Technical Field [0001] The im ention relates genera !h to rank adaptation for an open loop multi-antenna mode of wireless communication between a mobile station and a base station. Background [0002] Various wireless access technologies have been proposed or implemented to enable mobile stations to perform communications with other mobile stations or with wired terminals coupled to wired networks. Examples of wireless access technologies include GSM (Global System for Mobile communications) and UMTS (Universal Mobile Telecommunications S\stem) technologies, defined b\ the Third Generation Partnership Project (3GPP): and CDMA 2000 (Code Division Multiple Access 2000) technologies, defined by 3GPP2 [0003] As pan of the continuing evolution of wireless access technologies to improve spectral efficiency, to improve services, to lower costs, and so forth, new standards have been proposed. One such new standard is the Long Term Evolution (LTE) standard from 3GPP, which seeks to enhance the UMTS w ireless network. [0004] According to LTE, rank adaptation is afforded for channel-dependent precodinu (closed-loop) MIMO (multiple input, multiple output) wireless transmission using information that is fed back from the mobile station to a base station MIMO refers to the use of multiple antennas at the transmit side and or the receive side. Rank adaptation refers to selecting from among plural ranks, such as rank 1 and rank 2. for wireless communications between the mobile station and the base station "Rank 1" refers to use of just a single layer for the wireless channel that communicates data between the base station and mobile station With such single-lav er communications, the same signal us emitted from each of the transmit antennas (such that rcdundanc) is provided) w ith appropriate phase cd instead. [0022] Basically, a ""poor" channel condition (relative to predetermined or otherwise established criteria) will indicate that rank 1 communications is to be used, whereas a ""better" quality channel condition (telati\e to such criteria) would indicate that rank 2 communications can be used. In some implementations, the rank 1 scheme is a transmit diversity scheme in which the same information (redundant information) is sent over different paths (spatial beams) to a mobile station to increase the likelihood that the mobile station will receive the downlink data. For example, the transmit diversity format can be the spatial frequency block coding (SFBC) format [0023] Rank 2 communications pro\idc spatial multiplexing m which two antennas of the transmitter are used to transmit independent downlink data to be received bcorresponding antennas of the receiver Thus, in accordance with some embodiments, for a mobile station with high mobility- (high \ elocity). dynamic rank adaptation is provided for an open loop multi-antenna mode such that rank I communications or rank 2 communications can be utilized to increase both throughput and coverage gain. In other embodiments, additional ranks can be supported. [0024] Thus, according to some embodiments, for a slow moving mobile station, codebook based channel-dependent precoding (closed loop Ml MO) with rank adaptation can be used. For a fast moving mobile station, an open loop MIMO mode w ith rank adaptation can be used, where the rank selected is based on wireless channel conditions [0025] Fig 1 shows an exemplar) wireless network in which some embodiments of the imention can be provided. The wireless network includes a base station 100 ihat includes an antenna arra) or other assembly (multi-beam antenna) 102 for sending wireless signals along multiple paths 104. 106 (spatial beams) in a corresponding cell sector 1 OS. A cell scctoi is one >ection of sent b\ the mobile station to establish a call or other communication.-* .session. The sounding reference signal is a reference signal sent b\ the mobile station to enable the ba.se station to determine which beam (path) the mobile station w ill be using for uplink data. GPS or other position determination data can also be used for determining mobile station velocity. Signal(s) from the mobile .station us (are) monitored b\ the base station to determine approximate!} the velocitv of the mobile station. [0035] Based on the measured velocity, the base station classifies (at 304) the mobile station as a ""high-velocity" or "low-velocity" mobile station. The base station then signals (at 306) the mobile station type (high-velocity or low -velocity) to the mobile station The signaling sent to communicate the mobile station t\pe can be a highei layer signaling, such as lc\el 3 (or higher level) signaling. Using higher layer protocols (software) to measure mobile station velocity can be more efficient since the process of measuring mobile station vclocin can take time on the order of hundreds* of milliseconds, in .some implementations. [0036] According to the mobile station type recei\ed from the base station, the mobile station measures (at 30S> the S1NR, which in some embodiments, can be reported m the form of a channel quality indicator (CQl). The CQ1 information provides information regarding the quality of wireless signaling sent along a particular channel that is detected by the mobile station. For a high-velocity mobile, the measured CQI is the CQl corresponding to transmit diversity and a fixed or predetermined codeword. The fixed or predetermined codeword can be a predefined codeword in the codebook defined for the closed loop MIMO mode For a lou-velocit} mobile station, the measured CQI is the CQ] corresponding to a codeword in the codebook. [0037] The mobile station then selects (at 310) the MIMO mode with the largest S1NR. If closed-loop MIMO is selected, a rank and codebook index (V y.. PMI) are also derived and associated with the closed loop MIMO. If open loop MIMO mode is selected, the rank is .selected, but the codeword is fived U1.;;.. the codebook index is predefined). Note that in some implementations the .selection between the open loop MIMO mode and closed loop MIMO mode can be signaled by a higher layer softwarc (level 3 or higher) that also mea.siires or iccches an indication of the mobile .station velocitv. [0038] Next, the mobile station sends (at 312) a feedback message to the base station that contains a rank indicator along with the SINR information, The sending of feedback information at 312 can be performed periodically (or aperiodically). According to a first technique, the SINR information is provided in the form of CQI reports that are periodicalK lor aperiodicalh) sent by the mobile station to the base station. The rank indicator can be sent in each CQI report in this first technique Alternatively, according to a .second technique, the rank indicator can be .->ent e\ cry A'th CQI report, where N > 1 (.V is a configurable value). The rank indicator can be added as additional bit(s) to the CQI report, or alternatively, the rank indicator can replace existing bil(s) of the CQI report (a process referred to as puncturing the CQ1 report). [0030] The frequency of sending the rank indicator defines the rank adaptation interval Sending the rank indicator more frequently means that the raiu adaptation interval is shorter (which means that the rank is dynamically varied more frequently). [0040] In response to the rank indicator and the received SINR. the base station schedules (at 31-4) the downlink data transmission to the mobile station. The ba.se station determines the multi-antenna mode lo be used based on the velocity of the mobile station anc the rank feedback. Note that the base station can override the rank selected by the mobile station (as reported in the rank indicator received from the mobile station). The mode selected by the base station can either be closed loop MIMO mode or open loop MIMO mode with rank adaptation. [0041] The base station a!so determines the assigned channel type based on the mobile station category, where the assigned channel type i> either distributed or diversity channel assignment (for a high-velocity mobile station) or localized channel assignment (for a low-velocity mobile station). The switching between the closed loop MIMO mode and the open loop MIMO mode for downlink transmission can be a.vsociated with the channel type. [0042] Next, the base station signals (at 316) the selected MIMO mode to the mobile station mobile station, where the signaled MIMO mode can include rank information i for open loop MIMO) or rank and codebook index information (for closed loop MIMO). Other information can also be sent from the base station to the mobile station at 316 [0043] in some implementations, two downlink signaling channels can be used to provide the M1M0 mode. For example, a first signaling channel can indicate whether closed loop MIMO or open loop MIMO is to be used If closed loop MIMO is to be used, then a second signaling channel is used to indicate the rank and the codebook index. In some embodiments, if open loop MIMO is to be used, then the second signaling channel is used to indicate the rank (transmit dive:\sit\ or spatial multiplexing. [0044] The mobile station then decodes (at 318) the downlink data sent from the base station according to the mobile station category (high-velocity versus low-velocity) and the mode indicator sent from the base station, such as rank or rani, and codebook index. [0045] Fig. 4 is a message flow diagram of a process according to a different embodiment. In the Fig. 4 procedure, unlike the Fig. 3 procedure, it is the mobile station (rather than the base station) that measures the velocity of the mobile station The mobile station can measure (at 402) the mobile station's velocity by monitoring downlink transmissions, such as the downlink supplemental channel, downlink reference or pilot signal. or any other downlink transmission The mobile station then classifies (at 404) the mobile station as either a high-velocity or low-\eloeity mobile station according to the measured velocity. The mobile station then informs (at 406) the mobile station category to the base station, such as by using a higher layer (t\«.. level 3) signaling. [0046] The remaining tasks of Fig. 4 are identical to the tasks depicted in Fig. 3 (and are assigned the same reference numerals). [0047] At the mobile staton. rank adaptation (performed as part of MIMO mode selection in ta.sk 310 in Fig. 3 or 4) between rank 1 (e.g.. SFBCt and rank 2 U'.t1.. SM» may rely on the procedure a.s follows • Based on the downlink reference signal, the mobile station reccner estimates the channel and calcula:es effective SFNRs for both SFBC and SM. denoted as SIXR'^'1 and SI\:RlSX". respectively. • Based on S/A7?|S7'ff" ' and ,S\r:\7?'M". the mobile station computes both SFBC and SM capacities using a Shannon formula. where A/"'A' us the number of transmit antenna* • A comparison of the two computed capacities enables the mobile station decides whether SFBC or SM is to be used according to the following: [0048} The abo\e presides an example of how the mobile station can compare parameters computed based on SINR \alucs for rank 1 operation and rank 2 operation to determine whether to select rank 1 or rank 2. [0049] As discussed above, in the procedure of Fie. > or 4, as pait of scheduling the downlink transmission performed at 314. the base station can o-vcrride the rank selection made by the mobile station as follows1 • Count the number of decisions (AH> I1L') for SFBC fed back by the mobile station over a predefined observation time window • Count the number of decisions (A*M/)) for SM fed back by the mobile station over the predefined observation time window. • The base station selects SFBC when tfsn'' ' > V v'w'. otherwise the base station selects SM [0050] The selection b\ the base station of whether to use rank ] or rank 2 is thus based on a comparison of times the mobile station selected rank I in the predefined observation window. and the number of times the mobile station selected rank 2 in the predefined observation window—the base station w ill select the rank thai the mobile station picked most in the predefined observation window [005 I] In some embodiments, if the rank selected by the base station is greater than one. (hen the base station can employ a large delay cyclic delay diversity (CDD), which is a diversity scheme that transforms spatial diversity into frequency diversity to reduce inter- symbol interference The operation of the large delay CDD according to an embodiment is fun her defined js follows • For a transmitter with two antenna ports, the precoder for data resource element /, denoted by 1V{i), is selected from a codebook that includes codewords for ranks 1 and "* .An example of such a codebook is described m 3GPP TS ~*(~^ "*I 1 Version 8.30 A data resource element is an element of a resource block, discussed above A resource block includes a number of re.iourcc elements. The selected precoder applies the codeword lV(i). • For a transmitter with four antenna ports, the mobile station may assume that the base station cyclically assigns different precoders to different data resource elements on the physical dow nlink shared channel as follow s A different precoder is used every r data resource elements, where v denotes the number of transmission layers m the case of spatial multiplexing In particular, the precoder for data resource element /. denoted by ff"(/'). is selected according to ir(i>G. iT'l u here k is the precoder index uiven b\ A' = mod — j - 1.4 + 1. where A"^ 1.2....4. In and C;, C\ O, Cj are codewords from a codebook that contains entries for four ranks. An example of such a codebook is described in 3GPP TS 36.211. Version 8.30. [0052] By employing techniques according to some embodiments, rank adaptation is enabled for high-velocity mobile stations, for which an open loop multi-antenna mode is selected. System capacity can be enhanced while maintaining a desired level of w irelev- communications reliability. [0053] Instructions of such software are executed on the processor. The processor includes microprocessors, microcontrollers, processor modules or subsystems (including one or more microprocessors or microcontrollers), or other control or computing devices. A "'processor" can refer to a single component or to plural components. [0054] Data and instructions (of the software) are stored in respective storage devices, which are implemented as one or more computer-readable or computer-usable storage media The storage media include different forms of memon. including semiconductor memory devices such as dynamic or static random access memories (DRAMs or SRAMs). erasable and programmable read-only memories t'EPROMs). electrical) ciasable and programmable read-only memories (EEPROMs) and flash memories; magnetic disks such as fixed, floppy and removable disks: other magnetic media including tape, and optical media such as compact disks (CDs) or digital video disk* (DVDs). [0055] In the foregoing description, numerous details arc set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details. While the invention has been disclosed w ith respect to a limited number of embodiments, those skilled in the an will appreciate numerous modifications and variations therefrom. It is intended that the appended claims eo\ei Mich modification.-* and variation^ a^ fall within the true spirit and scope of the in\ ention What is claimed is: 1 A method, performed by a ba.se station, of rank adaptation for a mobile .station in a wireless network, comprising; receiving, by the base station from the mobile station, an indication of a rank to be used, wherein the rank to be used is based on a velocity of the mobile station. in response to determining that the velocity of the mobile station is above the predefined threshold, selecting an open loop multi-antenna mode to perform wireless communication between the mobile station and the base station; and selecting, based on at least one criterion, from among plural ranks for the wireless communication according to the open loop multi-antenna mode between the mobile station and the base station, wherein the selected rank overrides the rank to be used as specified by the indication 2. The method of claim 1. wherein determining the velocity is performed by one of the mobile station and base station 3. The method of claim 1, wherein receiving the indication of the rank to be used comprises receiving the indication of the rank to be used in association with a report of a wireless channel quality. 4. The method of claim 1. wherein selecting from among the plural ranks comprises selecting from among at least a first rank that emplovs transmit diversity and a second rank that employs spatial multiplexing. 5. The method of claim 1. further comprising" receiving, from the mobile station, an identifier for a codeword to use for coding data in wireiess communication according to the closed loop multi-antenna mode. 6 The method of claim 1. further comprising using, in the open loop multi-antenna mode, a piedetermined codeword or a codeword sequence to code data of the wireless communication 7 The method of claim 6. wherein the predetermined codeword is a predefined codeword in a codebook used for the closed loop multi-antenna transmission mode 8. The method of claim 1, wherein selecting from among the plural ranks based on the at least one criterion comprises counting corresponding numbers of times that the mobile station has selected respective ranks; and comparing the numbers to select from among the plural ranks. 9. The method of claim 8. wherein selecting from among the plural ranks comprises selecting the rank that the mobile station picked the most in a predefined time window 10 The method of claim 1. wherein selecting from among the plural ranks for the wireless communication according to the open loop multi-antenna mode comprises selecting a rank greater than rank 1. the nethod further comprising using cyclic delay diversity w hen the rank selected is greater than rank 1. 11. The method of claim 1. further comprising switching between a closed loop multi- antenna mode and the open loop multi-antenna mode 12 A base station comprising: an interface to perform wireless communication with a mobile station; and a processor to: determine whether the mobile station is a high-\elocit\ mobile station; in response to determining that the mobile station is a high-velocity mobile station, select a multi-antenna node of communication for downlink data transmitted to the mobile station that does not involve feedback from the mobile station that includes an identifier of a codeword to he used for coding the dow nhnk data; and apply rank adaptation to select from among a plurality of ranks for the communication of downlink data according to the multi-antenna mode of comnmnieation. wherein the applied rank adaptation overrides a rank selected b> the mobile station. 13 The base .station of claim 12. wherein the selected multi-antemia mode comprises an open loop multi-antenna mode. 14. The base .station of claim 13. wherein the processor i.s to apply a fixed codeword to the downlink data in the open loop multi-antenna mode. 15. The base station of claim 13, wherein the processor is to .select a closed loop multi- antenna mode in response to determining that the mobile station is not a high-velocity mobile station 16. The base station of claim 15, further comprising a higher layer softw are to signal selection of the open loop multi-antenna mode or closed loop multi-antenna mode To perform rank adaptation for a mobile station in a wireless network, it is determined whether the mobile station is fast moving In response to determining that the mobile station is fast moving, an open loop multi-antenna mode is selected to perform wireless communication between the mobile station and a base station Also, selection is made from among plural ranks for the wireless commumcation according to the open loop multi-antenna mode between the mobile station and the base station.