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Long Range Cell Coverage for LTE

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LTE is required to support communication with terminals moving at speeds of up to 350 km/h, or even up to 500 km/h depending on the frequency band. The primary scenario for operation at such high speeds is usage on high-speed trains – a scenario which is increasing in importance across the world as the number of high-speed rail lines increases and train operators aim to offer an attractive working environment to their passengers. These requirements mean that handover between cells has to be possible without interruption – in other words, with imperceptible delay and packet loss for voice calls, and with reliable transmission for data services.

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Long Range Cell Coverage for LTE

  1. 1. Long Range Cell Coverage for LTE Yi-Hsueh Tsai lucas@iii.org.tw 1
  2. 2. 1.2.1.4 Mobility and Cell Ranges • LTE is required to support communication with terminals moving at speeds of up to 350 km/h, or even up to 500 km/h depending on the frequency band. The primary scenario for operation at such high speeds is usage on high-speed trains – a scenario which is increasing in importance across the world as the number of high-speed rail lines increases and train operators aim to offer an attractive working environment to their passengers. These requirements mean that handover between cells has to be possible without interruption – in other words, with imperceptible delay and packet loss for voice calls, and with reliable transmission for data services. • These targets are to be achieved by the LTE system in typical cells of radius up to 5 km, while operation should continue to be possible for cell ranges of 100km and more, to enable wide-area deployments.
  3. 3. 5.4.1 Physical Layer Parameters for LTE • LTE aims at supporting a wide range of cellular deployment scenarios, including indoor, urban, suburban and rural situations covering both low and high UE mobility conditions (up to 350 or even 500 km/h). The cell sizes may range from home networks only a few meters across to large cells with radii of 100 kilometers or more. 3
  4. 4. 17.3.1.3 Step3: Layer2/Layer3 Message • If the UE successfully receives the RAR, the UE minimum processing delay before message 3 transmission is 5ms minus the round-trip propagation time. This is shown in Figure 17.3 for the case of the largest supported cell size of 100km. 4 Figure17.3: Timing of the message 3 transmission
  5. 5. 17.4.2.2 PRACH Formats • Four Random Access preamble formats are defined for Frequency Division Duplex (FDD) operation. Each format is defined by the durations of the sequence and its CP, as listed in Table17.1. The format configured in a cell is broadcast in the System Information. 5 Table17.1: Random access preamble formats.
  6. 6. 17.4.2.3 Sequence Duration • Maximum round-trip time. The lower bound for TSEQ must allow for unambiguous round-trip time estimation for a UE located at the edge of the largest expected cell (i.e. 100 km radius), including the maximum delay spread expected in such large cells, namely 16.67 µs. Hence 6
  7. 7. 17.4.2.4 CP and GT Duration • For formats 1 and 3, the CP is dimensioned to address the maximum cell range in LTE, 100 km, with a maximum delay spread of d≈16.67 µs. In practice, format 1 is expected to be used with a 3-subframe PRACH slot; the available GT in 2 subframes can only address a 77 km cell range. It was chosen to use the same CP length for both format 1 and format 3 for implementations implicitly. Of course, handling larger cell sizes than 100 km with suboptimal CP dimensioning is still possible and is left to implementation. 7
  8. 8. 18.2.2.1 Initial Timing Advance • After a UE has first synchronized its receiver to the downlink transmissions received from the eNodeB (see Section 7.2), the initial timing advance is set by means of the random access procedure described in Section 17.3. This involves the UE transmitting a random access preamble from which the eNodeB estimates the uplink timing and responds with an 11-bit initial timing advance command contained within the Random Access Response (RAR) message. This allows the timing advance to be configured by the eNodeB with a granularity of 0.52 µs from 0 up to a maximum of 0.67 ms,1 corresponding to a cell radius of 100km. 8
  9. 9. 18.2.2.1 Initial Timing Advance 9 Figure 10.13: Timing diagram of the downlink HARQ (SAW) protocol Figure10.14: Timing diagram of the uplink HARQ (SAW) protocol
  10. 10. 18.2.2.1 Initial Timing Advance • The timing advance was limited to this range in order to avoid further restricting the processing time available at the UE between receiving the downlink signal and having to make a corresponding uplink transmission (see Figures 10.13 and 10.14). In any case, a cell range of 100 km is sufficient for most practical scenarios, and is far beyond what could be achieved with the early versions of GSM, in which the range of the timing advance restricted the cell range to about 35 km. Support of cell sizes even larger than 100 km in LTE is left to the eNodeB implementation to handle. 10
  11. 11. 23.4.1 Accommodation of Transmit– Receive Switching • The LTE specifications support a set of guard period durations ranging (non- contiguously) from 1 to 10 OFDM symbols for the normal CP (or from 1 to 8 OFDM symbols for the extended CP). A duration of 1 OFDM symbol should be sufficient for many of the anticipated cellular deployments of LTE (up to around 2 km nominal cell radius for γ =2), whereas at the other end of the scale, guard period durations of the order of 700 µs support one-way propagation-path delays of the order of 100km. 11
  12. 12. References 1) LTE The UMTS Long Term Evolution from Theory to Practice Ed2 12

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