METHOD FOR RANGE EXTENSION IN WIRELESS COMMUNICATION SYSTEMS CROSS REFERENCE TO RELATED APPLICATIONS This application is related to U.S. Patent Application 11/741,068, filed on April 27,2007. This application also claims benefit of U.S. Provisional Patent Application 61/280,071 filed on October 29,2009. BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION This invention relates generally to communication systems, and, more particularly, to wireless communication systems. 2. DESCRIPTION OF THE RELATED ART Wireless communication systems typically include one or more base stations or access points for providing wireless connectivity to mobile units in a geographic area (or cell) associated with each base station or access point. Mobile units and base stations communicate by transmitting modulated radiofrequency signals over a wireless communication link, or air interface. The air interface includes downlink (or forward link) channels for transmitting information from the base station to the mobile unit and uplink (or reverse link) channels for transmitting information from the mobile unit to the base station. The uplink and downlink channels are typically divided into data channels, random access channels, broadcast channels, paging channels, control channels, and the like. Mobile units can initiate communication with the base station by transmitting a message on one or more of the random access channels (RACHs). Uplink random access channels are non-synchronized and therefore may be transmitted at any time relative to the synchronized downlink timing by any mobile unit within the coverage area of the base station. The receiver in the base station must therefore continuously monitor the random access channels and search the signals received on the random access channels for predetermined sequences of symbols (sometimes referred to as the RACH preamble) in random access channels transmitted by mobile units. To make the search process feasible, the format of the random access channels must be standardized. For example, conventional random access channels in the Universal Mobile Telecommunication Services (UMTS) Long Term Evolution (LTE) system are transmitted in a subframe during a transmission time interval (TTI) of 1 ms in 1.08 MHz bandwidth. The reception times for uplink random access of signals transmitted by mobile units near the center of the cell and mobile units near the edge of the cell can be offset by as much as the round-trip delay corresponding to the cell radius. The offset arises because the non-synchronized random access uplink signals for a particular subframe are transmitted relative to the arrival times of the corresponding synchronized downlink subframe. A mobile unit at the center of the cell receives the synchronized downlink subframe earlier than mobile unit at the edge of the cell (by approximately the one way delay corresponding to the cell radius) and uplink signals transmitted from central mobile units arrive at the base station earlier than uplink signals transmitted from edge mobile units (by approximately the one-way delay corresponding to the cell radius). Inter-symbol interference between random access channels associated with different subframes occurs if random access signals associated with one subframe overlap with a subsequent subframe and therefore interfere with random access signals associated with the subsequent subframe. Inter-symbol interference can be reduced by including a guard time in each random access channel subframe during which no uplink signal is transmitted to reduce or prevent inter-symbol interference. For example, the random access channel subframe can be divided into a 0.8 ms preamble and a 102.6 us cyclic prefix that includes a copy of a portion of the sequence of symbols in the preamble. The remaining 97.4 us in the transmission time interval is reserved as a guard time The coverage area of a base station is related to the duration of the cyclic prefix and the guard time. For example, the conventional guard time of approximately 0.1 ms corresponds to a round-trip delay for a signal that travels approximately 15 kilometers. Thus, a random access channel format that includes approximately 0.1 ms for the guard time is appropriate for reducing or preventing inter-symbol interference for coverage areas or cell sizes having a radius of up to approximately 15 kilometers. Similarly, the duration of the cyclic prefix is related to the size of the coverage area. For example, a cyclic prefix of approximately 0.1 ms is suitable for coverage areas having radii of up to approximately 15 kilometers. Although a range of 15 km may be considered sufficient for conventional wireless communication systems, the base station range of proposed wireless communications systems, such as the UMTS LTE, is expected to increase to at least 100 km in scenarios with good radio propagation conditions such as coverage in coastal areas. Proposals to extend the range of the random access channel supported by base stations include increasing the transmission time interval to 2 ms. For example, one proposal includes changing the structure of random access channels. In this proposal, the extended transmission time interval includes a 0.8 ms RACH preamble. The length of the cyclic prefix (CP) also increases in proportion to the desired coverage area. For example, every 0.1 ms of additional cyclic prefix length will account for additional 15 km coverage. The guard time also increases at the same rate as the cyclic prefix length. Thus, with the 0.8 ms RACH preamble, the time available for guard time and cyclic prefix is 2 ms - 0.8 ms = 1.2 ms. This RACH range extension proposal attempts to reduce the receiver complexity of the RACH preamble detection. However, the guard time and the cyclic prefix are considered pure overhead because no new information can be transmitted during these intervals. Increasing the guard time or the cyclic prefix length much beyond the current value of 0.1 ms is therefore not considered a desirable way to extend the range of cells because of the high resource cost. In other proposals, two partitions between cyclic prefix and guard time (or guard period) can be envisioned: In one case, the 1.2 ms portion of the subframe that is not allocated to the preamble could be evenly allocated to the cyclic prefix and the guard time so that the RACH coverage is extended to 90 km. Alternatively, the 1.2 ms portion of the subframe that is not allocated to the preamble could be unevenly distributed between the cyclic prefix length and the guard time. The uneven distribution of the allocated time to the cyclic prefix and the guard time could extend the coverage to the 100 km if the cyclic prefix length is equal to or greater than 0.667 ms. However, inter-symbol interference may occur when the cyclic prefix and guard time allocations are uneven in cases where the preamble is transmitted by a mobile unit near the cell edge. Moreover, the signal strength received from mobile units in the edge of an extended cell, e.g., mobile units that are as much as 90 or 100 km from the base station, may be very low, which may reduce the likelihood of detecting the preamble of the random access channel. SUMMARY OF EMBODIMENTS OF THE INVENTION The disclosed subject matter is directed to addressing the effects of one or more of the problems set forth above. The following presents a simplified summary of the disclosed subject matter in order to provide a basic understanding of some aspects of the disclosed subject matter. This summary is not an exhaustive overview of the disclosed subject matter. It is not intended to identify key or critical elements of the disclosed subject matter or to delineate the scope of the disclosed subject matter. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later. In one embodiment, a method is provided for range extension is wireless communication systems. One embodiment of the method includes determining whether a mobile unit is within a first range corresponding to a range of timing advances supported by a timing advance command. This embodiment also includes transmitting a plurality of timing advance commands to the mobile unit when the mobile unit is outside the first range so that the mobile unit can synchronize with the base station by combining information in the plurality of timing advance commands. In another embodiment, a method is provided for range extension wireless communication systems. This embodiment of the method includes receiving, at a mobile unit and from a base station, a plurality of timing advance commands when the mobile unit is outside a first range corresponding to a range of timing advances supported by a timing advance command. This embodiment also includes synchronizing the mobile unit to the base station by combining information in the plurality of timing advance commands. BRIEF DESCRIPTION OF THE DRAWINGS The disclosed subject matter may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: Figure 1 conceptually illustrates one exemplary embodiment of a wireless communication system; Figure 2 shows one exemplary embodiment of a random access channel; Figure 3 conceptually illustrates one exemplary embodiment of a transmitter that can be used to transmit RACH preambles such as the random-access channel shown in Figure 2; Figure 4 conceptually illustrates one embodiment of a receiver that can be used to detect a RACH preamble over a range associated with the structure of the RACH preamble; Figure 5 conceptually illustrates one exemplary embodiment of the base station that can determine whether a mobile unit is within a range supported by a timing advance command and/or a RACH preamble structure used by the mobile unit; Figure 6A conceptually illustrates one exemplary embodiment of a timing advance command that can be used to indicate values of the timing advance; Figure 6B conceptually illustrates two timing advance commands that can be combined to signal a timing advance that is larger than the maximum value of the timing advance that can be signaled using the bits in a single timing advance command; Figure 7 conceptually illustrates one exemplary embodiment of a method or transmitting timing advance commands to mobile units; and Figure 8 conceptually illustrates one exemplary embodiment of a method for receiving timing advance commands at a mobile unit. While the disclosed subject matter is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the disclosed subject matter to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the appended claims. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS Illustrative embodiments are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions should be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The disclosed subject matter will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present invention with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the disclosed subject matter. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. Generally, the present application describes techniques that can be used to extend the range of base stations within a wireless communication system. As discussed herein, the range of the base station is related to the structure of random-access preambles that mobile units transmit to signal their presence to the base station. For example, every 0.1 ms of additional cyclic prefix length (and corresponding guard time interval) in the preamble can add an additional 15 km coverage. However, the guard time and the cyclic prefix are considered pure overhead because no new information can be transmitted during these intervals. Increasing the guard time or the cyclic prefix length much beyond the current value of 0.1 ms is therefore not considered a desirable way to extend the range of cells because of the high resource cost. Other techniques may also be used to support range extension. For example, the Long Term Evolution (LTE) physical layer procedures and physical layer parameters may be designed based on target cell radius of 100 km. Cell range extension to beyond 100 km may be implemented in base stations and/or eNBs: In one embodiment, random-access (RA) preamble detection can be used: The CP length of the RA preamble is designed to support up to 100km and to allow efficient frequency-domain correlator implementation. To support range extension, the time-domain correlator may be used or, alternatively, the frequency-domain correlator could be used, with tradeoff in detection performance. For PUSCH transmission, limitations in the timing advance (TA) command range may cause the received uplink signals from remote users to lose time alignment and spill over to adjacent subframes, causing interference. For RA message 3, the reception timing of RA message 3 is configurable within a certain window size. The RA message 3 for users in different ranges may be scheduled at different subframes (TDM). Alternatively, FDM may be used. TDM/FDM separation may also be feasible for traffic channels. Over time, the uplink timing offset may be corrected by sending relative TA commands, e.g., when the base station determines that synchronized uplink transmissions are not properly aligned with the base station timing reference. In one embodiment, the eNB receiver can support special handling of the signals from remote users to determine their location relative to the eNB. For example, eNB may place different fast Fourier transform (FFT) windows on the received random-access signals, depending on the range of users (eg. 0-100km, 100-200km). The fundamental problem with the above solution is the expected capacity loss. The signals from remote users may spill over in 2 subframes provisioned by the scheduler at least in part because a single timing advance command does not have sufficient range to indicate the needed timing advance for distant users. It would be desirable to be able to align the received signals over a larger range, to avoid the capacity loss. Embodiments of the techniques described in the present application therefore do not rely on modifying the structure of random-access messages. The techniques described herein may also provide a time alignment mechanism that allows the mobile unit to receive the necessary timing advance information so that the mobile unit can quickly synchronize to the base station. Some embodiments may therefore reduce or eliminate spillover of uplink transmissions into multiple subframes. In one embodiment, base stations can determine whether a mobile unit is separated from the base station by a distance that is within a range of timing advances supported by a timing advance command. If so, the required timing advance for the mobile unit can be signaled using a single timing advance command. If not, the base station can transmit a plurality of timing advance commands to the mobile unit, which can combine the information in the timing advance commands to determine the timing advance used to synchronize to the base station. Figure 1 conceptually illustrates one exemplary embodiment of a wireless communication system 100. In the illustrated embodiment, a base station 105 provides wireless connectivity to mobile units 110 over air interfaces or wireless communication links 115. Techniques for establishing, maintaining, operating, de allocating, and/or tearing down wireless communication links 115 are known in the art and in the interest of clarity only those aspects of establishing, maintaining, operating, de-allocating, and/or tearing down the wireless communication links 115 that are relevant to the claimed subject matter will be discussed herein. Moreover, persons of ordinary skill in the art having benefit of the present disclosure should appreciate that the particular wireless communication systems shown in Figure 1 is intended to be illustrative and not to limit the claimed subject matter. For example, alternate embodiments of the wireless communication system 100 may include other numbers of base stations 105, and/or mobile units 115. The base station 105 and the mobile units 110 may initiate wireless communication over the wireless communication links 115 by exchanging random-access messages and timing advance commands. In the illustrated embodiment, the base station 105 and the mobile units 110 are configured to communicate over time-synchronized traffic or data channels. For example, frequency division duplex (FDD) and/or time division duplex (TDD) channels may use a frame structure for uplink and/or downlink transmissions in which each channel is divided into 5 ms or 10 ms frames that are each subdivided into subframes or slots, e.g. 0.5 ms timeslots. However, the base stations 105 and mobile units 110 may not initially be synchronized, at least in part because of the variable (and initially unknown) round-trip time delay between the base station 105 and each mobile unit 110. Signal processing time within the base stations 105 and/or the mobile units 110 may also contribute to the lack of synchronization. As discussed herein, the structure of a random access channel transmitted from a mobile unit 110 to the base station 105 corresponds to the range (R) of the base station 105. Figure 2 shows one exemplary embodiment of a random access channel 200. In this embodiment, the random access channel 200 includes a cyclic prefix 205 and a transmission sequence 210 that may include the random access channel (RACH) preamble. For example, a physical layer random access preamble (such as the random access channel 200) may consist of a cyclic prefix 205 of length TCP and a sequence part 210 of length TSEQ . Exemplary parameter values are listed in Table 1 and depend on the frame structure and the random access configuration. Higher layers in the protocol stack may control the preamble format. The preamble formats listed in Table 1 are defined in accordance with the standards and/or protocols set forth by the Third Generation Partnership Project (3GPP) and, in particular, in 3GPP TS 36.211 v9.1.0, entitled 3GPP Technical Specification Group Radio Access Network, Evolved Universal Terrestrial Radio Access (E-UTRA), Physical Channels and Modulation. The basic time unit (Ts) may also be defined in accordance with the 3GPP standards and/or protocols. Formats 1 and 3 in Table 1 may be used to achieve range of approximately 100 km because these formats use relatively longer cyclic prefixes. As used herein, the term "approximately" is used to indicate that under perfect conditions the round-trip delay corresponding to the duration of the cyclic prefix for these formats correspond to a range of 100 km. However, environmental conditions and other factors may cause the actual range achieved in practice to vary from this ideal value although persons of ordinary skill in the art would still refer to the range as being "100km" or "approximately 100km." In one embodiment that may be adopted in 3 GPP standards and/or protocols, the range of supported TA command may be limited to [0,..., 1282], e.g., in TS 36.213 and corresponding RAN2 specification TS 36.321. The range of TA command may be extended to [0 2047] without changing MAC PDU structure using embodiments of the techniques described herein. For example, in case of a random access (RA) response with an 11-bit timing advance command, TA indicates NTA values by index values of TA = 0, 1, 2,..., 1282, where an amount of the time alignment is given by NTA = TA xl6 and NTA is defined in the relevant 3GPP specifications. The Timing Advance Command field indicates the index value TA (0, 1,2... 1282) that may be used to control the amount of timing adjustment applied by a mobile unit. In one embodiment, the size of the Timing Advance Command field is 11 bits. Note that 20512 = 16*1282, as the applicable timing advance may be defined as steps of 16 Ts. The base station and the mobile unit can be synchronized using the timing advance so that transmission of the uplink radio frame number / from the mobile unit may start (A^TA + NTK offset )xTs seconds before the start of the corresponding downlink radio frame at the mobile unit, where 0