundling to send a single response to multiple processes or use ACK/NACK multiplexing to provide process-specific HARQ responses. While UL synchronous HARQ has eight HARQ processes in LTE FDD, the number of TD-LTE HARQ processes ranges from 1 to 7. Semi-persistent scheduling has additional constraints in LTE TDD. Due to the channel reciprocity in TD-LTE, the channel conditions in the UL and DL are likely to be similar. Such knowledge can be exploited by the eNodeB scheduler to make decisions about the DL packet scheduling by observing the UL.From the perspective of deployment, the availability of unpaired spectrum is needed for TD-LTE. Due to the tight timing synchronization requirements for TDD, eNodeBs would need a network synchronization mechanism such as GPS (Global Positioning System). LTE FDD may or may not use GPS. In addition to the inter-cell interference "management" schemes of LTE FDD such as adaptive modulation and coding, HARQ, UL power control, and ICIC (Inter Cell Interference Coordination), LTE-TDD can exploit GP to reduce inter-cell interference. In summary, TD-LTE utilizes unpaired spectrum and reuses many of the LTE FDD features and mechanisms. Differences between LTE FDD and LTE TDD are primarily due to the fundamental TDD/FDD difference and different UL:DL ratios supported by LTE TDD. Many of the LTE FDD and LTE TDD differences exist at the physical layer, allowing LTE FDD and LTE TDD to benefit from the same LTE ecosystem. Countries such as India and China may see early widespread deployments of TD-LTE. TDD-based legacy networks, such as TD-SCDMA (Time Division- Synchronous Code Division Multiple Access) in China, can evolve to TD-LTE to achieve higher spectral efficiency.UE measurement in LTE In cellular networks, when a mobile moves from cell to cell and performs cell selection/reselection and handover, it has to measure the signal strength/quality of the neighbor cells. We know that in UMTS, a UE measures RSSI, CPICH RSCP, and CPICH Ec/No; in 1xEV-DO, an MS measures Ec/Io; in WiMAX, an MS measures CINR/SINR on preamble. What does a UE measure in LTE?In LTE network, a UE measures two parameters on reference signal: RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality). The reference signal in LTE is similar to the pilot in WiMAX. RSRP is a RSSI type of measurement. It measures the average received power over the resource elements that carry cell-specific reference signals within certain frequency bandwidth. RSRQ is a C/I type of measurement and it indicates the quality of the received reference signal. It is defined as (N*RSRP)/(E-UTRA Carrier RSSI), where N makes sure the nominator and denominator are measured over the same frequency bandwidth; the carrier RSSI measures the average total received power observed only in OFDM symbols containing reference symbols for antenna port 0 (i.e., OFDM symbol 0 & 4 in a slot) in the measurement bandwidth over N resource blocks. The total received power of the carrier RSSI includes the power from co-channel serving & non-serving cells, adjacent channel interference, thermal noise, etc.RSRP is applicable in both RRC_idle and RRC_connected modes, while RSRQ is only applicable in RRC_connected mode. Now, let's check in which procedures RSRP and RSRQ are used. In the procedure of cell selection and cell reselection in idle mode, RSRP is used. In the procedure of handover, the LTE specification provides the flexibility of using RSRP, RSRQ, or both. It is implementation specific.Want more information? The 3GPP TS 36.214 specification can help.