Preamble group selection in random access of wireless networks

1. A method for selecting a transport block size (TBS) of a first uplink message (RACH Msg3) transmitted on a Physical Uplink Shared Channel (PUSCH) during a random access procedure in a User Equipment (UE) accessing a radio access network, comprising:

receiving a pathloss threshold parameter;

estimating a downlink pathloss value indicative of radio link conditions between the UE and a base station (eNB) serving the UE;

selecting from a set of TBS values a smaller value of TBS if the determined pathloss value is greater than an operating power level of the UE minus the pathloss threshold parameter;

sending a random access request containing an indication of the selected TBS;

wherein the operating power level of the UE is Pmax−P 0 — PRE −Δ PREAMBLE-Msg3 ;

wherein the pathloss is the downlink pathloss value estimate calculated in the UE;

wherein Pmax is the maximum allowed transmission power;

wherein P 0 — PRE is the initial target received preamble power at the serving eNB antenna port;

wherein Δ PREAMBLE-Msg3 is the offset between RACH preamble and the RACH Msg3; and

wherein the pathloss threshold parameter is configured by the network.

2. The method of claim 1 wherein the parameters Pmax, P 0 — PRE and Δ PREAMBLE-Msg3 , are configured by the network and broadcasted as system information on a broadcast channel.

3. The method of claim 2 wherein the parameters Pmax, P 0 — PRE and Δ PREAMBLE-Msg3 are also used to set the transmission power Pmsg3 of RACH Msg3 as follows:

P msg3=min{ P max;10 log 10 ( NRB+P 0 — PRE +Δ PREAMBLE-Msg3 +PL+ΔP};

wherein NRB is the size of a frequency allocation in resource blocks (RB) for RACH Msg3, PL is the downlink pathloss estimate determined by the UE and ΔP is an optional adjustment factor accounting for various contributors such as power rampup of the preamble during potential retries and MCS-dependent power offset.

4. The method of claim 1 wherein the threshold parameter is computed at the eNB as follows:

THRESHOLD=Δ TF — B +10 log 10 ( N RB — B )+margin

wherein N RB-B is the size of a frequency allocation in resource blocks (RB) for the RACH Msg3 associated with the larger TBS value;

wherein ΔTF-B is a MCS-dependent power offset in use with the RACH Msg3 associated with the larger TBS value; and

wherein the margin value is a parameter the network uses to tune the conservativeness of the selection process, allowing minimizing the rate of erroneous selection of the larger TBS value.

5. The method of claim 4 wherein the threshold parameter is coded on three bits and broadcasted as system information on the broadcast channel.

6. The method of claim 5 , wherein the threshold parameter is mapped on the three bit field, such that:

a field value of 0 indicates a parameter value (dB) of −∞,

a field value of 1 indicates a parameter value (dB) of 0,

a field value of 2 indicates a parameter value (dB) of 3,

a field value of 3 indicates a parameter value (dB) of 6,

a field value of 4 indicates a parameter value (dB) of 9,

a field value of 5 indicates a parameter value (dB) of 12,

a field value of 6 indicates a parameter value (dB) of 15, and

a field value of 7 indicates a parameter value (dB) of 18.

7. A method for selecting a transport block size (TBS) of a first uplink message (RACH Msg3) transmitted on a Physical Uplink Shared Channel (PUSCH) during a random access procedure in a User Equipment (UE) accessing a radio access network, comprising:

determining a pathloss threshold parameter by a base station (eNB) serving the UE;

broadcasting the pathloss threshold parameter to a UE being served by the eNB; and

receiving a random access request from the UE with an indication of a selected TBS, wherein a smaller value of TBS is selected from a set of TBS values by the UE if an estimated pathloss value indicative of radio link conditions between the UE and the eNB is greater than an operating power level of the UE minus the pathloss threshold parameter;

wherein a larger value of TBS is selected from the set of TBS values by the UE if the pathloss value is less than the pathloss threshold parameter and the TBS required to transmit the RACH Msg3 exceeds the smaller TBS value;

wherein the threshold parameter is computed by the eNB as follows:

THRESHOLD=Δ TF — B +10 log 10 ( N RB — B )+margin

wherein N RB-B is the size of a frequency allocation in resource blocks (RB) for the RACH Msg3 associated with the larger TBS value;

wherein ΔTF-B is a MCS-dependent power offset in use with the RACH Msg3 associated with the larger TBS value; and

wherein the margin value is a parameter the network uses to tune the conservativeness of the selection process, allowing minimizing the rate of erroneous selection of the larger TBS value.

8. The method of claim 7 wherein the threshold parameter is coded on three bits and broadcasted as system information on the broadcast channel.

9. The method of claim 8 , wherein the threshold parameter is mapped on the three bit field, such that:

a field value of 0 indicates a parameter value (dB) of −∞,

a field value of 1 indicates a parameter value (dB) of 0,

a field value of 2 indicates a parameter value (dB) of 3,

a field value of 3 indicates a parameter value (dB) of 6,

a field value of 4 indicates a parameter value (dB) of 9,

a field value of 5 indicates a parameter value (dB) of 12,

a field value of 6 indicates a parameter value (dB) of 15, and a field value of 7 indicates a parameter value (dB) of 18.

10. A User Equipment (UE) apparatus for accessing a radio access network, the UE comprising:

processing logic connected to a memory;

radio frequency (RF) transmission logic coupled to the processing logic; and

RF receiver logic couple to the processing logic, the RF receiving logic configured to receive a pathloss threshold parameter; and

wherein the processing logic is configured to select a transport block size (TBS) of a first uplink message (RACH Msg3) transmitted on a Physical Uplink Shared Channel (PUSCH) during a random access procedure while the UE is accessing a radio access network by:

estimating a downlink pathloss value indicative of radio link conditions between the UE and a base station (eNB) serving the UE;

selecting from a set of TBS values a smaller value of TBS if the determined pathloss value is greater than an operating power level of the UE minus the pathloss threshold parameter;

sending a random access request via the RF transmission logic containing an indication of the selected TBS;

wherein the processor is configured to select a larger value of TBS from the set of TBS values if the pathloss value is less than the operating power level of the UE minus the pathloss threshold parameter and the TBS required to transmit the RACH Msg3 exceeds the smaller TBS value;

wherein the operating power level of the UE is Pmax−P 0 — PRE −Δ PREAMBLE-Msg3 ;

wherein the pathloss is the downlink pathloss estimate calculated in the UE;

wherein Pmax is the maximum allowed transmission power;

wherein P 0 — PRE is the initial target received preamble power at the serving eNB antenna port;

wherein Δ PREAMBLE-Msg3 is the offset between RACH preamble and the RACH Msg3; and

wherein the pathloss threshold parameter is received by the RF receiver.

11. The UE of claim 10 wherein the parameters Pmax, P 0 — PRE and Δ PREAMBLE-Msg3 are also used by the processor to set the transmission power Pmsg3 of RACH Msg3 as follows:

P msg3=min{ P max;10 log 10 ( NRB+P 0 — PRE +Δ PREAMBLE-Msg3 +PL+ΔP};

wherein NRB is the size of a frequency allocation in resource blocks (RB) for RACH Msg3, PL is the downlink pathloss estimate determined by the UE and LP is an optional adjustment factor accounting for various contributors such as power rampup of the preamble during potential retries and MCS-dependent power offset.