Two-step uplink synchronization for pico/femtocell

This application claims priority under 35 U.S.C. §119 from U.S. Provisional Application No. 61/293,424, entitled “A Two-Step Scheme for Uplink Synchronization in Femtocell,” filed on Jan. 8, 2010; the subject matter of which is incorporated herein by reference.

The disclosed embodiments relate generally to wireless network communications, and, more particularly, to uplink synchronization for pico/femtocell in wireless mobile communication systems.

Picocell and Femtocell deployment is an important feature in the next generation (4G) wireless mobile communication systems, such as WiMAX 2.0 systems defined by IEEE 802.16m and LTE-Advanced systems defined by 3GPP Release 10. The deployment of pico/femtocell enhances indoor services of wireless mobile communication, off-loads traffic of macro and/or micro base stations, and compensates and reduces outdoor service coverage holes. Network timing synchronization becomes an important issue when pico/femtocells are deployed together with overlaying macro/microcells, especially in a co-channel development scenario. Network timing synchronization has to be kept so that radio signals from a pico/femto base station and an overlaying macro/micro base station over the air do not interfere with each other.

FIG. 1 (Prior Art) illustrates a hierarchical cell structure of macro/microcells and pico/femtocells in a cellular OFDM communication system 10 . Cellular OFDM communication system 10 comprises a macro/micro base station BS 11 , pico/femto base stations BS 12 and BS 13 , and mobile stations MS 14 and MS 15 . The two pico/femto base stations BS 12 and BS 13 have smaller cell coverage, while the overlaying macro/micro base station BS 11 has much larger cell coverage. In the example of FIG. 1 , BS 12 is located close to BS 11 while BS 13 is located far from BS 11 . Mobile station MS 14 is served by pico/femto BS 13 , and mobile station MS 15 is served by macro/micro BS 11 . MS 15 is located far from its serving BS 11 but relatively close to pico/femto BS 13 .

FIG. 2 (Prior Art) illustrates downlink and uplink subframes and transmission timing in cellular OFDM communication system 10 . As illustrated in FIG. 2 , in a time division duplex (TDD) system, macro base station BS 11 transmits data during downlink (DL) subframes and receives data during uplink (UL) subframes. Each DL subframe is followed by an UL subframe after a predefined transmit transition gap (TTG) time, and each UL subframe is followed by a DL subframe after a predefined receive transition gap (RTG) time. Pico/femto base station BS 12 is synchronized with overlaying BS 11 , and has approximately the same DL and UL transmission timing due to its physical proximity with BS 11 . Pico/femto base station BS 13 is also synchronized with overlaying BS 11 , but has a delayed DL and UL transmission timing due to DL propagation delay. For mobile station MS 15 that is served by BS 11 , it receives data during DL subframes and transmits data during UL subframes. After DL and UL synchronization with its serving BS 11 , each DL subframe of MS 15 synchronizes with each DL subframe of BS 11 with a DL propagation delay, while each UL subframe of MS 15 synchronizes with each UL subframe of BS 11 with an UL timing advance. Because of the DL propagation delay and the UL timing advance, it can be seen that the UL subframes between pico/femto BS 13 and MS 15 has a timing difference, as illustrated in FIG. 2 .

Therefore, it is necessary for pico/femto BS 13 to adjust its TTG to avoid possible uplink interference between a pico/femto mobile station (e.g. MS 14 ) and a nearby macro/micro mobile station (e.g. MS 15 ). Without adjustment of TTG in a pico/femtocell, the uplink receiving of the pico/femto base station may be interfered by uplink transmission of neighboring mobile stations served by an overlaying macro/micro base station due to non-aligned uplink subframes if the timing difference is not an integer number of OFDM symbols. For mobile stations that handover or are camped on in the pico/femtocell, however, they have no knowledge of any TTG adjustment. Without knowing the TTG adjustment value, there will be different understanding about the UL transmission time between mobile stations and the pico/femto BS, which would likely to result in incomplete receiving of uplink signals such as ranging signals at the pico/femto BS.

In current IEEE 802.16m and 3GPP systems, non-synchronized ranging channel (RCH) or random access channel (RACH) is utilized for initial and handover ranging in pico/femtocells. FIG. 3 (Prior Art) illustrates a non-synchronized ranging channel 31 and a data channel 32 in an IEEE 802.16m OFDM system. As illustrated in FIG. 3 , a pico/femto MS utilizes non-synchronized ranging channel 31 for ranging preamble transmission, and a macro/micro MS utilizes data channel 32 for data transmission. The non-synchronized ranging channel 31 has a long ranging cyclic prefix (RCP) length and guard time to accommodate the time difference of non-aligned uplink subframes between the pico/femto MS and the macro/micro MS. This solution, however, is associated with a few disadvantages. First, it requires a non-synchronous ranging channel to have a different CP length from a data channel in the same communication system. Second, the different CP lengths between non-synchronous ranging channel and data channel may result in interference with each other. As illustrated in FIG. 3 , since the time domain structure of the non-synchronous ranging channel 31 is different from the data channel 32 , the orthogonality in frequency domain between the two channels may be destroyed. When the macro/micro MS transmits data channel, and the pico/femto MS transmits ranging channel, ranging performance in pico/femtocell degrades significantly. Third, a non-synchronous ranging channel may have a different physical structure and code sequences than those of a synchronous ranging channel. Thus, without utilizing a unified synchronous ranging channel, hardware complexity and cost of a pico/femto BS may not be reduced.

A two-step uplink synchronization method is provided for uplink synchronization between a mobile station and a pico/femto base station that is deployed together with an overlaying macro/micro base station. In a first step, the pico/femto BS encodes and broadcasts UL transmission timing advance offset information via a broadcast channel. The MS decodes the received UL transmission timing advance offset information and advances its uplink timing for ranging or reference signal transmission based on the decoded offset value. In a second step, the MS and the pico/femto BS performs regular uplink synchronization and uplink access procedure. By using the two-step uplink synchronization method, a unified synchronous ranging channel may be used for ranging and UL access in pico/femtocells with reduced interference.

In one example, the UL transmission timing advance offset information indicates a round-trip propagation time of radio signals between the pico/femto base station and the overlay macro/micro base station. In one embodiment, the pico/femto BS encodes the UL transmission timing advance offset information into a number that is indicative of an UL transmission timing advance offset value. The MS receives the UL transmission timing advance offset information and decodes the UL transmission timing advance offset value accordingly. In an IEEE 802.16m WiMAX system, it is proposed that the MS advances ranging signal transmission timing based on the decoded offset value before conducting the regular UL synchronization procedure. In a 3GPP LTE/LTE-A system, it is proposed that the MS advances reference signal transmission timing based on the decoded offset value before conducting the regular UL access procedure.

Other embodiments and advantages are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims.

The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention.

FIG. 1 (Prior Art) illustrates a hierarchical cell structure of macro/microcells and pico/femtocells in a cellular OFDM communication system.

FIG. 2 (Prior Art) illustrates downlink and uplink subframes and transmission timing in a cellular OFDM communication system.

FIG. 3 (Prior Art) illustrates a non-synchronized ranging channel and a data channel in an IEEE 802.16m OFDM system.

FIG. 4 illustrates a hierarchical cell structure of macro/microcells and pico/femtocells in a cellular OFDM/OFDMA communication system 40 in accordance with one novel aspect.

FIG. 5 illustrates downlink and uplink subframes and transmission timing of base stations and mobile stations in a hierarchical cell structure.

FIG. 6 illustrates downlink and uplink subframes and transmission timing during an initial ranging procedure in a pico/femtocell.

FIG. 7 illustrates downlink and uplink subframes and transmission timing during a handover ranging procedure in a pico/femtocell.

FIG. 8 illustrates a flow chart of a method of two-step uplink synchronization in accordance with one novel aspect.

Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings.

FIG. 4 illustrates a hierarchical cell structure of macro/microcells and pico/femtocells in a cellular OFDM/OFDMA communication system 40 in accordance with one novel aspect. Cellular OFDM/OFDMA communication system 40 comprises a macro/micro base station BS 41 , pico/femto base stations BS 42 and BS 43 , and mobile stations MS 44 and MS 45 . In general, a pico/femto base station has smaller cell coverage, while a macro/micro base station has much larger cell coverage. In a hierarchical cell structure, a cell with smaller coverage (referred to as an underlying cell) is overlapped (either fully or partially) with a cell with larger coverage (referred to as an overlaying cell). In the example of FIG. 4 , macro/micro BS 41 is an overlaying BS that provides service for a larger cell 46 , while pico/femto BS 42 and BS 43 are underlying BSs that provide service for much smaller cells 47 and 48 . In addition, BS 42 is located very close to BS 41 with a short distance A, while BS 43 is located very far from BS 11 with a long distance B. Mobile station MS 44 is a pico/femto MS served by pico/femto BS 43 and mobile station MS 45 is a macro/micro MS served by macro/micro BS 41 . MS 45 is located relatively close to pico/femto BS 43 , thus is very far from its serving BS 41 with a long distance C.

In accordance with one novel aspect, a two-step uplink synchronization procedure is performed between MS 44 and BS 43 . In a first step, BS 43 encodes uplink (UL) transmission timing advance offset information and broadcasts the timing advance offset information to MS 44 . MS 44 decodes the UL transmission timing advance offset information and adjusts its UL transmission timing of a ranging or reference signal based on a decoded timing advance offset value. In a second step, MS 44 performs regular uplink synchronization and access with BS 43 . The two-step uplink synchronization procedure enables the use of a ranging or reference signal with normal cyclic prefix (CP) length for uplink synchronization and access, while prevents signals from pico/femto BS 43 and overlaying macro/micro BS 41 interfere with each other, as illustrated in the background section.

As illustrated in FIG. 4 , a mobile station (e.g. MS 44 ) served by a pico/femtocell comprises memory 51 , a processor 52 , an information decoding module 53 , an uplink access and timing adjustment module 54 , and a transceiver 55 coupled to an antenna 56 . Similarly, a pico/femto base station (e.g., BS 43 ) comprises memory 61 , a processor 62 , an information encoding module 63 , a signal detection and timing estimation module 64 , and a transceiver 65 coupled to an antenna 66 . In one or more exemplary embodiments, the functions described in the two-step UL synchronization procedure may be implemented in hardware, software, firmware, or any combination thereof by the different modules. The functions described above may be implemented together in a same module, or independently in separate modules. The details of the two-step UL synchronization procedure are now described below with accompanying drawings.

In wireless communication systems, such as defined by IEEE 802.16m specification, base stations and mobile stations communicate with each other by sending and receiving data carried in a series of superframes. Before a mobile station can access a base station, physical (PHY) layer synchronization and Media Access Control (MAC) layer random access are performed. Upon power-on, a mobile station first acquires downlink (DL) synchronization and adjusts its timing, frequency and power via synchronization channel (SCH) broadcasted by a serving base station. After DL synchronization, the mobile station acquires uplink (UL) PHY layer synchronization via ranging procedures and MAC layer random access via network entry procedures with the serving base station. When pico/femtocells are deployed together with macro/microcells in a hierarchical cell structure (e.g. wireless system 40 ), the DL and UL network timing synchronization has to be kept so that radio signals from a pico/femto BS (e.g. BS 43 ) and an overlaying macro/micro BS (e.g. BS 41 ) over the air do not interfere with each other.

FIG. 5 illustrates downlink and uplink subframes and transmission timing in a hierarchical cell structure (e.g., wireless system 40 ) in accordance with one novel aspect. In a Time Division Duplex (TDD) system, the DL subframe(s) are followed by the UL subframe(s) after a predefined transmit transition gap (TTG) time, and the UL subframe(s) are followed by the DL subframe(s) after a predefined receive transition gap (RTG) time. The TTG and RTG are reserved to avoid possible collision between downlink and uplink transmissions. In the example of FIG. 5 , macro/micro base station BS 41 transmits data during DL subframes and receives data during UL subframes, while pico/femto mobile station MS 45 receives data during DL subframes and transmits data during UL subframes. MS 45 is located far from its serving BS 41 , the propagation time of radio signals between BS 41 and MS 45 is referred to as a DL propagation delay. After DL and UL synchronization, the DL transmission timing of MS 45 is synchronized to BS 41 with the DL propagation delay, and the UL transmission timing of MS 45 is synchronized to BS 41 with an UL timing advance. For UL transmission, the UL timing advance relative to absolute UL timing of BS 41 is equal to the DL propagation delay.

For pico/femto base station BS 43 , because its cell coverage is overlapped with overlaying macro/micro base station BS 41 , network timing synchronization does not simply mean absolute timing synchronization between BS 43 and BS 41 . In a co-channel deployment scenario where BS 43 and BS 41 operate in the same frequency band, network timing synchronization means the timing synchronization of radio signals from BS 43 and BS 41 over the air. More specifically, because BS 43 is located far from BS 41 and relatively close to MS 45 , radio signals of mobile stations served by BS 43 (e.g., MS 44 ) is likely to interfere with radio signals of nearby mobile stations served by BS 41 (e.g. MS 45 ). For DL transmission, the receiving of MS 45 from BS 41 is likely to be interfered by the nearby BS 43 transmitting signals to MS 44 . On the other hand, for UL transmission, the receiving of BS 43 from MS 44 is likely to be interfered by the nearby MS 45 transmitting signal to BS 41 . Therefore, network timing synchronization in wireless system 40 means that the DL and UL transmission timing between pico/femto BS 43 and its nearby macro/micro MS 45 needs to be synchronized with each other to avoid interference.

As illustrated in FIG. 5 , in order for BS 43 to synchronize with MS 45 , the pre-defined TTG time and RTG time need to be adjusted. For example, the UL transmission timing of UL(RX) subframe 57 of BS 43 (e.g., depicted by a solid line) is determined based on an adjusted TTG time. In general, the TTG adjustment value is equal to the round-trip time of radio signals between BS 43 and BS 41 , such that the uplink receiving in UL(RX) subframe 57 of BS 43 is not interfered by the uplink transmission in UL(TX) subframe 58 of neighboring MS 45 served by BS 41 .

On the other hand, if pico/femto BS 43 and overlaying macro/micro BS 41 operate in different frequency bands, then such TTG adjustment may not be necessary, especially if the different frequency bands are separate from each other. For example, the UL transmission timing of the same UL(RX) subframe 57 of BS 43 (e.g., depicted by a dashed line) is determined based on the original pre-defined TTG time, as illustrated in FIG. 5 . In such a deployment scenario, it is up to the pico/femto base station to decide whether TTG adjustment is applied.

Therefore, due to downlink and uplink network timing synchronization requirement for a pico/femto base station, the pre-defined TTG reserved to avoid the collision of downlink and uplink transmission may need to be adjusted base on the deployment location of the pico/femto base station. However, for mobile stations which handover to a pico/femto cell or camping on in a pico/femtocell, there is no mechanism for them to know whether TTG adjustment is applied in the pico/femtocell. Without knowing this adjustment value, there will be different understandings about the uplink transmission timing between the mobile stations and the pico/femto BS.

FIG. 6 illustrates downlink and uplink subframes and transmission timing during an initial ranging in a pico/femtocell. In the example of FIG. 6 , a femto BS 1 is deployed together with an overlay macro BS in a co-channel deployment scenario. For the macro BS, its DL subframes are followed by UL subframes with a predefined TTG, and its UL subframes are followed by DL subframes with a predefined RTG. To prevent signals from femto BS 1 and macro BS interfering with each other, femto BS 1 applies the predefined TTG with a TTG adjustment value. The TTG adjustment value is equal to the round-trip time of radio signals between femto BS 1 and the macro BS. Femto BS 1 then adjusts its UL transmission timing for its UL frames based on the adjusted TTG. Suppose that a mobile station MS 1 is powered on inside the cell coverage of femto BS 1 . MS 1 does not know that a TTG adjustment value is being applied in the femtocell by femto BS 1 . Without knowing the TTG adjustment, MS 1 applies the same predefined TTG for its UL transmission timing. As a result, the UL transmission timing of MS 1 has a time difference as compared to the UL transmission timing of femto BS 1 . This time difference is equal to the TTG adjustment value.

When MS 1 performs initial ranging with femto BS 1 , such induced time difference results in incomplete receiving of ranging preamble signals at femto BS 1 if the length of the ranging channel cyclic prefix (RCP) is less than the time difference. As illustrated in FIG. 6 , in accordance with one novel aspect, if femto BS 1 informs its TTG adjustment to MS 1 before the initial ranging, MS 1 is then able to adjust its own UL transmission timing based on the same adjusted TTG value as femto BS 1 . MS 1 and femto BS 1 thus can successfully perform ranging followed by UL synchronization procedure.

FIG. 7 illustrates downlink and uplink subframes and transmission timing during a handover ranging in a pico/femtocell. In the example of FIG. 7 , a femto BS 2 is deployed together with an overlay macro BS, but the two BSs operate in separate frequency bands. For the macro BS, its DL subframes are followed by UL subframes with a predefined TTG, and its UL subframes are followed by DL subframes with a predefined RTG. For femto BS 2 , because it operates in a different frequency band from the overlaying macro BS, it may decide to use the same predefined TTG and RTG for its DL and UL transmission timing without applying any TTG adjustment. Suppose a nearby mobile station MS 2 is going to handover from the macro BS to femto BS 2 . MS 2 was originally served by the macro BS and its UL transmission timing was adjusted with a timing advance corresponds to a round-trip time of radio signals between the macro BS and MS 2 . MS 2 does not know that the pre-defined TTG is being applied in the femtocell by femto BS 2 . As a result, the UL transmission timing of MS 2 has a time difference as compared to the UL transmission timing of femto BS 2 . This time difference is equal to the timing advance applied by MS 2 before handover.

When MS 2 starts to perform handover ranging with femto BS 2 , such induced time difference results in incomplete receiving of ranging preamble signals at femto BS 2 if the length of the ranging channel cyclic prefix (RCP) is less than the time difference. As illustrated in FIG. 7 , in accordance with one novel aspect, if femto BS 2 informs its TTG adjustment value to MS 2 before the handover ranging, MS 2 is then able to adjust its own UL transmission timing based on the same TTG adjustment value as femto BS 2 . MS 2 and femto BS 2 thus can successfully perform ranging followed by UL synchronization procedure.

In addition to the problem of incomplete receiving of the ranging preamble signals, the non-aligned uplink transmission of ranging preamble signals also introduces serious interference to other mobile stations served by the femto BS. For example, the receiving of uplink data from other MSs at femto BS 1 or femto BS 2 would be interfered by the transmission of ranging preamble signals from MS 1 or MS 2 if the guard time of the ranging channel is less than the time difference.

While the use of a non-synchronous ranging channel with long RCP length and guard time may partially solve the problem of ranging and interference, a unified synchronous ranging channel is still preferred, for several reasons as explained in the background section. To enable the use of a unified synchronous ranging channel, in accordance with one novel aspect, a two-step uplink synchronization procedure is proposed such that a pico/femto BS with overlaying macro/micro BS is able to perform ranging and uplink synchronization successfully without interfering with other mobile stations.

Before describing the details of the two-step UL synchronization procedure, it should be noted that although the above examples were illustrated using TDD systems, the same principle could be applied in frequency division duplex (FDD) systems. In TDD systems, the term “TTG adjustment” is used. In FDD systems, however, a more generic term “transmission timing advance offset” may be used instead. It should be further noted that although the above examples were illustrates using IEEE 802.16m systems, the same principle could be applied in 3GPP systems. In IEEE 802.16m WiMAX systems, a mobile station transmits a ranging signal via a ranging channel (RCH) for UL synchronization. In 3GPP LTE/LTE-A systems, however, a user equipment (UE) transmits a reference signal via a random access channel (RACH) to request for UL access. The term ranging signal or reference signal, and the term ranging channel or random access channel, thus could be used interchangeably with regard to the process of uplink synchronization and access.

FIG. 8 illustrates a flow chart of a method of two-step uplink synchronization in accordance with one novel aspect. In the example of FIG. 8 , the two-step UL synchronization procedure is performed between a mobile station and a pico/femto base station that is deployed together with an overlay macro/micro base station in an IEEE 802.16m system. At the MS side, the MS first scans synchronization channel (SCH) preambles transmitted by neighboring base stations (step 81 ). The MS then establishes DL synchronization with a candidate cell (e.g., the pico/femtocell) in step 82 . During DL synchronization, the MS obtains and acquires Primary superframe header (P-SFH) and Secondary SFH (S-SFH) information. In step 83 , the MS determines whether the candidate cell belongs to a cell type of picocell or femtocell. If the answer is yes, then the MS obtains UL transmission timing advance offset information from SFH, and derives an UL transmission timing advance offset value from the offset information (step 84 ). The MS then advances its UL transmission timing for transmitting ranging signals based on the derived timing advance offset value (step 85 ). After UL transmission timing adjustment, the MS starts performing regular ranging and automatic adjustment with the pico/femto BS in step 86 . On the other hand, if the answer is no, then the MS starts to perform regular ranging and automatic adjustment and establishes UL synchronization with the pico/femto BS directly (step 86 ). During the regular ranging process, the MS encodes and transmits a ranging signal, receives and decodes a drift timing difference message sent from the pico/femto BS, automatically adjusts UL transmission timing based on the decoded drift timing difference, and re-transmits the ranging signal with the adjusted UL transmission timing. Finally, in step 87 , the MS receives a confirmation from the pico/femto BS and the uplink synchronization is completed.

At the pico/femto BS side, the pico/femto BS first determines an UL transmission timing advance offset value (e.g., a TTG adjustment value in TDD system) in step 91 . The pico/femto BS then encodes the timing advance offset value into UL transmission timing advance offset information and broadcasts the offset information via a broadcast channel (BCH) in step 92 . In step 93 , the pico/femto BS broadcasts ranging signal configuration to the MS. In step 94 , the pico/femto BS detects a ranging signal sent from the MS, the ranging signal is transmitted with an advanced UL transmission timing based on the broadcasted offset information. In step 95 , the pico/femto BS estimates the drift timing difference from the detected ranging signal. In step 96 , the pico/femto BS determines whether uplink timing is synchronized. If the answer is yes, then the pico/femto BS sends confirmation to the MS and the uplink synchronization is completed (step 98 ). On the other hand, if the answer is no, then the pico/femto BS sends a drift timing difference message to the MS and asks the MS to re-transmit the ranging signal (step 97 ). The pico/femto BS goes back to step 94 and repeats steps 94 - 97 until uplink timing is synchronized.

Although the UL transmission timing advance offset information illustrated above is broadcasted by the pico/femto base station, such information may also be sent by the overlaying macro/micro base station. When UE performs neighboring cell measurements and reports the measurement results back to serving base station, the serving base station determines the handover target and then send UE handover command along with some configuration information via dedicated signaling, where the UL transmission timing advance offset is included. In 3GPP specification, the dedicated signaling is done by radio resource control (RRC) messages. In IEEE 802.16 specification, the dedicated signaling is done by MAC management messages.

In one embodiment, the UL transmission timing advance offset information comprises an encoded number that is indicative of the UL transmission timing advance offset value (e.g., the round trip delay time from the pico/femto BS to the overlay macro/micro BS). Suppose a total number of k bits are used to signal the offset value, N RTO is a parameter provided at the pico/femto BS side that is used for the calculation of a sample number T RTO , and T RTO is a parameter applied at the MS side to advance the ranging signal transmission timing when the MS performs initial ranging or handover ranging in the pico/femtocell. Mathematically, N RTO and T RTO can be represented by the following equations:

N RTO =min([ RTD/ ( Tg− 2)],2 k −1)

T RTO =[N RTO *( Tg− 2)* Fs ](samples)

where RTD (μs) is the round trip delay from the pico/femto BS to the overlay macro/micro BS, Tg (μs) is the guard time, and Fs (MHz) is the sampling frequency at the MS side. The range of values is 0<=N RTO <=(2 k −1). Under this encoding method, at the pico/femto BS side, the round trip delay RTD is represented by a total number of N RTO time units, and each time unit is equal to (Tg−2) μs. The number N RTO is then sent by the pico/femto BS to the MS via a broadcast channel. At the MS side, the MS obtains N RTO from the broadcasted system information (SFH in IEEE 802.16m system), and then calculates a corresponding T RTO and the timing advance offset value accordingly. In other words, the UL transmission timing advance offset value is approximate to RTD, which is equal to N RTO *(Tg−2) μs, and the largest cell size this signaling method can cover is ((2 k −1)/2)*(Tg−2)*10 −6 *c, where c is the speed of radio signals. For example, if k=3 and Tg=91.4/8 μs, then the largest cell size this method can cover is equal to ((2 k −1)/2)*(Tg−2)*c=(7/2)*(91.4/8−2)*(10 −6 )*(3*10 8 )≈9.9 km.

Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. For example, while the main application of the two-step UL synchronization method is for OFDM/OFDMA systems, this method may also be applied in other systems such as macro/micro (TD-SCDMA) vs. pico/femto (3GPP LTE/LTE-A) or other system combinations with the same problem described in the background section. In addition, although the UL transmission timing advance offset information is broadcasted by the pico/femto base station, such information may also be sent by the overlaying macro/micro base station or other centralized network element via dedicated signaling or broadcast signaling. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.