Abstract:
In a cellular network, randomness is introduced into a transmitted signal at each transmitter, and the resulting received signal, which is the sum of all received signals, fluctuates more in time than a signal transmitted without the introduced randomness. While the introduction of randomness can diminish reception of some parts of the signal at the receiver, the transmitted signal can be encoded using forward error correction coding techniques, which allows the receiver to recover all of the signal information despite some diminished reception. Such randomization provides time diversity so that receivers can have more consistent performance. For broadcasted data, where users with the worst channel condition dictates the overall performance, having consistent performance across users can improve the overall network performance.

Description:
TECHNICAL FIELD 
     This disclosure relates to modulating signals that are broadcasted from multiple transmitters. 
     BACKGROUND 
     In a cellular communication system, a geographic area is divided into multiple cells  10 ,  12 ,  14  as shown in  FIG. 1 . At the center of each cell  10 , a base station  22  is located to serve mobile stations  28 ,  30  in the cell. Having small, multiple cells help increase coverage and capacity. Each cell can be further divided into sectors  16 ,  18 ,  20  by using multiple sectorized antennas to further increase capacity. Typically three sectors per cell are used. (Both conventionally and in this document, the term “sector” applies even when there is only one sector in a cell). In each cell, a base station serves one or more sectors and communicates with multiple mobile stations  28 ,  30  in the cell. The communication between the base stations  22 ,  24 ,  26  and the mobile stations uses analog modulation (such as analog voice) or digital modulation (such as digital voice or digital packet data) to transmit and receive such analog or digital information. The forward link or downlink refers to the direction of communication from the base station to the mobile station. The other direction, i.e., the direction of communication from the mobile to the base station, is called the reverse link or the uplink. 
     A certain amount of bandwidth (spectrum) is used for such communication between the base station and the mobile station. Two separate spectrums can be allocated for the forward and reverse links as in the frequency division duplexing (FDD) scheme or one spectrum can be multiplexed in time to carry traffic in both directions as in a time division duplexing (TDD) scheme. The minimum unit of bandwidth needed in a cellular wireless system can be referred to as a carrier. As the amount of data traffic is increased, the number of carriers can be increased to provide more capacity. A carrier in a sector can handle up to a certain amount of data traffic, which is referred to as the capacity per carrier per sector or simply capacity. In general, the capacity is different in the forward and in the reverse links. 
     A wireless communication link between a transmitter and a receiver can be categorized as line-of-sight (LOS) or non-line-of-sight (NLOS). LOS refers to the case when the receiving antenna sees the transmitter directly, i.e., there is a direct path between the two. In NLOS environment, the transmitting and the receiving antennas do not see each other directly, but have multiple different paths over which radio waves can travel due to reflection from objects such as buildings and trees. In a perfect LOS environment, as in free space, there will be no fading. As the number of objects obstructing a direct path between the transmitter and receiver (called “scatterers”) increases, fading increases due to interference from multiple paths. 
     When a transmitter transmits a signal such as a sinusoidal waveform, the resulting received signal is the sum of multiple copies of the same signal. However, each copy can have different amplitude and phase depending on the characteristic of the corresponding path, where amplitude typically decreases as the length of the path increases and the phase increases linearly in path length. In addition, different materials have different reflection and absorption characteristics that effect amplitude and phase differently. Thus, these multiple paths can create multiple copies which constructively and/or destructively interfere with each other at a receiver. 
     If there are many scatterers that reflect waves, the resulting signal fading can be modeled as Rayleigh fading where the envelope of the received signal follows a complex Gaussian distribution when the transmitted signal is a pure sinusoid. This technique can be useful in modeling a scattering environment in which there are multiple paths without any dominating path because the sum of independent, identically distributed channel gains tend to have a Gaussian-like distribution as there are many paths. This is a good model of many NLOS environment. However, often real-world RF channels have a mixture of LOS and NLOS components. In this case, fading can be modeled as Rician or Nakagami fading. Rician fading models a mixture of LOS and Rayleigh fading paths. Nakagami fading can model fading that is more severe or less severe than Rayleigh fading. 
     Multi-path fading can also distort the signal if the delay spread is not negligibly small compared to the symbol duration since the fading becomes frequency selective in such an environment. If the delay spread is negligibly small compared to the symbol duration, the fading becomes flat in frequency and is called the flat fading. Various known equalization techniques can be used to remove inter-symbol interference caused by the frequency-selective fading. 
     Fading can change over time when the transmitter, receiver, or some surrounding objects move. Since the phase of each path can change 360 degrees when the path length changes by the wavelength, the fading can change in a very small scale. For example, fading can change from constructive to destructive when the receiver moves merely a fraction of the wavelength. In 2GHz Personal Communications Service (PCS) band, for example, this corresponds to only 1 or 2 inches. Therefore, in a mobile communication system, the mobility of the terminal can cause the fading characteristics to change quickly. For example, a mobile terminal moving at a pedestrian speed in the PCS band will see the channel fading characteristics change at a rate of a few times a second. However, if the mobile terminal is in a moving vehicle, the channel fading characteristics can change as often as a few hundred times a second. Even when there is no mobility of a terminal, fading can change over time if there are moving objects in the paths. In a mobile communication system, the main cause of time-varying fading is often due to movement of the terminal. For example, if a mobile terminal stops moving during signal transmission, the fading characteristics of the channel can stay constant for a long time. Accordingly, a user will likely experience bad reception if the user&#39;s mobile terminal stops moving when it is in deep fade. 
     In a cellular communication system, a downlink is a broadcast channel whereas the uplink is a multiple access channel from a single sector&#39;s perspective since a sector needs to be able to handle multiple mobile terminals. Therefore, to carry user data in the downlink, multiplexing is used. There are many forms of multiplexing, e.g., time-division multiplexing (TDM), frequency-division multiplexing (FDM), orthogonal frequency-multiplexing (OFDM) and code-division multiplexing (CDM). CDM, FDM, and static TDM (i.e., TDM where channel allocation does not change during the entire duration of a call) are typically used for low and fixed rate communication including voice. 
     Recently many efforts have been made in standardizing high speed data communication in cellular environment, which includes CDMA2000 and wide band CDMA (WCDMA). For example, 1× Evolution Data Only protocol (1×EVDO) described in “CDMA2000 High Rate Packet Data Air Interface Specification,” 3GPP2 C.S0024, which is referred to in this document as “3GPP2” and is fully incorporated herein by reference, supports data rates up to 3.1 Mbps in the downlink and 1.8 Mbps in the uplink. When the data rate becomes higher and higher, it becomes more important to use an efficient multiplexing scheme. An often good choice for such high speed applications is TDM because it can maximize burst throughput for each user and thus minimize latency. 
     Furthermore, when TDM is used, a smart scheduler in the downlink can take advantage of the time varying channel conditions to give higher scheduling priority to users whose channel condition has temporarily improved. The resulting gain from using such a smart scheduler is referred to as a “multi user diversity” gain since the gain becomes higher as there are more users. Multi user diversity gain is described in more detail in “Information capacity and power control in single-cell multi-user communications”, R. Knopp and P.A. Humblet, Proceedings of International Conference on Communications (ICC), 1995, Seattle, Wash., pp. 331-335, June 1995. 
     There are additional benefits in using TDM for downlink. One benefit is that it simplifies resource allocation since the resource is only one dimensional, i.e., time slots. In other multiplexing schemes, resource management can be a two-dimensional problem since code or frequency space also needs to be shared among users. This adds more complexity and tends to be less efficient than TDM. Another benefit of TDM for downlink is that it allows for dynamic scheduling of different types of contents to different users. For example, it can support mixture of unicast and broadcast services, where a unicast packet is received by a single user while a broadcast packet can be received by multiple users simultaneously. 
     Broadcasting and multicasting (BCMC) have been recently standardized in 3GPP2 and an enhanced version is being standardized in “Enhanced Broadcast and Multicast,” 3GPP2, QUALCOMM™. BCMC allows transmitting the same data from one or more sectors to mobiles. This is useful in sending, for example, TV or radio-like programs to a large number of mobile terminals in a large geographic area. In BCMC, no feedback channel is available on the channel condition. Therefore, adaptive modulation is not possible and the modulation needs to be done for the user who has the worst channel condition. Thus, broadcast typically does not perform as well as unicast in terms of total throughput. However, it is possible to improve the broadcast performance by taking advantage of the fact that multiple sectors can transmit the same information. Unlike in the unicast where each sector transmitter transmits different signals thus creating inter-sector interference to all neighbor sectors, transmitting the same signal from multiple sectors does not necessarily create such inter-user interference. However, the signals from multiple sectors are combined at the receiving antenna, thus creating self-interference. This interference can be either constructive or destructive depending on the phase of the signals. 
     SUMMARY 
     In downlink broadcasting, transmission of the same signal from multiple transmitters can create patterns of constructive and destructive interference. This can be a severe problem especially between neighboring sectors belonging to the same base station where the antennas belonging to different sectors are closely co-located. The multiple signal paths will destructively interfere in certain areas where the signal strength will be weak. These weak spots are especially problematic for stationary users since their channel conditions usually do not change much over time. Therefore the user can experience a very bad channel condition for a long time. This problem is exacerbated in a strong LOS environment. 
     In some cases, outer codes such as Reed-Solomon codes are used to correct errors caused by such weakened signal. However, this assumes the fading changes over time and the outer-code spans enough time to cover the time-varying fading. However, this may not be always the case. For example, the fading can change very slowly or may not change much as in a strong LOS case. 
     Assuming a set of packets that constitute a unit of broadcast transmission are transmitted over multiple blocks of transmission units, it can be beneficial to modulate each block with a random time-varying phase rotation pattern, where the random modulation pattern is independent from sector to sector and the phase rotation is fixed for the duration of each block. The block of transmission also contains a pilot signal, which is also modulated. For such patterns, M-sequences can be used with different offset for different sectors. Each sector is configured with a set of parameters to specify the modulation pattern. The phase rotation can be one of N equally spaced points on the unit circle in the complex plane, where N is an integer&gt;=2. When multiple transmitters are transmitting the same broadcast contents, the self-interference pattern will vary randomly over multiple blocks providing diversity. By also employing an outer-code such as Reed-Solomon code, the original set of broadcast packets can be recovered by correcting errors caused by such interference. By making self-interference time varying, the system helps to prevent persistent destructive interference. This is because if the time span of the coding block of the outer code is much greater than the time scale of the modulated pattern, then the performance after the error correction by the outer code is dictated mostly by the average channel condition during the time span. In cases where it is not easy to design a single outer code with a wide time span, we can use multiple outer codes interleaved in time to get similar benefit. 
     Modulation of a broadcast transmission can also be more generally accomplished using a randomly time-varying complex number, where the random pattern is independent between sectors. Special cases include the complex number being a real number or the complex number being zero or a constant. 
     By introducing the randomness in broadcast transmission, coverage is typically increased over a system without introduction of time-varying self interference, i.e., more users can receive broadcast packets with higher quality, i.e., less packet errors. 
     In one aspect, the invention features a method for transmitting a sequence of symbols that includes generating a first sequence of symbols, sending the generated sequence to multiple transmitters, encoding the sequence at each transmitter, multiplying the encoded sequence by a second sequence of symbols at each transmitter, wherein the second sequence of symbols is different at each transmitter, modulating the resulting multiplied sequence, and transmitting the modulated symbols at each transmitter. 
     Embodiments may include one or more of the following features. The encoding step may include the application outer forward error correction coding technique (e.g., convolutional coding or block coding such as Reed-Solomon coding) to the sequence of symbols. This resulting encoded sequence of symbols may be interleaved and then encoded again by applying an inner forward correction coding technique (e.g., block coding, convolutional coding, turbo coding, or low-density parity-check coding). 
     Prior to multiplying the first and second sequence, the method may also include applying an inverse Fourier transform function on the encoded data and appending a cyclic prefix to the encoded data. 
     The second sequence may be a pseudo-random sequence (e.g., an M-sequence) generated at each transmitter. This pseudorandom sequence may be a sequence of complex numbers or a real numbers. In a sequence of complex numbers, each of the complex numbers may have the same amplitude (e.g., an amplitude equal to one). The sequence of complex numbers may be a sequence of N values at N equally spaced points on a unit circle. Each transmitter may generate its second sequence based on pre-configured information (e.g., pseudorandom noise offset for the sector) stored at the transmitter. 
     The modulation step may include modulating the sequence using quadrature amplitude modulation (QAM), binary phase shift keying (BPSK), or quadrature phase shift keying (QPSK). 
     The transmitters may transmit the modulated sequence using time division multiplexing to multiplex a second signal with the modulated symbols transmitted at each transmitter. The second signal may be a pilot signal or a unicast signal. 
     In another aspect, the invention features a method for simultaneously broadcasting blocks of symbols from a first and second transmitter in a cellular system that includes rotating a phase angle of each block of symbols according to a first pattern to produce a first sequence of rotated blocks of symbols and transmitting the first sequence of rotated blocks of symbols from a first transmitter. The method also includes rotating a phase angle of each block of symbols according to a second pattern to produce a second sequence of rotated blocks of symbols, and transmitting the second sequence of rotated blocks of symbols from the second transmitter at the same time the first sequence is being transmitted from the first transmitter. 
     Embodiments may include one or more of the following features. The first and second patterns may each comprise a series of complex numbers that vary with time. The first and second patterns may be each a series of pseudorandom numbers. The step of rotating a phase angle may include multiplying a block of data with the complex number. 
     In another aspect, the invention features a transmitter for broadcasting a sequence of symbols in a cellular network comprising multiple transmitters. The transmitter includes an encoder for encoding the sequence of symbols, a multiplier configured to multiply the encoded sequence by a second sequence of symbols (where the second sequence of symbols is determined according to a pattern that is different from patterns used by other transmitters in the network), a modulator configured to modulate the multiplied sequence, and an antenna for transmitting the modulated sequence. 
     Various embodiments may include one or more of the following features. The encoder may include an outer encoder configured to apply an outer forward error correction code to the sequence of symbols and an inner encoder configured to apply an inner forward error correction code to the sequence of symbols. The apparatus may also include an interleaver circuit configured to interleave the encoded sequence of symbols. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram of an example of a cellular system. 
         FIG. 2  is a diagram of a TDM structure within a time slot. 
         FIG. 3  is a diagram of unicast and broadcast data that have been time-division multiplexed. 
         FIG. 4  is a diagram of a CDM symbol within a data burst. 
         FIG. 5  is a diagram of an OFDM structure for a data burst. 
         FIG. 6  is a diagram of functional components of a transmitter. 
         FIG. 7  is a diagram of an example of a cellular system in accordance with or more embodiments of the invention and including, e.g.,  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     In a cellular communication system, a geographic area is divided into multiple cells  310 ,  312 ,  314  as shown in  FIG. 7 .  FIG. 7  is a diagram of an example of a cellular system in accordance with or more embodiments of the invention and including, e.g.,  FIG. 6 . At the center of each cell  310 , a base station  322  is located to serve mobile stations  328 ,  330  in the cell. Having small, multiple cells help increase coverage and capacity. Each cell can be further divided into sectors  316 ,  318 ,  320  by using multiple sectorized antennas to further increase capacity. Typically three sectors per cell are used. (Both conventionally and in this document, the term “sector” applies even when there is only one sector in a cell). In each cell, a base station serves one or more sectors and communicates with multiple mobile stations  328 ,  330  in the cell. The communication between the base stations  322 ,  324 ,  326  and the mobile stations uses analog modulation (such as analog voice) or digital modulation (such as digital voice or digital packet data) to transmit and receive such analog or digital information. The forward link or downlink refers to the direction of communication from the base station to the mobile station. The other direction, i.e., the direction of communication from the mobile to the base station, is called the reverse link or the uplink. 
     In a TDM scheme, a time slot is the minimum transmission unit for data. For various purposes, it is sometimes desirable to transmit other information in addition to payload data in the same time slot. For example, a pilot signal can be transmitted to aid the mobile&#39;s estimation of the channel.  FIG. 2  shows an example of TDM structure within a time slot, where payload data  120 ,  124  and pilot data  122  are time-division multiplexed within the same time slot. As shown, the payload data is spread into two parts  120  and  124  in this example. 
     Referring to  FIG. 3 , a series of five time slots includes some time slots that are used for unicasting data  100 ,  104  and others that are used for transmitting broadcast data  102 ,  106 ,  108 . In one implementation, a system modulates each block of broadcast data  102 ,  106 ,  108  with a random time-varying phase rotation, where different random modulation patterns are used among the sectors in the system and the phase rotation is fixed for the duration of each block. 
     Referring to  FIG. 6 , a transmitter includes an outer encoder  200 , an interleaver  202 , an inner encoder  204 , a mapping function f( )  206 , a rotation function  208 , a modulator  210  and an amplifier  212 . A broadcast signal, X(t) (which is a sequence of T broadcast symbols, where t is the time index and where it is assumed that t is between 0 and T−1) is encoded by the outer encoder  200 . In one particular implementation, block codes such as Reed-Solomon codes are used by the outer encoder to encode X(t). The purpose of the outer coder is to correct errors not corrected by the inner coding  204 . For example, when there is time-varying fading some symbols are prone to more errors. Outer coding can correct some of the errors. It should be noted that not all T symbols need to be encoded together. In one implementation, T incoming symbols are divided into K equal-sized subsets and outer encoding is done for each subset. For example, {X(0), . . . X(T/K−1)} is first encoded by the outer coder and then {X(T/K), . . . ,X(2*T/K−1)} is encoded. In general, the subsets can have different sizes. The output of the outer coder, Y(t) now has T′ symbols, where T′&gt;=T. T′−T additional symbols are added by the outer encoder to give redundancy. Thus, the first subset of input symbols {X(0), . . . X(T/K−1)} is encoded to {Y(0), . . . ,Y(T′/K−1)} and the second subset of input symbols {X(T/K), . . . X(2*T/K−1)} is encoded to {Y(T′), . . . ,Y(2*T′/K−1)} and so on. For simplicity, this document will denote T′/K as A. 
     The outer encoder&#39;s output, Y(t), is interleaved by the interleaver  202 . The interleaver  202  reorders Y(t) before applying the inner encoder  204  and generates output Y′(t). In one implementation, interleaving is done such that no symbols belonging to the same outer-encoding block belong to the same block of the inner coding. 
     The inner encoder  204  then encodes groups of symbols, where each group has L symbols and where L is a fraction of K. For example, if L=K/2, then the inner encoder first encodes {Y(0), Y(A), . . . , Y((L−1)*A)} and produces {Z(0), Z(1), . . . , Z(L′−1)}, where L′&gt;L and L′−L is the redundancy introduced by the inner coder. In one implementation, the inner-encoder employs convolutional or turbo codes, which encodes {Y(1), Y(A+1), . . . , Y((L−1)*A+1)} as {Z(L′), Z(L′+1), . . . , Z(2*L′−1)}. Encoding is continued until {Y(A−1), . . . Y((L−1)*A+A−1)} is encoded. After encoding the first group, the inner encoder encodes the next group to encode {Y(T′/2), Y(T′/2+A), . . . , Y(T′/2+(L−1)*A)} as {Z(T′/2), Z(T′), . . . , Z((T′−1)*(L−1))} and so on. The output of the inner encoder, Z(t), has now B=L′*T′/L symbols. 
     The output symbols Z(t) are applied to a mapping function f( )  206  that maps {Z(0), . . . , Z(B−1)} into {U(0), . . . , U(C−1)}, where C is the number of symbols in U(t). The mapping function f( ) can be an interleaver, an OFDM modulator, or a combination of the two. In another implementation other invertible mapping functions, such as a Walsh-Hadamard transform function, are used in lieu of OFDM. As shown in  FIG. 4 , a CDM symbol composed of N codes can be transmitted within a data burst, where a data burst can contain multiple CDM symbols. If OFDM is used, then it is done such that the input sequence Z(t) is divided into one or more subsets of equal size and the inverse fast Fourier transform (FFT) is performed for each subset. Cyclic prefix is then appended for each subset. The OFDM symbols are serialized and become U(t). Due to cyclic prefix, the number of output symbols is generally greater than that of the input symbols. Interleaving can be applied either before or after OFDM (or both) for randomization. When OFDM is used, some frequency bins can be used to transmit known pilot signals to enable the receiver estimate the channel. U(t) is then divided into D groups of equal size and each group of symbols are transmitted within a single time slot. Therefore, it takes total of D time slots to transmit the original T input symbols. It is preferable to set D to have a value of 10 or greater in order for error correction by the outer coder work properly. 
       FIG. 5  illustrates how OFDM symbols can be organized in a data burst within a time slot, where {F(1,1), . . . ,F(1,N)} are the output of the first inverse fast Fourier transform (IFFT) block and {F(2,1), . . . ,F(2,N)} are the output of the second IFFT block, and so on. Between two IFFT blocks, cyclic prefix can be inserted (not shown in  FIG. 5 ). 
     The description so far applied to one sector. When multiple sectors broadcast the same data simultaneously, they all receive the same input X(t) from a single source and perform the same operations including outer coding  200 , interleaving  202 , inner encoding  204 , and the mapping f( )  206 . However, each sector uses a different pattern for rotating  208  the sequence of symbols in each time slot, where the pattern is stored in each sector. In one embodiment, each sector can use a pre-stored information to generate the pattern. For example, pseudorandom noise offset (PN offset) of CDMA can be used as the pre-stored information. In one implementation, rotation is done per time slot, i.e., V(t)=U(t)*R(t), where R(t) is a complex number with the unit amplitude and R(t) is the same for all symbols in a time slot. More generally, rotation can be done per group of time slots. The sequence {R(t)|t&gt;=0} is referred to as the rotation pattern. In one implementation, each sector uses a unique pattern for R(t). Other implementations may reuse one or more of the patterns if the two sectors are separated far enough apart that it is unlikely that a mobile terminal will receive signals from both of them simultaneously. 
     The rotated symbols V(t) are modulated  210  and amplified  212  and then transmitted through an antenna. Modulation  210  can be performed using QAM, BPSK, or QPSK. If the data does not contain a pilot signal, a pilot signal may be time division multiplexed with the data (as shown in  FIG. 2 ) and rotated in the same way as the data. In another embodiment, the pilot signal may be frequency division multiplexed with data or it can be sent using a group of OFDM symbols. 
     In one embodiment, the rotation angles are quantized. For example, quantization can be one of the points in 4-PSK (phase shift keying), or 8-PSK. More generally, they can be a set of points equally spaced on the unit circle in the complex plane. For the rotation pattern, a pseudo-random sequence, such as a maximum length sequence (M-sequence) , can be used to generate a sequence of random numbers, i.e., t bits are generated by the random number generator in each time slot and they choose one of 2 t  points in 2 t -PSK modulation points. If some time slots are not used for broadcast, then random number generation can be temporarily suspended during the time slot. Different sectors can use the same M-sequence with different time offset. 
     When a mobile terminal receives the broadcast signal from multiple sectors, its received signal is the sum of all transmitted signals (each signal experiences different channel gain). The mobile terminal does not know which signal is from which sector, but it only sees an aggregated signal. By using the aggregated pilot signal received at the antenna, which is the sum of multiple received pilot signals, the mobile terminal estimates the channel in each time slot. Using the channel estimation information provided by the pilot, the mobile terminal can coherently detect data symbols in each time slot. Rotation does not affect the mobile terminal&#39;s detection operation since the rotation stays the same for the whole duration of the time slot. 
     While the above example uses complex number of unit amplitude to rotate the symbols, it should be understood that any sequence of complex numbers can be used to rotate the sequence of the symbols in each time slot. 
     By randomly rotating the signal, a mobile terminal is likely to see more fluctuation in the combined received signal, which can be better than having a poor signal quality for a long time (e.g., in the case of a stationary terminal) due to destructive interference. Moreover, since outer coding is used, errors due to such fluctuation in the combined received signal can be corrected using known techniques. By modulating broadcast signals differently in different transmitters, a system achieves a more even user throughput than in a system without such modulation. In broadcast transmissions, where the encoding rate is governed by the user who has the worst channel condition, a more even user throughput can be particularly advantageous. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention, and, accordingly, other embodiments are within the scope of the following claims.