Abstract:
A med and apparatus is provided that allows M transceivers to transmit/receive using M2 N  distinct beams using passive beam steering. This provides for the use of arbitrary narrow beams with a number of transceivers that is a fraction of the number of beams but ensures 360° coverage. In other words it permits significant improvements in the link budget with a minimal rise in the cost of the BS. The apparatus includes M distribution switches applied 2 N  passive beam forming networks each coupled to M antennas. The method and apparatus ate compatible with TDMA in the downlink and in the uplink.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to wireless communication systems and is particularly concerned with beam steering.  
       BACKGROUND OF THE INVENTION  
       [0002]     An essential part of any wireless link is the design of the antenna and the choice of its beam width (or angle) and its gain. In general antennas with narrower beam provide higher gains.  
         [0003]     The gain of the antenna contributes twice in the, link budget: both at transmission and at reception. At transmission, the effective incident radiated power (EIRP) [dBm] is the sum between the antenna gain G T  [dBi] and the transmitter power P [dBm]. 
 
 EIRP  [dBm]=P [dBm]+G T  [dBi]
 
         [0004]     At reception, the signal level S[dBm] at input of the receiver is the sum between the antenna gain G R  and the transmitted EIRP minus the path loss PL [dBi]. 
 
 S  [dBm]=G R  [dBi]+ EIRP  [dBm]− PL  [dBi]
 
         [0005]     The link budget and consequently the coverage can be improved by raising the transmitter power P or by raising the antenna gains G T  or G R . For a transceiver that use the same antenna to transmit and receive, i.e. G T =G R , increasing the antenna gain has positive effects on both transmission and reception while increasing the power improves only the transmission. For symmetric links (all participant systems have the same P and G T =G R ), increasing the antenna gain has double effect than increasing the transmitter power P.  
         [0006]     The EIRP in each frequency band is usual limited by regulatory bodies like Federal Communications Commission in USA. In such cases, the only way to improve the link budget and the coverage is to raise the gain of the antenna at the receiver G R .  
         [0007]     When EIRP is limited, rising the antenna gain at the transmitter G T  has to be associated with a corresponding reduction in the power of the transmitter P and implicitly a reduction in the cost of the power amplifier (PA).  
         [0008]     Antennas with narrower beams provide more spatial selectivity, which in turn, improves the system immunity to interference.  
         [0009]     With current technologies, the advantages of using high-gain, narrow-beam antennas are offset in the design of a base-station (BS) by the price of the transceivers needed to obtain 360° coverage. For example, a 23 dBi pencil-beam (same beam width in the vertical and horizontal plane) antenna will have a beam with of only 14°. Thus, in order to ensure 360° coverage with current technologies, we would need 26 antennas and consequently 26 transceivers.  
         [0010]     It is known in wireless systems to use beam forming to emulate a high gain antenna using multiple low-gain antennas. This is achieved using a system as depicted in  FIG. 1 . A wireless system  10  includes a transceiver  12  coupled to a phase-delay passive network,  14  coupled to a plurality of antennas  16  as in the system of  FIG. 1 . A phase-delay network is inserted between the transceiver and the antennas.  
         [0011]     In operation, at transmission, the phase-delay network  10  distributes the signal from the transceiver  12  to all antennas  16 . At reception the network combines the signal received from all antennas  16  and passes the resulting signal to the transceiver  12 . The phase and delay for each antenna are established in accordance with the position of the antennas such that the desired beam width and direction are obtained.  
         [0012]     An extension of the passive beam forming uses of several transceivers  12  with multiple-input phase-delay network. It has been shown that such a network can be implemented and produces beams with gain higher than of the constituent antennas if: 
        1. The number of transceivers does not exceed the number of antennas.     2. The transceivers operate on close but different frequencies to avoid cross-talk between beams.        
 
         [0015]     Referring to  FIG. 2 , there is illustrated a known wireless system for active beam steering. The wireless system  20  includes a transceiver common part  22  coupled to an electronically controlled phase delay active network  24  coupled to a plurality of transceiver RF parts  26  each coupled to a corresponding one of a plurality of antennas  28 .  
         [0016]     Active beam steering is another extension of beam forming, in which the phase-delay network is electronically controlled. By trimming phases and delays, the resulting beam can be steered into the desired direction.  
         [0017]     Both known beam forming of  FIG. 1  and steering of  FIG. 2  require precise amplitude, phase and delay control in the phase-delay network. They also require precise alignment of the antennas and precise amplitude, phase and delay matching between RF cables. In practical systems, the precision of these elements is the most important factor that limits the achievable antenna gain. Precision is especially hard to maintain with beam steering where phase and delay parameters are variable. Practical implementations of beam steering use phase-delay networks implemented in base-band processors to ensure precise delay and phase control. Therefore in active beam steering systems the RF part of the transceiver is replicated for each antenna, as shown in  FIG. 2 .  
         [0018]     Active beam steering systems are very expensive because they require replication of the RF subunit for each antenna when multiple antennas are used to achieve a single beam.  
         [0019]     Even with the phase-delay network implemented in base-band, the active beam-steering systems require precise amplitude, phase and delay matching between RF subunits. In practice, errors occur and this seriously limits the maximum achievable antenna gain.  
         [0020]     A further concern is that the active beam steering system of  FIG. 2  offers no upgrade path. In order to add a second beam, one must add an entire new system with multiple RF subunits and multiple antennas in addition to the new transceiver. This could be an important limitation during wireless system deployment.  
         [0021]     Active beam steering may not be compatible with current standards for wireless broadband access. In  FIG. 3 , an example of an air interface for a wireless system illustrated in a functional block diagram. The air interface  30  includes a downlink portion  32  and an uplink portion  34 . The downlink portion begins with a broadcast segment  36  followed by a plurality of unicast segments  38 . The uplink portion  34  includes a contention window  40  and a plurality of scheduled uplinks  42 .  
         [0022]     As shown in  FIG. 3 , these standards, e.g. IEEE802.16, employ downlink broadcast messages that must be sent from the base-station (BS) to all subscriber stations (SS) at the same time. They also employ uplink contention windows during which BS has to “listen” for new SSs without knowing the direction in which it must steer the beam. In order to support these features, the beam must be made 360° wide during these periods. This may not be acceptable or even possible because, for example, enlarging the beam from 22° to 360° causes a reduction of the antenna gain of at least 12 dB.  
       SUMMARY OF THE INVENTION  
       [0023]     Accordingly the present invention provides a method and apparatus that allows M transceivers to transmit/receive using M2 N  distinct beams using passive beam steering.  
         [0024]     Advantageously the present invention allows use of arbitrary narrow beams with a number of transceivers that is a fraction of the number of beams but ensures 360° coverage. In other words it permits significant improvements in the link budget with a minimal rise in the cost of the base station.  
         [0025]     Advantageously the present invention entails a method that does not require precise positioning of the antennas and does not require amplitude, phase or delay matching in the RF cabling.  
         [0026]     Advantageously the present invention entails a method that does not require replication of the RF stages.  
         [0027]     Advantageously the present invention entails a method and apparatus that allows easy, hot upgrade from M to M+1, M+2 and so on up to M2 N  transceivers.  
         [0028]     Advantageously the present invention entails a method and apparatus that allows hot downgrade from any number of transceivers grater than M+1 down to M transceivers. Also downgrade paths can be used to provide a fail-safe system.  
         [0029]     Advantageously the present invention provides for both upgrades and downgrades to be performed without affecting the antenna or the beam gain as seen by each subscriber station. In other words upgrades and downgrades are performed without affecting the RF link budget.  
         [0030]     Advantageously the present invention entails a method and apparatus that are described as applied at RF but it can also be seamlessly applied at IF or base-band. However the cost of the system is minimized when invention is applied at RF.  
         [0031]     Advantageously the present invention entails a method that is compatible with existing-wireless broadband access standards and that supports broadcast messages in the downlink and contention windows in the uplink without changing the antenna gain and the link budget. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings.  
         [0033]      FIG. 1  illustrates a known wireless communications system with passive beam forming;  
         [0034]      FIG. 2  illustrates a known wireless communications system with active beam steering; and  
         [0035]      FIG. 3  illustrates a known air interface for a wireless communications system;  
         [0036]      FIG. 4  illustrates a wireless communications system in accordance with an embodiment of the present invention;  
         [0037]      FIGS. 5   a  and  5   b  illustrate examples of grouping for M2 N =16, for the wireless system of  FIG. 4 ;  
         [0038]      FIG. 6  illustrates in further detail the distribution switch of  FIG. 4 ;  
         [0039]      FIG. 7  illustrates all useful configurations for the switch of  FIG. 6 ;  
         [0040]      FIG. 8  illustrates an 8-way distribution switch;  
         [0041]      FIG. 9  illustrates the upgrade-downgrade paths for the distribution switch of  FIG. 8 ;  
         [0042]      FIG. 10  illustrates an implementation of the cross switch of  FIGS. 6 and 8 ;  
         [0043]      FIG. 11  illustrates an implementation of the straight-switch of  FIGS. 6 and 8 ;  
         [0044]      FIG. 12  graphically illustrates operation of the transceiver using TDMA; and  
         [0045]      FIG. 13  illustrates in a block diagram the downlink and uplink detail for one beam. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0046]     Referring to  FIG. 4 , there is illustrated a wireless system in accordance with an embodiment of the present invention. The wireless system  50  includes a plurality of transceivers  52   a - m  coupled to a corresponding plurality of distribution switches  54   a - m.  Distribution switches  54   a - m  each having 2 N  outputs for coupling to corresponding inputs of 2 n  passive beam forming networks  56  each passive beam-forming network  56  is connected to a plurality M of antennas  58 . Each of the plurality of transceivers  56   a - 56   m  may include 2 N  transceivers.  
         [0047]     The system of  FIG. 4  thus uses M2 N  high-gain antennas  58  that are first grouped in 2 N  groups of M antennas each. Each group of M antennas is processed by one beam-forming network  56  to form M high-gain beams. Note, that an embodiment of the invention may be applied without the beam-forming network, in which the beam width and gain are equal to the antenna angle and gain. However, in most cases when a large number of antennas are used the beam-forming network  56  will be used to reduce significantly the cost of the antenna system.  
         [0048]     In operation, the resulting M2 N  beams operate on M different frequencies to ensure proper operation of the beam-forming network.  
         [0049]     Each group of 2 N  beams operating on the same frequency is processed through a distribution switch  54  that allows 1, 2, 3, and up to 2 N  transceivers  52  to control the 2 N  beams.  
         [0050]     The passive beam steering permits a top-down approach to the design of an upgradeable base stations (BS). The designer chooses the beam angle (width) BA based on the performance of the beam forming technology and the antenna availability. The designer also chooses the minimum overlapping angle OA between adjacent beams. Then, 360°/(BA−OA) gives the minimum number of sectors needed in the system. The designer chooses M and N such that: 
 
 M 2 N ≧360°/( BA−OA ) 
 
         [0051]     The antenna system of  FIG. 4  provides M2 N  beams circularly placed at angles of 360°/M2 N  one to each other. The beams are divided into M groups: G 1 , G 2 , . . . , G M , each having 2 N  beams. If beams are numbered in circular order from 1 to M2 N , then G 1  will contain beams B 1 1=1, B 1 2=M+1, B 1 3=2M+1, . . . , while G 2  will contain beams B 2 1=2, B 2 2=M+2, B 2 3=2M+2, . . . , etc. Each group of antennas operates on the same frequency and different groups will operate on different frequencies.  
         [0052]     Referring to  FIGS. 5   a  and  5   b  there are illustrated examples of grouping for M2 N =16. Note that M=8, N=1 and M=16, N=0 are also possible solutions.  FIG. 5   a  shows M=4, N=2 and  FIG. 5   b  shows M=2, N=3.  
         [0053]     Each group of beams is processed by one distribution switch  54  that allows 1, 2, . . . , or 2 N  transceivers  52  to cover all subscriber-stations in all 2 N  beams. This is achieved using time-division-multiple-access (TDMA).  
         [0054]     Referring to  FIG. 6 , there is illustrated in further detail the distribution switch of  FIG. 4 . The four way distribution switch.  54 , for N=2, includes a plurality of inputs  60   a - 60   d  for coupling to corresponding transmitters T 1 -T 4  and a plurality of outputs  62   a - 62   d  for coupling to corresponding beams B 1 -B 4 . The four way distribution switch  54  includes first and second cross switches  64  and  66  coupled in series between inputs  60   a  and  60   b  and outputs  62   a  and  62   b.  A third cross switch  68  coupled to outputs  62   c  and  62   d  having a first input coupled to a second output of cross switch  64 . The distribution switch  54  also includes straight switches  70  and  72 . Straight switch  70  coupled to input  60   c  and  72  coupled to input  60   d.  Straight switch  70  has an output coupled to a second input of cross switch  66  and straight switch  72  has an output coupled to a second input of cross switch  68 .  
         [0055]     The distribution switch is important because it connects one group of 2 N  beams to one transceiver or 2 transceivers or so on up to 2 N  transceivers. To understand its operation we use an example for N=2, then we show how it can be extend to N=3, 4, etc.  
         [0056]      FIG. 6  shows the structure of the  4  -way distribution switch (i.e. N=2). In operation, it connects 4 beams B 1 , B 2 , B 3  and B 4  to one, two, three or four transceivers: T 1 , T 2 , T 3 , T 4 . The distribution switch is built with 3 cross-switches: XS 20 , XS 10  and XS 11 ( 64 ,  66 ,  68 ), and two straight switches SS 21   a  and SS 21   b  ( 70 , 72 ).  
         [0057]     The cross switches ( 64 ,  66 ,  68 ) can be configured in two modes: 
        1. Straight: port A connects port C and port B connects port D     2. Cross: port A connects port D and port B connects port C        
 
         [0060]     The straight switches ( 70 , 72 ) can be used to introduce additional isolation when either T 3  ( 60   c ) or T 4  ( 60   d ) are not in use, or they can be simple shorts connecting their port A with port B. More details can be found below, where the construction of these switches is described. Both straight-switches and cross-switches introduce substantially no insertion loss (except those due to imperfections).  
         [0061]     When deploying the system, the service provider may initially decide that a single transceiver  52   a   1  is enough to cover all four beams. The transceiver is connected to T 1 ( 60   c ) and the BS controller instructs the distribution switch  52  that T 1  can manipulate all cross switches. Therefore, T 1  covers all four beams: B 1 , B 2 , B 3  and B 4  using the following configurations:  
                                                         Configuration                XS20   XS10   XS11   Mode   Description               Straight   Straight   —   Tx or   T1 transmits/receives through B1                   Rx       Straight   Cross   —   Tx or   T1 transmits/receives through B2                   Rx       Cross   —   Straight   Tx or   T1 transmits/receives through B3                   Rx       Straight   —   Cross   Tx or   T1 transmits/receives through B4                   Rx                  
 
         [0062]     When the service provider (sp) determines that the single transceiver  52   a   1  is overloaded, i.e. the data bandwidth provided by one transceiver is not enough,. the sp can upgrade the system to two transceivers. The second transceiver  52   a   2  is added to port T 2  without interfering with the operation of the existing transceiver  52   a   1 . The BS controller configures XS 20  ( 64 ) as straight (A connects C and B connects D) and instructs the distribution switch  54   a  to allow T 1  ( 60   a ) to control XS 10  ( 66 ) and T 2  ( 60   b ) to control XS 11  ( 68 ). Therefore, T 1  ( 60   a ) covers two beams: B 1  and B 2 , and T 2  ( 60   b ) covers the other two beams: B 3  and B 4 .  
         [0063]     Depending on the service growth, the service provider may need to further upgrade the system. If T 1  ( 60   a ) is overloaded, a third transceiver  52   a   3  can be added at port T 3  ( 60   c ); the BS controller configures XS 10  ( 66 ) as straight and will leave T 2  ( 60   b ) to control XS 11  ( 68 ) (XS 20 ( 64 ) was already configured straight); T 1 ( 60   a ) covers beam B 1 , T 3  ( 60   a ) covers B 2 , and T 2  ( 60   b ) covers B 3  and B 4 . If T 2  ( 60   b ) is overloaded, a transceiver can be added at port T 4  ( 60   d ); the BS controller configures XS 11 ( 68 ) as straight and leaves T 1 ( 60   a ) to control XS 10  ( 66 ); T 1 ( 60   a ) covers B 1  and B 2 , T 2 ( 60   b ) covers B 3 , and T 4 ( 60   d ) covers B 4 . Finally, if all four transceivers are used, the BS controller configures all 3 cross switches ( 64 , 66 , 68 ) as straight and does not let any transceiver to control any cross switch. Then, T 1 ( 60   a ) covers B 1 , T 2  ( 60   b ) covers B 3 , T 3 ( 60   c ) covers B 2  and T 4 ( 60   d ) covers B 4 .  
         [0064]     The same paths used to upgrade to more transceivers can also be used to downgrade to fewer transceivers. The distribution switch  54  offers many other configurations that can be used for making the system  50  fail safe.  
         [0065]     Referring to  FIG. 7  there is illustrated all useful configurations that can be obtained with the 4-way distribution switch. The five white blocks show the configurations discussed above, i.e. the upgrade-downgrade paths. The shaded configurations are not recommended for upgrade/downgrade; which provides the same functionality as the white, but for non-shaded configurations there is less upgrade/downgrade flexibility. However, shaded configurations can be used to provide back-off possibilities in the event that one or more transceivers fail. With two or more transceivers installed in -the system, if any of the transceivers fails, the distribution switch can always be reconfigured such that the remaining transceivers cover all beams. When all transceivers are installed, the system becomes immune to failure of any two transceivers.  
         [0066]     Referring to  FIG. 8  there is illustrated an 8-way distribution switch (N=3). The 8-way switch includes eight inputs  60   a, . . .    60   i  for transceivers T 1 , . . . T 8  and eight outputs  62   a, . . .    62   i  for beams B 1 , . . . B 8 . Between inputs  60   a  and  60   b  and outputs  62   a  and  62   b  are three cross switches  74 ,  64 , and  66 , each having first and second inputs (A, B) and first and second outputs (C, D) series connected at first inputs/outputs. A fourth cross switch  68  has its first and second outputs series connected to the outputs  62   c  and  62   d  cross switch  80  has its first and second outputs coupled to the outputs  62   e  and  62   f.  A seventh cross switch  82  has its first and second outputs coupled to outputs  62   g  and  62   h,  respectively. The input  60   b  is connected to the second input (B) of the cross switch  74 , whose second output (D) is connected to the first input (A) of cross switch  78 . The input  60   c  is coupled via a straight switch  90  to the second input (B) of cross switch  64 , whose second output (D) is connected to the first output (A) of cross switch  68 . The input  60   d  is coupled via a straight switch  92  to the second input (B) of cross switch  78 , whose second output (D) is connected to the first input (A) of cross switch  82 . The input  60   e  is coupled via straight switches  94  and  96  to the second input (B) of cross switch  66  whose second output (D) is connected to the output  62   b . The input  60   f  is coupled via the straight switches  98  and  100  to the second input (B) of cross switch  68 . The input  60   g  is coupled via the straight switches  102  and  104  to the second input (B) of cross switch  80 . The input  60   h  is coupled via the straight switches  106  and  108  to the second input (B) of cross switch  82 .  
         [0067]     The 8-way distribution switch is constructed with two 4-way distribution switches, whose T 1  ports are passed through the cross-switch XS 30 ( 74 ) to obtain the T 1 ( 60   a ) and T 2 ( 60   b ) ports for the 8-way distribution switch. The other three T ports in each of the 4-way switches are passed through straight-switches to obtain the T 3 . . . T 8  ports for the 8-way switch. Using the same rule, two 8-way switches can construct a 16-way distribution switch (N=4) and so on.  
         [0068]     Referring to  FIG. 9  there is illustrated the upgrade-downgrade paths for the 8-way distribution switch of  FIG. 8 . The switch can connect any number of transceivers between 1 and 8 ( 60   a - 60   h ). The service provider has the option of upgrading the system only when needed. If a transceiver is overloaded and covers two or more beams, its payload can always be split with a newly added transceiver. Both the upgrades and the downgrades do not require system shutdown and can be performed without any interruption of the ongoing communications.  
         [0069]     When using more than one transceiver, if one transceiver fails, the switch can be reconfigured such that all beams are covered.  
         [0070]     Similarly a 2 N -way distribution switch can be built that allows transceivers T 1 , T 2  to cover 1, 2, 4, . . . , 2 N  beams, transceivers T 3 , T 4  to cover 1, 2, . . . , 2 N−1 , T 5 , T 6 , T 7 , T 8  to cover 1, 2, . . . , 2 N−2  and so on. The fail-safe feature comes from the fact that. for each sub-tree there are two transceivers that can cover the entire sub-tree.  
         [0071]     Based on the structure of the switch, the number of beams that a particular transceiver covers in any configuration is always a power of 2. This helps with the development of the algorithms that will reside in each transceiver and will ensure coverage of the required number of beams.  
         [0072]      FIG. 10  shows a possible implementation of the cross-switch  64  of  FIGS. 6 and 8  using two single-pole-dual-terminal (SPDT) RF/IF switches ( 112 ,  114 ). When both SPDT switches are in ‘0’ position, the cross-switch is in straight mode. When both SPDT switches are in ‘1’ position, the cross-switch is in cross mode.  
         [0073]     Depending on the performance required for the straight-switches in terms of insertion-loss and isolation, the straight-switch can be:  
         [0074]     1. a simple short (switch is always closed)  
         [0075]     2. an single-pole-single-terminal (SPST) RF/IF switch with no impedance matching  
         [0076]     3. an SPST RF/IF switch with impedance matching.  
         [0077]      FIG. 11  shows a possible implementation of the straight-switch  70  of  FIGS. 6 and 8  as an SPST switch  122  with impedance matching. The implementation uses a 4-terminal dual-pole-dual-terminal (DPDT) RF/IF switch  122  as switching element. With the DPDT switch, if terminal  1  is connected to  4 , then the straight-switch is closed (ports A and B are connected); if terminal  1  connects to  3  and terminal  2  to  4 , then ports A and B are disconnected and each of them is terminated to ground with an impetitive ( 124 , 126 ) Z 0  (e.g. 50Ω). To obtain an SPST switch without impedance matching, the two termination impedances Z 0  are removed from the circuit and the DPDT switch is replaced by a simple SPST switch (placed between terminals  1  and  4 ).  
         [0078]     In order to cover 2 n  beams: B 1 , B 2 , . . . , B 2   n , a transceiver T accesses the beams using time-division-multiple-access (TDMA). To implement this, T emulates one media-access-control (MAC) layer for each beam. All MACs operate with the same frame length but the frames are shifted in time. Each MAC produces its own downlink (DL) and its own uplink (UL). For maximum efficiency T concatenates all 2 n  downlinks in a long DL and all uplinks in a long UL. The operation is depicted in  FIG. 12  where T denotes the signal at the transceiver and Bi denotes the signal going to or expected to come from beam Bi. The downlink and uplink details for one beam are shown in  FIG. 13  in a block diagram.  
         [0079]     Note that it not necessary to group the uplink bursts by beam. The system will have the same performance if the uplink bursts are not grouped by beam. However, since the downlink on each beam uses time-division-multiplexing (TDM), i.e. all downlink packets are concatenated in a single RF burst, it is more efficient to group the downlink packets by beam.  
         [0080]     According to current standards for broadband wireless access, each subscriber station (SS) synchronizes on the beginning of the downlink and considers this to be the beginning of a MAC frame. Each SS checks for the MAC frame length as it is announced by the base-station (BS). Due concatenation of the downlinks, the MAC frame on each beam starts at a different moment. We see in  FIG. 12 , that the beginning of MAC frame for beam B 2  is delayed by the duration of the downlink for B 1  and that the beginning of MAC frame for B 3  is delayed by the duration of B 1  plus B 2 , and so on. As long as the DL sizes on individual bearns are preserved, the MAC frame lengths are constant and equal. Every time the DL size is changed for one beam, all subsequent beams will have a different MAC frame size for one frame, and then, if no more changes occur, they return to the nominal MAC frame size.  
         [0081]     In order to support dynamic bandwidth allocation, i.e. to allow variable DL sizes, a MAC management message is sent on each beam every time the MAC frame size for that beam needs to be temporarily changed. The message encodes the difference between the desired MAC frame size and the nominal MAC frame size. This MAC management message is already used by different standards to allow time alignment (synchronization) between base-stations.  
         [0082]     The system may also work with fixed bandwidth allocation such that the use of the above-mentioned MAC management message in not needed.  
         [0083]     A second method of TDMA access is to produce both the downlink and the uplink for a beam before moving to another beam. With this, there are two distinct arrangements: 
        1. All uplinks and downlinks are scheduled within the same MAC frame.     2. Each MAC frame for the transceiver is dedicated to a single beam, and the beams are circularly accessed one by one during 2 n  MAC frames. The BS communicates to all SS&#39;s a MAC frame that is 2 n -times larger than the actual MAC frame.        
 
         [0086]     One difference between the two arrangements is that, the first allows variable bandwidth distribution between beams while the second does not. However, the second approach allows different transceivers operating with different number of beams to be synchronized without any MAC management message, which does not apply to the first arrangement