Abstract:
A system is configured to perform Spatial Division Multiple Access. The system includes at least one transmitter or receiver capable of polarization alignment. The transmitter includes a baseband precoder configured to precode a signal, an array of sub-array antennas and a plurality of radio frequency (RF) chains. Each RF chain is coupled to a respective antenna sub-array of the array of antennas. The transmitter is configured to perform a method that includes precoding, by a baseband precoder, a signal for spatial division multiple access (SDMA). The method also includes applying, by each of the plurality of radio frequency (RF) chains, a phase shift and beamforming weight to the signal and transmitting the phase shifted and weighted signal by an array of sub-array antennas.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
     The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/595,623 filed Feb. 6, 2012, entitled “LOW COMPLEXITY SPATIAL DIVISION MULTIPLE ACCESS (SDMA) IN A MILLIMETER WAVE MOBILE COMMUNICATION SYSTEM”. The content of the above-identified patent documents is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present application relates generally to millimeter wave communication networks and, more specifically, to low complexity Spatial Division Multiple Access (SDMA) in a millimeter wave communication network. 
     BACKGROUND 
     Mobile communication has been one of the most successful innovations in the 20 th  century. In recent years, the number of subscribers to mobile communication services has exceeded 4.5 billion and is growing fast. At the same time, new mobile communication technologies have been developed to satisfy the increasing needs and to provide more and better mobile communication applications and services. Some examples of such systems are Code Division Multiple Access 2000 (cdma2000) 1xEV-DO systems developed by 3GPP2, WCDMA, HSPA, and Long Term Evolution (LTE) systems developed by 3 rd  Generation Partnership Project (3GPP), and mobile WiMAX systems developed by the Institute of Electrical and Electronics Engineers (IEEE). As more and more people become users of mobile communication systems, and more and more services are provided over these systems, there is an increasing need of a mobile communication system with larger capacity, higher throughput, lower latency, and better reliability. 
     Millimeter waves are radio waves with wavelength in the range of 1 mm-10 mm, which corresponds to radio frequency of 30 GHz-300 GHz. Per definition by the International Telecommunications Union (ITU), these frequencies are also referred to as the Extremely High Frequency (EHF) band. These radio waves exhibit unique propagation characteristics. For example, compared with lower frequency radio waves, they suffer higher propagation loss, have poorer ability to penetrate objects, such as buildings, walls, foliage, and are more susceptible to atmosphere absorption, deflection and diffraction due to particles (e.g., rain drops) in the air. Alternatively, due to their smaller wave lengths, more antennas can be packed in a relative small area, thus enabling high-gain antenna in small form factor. In addition, due to the aforementioned deemed disadvantages, these radio waves have been less utilized than the lower frequency radio waves. This also presents unique opportunities for new businesses to acquire the spectrum in this band at a lower cost. The ITU defines frequencies in 3 GHz-30 GHz as Super High Frequency (SHF). However, some higher frequencies in the SHF band also exhibit similar behavior as radio waves in the EHF band (i.e., millimeter waves), such as large propagation loss and the possibility of implementing high-gain antennas in small form factor. 
     Vast amount of spectrum are available in the millimeter wave band. For example, the frequencies around 60 GHz, which are typically referred to as 60 GHz band, are available as unlicensed spectrum in most countries. In the United States, 7 GHz of spectrum around 60 GHz (57 GHz-64 GHz) is allocated for unlicensed use. On Oct. 16, 2003, the Federal Communications Commission (FCC) issued a Report and Order that allocated 12.9 GHz of spectrum for high-density fixed wireless services in the United States (71-76 GHz, 81-86 GHz, and 92-95 GHz excluding the 94.0-94.1 GHz for Federal Government use). The frequency allocation in 71-76 GHz, 81-86 GHz, and 92-95 GHz are collectively referred to as the E-band. It is the largest spectrum allocation ever by FCC—50 times larger than the entire cellular spectrum. 
     SUMMARY 
     A transmitter is provided. The transmitter includes a baseband precoder configured to precode a signal. The transmitter also includes an array of sub-array antennas and a plurality of radio frequency (RF) chains configured to apply a phase shift and beamforming weight to the signal. Each RF chain is coupled to a respective antenna sub-array of the array of antennas. 
     A method for spatial division multiple access is provided. The method includes precoding, by a baseband precoder, a signal for spatial division multiple access (SDMA). The method also includes applying, by each of a plurality of radio frequency (RF) chains, a phase shift and beamforming weight to the signal. The method further includes transmitting the phase shifted and weighted signal by an array of sub-array antennas. Each RF chain is coupled to a respective antenna sub-array of the array of antennas. 
     Before undertaking the DETAILED DESCRIPTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
         FIG. 1  illustrates dynamic beamforming according to embodiments of the present disclosure; 
         FIG. 2  illustrates a two-dimensional array according to embodiments of the present disclosure; 
         FIG. 3  illustrates a transmit beamforming according to embodiments of the present disclosure; 
         FIG. 4  illustrates a receive beamforming according to embodiments of the present disclosure; 
         FIG. 5  illustrates a geometry of planar arrays according to embodiments of the present disclosure; 
         FIG. 6  illustrates digital beamforming according to embodiments of the present disclosure; 
         FIG. 7  illustrates analog beamforming according to embodiments of the present disclosure; 
         FIG. 8  illustrates Radio Frequency (RF) beamforming according to embodiments of the present disclosure; 
         FIG. 9  illustrates a hybrid beamforming architecture according to embodiments of the present disclosure; 
         FIG. 10  illustrates a millimeter wave (mmW) mobile communication system according to embodiments of the present disclosure; 
         FIG. 11  illustrates a SDMA system according to embodiments of the present disclosure; 
         FIG. 12  illustrates a SDMA architecture according to embodiments of the present disclosure; 
         FIGS. 13-16  illustrate beamforming gain according to embodiments of the present disclosure; and 
         FIGS. 17A and 17B  illustrate antenna array types according to embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 through 17B , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless communications system. 
     Millimeter wave wireless communication using component electronics has existed for many years. Several companies have developed or are developing millimeter wave communication systems that can achieve giga-bps data rate. For example, Asyrmatos Wireless developed a millimeter wave communication system that enables 10 Gbps data transfer over distances of several kilometers. The Asyrmatos transceiver is based on photonics, which provides flexibility of operating in a variety of millimeter wave bands such as 140 GHz (F-Band), 94 GHz (W-Band), 70/80 GHz (E-Band), and 35 GHz(Ka-Band). As another example, GigaBeam Corp. developed multigigabit wireless technologies for the 70 GHz and 80 GHz band. However, these technologies are not suitable for commercial mobile communication due to issues such as cost, complexity, power consumption, and form factor. For example, GigaBeam&#39;s WiFiber G-1.25 gigabit per second wireless radio requires a two-foot antenna to achieve the antenna gain required for the point-to-point link quality. The component electronics used in these systems, including power amplifiers, low noise amplifiers, mixers, oscillators, synthesizers, waveguides, and the like, are too big in size and consume too much power to be applicable in mobile communication. 
     Many engineering and business efforts have been and are being invested to utilize the millimeter waves for short-range wireless communication. A few companies and industrial consortiums have developed technologies and standards to transmit data at giga-bps rate using the unlicensed 60 GHz band within a few meters (up to 10 meters). Several industrial standards have been developed, e.g., WirelessHD technology, ECMA-387, “High Rate 60 GHz PHY, MAC and HDMI PAL”, December 2008, and IEEE 802.15.3c, “Wireless Medium Access Control (MAC) and Physical Layer (PHY)Specifications for High Rate Wireless Personal Area Networks (WPANs): Millimeter-wave based Alternative Physical Layer Extension Amendment”, October, 2009, with a couple other organizations also actively developing competing short-range 60 GHz giga-bps connectivity technology, such as the Wireless Gigabit Alliance (WGA) and the IEEE 802.11 task group ad (TGad) in Perahia, E.; Cordeiro, C.; Minyoung Park; Yang, L.L.; , “IEEE 802.11ad: Defining the Next Generation Multi-Gbps Wi-Fi,” Consumer Communications and Networking Conference (CCNC), 2010 7th IEEE , vol., no., pp. 1-5, 9-12 Jan. 2010, the contents of each are hereby incorporated by reference. Integrated circuit (IC) based transceivers are also available for some of these technologies. For example, researchers in Berkeley Wireless Research Center (BWRC) and Georgia Electronics Design Center (GEDC) have made significant progress in developing low-cost, low-power 60 GHz RFIC and antenna solutions. In Doan, C.H.; Emami, S.; Niknejad, A.M.; Brodersen, R.W.; “Millimeter-wave CMOS design,” Solid-State Circuits, IEEE Journal, vol.40, no.1, pp. 144- 155, Jan. 2005, the contents of which are hereby incorporated by reference, researchers from BWRC show that 60 GHz power amplifiers can be designed and fabricated in 130 nm bulk “digital” CMOS. A core team of researchers from BWRC co-founded SiBeam Inc. in 2004 and developed CMOS based RFIC and baseband modem for the WirelessHD technology. It is worth mentioning that the common view is that the biggest challenge of short-range 60GHz connectivity technology is the RFIC. As such, much of the engineering efforts have been invested to develop more power efficient 60 GHz RFICs. Many of the designs and technologies can be transferred to RFIC design for other millimeter wave bands, such as the 70-80-90 GHz band. Although the 60 GHz RFIC today still suffers from low efficiency and high cost, the advancement in millimeter wave RFIC technology points to the direction of higher efficiency and lower cost, which can eventually enable communication over larger distance using millimeter wave RFICs. 
     In order to overcome the propagation loss at millimeter waves beamforming can be employed. Beamforming is particularly beneficial at millimeter waves as more antennas can be packed in a relative small area, thus enabling high-gain beamforming. 
     Beamforming is a signal processing technique used for directional signal transmission or reception in a wireless system. The spatial selectivity is achieved by using adaptive receive/transmit beam patterns. When transmitting, a beamformer controls the phase and relative amplitude of the signal at each transmitter antenna to create a pattern of constructive and destructive interference in the wavefront. The receiver combines information from different antennas in such a way that the expected pattern of radiation is preferentially observed. The improvement compared with an omnidirectional reception/transmission is known as the receive/transmit gain. For example, with N transmit antennas, a transmit beamforming gain of 10×log 10 (N) dB can be achieved. This is assuming that the total transmit power from the N antennas is the same as the transmit power from a single omnidirectional antenna. Similarly, with M receive antennas, a receive beamforming gain of 10×log 10 (M) dB can be achieved. When both transmit and receive beamforming is performed with N transmit and M receive antennas a total combined beamforming gain of 10×log 10 (N×M) dB can be achieved. 
       FIG. 1  illustrates dynamic beamforming according to embodiments of the present disclosure. The embodiment of the dynamic beamforming shown in  FIG. 1  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     A transceiver  100  with a uniform linear array (ULA) performs dynamic beamforming by adjusting weights  105  that are based on phase control. By using appropriate phase adjustments to signals transmitted (or received) from multiple antennas  110 , a beam  115  can be steered in a particular direction. 
       FIG. 2  illustrates a two-dimensional (2D) array according to embodiments of the present disclosure. The embodiment of the 2-D array  200  shown in  FIG. 2  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     With an ULA, a transmitter can steer a beam in a single plane containing the line of the antenna elements&#39; centers. In order to steer the beam in any direction, such as horizontal and vertical steering from a base station, the transmitter employs a 2-D antenna array  200  as shown. The array grid  205  can have equal or unequal row spacings (d x )  210  and column spacings (d y )  215 . 
       FIG. 3  illustrates a transmit beamforming according to embodiments of the present disclosure. The embodiments of the transmit beamforming  300  shown in  FIG. 3  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     A transmitter applies a beamforming weight or gain g i    305  to the signal  310  transmitted from the ith transmit antenna. The transmitter applies the gain  305  to adjust the phase and relative amplitude of the signal  310  transmitted from each of the transmit antennas  315 . The signal  310  can be amplified  320  separately for transmission from each of the transmit antennas  315 . In certain embodiments, a single amplifier  320  is used regardless of the number of transmit antennas  315 . In certain embodiments, the transmitter includes a smaller number of amplifiers  320  than the number of transmit antennas  315 . That is a smaller number of amplifiers  320  than the number of transmit antennas  315  is used. In certain embodiments, the beamforming weights or gains  305  are applied before signal amplification  320 . In certain embodiments, the beamforming weights or gains  305  are applied after signal amplification  320 . 
       FIG. 4  illustrates a receive beamforming according to embodiments of the present disclosure. The embodiments of the receive beamforming  400  shown in  FIG. 4  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     Each received signal from each receive antenna  405  is amplified by a low-noise amplifier (LNA)  410 . The receiver applies a beamforming weight or gain gi  415  to the signal  420  received and amplified from the ith receive antenna  405 . The receiver uses the gain  415  to adjust the phase and relative amplitude of the signal  420  received from each of the transmit antennas  405 . The phase and amplitude adjusted signals are combined to produce the received signal  420 . The receive beamforming gain  415  is obtained because of coherent or constructive combining of the signals from each receive antenna. 
       FIG. 5  illustrates a geometry of planar arrays according to embodiments of the present disclosure. The embodiment of the geometry shown in  FIG. 5  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     The number of antenna elements  505  in the columns and rows determine the beam steering capability along the y-axis  510  and x-axis  515  respectively. For example, with more antennas  505  along the y-axis  510 , the beam  520  can be steered  530  with greater granularity and flexibility along the y-axis  510 . A higher number of antennas  505  along the x-axis  515  also determines the beamwidth  535  along the x-axis  515  with narrower beams  520  and with increasing number of antennas. Therefore, the planar array geometry for a particular application can be selected based on the beamwidth and beam steering requirements. 
       FIG. 6  illustrates digital beamforming according to embodiments of the present disclosure. The embodiment of the digital beamforming  600  shown in  FIG. 6  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     In the example shown in  FIG. 6 , a transmitter  605  uses digital beamforming techniques to transmit a signal. A receiver  610  uses corresponding digital beamforming techniques to receive the signal. 
     Different beamforming architectures that enable different tradeoffs between performance, complexity and flexibility are possible. For example, the digital beamforming approach 600 enables optimal capacity for all channel conditions while requiring very high hardware complexity with M (N) full transceivers. This architecture also results in very high system power consumption. 
     The beamforming weights  615  at the transmitter  605  W 0   t −W (M−1)   t  are applied before signal conversion to analog, that is, before the Digital to Analog (DAC) conversion block  620 . The beamforming weights  625  at the receiver  610  W 0   r −W (M−1)   r  are applied after signal is converted to digital using an Analog to Digital (ADC) converter  630 . 
       FIG. 7  illustrates analog beamforming according to embodiments of the present disclosure. The embodiment of the analog beamforming  700  shown in  FIG. 7  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     In the example shown in  FIG. 7 , a transmitter  705  uses analog beamforming techniques to transmit a signal. A receiver  710  uses corresponding analog beamforming techniques to receive the signal. 
     Analog baseband beamforming  700  reduces the number of data converters (ADC/DAC) providing intermediate complexity and power consumption while losing some flexibility in beamforming control. The beamforming weights  715  at the transmitter  705  W 0   t −W (M−1)   t  are applied after signal conversion to analog, that is, after the Digital to Analog (DAC) conversion block  720 . The beamforming weights  725  at the receiver  710  W 0   r −W (M−1)   r  are applied before signal is converted to digital using an Analog to Digital (ADC) converter  730 . 
       FIG. 8  illustrates Radio Frequency (RF) beamforming according to embodiments of the present disclosure. The embodiment of the RF beamforming  800  shown in  FIG. 8  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     In the example shown in  FIG. 8 , a transmitter  805  uses analog beamforming techniques to transmit a signal. A receiver  810  uses corresponding analog beamforming techniques to receive the signal. 
     The RF beamforming  800  reduces the number of mixers required in addition to reducing the number of data converters (ADC/DAC) therefore providing lowest complexity and power consumption. However, this reduction in complexity comes at the expense of reduced flexibility in beamforming control as well as the limited options for multiple access to serve multiple users simultaneously. The beamforming weights  815  at the transmitter  805  W 0   t −W (M−1)   t  are applied after signal up-conversion to RF frequency, that is, after the mixer block  820 . The beamforming weights  825  at the receiver  810  W 0   r −W (M−1)   r  are applied before the signal is down-converted from RF, that is, before the mixer block  830 . 
     Current peer-to-peer (P2P) millimeter wave standards, such as WirelessHD technology, ECMA-387, IEEE 802.15.3c, and IEEE 802.11ad, employ adaptive antenna arrays both at the transmitter and the receiver. However, the antenna arrays for these systems are used for transmissions to a single user at a time thereby lacking support for serving multiple users simultaneously using Spatial Division Multiple Access (SDMA). 
       FIG. 9  illustrates a hybrid beamforming architecture according to embodiments of the present disclosure. Hybrid beamforming refers to beamforming using the combination of digital precoding in the baseband and RF/analog precoding using phase shifters. The embodiment of the hybrid beamforming architecture  900  shown in  FIG. 9  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     Hybrid architectures with digital and analog beamforming have also been considered in the past. In these architectures, all RF chains are connected to all antennas using combiners with the number of input equal to the number of RF chains so that the signals are sent from all antennas. In the hybrid beamforming architecture, U is the number of users and S is the number of streams per user. This information (U*S) is sent to a digital baseband precoder  905  of size [US×K], where K is the number of RF chains  910 . Each RF chain  910  is connected to the same set of N antennas  915  through a combiner with K inputs. The digital precoder  905 , also referred to as a baseband precoder provides a precoded version of the information (U*S) to each of the RF chains  910 . The digital precoder  905  can also include a digital weighting of the information (U*S), i.e., a weighting at the baseband. Each RF chain  910  also provides analog weighting, i.e., a weighting at the RF. A phase shift is applied by phase shifters  920 . After the phase shift is applied, the signals from each RF chain  910  are combined by combiners  925 , which are each coupled to a respective antenna  915 . 
     A receiver receives signals via antennas. Each antenna is coupled to a respective combiner that separates the signal to be processed by a corresponding RF chain. A phase shift also is applied to the signals for each RF chain. Each RF chain processes the signals and applies an RF weighting. The signals for each RF chain are received by a baseband combiner that applies a digital weighting and processes the signal. 
     Embodiments of the present disclosure illustrate an antenna array system and associated apparatus and methods that provide spatial division multiple access (SDMA) for millimeter wave mobile communications. Although certain embodiments are disclosed in the context of communication with millimeter waves, the embodiments are certainly applicable in other communication medium, e.g., radio waves with frequency of 3 GHz-30 GHz that exhibit similar properties as millimeter waves. In some cases, the embodiments are also applicable to electromagnetic waves with terahertz frequencies, infrared, visible light, and other optical media. 
     Millimeter waves suffer larger propagation loss than radio waves with lower frequencies. This larger propagation loss can become pronounced when millimeter waves are deployed for local-area (10 m˜100 m) or wide-area (&gt;100 m) communication. To compensate for the large propagation loss, antennas with high antenna gains are often used in millimeter wave communication. In recent years, a number of cost-effective antenna and RFIC solutions became available for millimeter wave communication. In addition, due to the small wavelength of millimeter waves (e.g., λ=5 mm for 60 GHz carrier frequency), the antenna size and separation can be made very small (around λ/2) for beamforming purposes. The small size and separation of millimeter wave antennas allow a large number of antennas in a small area, which enables high gain antenna implementation in a relatively small area. 
     For the purpose of illustration, certain embodiments are illustrated using only base stations and mobile stations. However, the mobile communication technology has evolved such that a person with ordinary skill of the art understands that other advanced system topologies, such as relay communication among base stations, direct communication among mobile stations, and different kinds of cooperative communication, can also be supported. The embodiments in this disclosure apply in such communication systems. 
       FIG. 10  illustrates a millimeter wave (mmW) mobile communication system according to embodiments of the present disclosure. The embodiment of the mmW mobile communication system  1000  is shown for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     In certain embodiments, the mmW mobile communication system  1000  provides communication both from the base station (BS)  1005  to mobile station (MS)  1010  as well as base station  1005  to base station  1005  communication. The base station  1005  to base station  1005  communication can be performed using the same time-frequency resources as for the base station  1005  to mobile station  1010  communication. This is enabled by non-interfering narrow beams enabled by large antenna arrays at mmW frequencies. Another advantage of antenna array based backhaul communication between base stations is that an adaptive non-line-of-sight (NLOS) operation can be enabled for backhaul  1015  in case the LOS is blocked by an obstruction. 
       FIG. 11  illustrates a SDMA system according to embodiments of the present disclosure. The embodiment of the SDMA system  1100  shown in  FIG. 11  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     The SDMA system  1100  of  FIG. 11  includes multiple streams (S 1 (t), S 2 (t), . . . S k (t)) going into a digital precoder  1105 . The output of the precoder  1105  is sent to separate RF chains  1110 , where there is a second level of analog precoding with a phased antenna array. Each RF chain  1110  includes a digital to analog converter (DAC)  1115 , beamforming weighting  1120 , and power amplifiers (PA)  1125  coupled to antenna arrays  1130 . The significance of this architecture is that the arrays for each RF chain  1110  (called “sub-arrays”) are independent and are not interconnected with each other. Thus, each RF chain is connected to one sub-array, leading to an array of sub-arrays structure for the antennas. Furthermore, in certain embodiments, the antennas  1130  in this SDMA system  1100  structure can have uniform spacing with each other and can be considered to operate as a single unit. The SDMA system  1100  architecture can flexibly switch between single user and multiple user systems using the digital precoder(s)  1105  with effective utilization of all the antennas  1130  and phase shifters and providing beamforming gains  1120 . 
     A receiver in the SDMA system  1110  includes a comparable arrangement. That is, the receiver receives a signal from the transmitter via a plurality of chains. In each RF chain, the signal is received by at least one of a number of antennas  1135  amplified by LNAs  1140 , beamforming weights applied  1145 , converted by Analog to Digital Converters (ADC)  1150  and processed by SDMA processing circuitry  1155 . 
     In the embodiments illustrated, there are U users and S streams per user. In addition, the number of RF chains is K while the total number of antennas per RF chain be N. 
       FIG. 12  illustrates an SDMA architecture according to embodiments of the present disclosure. The SDMA architecture  1200  shown in  FIG. 12  is without interconnection between RF chains and antennas. The embodiment of the SDMA architecture  1200  without interconnection between RF chains and antennas shown in  FIG. 12  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     The SDMA architecture  1200  greatly simplifies the implementation of the hybrid beamforming architecture  900  shown in  FIG. 9 . For example, if the number of antennas  1205 , for comparison, is maintained as N, the number of phase shifters and combiners is reduced. That is, the number of phase shifters reduces to N/K and there is no need for any combiners. Alternately, by maintaining the same number of phase shifters, this SDMA architecture  1200  provides a tradeoff for each combiner with an antenna, providing K times more antennas compared to  FIG. 9 . 
     Additionally, the SDMA architecture  1200  provides for reduced interconnect complexity. For example, in the system of  FIG. 9 , each RF chain  910  is coupled to each antenna  915 . Therefore, at least RF chain  910  is disposed a considerable distance from a respective antenna  915 . The path length from the furthest RF chain  910  to the respective antenna introduces losses and constraints on the system. 
     While the traditional architecture provides a beamforming gain of N per RF chain, the new SDMA architecture  1200  can provide a beamforming gain between N and N*K (assuming each combiner is replaced by an antenna), where the gain is dependent upon the direction of transmission and reception. The beamforming gain is K*N when all the antennas are pointed in the same direction. 
       FIG. 13  illustrates beamforming gain according to embodiments of the present disclosure. In the transmitter  1300  system shown in  FIG. 13 , users are scheduled in different directions. The embodiment of the transmitter  1300  system and beamforming gain shown in  FIG. 13  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     Analog beamforming  1305  is used to shape the general direction of the beam and digital beamforming  1310  operates within the shape decided by the analog beamforming. For example, in  FIG. 13 , when all users are pointing in different directions, if the analog beamforming  1305  for each RF chain points in a different direction, the beamforming gain for each direction will be at least equal to 10*log 10(N), where N is the number of antennas per RF chain. That is, the beamforming gain for each direction can be defined according to Equation 1:
 
10*log 10( N )≦Beamforming gain≦10*log 10( N*K )  [Eqn. 1]
 
     When the users are pointed in substantially different directions, the digital precoder  1315  does not have a significant impact on the beamforming gain. However, the digital precoder  1315  weights may be used for power allocation or for compensating for the channel response, for example. Both the amplitude and phase of the digital precoder  1315  can be changed to attain the desired functionality. 
       FIG. 14  illustrates beamforming gain according to embodiments of the present disclosure. In the transmitter  1400  system shown in  FIG. 14 , all users are scheduled in the same direction. The embodiment of the transmitter  1400  system and beamforming gain shown in  FIG. 14  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     When all users are in the same direction, a beamforming gain of 10*log 10(K*N) is obtained for the system for all users. However, this assumes that the users are able to cancel the signals from other users using digital beamforming (precoding) to separate the users in space and using interference cancellation techniques to suppress any residual interference. 
       FIG. 15  illustrates beamforming gain according to embodiments of the present disclosure. In the system shown in  FIG. 15 , the transmitter  1500  employs a single-user configuration according to embodiments of the present disclosure. The embodiment of the transmitter  1500  system and beamforming gain shown in  FIG. 15  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     In certain embodiments, the antennas are flexibly configured to act as a single-user system by using a unitary precoding matrix  1505  and sending the same signal to all RF chains  1510 . In this case, all the antennas  1515  in the system are used to get a beamforming gain of 10*log 10(N*K) by sending the same signal to all precoders  1520  and by using a unitary precoder  1505  of size K×K, where K is the number of RF chains  1510 . 
       FIG. 16  illustrates beamforming gain according to embodiments of the present disclosure. In the system shown in  FIG. 16 , the transmitter  1600  employs a single-user multiple-streams configuration according to embodiments of the present disclosure. The embodiment of the transmitter  1600  system and beamforming gain shown in  FIG. 16  is for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     In certain embodiments, multiple streams can be sent to a single user as shown in  FIG. 16 . In certain embodiments, the transmitter  1600  can transmit a combination of multiple streams to a single user as well as spatially multiplexing the signals several users. 
       FIGS. 17A and 17B  illustrate antenna array types according to embodiments of the present disclosure. The embodiments of the array types shown in  FIGS. 17A and 17B  are for illustration only. Other embodiments could be used without departing from the scope of this disclosure. 
     In certain embodiments, as illustrated herein above, the antenna array is configured as a uniform linear array  1705 . The uniform linear array  1705  includes N antennas  1710  per RF chain  1715  per user  1720 . In certain embodiment, the antenna array is configured as another array structures such as a 2-D planar array  1725 . 
     Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.