LTE-TDD 커버리지

 

TDD 대표적인 단점은 커버리지이다.

먼저 TDD이기 때문에, UL에 항상 연결되어있는 FDD와 달리 TDD는 DL:UL 비율이 1:1로 설정된 경우 UL에 할당되는 time frame이 전체의 약 50% 정도로, 신호가 약 3dB정도 감쇄된다고 한다. (3dB 신호 감쇄는 곧 신호의 세기가 절반으로 떨어진다) 신호가 2배 약하게 되면, 커버리지 역시 좁아지게 된다. 커버리지는 최소한의 신호 세기가 보장되는 영역이라고 볼수 있기 때문.

이상적인 환경에서는 이러한 점이 단점으로 강조되지만  실제 상황에서는  FDD와 크게 차이가 없다고 한다.

추가적으로 일반적인 TDD 주파수 대역은 2GHz 이상의 고주파 대역이라는 점 또한 LTE TDD의 커버리지가 작은 이유 중 하나이다. 주로 2.3GHz, 2.5GHz, 2.6GHz등에서 도입되는데, 높은 주파수의 신호는 일단 멀리 가지 못하고 건물등을 투과하기도 어렵다는 단점이 있다.

 

<출처 : http://m.blog.naver.com/johnlee08/166405782 >

 

Assuming same transmit power . The main reason for reduced coverage is that the uplink device power is used part of the time for TDD but continuously for FDD . As an example, an 1:1 UL/DL allocation means a duty cycle of ~50% in the uplink which contributes to a ~3dB reduced link budget; In capacity driven deployments, there is no coverage advantage.

 

The commonly heard argument for FDD is one of better coverage.
FDD systems transmit continuously and therefore require lower peak power at the transmitter for any given link budget than a TDD system occupying the same bandwidth. For example, let us compare a 2 x 5MHz FDD with a 10MHz TDD system. The TDD system transmits for half the time (assuming a symmetric uplink/downlink) whereas the FDD system transmits continuously. Therefore the TDD system requires 3dB more power to achieve the same performance in terms of energy per bit at the receiver. If maximizing range for a given peak ERP is the objective, FDD appears to have an advantage. Remember that, over the complete Tx/Rx cycle, both systems are consuming the same power – TDD uses twice as much power for half the time.
However, this argument applies in the extreme, and rather unlikely, case of a single user at cell edge consuming the total bandwidth of the cell. In more practical situations the coverage advantage of FDD over TDD is reduced or even eliminated - “in a commercial environment where the cells have more than 1 user, the performance of FDD and TD-LTE, as demonstrated with 5 percentile edge throughput performance, will be very similar as the FDD device will not be able to access the full frame 10 UL resource blocks like he did on an unloaded single user network”

 

<출처 : http://www.accelleran.com/sites/default/files/AccelleranWP101_0.pdf >