Random access in time division duplex communication systems
A mobile terminal is operated in a cellular communications system that comprises a base station operating in a time division duplex mode. The mobile terminal detects a downlink transmission from the base station and determines therefrom a future moment in time at which an end part of a downlink transmission just prior to a random access window will become detectable at the mobile terminal. An uplink random access transmission is begun at a moment in time that is before the future moment in time. The moment in time that is before the future moment in time can be a fixed amount of time before the future moment in time. If the mobile terminal is not near the edge of the cell, the base station may consequently receive only a partial uplink random access transmission, but this does not detrimentally affect performance.
1. A method of operating a mobile terminal in a cellular communications system that comprises a base station operating in a time division duplex mode, the method comprising:
ascertaining a need to perform a random access operation;
in response to said ascertaining, detecting a downlink transmission from the base station and locating therefrom the timing of an upcoming random access window and determining a future moment in time at which an end part of a sub-frame of a downlink transmission just prior to the upcoming random access window will become detectable at the mobile terminal; and
skipping reception of at least a last part of the sub-frame of the downlink transmission just prior to the upcoming random access window and beginning an uplink random access transmission at a moment in time that is before the future moment in time.
2. The method of claim 1 , wherein the moment in time that is before the future moment in time is a fixed amount of time before the future moment in time.
3. The method of claim 2 , wherein the fixed amount of time corresponds to a maximum possible round trip delay between the base station and the mobile terminal.
4. The method of claim 1 , wherein the uplink random access transmission comprises a body and a cyclic prefix, the cyclic prefix comprising an end portion of the body.
5. A method of operating a base station in a time division duplex mode in a cellular communications system, the method comprising:
during a random access window that begins directly after a downlink transmission directed to one or more mobile terminals, receiving at least a portion of an uplink random access transmission from one of the one or more mobile terminals,
wherein a duration of the random access window, T RA — window satisfies the following relationship:
T PR ≦T RA — window <T PR +RTD max ,
where T PR is an entire duration of the uplink random access transmission, and RTD max is a maximum possible round trip delay between the base station and any one of the one or more mobile terminals,
wherein T RA — window satisfies:
T RA — window =T PR +T UD ,
wherein T UD is a duration of an uplink-downlink switching time.
6. A method of operating a base station in a time division duplex mode in a cellular communications system, the method comprising:
during a random access window that begins directly after a downlink transmission directed to one or more mobile terminals, receiving at least a portion of an uplink random access transmission from one of the one or more mobile terminals,
wherein a duration of the random access window, T RA — window satisfies the following relationship:
T PR ≦T RA — window <T PR +RTD max ,
where T PR is an entire duration of the uplink random access transmission, and RTD max is a maximum possible round trip delay between the base station and any one of the one or more mobile terminals,
wherein:
the uplink random access transmission comprises a preamble body and a cyclic prefix;
the preamble body has a duration of T PRE — BODY ; and
the cyclic prefix has a duration of T CP ,
whereby T PR =T CP +T PRE — BODY .
7. The method of claim 6 , comprising:
generating a frequency-domain representation of the received at least a portion of the uplink random access transmission;
generating a frequency-domain signal by multiplying the frequency-domain representation of the received at least a portion of the uplink random access transmission by a frequency-domain representation of a known preamble sequence;
generating a time-domain signal from the frequency-domain signal; and
ascertaining whether the uplink random access transmission represented a random access preamble by comparing one or more elements of the time-domain signal with a threshold value.
8. The method of claim 7 , wherein the threshold value is scaled to indicate the presence of the random access preamble when only a portion of the uplink random access transmission matches a corresponding portion of the known preamble sequence.
9. The method of claim 6 , comprising:
using a sliding correlator to generate a correlation value indicative of a maximum degree of correlation between the received at least a portion of the uplink random access transmission and a known preamble sequence; and
ascertaining whether the uplink random access transmission represented a random access preamble by comparing the correlation value with a threshold value.
10. The method of claim 9 , wherein the threshold value is scaled to indicate the presence of the random access preamble when only a portion of the uplink random access transmission matches a corresponding portion of the known preamble sequence.
11. An apparatus in a mobile terminal that interacts with a cellular communications system that comprises a base station operating in a time division duplex mode, the apparatus comprising:
logic circuitry configured to ascertain a need to perform a random access operation;
logic circuitry configured to detect, in response to said ascertaining, a downlink transmission from the base station and to locate therefrom the timing of an upcoming random access window and to determine a future moment in time at which an end part of a sub-frame of a downlink transmission just prior to the upcoming random access window will become detectable at the mobile terminal; and
logic circuitry configured to skip reception of at least a last part of the sub-frame of the downlink transmission just prior to the upcoming random access window and to begin an uplink random access transmission at a moment in time that is before the future moment in time.
12. The apparatus of claim 11 , wherein the moment in time that is before the future moment in time is a fixed amount of time before the future moment in time.
13. The apparatus of claim 12 , wherein the fixed amount of time corresponds to a maximum possible round trip delay between the base station and the mobile terminal.
14. The apparatus of claim 11 , wherein the uplink random access transmission comprises a body and a cyclic prefix, the cyclic prefix comprising an end portion of the body.
15. An apparatus in a base station that operates in a time division duplex mode within a cellular communications system, the apparatus comprising:
logic circuitry configured to perform a downlink transmission directed to one or more mobile terminals; and
logic circuitry configured to receive at least a portion of an uplink random access transmission from one of the one or more mobile terminals during a random access window that begins directly after the downlink transmission,
wherein a duration of the random access window, T RA — window satisfies the following relationship:
T PR ≦T RA — window <T PR +RTD max ,
where T PR is an entire duration of the uplink random access transmission, and RTD max is a maximum possible round trip delay between the base station and any one of the one or more mobile terminals,
wherein T RA —window satisfies:
T RA — window =T PR +T UD ,
wherein T UD is a duration of an uplink-downlink switching time.
16. An apparatus in a base station that operates in a time division duplex mode within a cellular communications system, the apparatus comprising:
logic circuitry configured to perform a downlink transmission directed to one or more mobile terminals; and
logic circuitry configured to receive at least a portion of an uplink random access transmission from one of the one or more mobile terminals during a random access window that begins directly after the downlink transmission,
wherein a duration of the random access window, T RA — window satisfies the following relationship:
T PR ≦T RA — window <T PR +RTD max ,
where T PR is an entire duration of the uplink random access transmission, and RTD max is a maximum possible round trip delay between the base station and any one of the one or more mobile terminals,
wherein:
the uplink random access transmission comprises a preamble body and a cyclic prefix;
the preamble body has a duration of T PRE — BODY ; and
the cyclic prefix has a duration of T CP ,
whereby T PR =T CP T PRE — BODY .
17. The apparatus of claim 16 , comprising:
logic circuitry configured to generate a frequency-domain representation of the received at least a portion of the uplink random access transmission;
logic circuitry configured to generate a frequency-domain signal by multiplying the frequency-domain representation of the received at least a portion of the uplink random access transmission by a frequency-domain representation of a known preamble sequence;
logic circuitry configured to generate a time-domain signal from the frequency-domain signal; and
a comparator that ascertains whether the uplink random access transmission represented a random access preamble by comparing one or more elements of the time-domain signal with a threshold value.
18. The apparatus of claim 17 , wherein the threshold value is scaled to indicate the presence of the random access preamble when only a portion of the uplink random access transmission matches a corresponding portion of the known preamble sequence.
19. The apparatus of claim 16 , comprising:
a sliding correlator that generates a correlation value indicative of a maximum degree of correlation between the received at least a portion of the uplink random access transmission and a known preamble sequence; and
a comparator that ascertains whether the uplink random access transmission represented a random access preamble by comparing the correlation value with a threshold value.
20. The apparatus of claim 19 , wherein the threshold value is scaled to indicate the presence of the random access preamble when only a portion of the uplink random access transmission matches a corresponding portion of the known preamble sequence.