E-RGCH (Enhanced Relative Grant Channel)
Enhanced Relative Grant Channel (E-RGCH) is a type of control channel in the Universal Mobile Telecommunications System (UMTS) used for scheduling uplink transmissions. UMTS is a third-generation mobile communication technology that uses Wideband Code Division Multiple Access (WCDMA) as the air interface. The E-RGCH channel was introduced in Release 7 of the 3GPP (Third Generation Partnership Project) specifications to improve the efficiency of uplink transmissions.
The uplink transmission in UMTS uses a technique called Code Division Multiple Access (CDMA). In CDMA, each user is assigned a unique code to transmit their data. Since all users are transmitting at the same time and frequency, the base station needs to separate the signals of different users using their unique codes. However, due to the nature of CDMA, the uplink transmission suffers from a problem called "near-far" effect, where the signal from a strong user can overpower the signal from a weak user. This can lead to degradation in the overall system performance.
To mitigate the near-far effect, UMTS uses a technique called power control, where the base station adjusts the transmission power of each user to ensure that their signals arrive at the same power level at the base station. However, power control requires a lot of overhead signaling, which can reduce the overall system capacity. Moreover, power control is not very effective in scenarios where the uplink traffic is highly variable, such as in bursty data applications.
The E-RGCH channel addresses these issues by providing a more efficient way to schedule uplink transmissions. The E-RGCH channel is a common control channel that is used by all users to request uplink resources. The base station uses the E-RGCH channel to allocate uplink resources to users based on their relative grant values. The relative grant value is a measure of the user's priority to access uplink resources, and it is calculated based on various factors such as the user's Quality of Service (QoS) requirements and the amount of data waiting to be transmitted.
The E-RGCH channel operates in the same frequency band as the uplink traffic channels. It is a time-multiplexed channel that consists of 15 time slots, each of which is 2 ms long. The E-RGCH channel is transmitted continuously by the base station and is received by all users. Each user can request uplink resources by transmitting a request message on the E-RGCH channel. The request message contains the user's relative grant value and the amount of data waiting to be transmitted.
The base station uses the information in the request message to allocate uplink resources to users. The allocation process is done in two steps. In the first step, the base station calculates the available uplink resources and the total relative grant value of all the users. In the second step, the base station allocates uplink resources to each user based on their relative grant value.
The E-RGCH channel provides several advantages over traditional power control-based scheduling. First, it reduces the overhead signaling required for power control, which increases the overall system capacity. Second, it provides a more flexible and efficient way to allocate uplink resources, which is particularly beneficial in scenarios with bursty data traffic. Third, it improves the overall system performance by mitigating the near-far effect and ensuring that all users have equal access to uplink resources.
In conclusion, Enhanced Relative Grant Channel (E-RGCH) is a control channel in UMTS that is used to schedule uplink transmissions. It provides a more efficient and flexible way to allocate uplink resources, which is particularly beneficial in scenarios with bursty data traffic. The E-RGCH channel improves the overall system performance by mitigating the near-far effect and ensuring that all users have equal access to uplink resources. The E-RGCH channel also reduces the overhead signaling required for power control, which increases the overall system capacity.