Packet data latency is a key performance metric in today's communication systems. Patent data latency is regularly measured by vendors, operators, and end-users (e.g., via speed test applications). Latency is measured throughout the lifetime of a radio access network system. For example, latency is measured when verifying a new software release or system component, when deploying a system, and after the system is put in commercial operation.
From the beginning, the long term evolution (LTE) radio access technology was designed with low latency in mind. As a result, today LTE has better packet data latency than previous generations of 3rd Generation Partnership Project (3GPP) radio access technologies. A wide range of end-users recognize LTE as a system that provides faster access to internet and lower data latencies than previous generations of mobile radio technologies.
Since the introduction of LTE in 2009, several improvements have been developed, such as Carrier Aggregation (CA), 8×8 multiple-input multiple-output (MIMO) operation, and so on. The main target of the improvements has been increasing the maximum data rates of the system. To get the full benefit of these data rate enhancements, enhancements to reduce latency should be an important part of the future evolution track of LTE. An ongoing 3GPP study item aims to shorten the packet data latency over the LTE air interface. One of the discussed options is to shorten the transmission time interval (TTI) length, which is currently 14 Orthogonal Frequency Division Multiplexing (OFDM) symbols.
Certain embodiments of the present disclosure relate to random access procedures for latency reduction. As background, FIGS. 1-2 illustrate prior random access procedures in LTE. During initial access, a user equipment (UE) seeks access to the network in order to register and commence services. The random access procedure serves as an uplink control procedure to enable the UE to access the network and acquire proper uplink timing (synchronize uplink). Since the initial access attempt cannot be scheduled by the network, the initial random access procedure is by definition contention based. Collisions may occur and an appropriate contention-resolution scheme should be implemented. Including user data on the contention-based uplink is not spectrally efficient due to the need for guard periods and retransmissions. Therefore, the LTE specification separates the transmission of the random access burst (preamble), whose purpose is to obtain uplink synchronization, from the transmission of user data.
Other reasons for initiating the random access procedure, beyond initial network access or establishing a radio link (i.e., moving radio resource control (RRC) state from RRC_IDLE to RRC_CONNECTED) include performing handover to establish uplink synchronization to a new cell, establishing uplink synchronization when the UE needs to transmit in the uplink (e.g., data or hybrid automatic repeat request (HARQ) feedback) when it has lost uplink synchronization while in RRC_CONNECTED, and when no scheduling request resources have been configured on the Physical Uplink Control Channel (PUCCH) and the UE wants to transmit data in the uplink.
When uplink data arrives and the UE wants to transmit, it needs to be in RRC_CONNECTED mode, have its uplink synchronized (assigned MAC time alignment timer has not expired), and have scheduling request resources configured. If any of these requirements is not met, the UE initiates the random access procedure. The goal of the procedure is to acquire proper uplink timing in order for the UE to be able to send uplink data.
FIGS. 1-2 outline basic random access procedures. The figures illustrate messages communicated between a UE and a network node, such as an enhanced Node B or “eNB.” FIG. 1 illustrates a contention based random access procedure in the case of initial access. At step 10, the UE sends a random access preamble to the network node. In LTE, the random access preambles are transmitted over the Physical Random Access Channel (PRACH). The transmission of preambles is limited to certain time and frequency resources. The time and frequency resources are configured by upper layers (in system information). For Frequency Division Duplex (FDD—frame structure format 1), the PRACH frequency can currently vary from every subframe to once in every other radio frame (i.e., once in every 20 ms).
The PRACH resource has bandwidth corresponding to 6 physical resource blocks. The length of the PRACH preamble in time depends on the preamble format being used. For example, the basic format 0 fits into one subframe (1 ms) and can be used in cell sizes up to 15 km. For FDD, only one random access region per subframe can be configured. There are 64 different preamble sequences available in each cell. The preambles can be divided into (two) subsets, and the UE selects one sequence from one subset uniformly at random before performing the preamble transmission. The configuration of the PRACH resources in a cell is done by RRC protocol, and the configuration is the same for all UEs in a cell.
At step 12, the network node sends the UE a random access response. In LTE, the random access response can be sent using the Physical Downlink Shared Channel (PDSCH). The random access response includes an initial assignment of uplink resources. At step 14, the UE sends the network node an RRC Connection Request. The message is sent using the uplink resources assigned by the network node in step 12. The message requests to establish a connection at the radio resource control (RRC) layer. In LTE, the RRC Connection Request can be sent on the Physical Uplink Shared Channel (PUSCH). At step 16, the network node sends the UE an RRC Connection Setup message in order to establish the RRC connection.
FIG. 2 illustrates an example of contention free random access in the case of initial access. At step 20, the network node sends a random access preamble assignment to the UE. Assignment of the random access preamble by the network node allows the network node to coordinate the allocation of random access preambles among a number of UEs so that the random access procedure can be contention-free. At step 22, the UE sends the network node the random access preamble that was assigned to the UE in step 20. For simplicity, FIG. 2 illustrates an example in which the UE sends the random access preamble to the same network node that assigned the random access preamble. However, it is possible for the UE to send the random access preamble to a different network node (i.e., a network node other than the one that assigned the preamble), for example, in the case of a handover. At step 24, the network node sends the random access response. As described with respect to FIG. 1, the UE and network node may establish an RRC Connection after the network node has sent the random access response.
Random access procedures can introduce latencies. For example, in the contention-based random access procedure described with respect to FIG. 1, the UE may have to wait for a PRACH opportunity before sending a preamble. The wait depends on the periodicity of the PRACH. As an example, the wait may be 0.5 TTI. Preamble transmission may require 1 TTI. The network node receives the preamble and processes the preamble. Processing may introduce a delay that depends on the implementation of the network node. The delay may be on the order of 3 TTI. The network node then sends the random access response to the UE via the PDSCH. The UE listens during the random access response window and receives the response after 1 TTI. The UE decodes the uplink grant and performs L1 encoding of uplink data. The UE processing delay may be on the order of 5 TTIs. The UE sends uplink data to the network node, which may require an additional 1 TTI. Thus, the total time for the random access procedure in the example is 11.5 TTIs.