Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/7097—Interference-related aspects
  • H04B1/7103—Interference-related aspects the interference being multiple access interference
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J11/00—Orthogonal multiplex systems, e.g. using WALSH codes
  • H04J11/0023—Interference mitigation or co-ordination
  • H04J11/0026—Interference mitigation or co-ordination of multi-user interference
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
  • H04L25/03006—Arrangements for removing intersymbol interference
  • H04L25/03178—Arrangements involving sequence estimation techniques
  • H04L25/03305—Joint sequence estimation and interference removal
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/0001—Arrangements for dividing the transmission path
  • H04L5/0003—Two-dimensional division
  • H04L5/0005—Time-frequency
  • H04L5/0007—Time-frequency the frequencies being orthogonal, e.g. OFDM(A) or DMT
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/003—Arrangements for allocating sub-channels of the transmission path
  • H04L5/0058—Allocation criteria
  • H04L5/006—Quality of the received signal, e.g. BER, SNR, water filling
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/003—Arrangements for allocating sub-channels of the transmission path
  • H04L5/0078—Timing of allocation
  • H04L5/0085—Timing of allocation when channel conditions change
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W36/00—Hand-off or reselection arrangements
  • H04W36/06—Reselecting a communication resource in the serving access point
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W74/00—Wireless channel access
  • H04W74/08—Non-scheduled access, e.g. ALOHA
  • H04W74/0833—Random access procedures, e.g. with 4-step access
  • H04W74/0841—Random access procedures, e.g. with 4-step access with collision treatment
  • H04W74/085—Random access procedures, e.g. with 4-step access with collision treatment collision avoidance
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B2201/00—Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
  • H04B2201/69—Orthogonal indexing scheme relating to spread spectrum techniques in general
  • H04B2201/707—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
  • H04B2201/7097—Direct sequence modulation interference
  • H04B2201/709709—Methods of preventing interference
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
  • H04L25/03006—Arrangements for removing intersymbol interference
  • H04L2025/0335—Arrangements for removing intersymbol interference characterised by the type of transmission
  • H04L2025/03375—Passband transmission
  • H04L2025/03414—Multicarrier
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
  • H04L25/03006—Arrangements for removing intersymbol interference
  • H04L2025/0335—Arrangements for removing intersymbol interference characterised by the type of transmission
  • H04L2025/03426—Arrangements for removing intersymbol interference characterised by the type of transmission transmission using multiple-input and multiple-output channels
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/0001—Arrangements for dividing the transmission path
  • H04L5/0014—Three-dimensional division
  • H04L5/0016—Time-frequency-code
  • H04L5/0021—Time-frequency-code in which codes are applied as a frequency-domain sequences, e.g. MC-CDMA
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W16/00—Network planning, e.g. coverage or traffic planning tools; Network deployment, e.g. resource partitioning or cells structures
  • H04W16/14—Spectrum sharing arrangements between different networks
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W24/00—Supervisory, monitoring or testing arrangements
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W80/00—Wireless network protocols or protocol adaptations to wireless operation
  • H04W80/02—Data link layer protocols
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W84/00—Network topologies
  • H04W84/02—Hierarchically pre-organised networks, e.g. paging networks, cellular networks, WLAN [Wireless Local Area Network] or WLL [Wireless Local Loop]
  • H04W84/10—Small scale networks; Flat hierarchical networks
  • H04W84/12—WLAN [Wireless Local Area Networks]

Definitions

• Mulitcarrier Code-Division Multiple Access has recently gained significant attention and has become a promising candidate for future wireless high capacity communication networks.
• multicarrier techniques are robust against multipath fading; provide high spectral efficiency, and interference rejection capabilities.
• MC-CDMA has several other advantages, such as spectral diversity and immunity against frequency selective fading and impulse noise.
• each symbol 1, 2, 3, 4 of a MC-CDMA data stream of one user is multiplied by each element of the same spreading code and is then placed in several narrow band subcarriers 6a, 6b, 6c, 6d. Multiple chips 8 are not sequential, but instead transmitted in parallel on different subcarriers (e.g. 6a).
• MC-CDMA one single data symbol is spread in frequency.
• SF spreading factor
• a station that is ready to transmit 20 has to select a code- channel (CChI, CCh2, CCh3, CCh4). For this code channel selection two methods are possible.
• the first is to select a code-channel before every packet transmission. Initially this selection is done randomly. For later transmissions, the station does not select code- channels that have already been reserved by other stations (according to the standard the considered station has set a Network Allocation Vector (NAV) for an occupied channel).
• NAV Network Allocation Vector
• the second method of code channel selection comprises of selecting the code-channel with the least traffic and keeping this code-channel for the entire duration of the connection.
• Figure 2 is an example of an RTS/CTS access mechanism. Before accessing a medium, a station should detect whether the medium is idle for a time duration called a Distributed Inter-Frame Space (DIFS). Once the station detects that the medium is idle, the station signals the intended data transfer by transmitting a RTS packet 22, as shown.
• DIFS Distributed Inter-Frame Space
• AU stations that receive the RTS control packet, and are not the intended receivers e.g. STA 3 and STA4
• set their NAV timer 24 interrupt their backoff down counts, and defer from the medium in order not to interfere with the transmission.
• the receiver e.g. STA2
• SIFS Short Inter-Frame Space
• the receiver of the RTS is busy the RTS transmission is repeated after a new backoff.
• Mobile stations which receive the CTS set their NAV timer as well.
• the sender 20 can now transmit its data packet 32 after the SIFS 30.
• the receiver e.g.
• STA2 acknowledges a successful reception by an Acknowledgement (ACK) 34 after a SIFS 36 time after the end of the data frame 32.
• DCF Distributed Coordination Function
• MUD Multi-User Detector
• the demodulator outputs ym are multiplied with a decision variable wm, which is used for optimizing the decision of the detector on the transmitted symbol and mitigate the effects of the channel.
• wm decision variable
• the optimum weight matrix for a given set of delays ⁇ K and fading parameters ⁇ km is selected to minimize the mean square error of the detector:
• a MUD is not only applied in many CDMA systems, but may also be applied in other systems such as, for example, in MIMO systems. Further information regarding the background of the invention may be found in the following references which are hereby incorporated by reference as needed: G. Orfanos, J. Habetha, L Liu, "MC-CDMA based IEEE 802.11 wireless LAN,” Proc. IEEE MASCOTS 2004, Oct. 2004; J. Proakis, Digital Communications. McGraw-Hill, Singapore, 4th Ed., 2001; S. Hara, R. Prasad, "Overview of multicarrier CDMA," IEEE Comm. Magazine, vol. 35, issue 4, pp. 104-108, April 1997; B. Walke, Mobile Radio Networks.
• Embodiments of the invention include a method of dynamically deciding to transmit in parallel with current transmissions or selecting another frequency channel in a communications network including a plurality of devices including a sender of a data stream indicating at least its current transmit power to the receiver(s) of the stream, a receiver of the stream of data packets calculating the mean interference level of at least a single previous data packet by means of a Multi-User Detector, the receiver calculating the maximum achievable SINR according to its interference situation, the maximum allowed transmission power and the pathloss from the sender to the receiver, the receiver decides for a frequency change if the maximum achievable SINR calculated above is smaller than the intended for the transmission, the receiver performing the change to another frequency channel with a certain probability P, in order to avoid interfering stations to change the channel in parallel; or transmitting in parallel with current transmissions with a probability 1-P, the receiver notifying the sender of the frequency change by using a small value in the duration field of the frame; and the receiver encoding the frequency channel id in the
• an embodiment of the invention includes a sender and receiver that use a packet, prior to the data frame and/or the data frame itself, to exchange information about transmit power and interference situation at the current station, the signaling packets are a Ready-to-Send (RTS) and a Clear-to-Send (CTS) packet according to the standard IEEE 802.11 or equivalent packets according to later revisions of the standard (like e.g.
• RTS Ready-to-Send
• CTS Clear-to-Send
• the probability P of performing the frequency change is raising with the amount of packets triggering the frequency change procedure at the receiver or the sender, and the communications network is using Multicarrier Code Division Multiple Access (MC-CDMA), Multiple Input Multiple Output (MIMO), Orthogonal Frequency Division Multiple Access (OFDMA) technology, a Time Division Multiple Access (TDMA), a Frequency Division Multiple Access (FDMA), or a combination thereof.
• MC-CDMA Multicarrier Code Division Multiple Access
• MIMO Multiple Input Multiple Output
• OFDMA Orthogonal Frequency Division Multiple Access
• TDMA Time Division Multiple Access
• FDMA Frequency Division Multiple Access
• FIGURE 1 is MC-CDMA system with a spreading factor (SF) of 4 that may include embodiments of the present invention
• FIGURE 2 is a MC-CDMA based MAC protocol with a RTS/CTS access mechanism that may include embodiments of the present invention
• FIGURE 3 is a block diagram of a MMSE receiver that may include embodiments of the present invention.
• FIGURE 4 is an exemplary MAC RTS MAC frame format according to an embodiment of the invention.
• FIGURE 5 is an exemplary MAC CTS MAC frame format according to an embodiment of the invention
• FIGURE 6 is a flow chart and exemplary algorithm of an exemplary frequency adaptation in accordance with an embodiment of the invention.
• Code division multiple access (CDMA) networks suffer from near-far-effects. Near- far-effects occur when, for example, a receiver A is blocked by a near transmitter B if both operate at the same time in the same frequency channels, using different spreading codes.
• Embodiments of the present invention provide a mechanism or means to control the amount of parallel transmissions in a MC-CDMA WLAN in order to avoid near-far-effects. By providing a mechanism or means to control parallel transmissions of different stations in a MC-CDMA WLAN the performance of the communication network is improved thus providing a more efficient use of the transmission channels, and results in a higher throughput and less delays of signals.
• the embodiments of the present invention determine the interference level and thereby the SINR by means of a MUD.
• the interference level and/or the SINR is used by the stations to determine whether they should divert their communication link to another frequency channel, or continue in the same frequency channel, in parallel to the current links.
• the sender and receiver explicitly exchange information about transmit power using frames like RTS and CTS.
• the probability P of performing the frequency change increases with the amount of packets triggering the frequency change.
• a received signal in an asynchronous MC-CDMA system can be described by the following equation:
• the Rayleigh fading process for the m-th subcarrier and k-th user is represented as:
• SINR at the MUD detector can be given from the following expression:
• the interference during the last received frame is weighted with 50% since the last received frame is closest to an actual value. Using this weighting, channel changes due to a single packet having deep fading can be avoided. Furthermore, the algorithm is fast enough to overcome the near-far-effect problem.
• the weight factor of 50% is an example value that might be adapted to different operating conditions of the WLAN.
• RTS 50 and CTS 51 frames are extended (beyond the IEEE 802.1 Ia standard) with two additional fields, TxP ⁇ w 52 and Ifi > ow54, as depicted in Figure 4 and Figure 5.
• Figure 4 is an extended RTS frame 50.
• Figure 5 is an extended CTS frame 51.
• TxPow 52 the transmit power of the current frame is encoded and IfPow 54 carries information about the last estimate of mean interference for this station on the channel on which the data transfer takes place.
• the length of each field consists of one byte.
• Table 1 Parameters for the adaptation algorithm
• Figure 6 is a frequency adaptation algorithm or flowchart 60 in accordance with an embodiment of the present invention.
• Figure 6 provides an overview of the frequency adaptation algorithm, which is based on the RTS-CTS-Data-Ack transmission cycle.
• Station 1 (Sl) 62 denotes the transmitter and station 2 (S2) 64 the corresponding receiver.
• Further variables needed for the algorithm are defined in Table 1.
• AU values are given in dB.
• Station Sl transmits an RTS frame 66, using the extended frame format of Figure 4.
• the current values of P ⁇ and Pj. 1 are set and sent.
• Station S2 receives the RTS frame with power P ⁇ s and decodes the values of P ⁇ and P ⁇ .
• S2 can now calculate the pathloss L between Sl and S2:
• S2 calculates the maximum possible SINR for packet receptions under consideration of its actual mean interference estimate P ⁇ 2 , the pathloss L and the maximum allowed transmission power: m aSx Imfta R - j P X L 7 P F S2 (7)
• Station S2 can now decide whether the packet transfer can take place under the current interference situation of S2 (P 1 / ).
• m aStf MtRiSnI N R where min 57NJ? denotes a set threshold that is needed, according to the used PHY- mode, and the QoS target of the link, for example in the form of a maximum tolerable Packet Error Rate (PER).
• PER Packet Error Rate
• S2 64 initializes a frequency change with a certain probability.
• the extended CTS frame 72 is send to Sl 62 using and the robust BPSK 1 A PHY-mode.
• the fields P ⁇ and Pf/ are filled with the current values of S2.
• the Duration field in the CTS frame is used to denote the duration of the data transfer after the end of the CTS frame in microseconds.
• S2 64 can use the duration field to denote a frequency change and to encode the new frequency channel id, since the small values in duration field are not used by the standard.
• S2 64 After the transmission of the CTS frame 72, S2 64 changes to the new frequency channel 74 and resets all its transmission relevant parameters: lfi tp rf P x (S) P? F 2 ⁇ o i ⁇ 4 -Bi m (9)
• the probabilistic change of the frequency channel is needed in order to prohibit unnecessary frequency changes and collisions due to near-far-effects in the new frequency channel.
• situations can occur wherein more than one station's receiver is blocked by transmissions from other stations or wherein stations are blocking transmissions of each other. If a link between stations is diverted to another frequency channel, the interference situation is changed immediately, and it might then be possible for all or most of the receivers to work free of near-far-effects. If this is not the case further frequency changes can be initiated.
• the above probabilistic change may be implemented as follows: Each station that calculates m aSrf MiRi S ⁇ I N R changes its frequency channel with a certain probability, which in this exemplary embodiment is set to 25%. The selection of the new frequency channel is randomly determined from the set of the system's other frequency channels.
• Sl 62 receives the CTS frame with Rx-Power PTM and decodes from the frame the values of P ⁇ and P 1 ⁇ . Accordingly Sl calculates the pathloss between Sl and
• Sl compares the above calculated value of max 57NJ? with min 57NJ? to decide about a frequency change. This operation is not needed, since the information about the frequency change has already been enclosed in the Duration field of the extended CTS frame by S2, but it provides another control mechanism for the algorithm.
• Sl 62 changes to the new frequency channel 78 and resets its P ⁇ and Pj. 1 parameters according to eq. (7) and (8). Afterwards, Sl 62 initiates a new transmission with a RTS packet in the new channel.
• the data transfer 80 can take place in the current frequency channel.
• S2 64 sends an extended CTS frame 72, as shown in Fig. 2, with the actual values of P ⁇ and Pf/ .
• the Duration field contains the duration of the data transfer duration after the end of the CTS frame, as described in the standard.
• Sl 62 will transmit the actual data packet 80 according to the protocol.
• the RTS retransmit timer of the MAC protocol of Sl will reach zero and Sl 62 will start a retransmission attempt with a new RTS packet by previously increasing the CW by a factor of two.
• the received signal at the ra-th subcarrier can be expressed as :
• the demodulator outputs y n are multiplied with a decision variable w n , which is used for optimizing the decision of the detector on the transmitted symbol and mitigate the effects of the channel:
• embodiments of the invention provide a method for a station in a MC- CDMA based WLAN or other CDMA based wireless system, to dynamically decide to transmit in parallel with current transmissions or to select another frequency channel in the communication system.
• Embodiments of the invention are well suited for wireless systems based on CDMA using a MUD.
• an estimate of the interference of signals on the same frequency channel is determined using the MUD.
• Stations can decide, in a distributed manner, if they should divert their communication link to another frequency channel, or to continue in the same frequency channel in parallel to the current communication links in the same frequency channel.
• Embodiments of the present invention can be applied to other systems with parallel streams of communications, such as Multiple Input Multiple Output (MIMO) and Orthogonal Frequency Division Multiple Access (OFDMA) systems.
• MIMO Multiple Input Multiple Output
• OFDMA Orthogonal Frequency Division Multiple Access
• the interference estimate of embodiments of the inventions does not have to be provided by a MUD, but instead may be calculated by channel measurements made while a station or mobile stations is in an idle state.
• an exemplary protocol for implementing embodiments of the invention may be based on the Medium Access Control (MAC) protocol of the IEEE 802.1 Ia WLAN standard with some modifications needed to support an exemplary CDMA Physical Layer (PHY Layer).
• MAC Medium Access Control
• PHY Layer Physical Layer

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