Cyclic Prefix (CP)

A cyclic prefix (CP) is a copy of the end of an orthogonal frequency division multiplexing (OFDM) symbol inserted at the beginning. Each CP serves as a guard interval between OFDM symbol

Radio signals are transmitted between an eNodeB and a UE through multiple propagation paths. The maximum transmission delay difference between paths is the delay spread. In typical eNodeB deployment scenarios (for example, urban areas), the delay spread is generally short. However, in some areas with special channel environments (for example, valleys) or special network topologies (for example, roads covered with several RRUs serving one cell), the delay spread is long. In addition, the delay spread increases with the cell radius.

To mitigate intersymbol interference (ISI) and inter-carrier interference (ICI) caused by delay spread in the OFDM system, the CP technique is introduced.

The symbol energy that can be captured by an OFDM receiver depends on the CP length:

• If the CP length is longer than the multipath delay spread of an OFDM symbol, the OFDM receiver can capture all the energy of the symbol.

• If the CP length is shorter than the multipath delay spread of an OFDM symbol, the OFDM receiver can capture only some energy of the symbol.

If the multipath components of an OFDM symbol whose multipath delay spread is longer than the CP are incorrectly received as a subsequent OFDM symbol, interference is generated to the actual subsequent OFDM symbol. This causes ISI. If the interfered OFDM symbol then mistakenly captures some multipath components of adjacent symbols, the orthogonality between subcarriers is lost after fast Fourier transformation (FFT). This causes ICI. To avoid ISI and ICI, a sufficiently long CP is required. However, an unnecessarily long CP must be avoided because it causes high system overhead and a loss of spectral efficiency.

To balance system performance, system overhead, and data transmission capability, normal CP and extended CP were defined in section 5.6 "SC-FDMA baseband signal generation" and section 6.12 "OFDM baseband signal generation" of 3GPP TS 36.211 V8.9.0 for non-RACH channels to meet different multipath delay spread requirements.

Normal CP

Normal CP is a basic feature and is enabled by default. It applies to scenarios where the delay spread is short.

Benefits

Extended CP

Extended CP applies to scenarios where the delay spread is long. When multipath interference is severe, system performance deteriorates significantly if the multipath delay spread is longer than a normal CP. In an OFDM system, extended CP enhances multipath delay spread resistance in large-cell-radius scenarios and multipath interference resistance. However, extended CP increases the system overhead and consumes certain number of transport resources.

The timeslot consists of six downlink OFDM symbols or uplink SC-FDMA symbols. The CP length of each symbol is approximately 16.7 μs.

Structure of a timeslot in the extended CP case (with a subcarrier spacing of 15 kHz)

an extended CP is longer than a normal CP. The permissible multipath delay at the receiving end increases with the CP length. The use of extended CP, however, causes more system overhead than the use of normal CP. The system overhead for the extended CP case is calculated using the following formula:

Benefits

The CP technique mitigates ISI and ICI caused by multipath delay, ensuring network coverage in E-UTRAN cells.

The deployment suggestions of extended CP are as follows:

• It is recommended that extended CP be activated to improve uplink and downlink signal to interference plus noise ratios (SINRs) if the multipath delay spread exceeds the normal CP length of 4.7 μs in the area (the time difference is greater than the length of a normal CP, 4.7 μs), for example, where there are high buildings or mountains in the distance that will reflect signals. This condition can be interpreted as follows: There is at least 1.5 km difference between the line of sight (LOS) path of a signal arriving at a UE and the reflected path of the same signal last arriving at the UE.

• It is difficult to obtain the actual multipath delay spread. To evaluate whether to use extended CP, operators can estimate whether the multipath delay spread is longer than the normal CP length based on the building distribution and landform.

• UEs may not support extended CP. Enabling this function may cause exceptions of such UEs. It is not recommended that this function be enabled if there are such UEs on the live network. The extended CP capability of UEs depends on UEs themselves.

Thank You.